OT: DOF of the same lens between different formats

Will · 11-08-2005

hi guys,

I got myself a CZJ 180mm, so if I use it on a 6x6, 135, APS-SLR, does the DOF change?

I read it on the RD-1 forum not so long ago, a 35mm F/1.4 used on the RD-1 the Angle of View changes, that's easily understandable, but how about the DOF?

Is there any formular to calculate the DOF when let's say the CZJ 180mm on the different formats?

Cheers

Will

tetrisattack · 11-08-2005

Yes and no.

Obviously, the projected image will be the same regardless of format, so the absolute DOