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Shallow DOF in 50 mm affected by 28 mm lens?
Old 04-18-2016   #1
Daneinbalto
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Shallow DOF in 50 mm affected by 28 mm lens?

From what is said, shallow depth of field is less easily obtained on a 28 mm lens than a 50 mm lens. But what happens to DOF if you apply a 50 mm crop to a shot captured with a 28 mm lens? If you compare two shots taken of the same subject, at the same distance, with the same f/stop, one with a native 50 mm lens and the other with a 28 mm lens but cropped to correspond to the 50 mm shot, will the DOF be different? Someone seemed to think the depth of field would be greater in the latter situation because the shot originated from a 28 mm lens.
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Old 04-18-2016   #2
benmacphoto
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I believe the DOF will be different if you take a shot with a 28mm lens and crop it to correspond to a 50mm lens shot with the same f stop and same distance.
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Old 04-18-2016   #3
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Think about it this way: DOF isn't mysterious, it's a function of the size of the aperture as seen by the subject. Taking in rays coming from different angles, therefore bluring the background, that produces the OOF in the background.

So, your 28mm lens at f/2 has a 14mm aperture. Make the shot and crop, it still has a 14mm aperture. At half size, that will be like a 50mm lens at f/4.

Your 50mm lens at f/2 has a 25mm aperture. Maybe not exactly, but close enough.

So, the cropped image from the 28mm f/2 will have more DOF than the 50mm at f/2.

Does that help?

More: Depth of Field is subjective, depends on lots of things, it's not just a technical consideration.
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Old 04-18-2016   #4
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http://toothwalker.org/optics/dof.html

read this!
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Old 04-18-2016   #5
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Quote:
Originally Posted by ColSebastianMoran View Post

So, the cropped image from the 28mm f/2 will have more DOF than the 50mm at f/2.
This is how I understand it.

But you can take the cropped image and the uncropped image and make prints of the same size, and view them from the same distance, then the perceived depth of field from the cropped image shrinks. Though it may not look equivalent to the uncropped print.
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Old 04-18-2016   #6
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Quote:
Originally Posted by TXForester View Post
This is how I understand it.

But you can take the cropped image and the uncropped image and make prints of the same size, and view them from the same distance, then the perceived depth of field from the cropped image shrinks. Though it may not look equivalent to the uncropped print.
I agree with this. DOF is a zone of acceptable unsharpness. Cropping and enlarging magnifies the image that remains after cropping, and the unsharpness gets magnified along with it. So what was acceptable unsharpness in the uncropped image can become unacceptable unsharpness after cropping and magnifying.
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Old 04-18-2016   #7
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I think also the diameter of the lens opening - not the aperture but the actual size of the element on the front of the lens will affect the size of the circle of confusion co-relating to the depth of field and angle effect of the image. Correct me if I'm wrong anyone?
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Old 04-19-2016   #8
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The depth of field is determined by the lens optical characteristics. Cropping a shot taken with a 28mm lens to look like a 50mm does not change the DOF as it cannot - the DOF is something that is determined by factors such as the focal length of the lens, the f stop used, whether it was designed for a full frame or other camera, the distance to the point of focus etc. All of these are fixed at the moment the shutter button is pressed. Nothing you do with the shot afterwards changes it or possibly can change it. The image is already in the camera.

Also based on the above if you take a 28mm lens from a full frame camera and put it on a small format camera using an adapter it will have the same DOF (with minor variations) on either camera. (All the smaller camera is doing in this case is cropping the image because the sensor is smaller ( it does not change thee characteristics of the lens.)

But if you take a 28mm lens designed for a small format camera and keep all of those other factors the same it would be expected to have a different DOF from a 28mm designed for a full frame camera as optically its different. Other things being equal, DOF is determined largely by the size of the aperture being used and the fact is that lenses designed for such a camera are smaller than those designed for their larger format brothers. This results in a larger DOF at each distance, each f stop etc.
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Old 04-19-2016   #9
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Quote:
Originally Posted by teddy View Post
I think also the diameter of the lens opening - not the aperture but the actual size of the element on the front of the lens will affect the size of the circle of confusion co-relating to the depth of field and angle effect of the image. Correct me if I'm wrong anyone?
Sorry, you are wrong. It's the effective aperture versus the lens strength. The most limiting lens is usually inside.
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Old 04-19-2016   #10
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Quote:
Originally Posted by ColSebastianMoran View Post
Think about it this way: DOF isn't mysterious, it's a function of the size of the aperture as seen by the subject. Taking in rays coming from different angles, therefore bluring the background, that produces the OOF in the background.

So, your 28mm lens at f/2 has a 14mm aperture. Make the shot and crop, it still has a 14mm aperture. At half size, that will be like a 50mm lens at f/4.

Your 50mm lens at f/2 has a 25mm aperture. Maybe not exactly, but close enough.

So, the cropped image from the 28mm f/2 will have more DOF than the 50mm at f/2.

Does that help?

More: Depth of Field is subjective, depends on lots of things, it's not just a technical consideration.
This is pretty close - though as has been said the enlargement of the image may alter the DOF issues - usually exacerbate them not reduce them - it's more common for slightly out of focus images to LOOK in focus with smaller magnifications.
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Old 04-19-2016   #11
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Quote:
Originally Posted by Scrambler View Post
This is pretty close - though as has been said the enlargement of the image may alter the DOF issues - usually exacerbate them not reduce them - it's more common for slightly out of focus images to LOOK in focus with smaller magnifications.
I agree with this. Its what I meant in my post about "minor variations". ie a bigger enlargement (e.g. to get the same sized print from a smaller format camera/ sensor) will result in a marginally smaller DOF as it exacerbates any blur in the image
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Old 04-19-2016   #12
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It's quite easy to visualize this. A medium format camera, and a 110 camera, both having lenses giving the same angle of view for the respective formats, would have vastly different results in depth of field.
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Old 04-19-2016   #13
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DOF with a 28mm lens will be deeper than with a 50mm at the same aperture shot at the same distance.
If you get pretty close, you can achieve shallow DOF also with a 28.
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Old 04-19-2016   #14
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Just use this and compare:
http://www.dofmaster.com/dofjs.html
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Old 04-19-2016   #15
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This works the same way on smaller sensors. Thats's why you get more DOF on an Olympus pen with a 25mm f2 lens than a Sony A7 with a 50mm f2
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Old 04-19-2016   #16
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Are we talking about the Q? If you crop to 50mm (in camera), it is still a 28mm lens that is cropped. Same depth of field.
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Old 04-19-2016   #17
teddy
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Quote:
Originally Posted by Scrambler View Post
Sorry, you are wrong. It's the effective aperture versus the lens strength. The most limiting lens is usually inside.
Ok, I stand corrected, but wouldn't the front pupil affect the character (or the look) of the depth of field? This is probably besides the point.
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Empirical data needed
Old 04-19-2016   #18
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Empirical data needed

Would anyone care to do the experiment? Take a shot with a 50 mm lens at f/1.4 of a scene well suited to illustrate DOF. Then take the same shot (from the same vantage point and with the same camera) with a 28 mm lens at f/1.4. Crop the 28 mm shot to correspond to the 50 mm shot. Enlarge the crop to have the same size as the native 50 mm shot and let us compare. That would illustrate how much, if at all, the DOF of the 50 mm crop from the Q differs from a shot using a native 50 mm lens. For a proof of principle, the lenses don't have to be exactly 28 mm and 50 mm. If the effect is there it will be exaggerated if comparing two lenses that are further from each other in focal length.
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Old 04-19-2016   #19
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Quote:
Originally Posted by Daneinbalto View Post
Would anyone care to do the experiment? Take a shot with a 50 mm lens at f/1.4 of a scene well suited to illustrate DOF. Then take the same shot (from the same vantage point and with the same camera) with a 28 mm lens at f/1.4. Crop the 28 mm shot to correspond to the 50 mm shot. Enlarge the crop to have the same size as the native 50 mm shot and let us compare. That would illustrate how much, if at all, the DOF of the 50 mm crop from the Q differs from a shot using a native 50 mm lens.
Of course it is different. A 28mm lens has more depth of field at a given aperture than a 50mm lens. A 50mm crop function on a Leica Q is still made from a 28mm lens. The crop has a 28mm depth of field it's just cropped to a 50mm field of view.
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Old 04-19-2016   #20
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Quote:
Originally Posted by jsrockit View Post
Of course it is different. A 28mm lens has more depth of field at a given aperture than a 50mm lens. A 50mm crop function on a Leica Q is still made from a 28mm lens. The crop has a 28mm depth of field it's just cropped to a 50mm field of view.
That's nice but it's the same theoretical argument that has been stated before, not an empirical one.
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Old 04-19-2016   #21
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Quote:
Originally Posted by Daneinbalto View Post
That's nice but it's the same theoretical argument that has been stated before, not an empirical one.
No, it's based on actual photography... and doing it for many years. If you use a 28mm f/2 and a 50mm f/2 at the same aperture (obviously towards the wide open end of the spectrum), you'd have to be blind to not notice the difference. The Q has a 28mm lens and cropping it does not change the fact that it is a 28mm lens with 28mm lens characteristics.
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Old 04-19-2016   #22
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Quote:
Originally Posted by Daneinbalto View Post
That's nice but it's the same theoretical argument that has been stated before, not an empirical one.


I don't think this question would require empirical testing. jsrockit is spot on in that, a 50-ish crop of a 28mm lens would retain its innate 28mm DoF properties.
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Empirical data
Old 04-19-2016   #23
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Empirical data

This was a quick and dirty experiment with my digital camera zoom lens. One shot at focal length 35 mm (56 mm equivalent). The other at 17 mm (28 mm equivalent), which was then cropped and enlarged. Focus is on object on the grass. Due to the slowness of the lens at 35 mm focal length the best I could do was f/5. Which is which? I realize the limitation of doing this experiment at f/5. Would anybody care to try at f/2 or the like? My interchangeable lenses are for film.
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Old 04-19-2016   #24
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Here we go yet again ... another confused thread regarding depth of field (DOF). What's a person to think about these apparently contradictory comments?

The OP has asked: "...what happens to DOF if you apply a 50 mm crop to a shot captured with a 28 mm lens? If you compare two shots taken of the same subject, at the same distance, with the same f/stop, one with a native 50 mm lens and the other with a 28 mm lens but cropped to correspond to the 50 mm shot, will the DOF be different?"

As with most DOF questions, the OP's question is not well-defined, not complete. Thus the question can have different correct answers whenever different responders complete the question with different unstated assumptions in getting their answer. But not all proposed answers are correct, however, and figuring out which is correct is another problem.

DOF is simple and well-defined and always yields a unique answer to any well-defined DOF question. The following is my attempt to explain it with very transparent concepts and very minimal mathematics. I've been working on this explanation for some time and now seems like a good time to try it out on you guys. Please let me know what you think of it.

Anyone who understands my explanation will be able to determine when DOF questions are well-defined and will be able to determine/calculate the unique answer to the well-defined ones. After presenting my explanation, I will use it to treat the OP's question and will present four examples that show how the answer can change dramatically as unstated assumptions change.

--- Mike


What IS depth of field? DOF exists only as a perception by a human viewer of a photographic image (negative or slide or print or electronic display). Consider a film negative, for simplicity. The objects recorded in the negative will be perfectly in-focus for only the single plane at the focus distance from the camera. Objects at all other distances from the camera will be out of focus, more so as their distance differs from the plane of focus. The image on the negative of every out-of-focus point will be blurred, becoming a blur circle instead of a point image, and will have a diameter that grows as the point is further from the plane of focus. Large out-of-focus objects will consist of a myriad of out-of-focus points which overlap and interfere with one another to make such large objects blurry. This much is a physical consequence of the way that photographic lenses work and films record lens images.

Human vision is limited in resolution; there's a limit to the size of things that one can see, blur circles included. So if a negative is viewed by a human from the closest distance at which he can focus his eyes, then the human will be able to resolve the large blur circles of some out-of-focus objects, and these objects will be perceived as out-of-focus. All the objects whose blur circles the human cannot resolve will be perceived as in-focus and the distances from the camera of these in-focus objects are within the DOF. The DOF thus is defined as the difference in distance from the camera between the nearest and the farthest objects which are perceived to be in-focus. If the human moves back from the negative, he will not be able to resolve some of the smaller blur circles that he could previously see, and the DOF will increase.

Now enlarge the negative to make a print. The blur circles will also be enlarged. This makes them easier for a human to resolve from the same close viewing distance, so the DOF will decrease. If the viewer moves back from the print, he will again not resolve some of the smaller blur circles and the DOF will increase.

So the DOF value depends on which blur circles in a photographic image a human viewer can perceive. Thus DOF is not a fixed property of a negative (or digital image). DOF is somehow related to the camera, a lens and an image (film or digital) they have produced. But DOF is also related to the human viewer and the viewing conditions. How to sort all this out?

We can identify exactly six independent factors which TOGETHER determine the DOF in a given situation. The six independent factors are the following, and we also list the INDIVIDUAL effect of changes to each factor on DOF when all the others are kept fixed.
1. focal length of the lens; longer focal length yields smaller DOF
2. f-stop of the lens; larger f-stop (smaller f-number) yields smaller DOF
3. focus distance setting of the lens; nearer focus distance yields smaller DOF
4. magnification of the viewed image; higher magnification yields smaller DOF
5. viewing distance to the viewed image; closer viewing distance yields smaller DOF
6. visual acuity of the viewer's eyes; people with 20/20 vision will experience a smaller DOF than all people with poorer vision and greater DOF than all people with better vision

The DOF involves all six of these factors. Change any one of them alone and the DOF changes. When you change more than one factor, the DOF can either increase or decrease depending on the degree to which the individual changes complement or oppose one another. Multiple changes can be complicated to treat. In many instances one cannot simply reason with just English (or any other language) to get a verifiably correct answer. Instead, one MUST use a mathematical treatment, a key result of which is given by this equation.

DOF = (2 * F^2 * f * D^2 * M * d *a) / (F^4 * M^2 - (f * D * d * a)^2)

where the value of each of the six factors is represented by a letter
F is the focal length of the lens
f is the f-number (1.4 or 8, for example)
D is the focus distance to the plane in focus (in the same units as F)
M is the magnification of the viewed image (M=1 for just a negative)
d is the viewing distance
a is human visual acuity (commonly about 0.00058, corresponding to 1/30)
and * means multiply while x^2 = x * x, and x^4 = x * x * x * x
and (f * D * d * a)^2 means to multiply the four together and then square that

This equation is derived using simple geometrical optics. You can solve most common DOF questions by putting the appropriate values into this equation and calculating the result. But you must know the numerical values of all six factors to get any result at all; the question at issue must be well-defined.
Comments about the equation:
1. The equation applies to general photography for which the focus distance is much greater than the lens focal length (D>>F); the correct equation for macro work is slightly but importantly different and a bit more complicated.
2. The equation doesn't handle D = infinity very transparently; besides, if D=infinity then DOF=infinity, too, and no equation is needed.
3. If the calculated result of the equation is negative, that means that the DOF stretches to infinity.
4. Other DOF factors have been defined and used in other DOF equations, but the equation shown can accomplish everything that those others can and with fewer misunderstandings. For example DOF equations are often written in terms of something called the hyperfocal distance H, but the hyperfocal distance is just a special combination of our factors: H = F^2*M/(f*d*a). Similarly, the oft-defined circle of confusion c is just c = d*a/M.

Now apply this equation to the OP's question. We're going to calculate two values of DOF, one for the 50mm lens and one for the 28mm lens and then compare them. The OP told us that the f-number and focus distance are to be the same for both lenses but we weren't given values. Let's take f=4 and D = 3meter.

We already know that we need more information, and that the results of the comparison will depend on it. Let's assume a full frame film/sensor and magnify both of the negatives eight times (M=8) to make two prints that are 0.192m x 0.288m and we agree to view both from a distance equal to their common diagonal (d=0.346m). Further, we'll use the "standard" visual acuity (a=0.00058). Then our equations become

DOF(50) = (2 * 0.050^2 * 4 * 3^2 * 8 * 0.346 * 0.00058) / (0.050^4 * 8^2 - (4 * 3 * 0.00058 * 0.346)^2)

DOF(28) = (2 * 0.028^2 * 4 * 3^2 * 8 * 0.346 * 0.00058) / 0.028^4 * 8^2 - (4 * 3 * 0.346 * 0.00058)^2

and we can calculate the results which are

DOF(50) = 0.733 meter
DOF(28) = 2.08 meter

Comparing the two results, we find that the DOF of the 28mm lens is much greater than that of the 50mm lens. That is expected, because the only difference is focal length, and we already know generally how that affects DOF. Cropping in this example is just a matter of either masking the projected 28mm negative in the easel or trimming the print to show just the part of the 28mm negative that is included as the entire 50mm negative. After cropping, the 28mm print will be about 146mm x 219mm, which is smaller than the 50mm print. It's smaller, because the portion of a 28mm negative that matches an entire 50mm negative is only about 76% of each side of the 28mm negative (and 0.76^2 = 58% of the 28mm negative's area). But we still view it from 0.346m.

Next consider making the just cropped portion of the 28mm print the same size as the 50mm print when the 50mm is magnified 8 times. This will require a magnification for the 28mm of 8/0.76 = 10.5; all other factors retain the same values. In this case, the equation give

DOF(50) = 0.733 meter
DOF(28) = 1.919 meter

The 28mm print still has much greater DOF than the 50mm print.

Next suppose we magnify the 28mm negative by 20.8 while keeping all the other factors the same as before. Then the results are

DOF(50) = 0.733 meter
DOF(28) = 0.733 meter

Now the 50mm DOF is the same as the 28mm DOF. Again, cropping amounts to masking or trimming the 28mm image to match the extent of the 50mm image, and the print size will be about 0.380m x 0.570m, almost twice as large as the 50mm print but showing the same scene. Again we view both from 0.346m.

If we magnify the 28mm negative by 25 while keeping all else constant, the results will be

DOF(50) = 0.733 meter
DOF(28) = 0.608 meter

and the 50mm DOF is greater than the DOF of the 28mm.

To make the 28mm print have the same DOF as the 50mm, one must magnify the 28mm negative 2.6 times more than the 50mm negative. The mathematical reason why one must magnify the 28mm negative so much more is that the DOF depends on F^2 * M and (F^2 * M)^2. Thus if the focal length changes by a factor of 50/28 = 1.79 as in the OP's question, then the magnification must change by a factor of 1.79^2=3.19 to keep the product of these two factors constant. But since the denominator of the equation includes a second, negative term not involving F and M, this calculated 3.19 factor actually only needs to be 2.6 to accomplish equal DOFs.

So, in summary, the OP's question was ambiguous and the DOF of the 28mm can be greater than, the same as, or smaller than the DOF of the 50mm, depending on the values of the factors which the OP didn't specify but which are necessary for a well-posed question that this or any other DOF equation can answer. One can calculate comparisons for many other interesting situations involving these two lenses or for any other well-defined situation using the equation given.
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Old 04-19-2016   #25
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Even though I know it's pointless to experiment with this idea, let me fetch my tripod. It may be amusing.
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Old 04-19-2016   #26
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Well would you look at that!
See if you can tell which one was taken with a 50mm lens, and which one was taken with a 28mm lens.
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Old 04-19-2016   #27
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Quote:
Originally Posted by tunalegs View Post
Well would you look at that!
See if you can tell which one was taken with a 50mm lens, and which one was taken with a 28mm lens.
The difference is obvious.

Sorry for persevering about empirical evidence, but before Galileo a lot of smart people would tell you that a heavy ball drops faster than a light ball.

I realize that mrmeadows' post is based on theories that are firmly rooted in empirical evidence but sometimes "a picture says more than a thousand words."

Especially if you want to know what you can get from the 50 mm crop in the Q compared to another camera with a native 50 mm lens, which was the motivation behind my question. Thank you to all who helped clear this up.
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Old 04-19-2016   #28
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Is it correct to assume that the DOF in 50 mm crop mode on the Q corresponds to that of a camera with a smaller sensor (crop factor 50/28 = 1.8), equipped with a 28 mm lens. Assuming same distance to subject, same f/stop, same print size?

Last edited by Daneinbalto : 04-19-2016 at 18:12. Reason: Corrected "equipped with a 50 mm lens" to "...28 mm..."
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Old 04-19-2016   #29
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I just pulled the trigger on a Ricoh GR II as I wanted a big-sensor, jeans-pocketable camera. My ideal FOV would be 40mm -- why no company has filled this niche yet is beyond me. Anyway, me and 28mm aren't always the best match, so I too was trying to figure out what happens as you use the 35/50mm crop modes. The GR has a 18mm lens and APS-C size sensor -- which gives you ~28mm equiv. FOV. As you crop, an easy way to visualize it would be like having a 35mm equiv. FOV using a m43 size sensor, or a 50mm equiv. FOV using a 1" size sensor. Granted those would be using an aperture of F2.8 so the roughly equivalent in FF in total would be (from this thread):

NO CROP -- 28mm FOV, F4 DOF, 16mp, APS-C sensor area
35mm CROP MODE -- 35mm FOV, F5.6 DOF, 10.3mp, m43 sensor area
50mm CROP MODE -- 47mm FOV, F7-ish DOF, 5.6mp, 1" sensor area

That didn't sound terribly impressive compared to FF. However, when compared to the other high end pocketable P&S cameras like the RX100, it was enough to get me to pull the trigger on the GR II yesterday!

Equivalent apertures and FOV of 1" sensor cameras throughout the zoom range (from this DPReview article):



Back of the envelope, yor your FF Leica Q it would look something like:

NO CROP -- 28mm FOV, F1.7 DOF, 24mp (FF sensor area)
35mm CROP MODE -- 35mm FOV, F2.4-ish DOF, 15.3mp (50%>APSC sensor area)
50mm CROP MODE -- 50mm FOV, F4-ish DOF, 8.4mp (between m43/APSC sensor area)

Hopefully that was close and hope that helps
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Old 04-19-2016   #30
Daneinbalto
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So for a given crop, you can calculate an associated increase in apparent f/stop? And presumably the number is constant across the range of f/stops?
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Old 04-19-2016   #31
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Yes, most people estimate the conversions for DOF and FOV as:

m43->FF roughly 2 f-stops DOF, 2x listed focal length -- ex. Olympus 12mm/F2 acts like a 24mm/F4 FF lens.
APS-C->FF roughly 1 f-stop DOF, 1.5x listed focal length -- ex. Fuji 35mm/F1.4 acts like a 50mm/2 FF lens.

I'm not sure about DOF impact of 1 inch sensors, but judging from DPReviews chart the F2.8 RX10 gives a FF-equivalent DOF of roughly F7.7-7.8, or about 2.8 stops above the listed f-stop. The crop factor for 1 inch sensors is 2.7x listed focal length. So a Nikon 10mm/F2.8 would give similar characteristics to a 27mm/F7.7 FF lens.

Roughly
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Old 04-19-2016   #32
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This thread should probably be a sticky. I think Mr. Mike M. has nailed it.
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Old 04-20-2016   #33
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Obviously the general facts are clear but I don't follow the logic why the print size/magnification has any influence on the DOF of the original shot ...???

If I capture a scene with a 3D object and there is a 2 inch DOF in sharp focus, then this will not change when the shot is magnified to whatever size ... what am I missing here?
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Old 04-20-2016   #34
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Quote:
Originally Posted by icebear View Post
Obviously the general facts are clear but I don't follow the logic why the print size/magnification has any influence on the DOF of the original shot ...???

If I capture a scene with a 3D object and there is a 2 inch DOF in sharp focus, then this will not change when the shot is magnified to whatever size ... what am I missing here?
It is because, as mmeadows said, "DOF exists only as a perception by a human viewer". If you size down a print and look at it at the same distance its DOF will appear wider. Think cell phone compared to a large screen or print.

With regard to the 50 mm software crop mode on the full-frame sensor, 28 mm lens Q, an apt analogy might be to a camera with a smaller sensor (crop factor 1.8, or somewhere between micro 4/3 and APS-C), which would capture a 50 mm-equivalent field of view with a 28 mm lens. As Harry Caul stated, f/1.7 would appear as if taken somewhere around f/4 with a native 50 mm lens.
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Old 04-20-2016   #35
Rob-F
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Quote:
Originally Posted by icebear View Post
Obviously the general facts are clear but I don't follow the logic why the print size/magnification has any influence on the DOF of the original shot ...???

If I capture a scene with a 3D object and there is a 2 inch DOF in sharp focus, then this will not change when the shot is magnified to whatever size ... what am I missing here?
DOF is about the degree of acceptable unsharpness. If we start with a 5x7 print that has foreground or background details not in the plane of sharp focus, the unsharpness of those details may not be evident in the 5x7. But if we blow it up large enough, there will be a point where the sharpness difference between the in- and out-of-focus areas becomes visible.

If you want to check a 35mm slide or negative for sharpness, you will need a loupe or slide projector because the slide is too small to see without magnification. It's the same idea. Whether you use a loupe or just make a bigger print, you had to blow up the picture to see what parts are sharp and what parts aren't.
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Old 04-20-2016   #36
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Quote:
Originally Posted by Rob-F View Post
DOF is about the degree of acceptable unsharpness. If we start with a 5x7 print that has foreground or background details not in the plane of sharp focus, the unsharpness of those details may not be evident in the 5x7. But if we blow it up large enough, there will be a point where the sharpness difference between the in- and out-of-focus areas becomes visible.

If you want to check a 35mm slide or negative for sharpness, you will need a loupe or slide projector because the slide is too small to see without magnification. It's the same idea. Whether you use a loupe or just make a bigger print, you had to blow up the picture to see what parts are sharp and what parts aren't.
But this isn't about changing the depth of field. It is changing the perception of depth of field. The info was always there... it's just that making the image larger starts to show all limitations of an image (be it sharpness, focus, depth of field, resolution, etc).
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Old 04-20-2016   #37
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Quote:
Originally Posted by Daneinbalto View Post
It is because, as mmeadows said, "DOF exists only as a perception by a human viewer". If you size down a print and look at it at the same distance its DOF will appear wider. Think cell phone compared to a large screen or print.....
If you photograph a box of matches that sits diagonal in the perfect DOF so that the front and back corner of the box will be tack sharp and matches spilled in the foreground and a candle in the background will be OOF then you can change the size of the print or the viewing distance any way you want. The corners of the box will remaining sharp at any size or any viewing distance- given there are no technical screw ups in enlarging the print or screen view. DOF is a function of focal length, aperture and subject distance given the plane of film/sensor is fixed in the camera. The viewing distance or printed size of the captured image is not relevant.
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Old 04-20-2016   #38
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Klaus:

You said: "If you photograph a box of matches that sits diagonal in the perfect DOF so that the front and back corner of the box will be tack sharp and matches spilled in the foreground and a candle in the background will be OOF then you can change the size of the print or the viewing distance any way you want. The corners of the box will remaining sharp at any size or any viewing distance- given there are no technical screw ups in enlarging the print or screen view."

What you said would be true ... if you could do it. It is always true, as you say, that the objects in a negative that are "tack sharp" will stay tack sharp at all magnifications and viewing distances for all viewers. But the facts of physical optics dictate that you cannot make the front and back corners of the box of matches both tack sharp with a typical camera and lens, if the corners are at different distances from the film plane. For such a camera, there will always be exactly one plane at one distance from and parallel to the the film plane that will be tack sharp. We can choose either of the corners at different distances to be tack sharp, if we place it in the single plane of focus. Then the other corner will always be out-of-focus, just as everything else not in the unique plane of focus will be out-of-focus. To do what you have in mind would require tilting the optic axis of the lens from being perpendicular to the film plane. Tilting the optic axis will tilt the plane of focus, so that the corners of the box of matches can be at different distances yet both still be in the unique plane of focus. This principle is named the Scheimpflug Principle (see Wikipedia for detail). The ability to do this is one of the strengths of view cameras which can tilt and shift their lens relative to the film plane. For negatives made with tilt, the DOF whould still be measured perpendicular to the plane of focus, just as it is when that plane is not tilted.

You then said: "DOF is a function of focal length, aperture and subject distance given the plane of film/sensor is fixed in the camera. The viewing distance or printed size of the captured image is not relevant."

This statement is false. The plane of focus and the sizes of all the BLUR CIRCLES of all the out-of-focus objects in a negative are fixed at the time of exposure by the factors that you cite, but the DOF is never fixed at the time of exposure by these factors. The DOF is a perception issue, not just a hardware issue, and it depends on whether a viewer can detect the blur circles when he views the negative or a magnified print or other image made from the negative. If the viewer cannot detect the blur circles of an object, then the viewer cannot detect that the object is actually out-of-focus and it will appear to him to be in focus and to be within in the DOF. Only those blur circles that the viewer can detect will cause him to recognize that the corresponding object is actually out-of-focus. Detecting the the blur circles depends on how the sizes of the blur circles compare to the acuity of the viewer's vision at the viewing distance, so DOF depends on magnification, viewing distance and visual acuity in addition to the three hardware factors. If your statement were true, then the DOF of EVERY photograph would be ZERO, because only the objects exactly in the plane of focus are tack sharp and a plane has no depth; therefore there couldn't be any DOF at all.

Hope this response helps.

--- Mike
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Old 04-20-2016   #39
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LOL, yeah, helps a lot .
For all practical purpose of handheld photography I go by what I tried to explain.
My definition of tack sharp is: sharp at 100% view on my calibrated 27" NEC, that's good enough in my book.

For adcademic purpose of this discussion, your are correct
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Old 04-20-2016   #40
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All I know is that this discussion makes me want to run in circles of confusion.
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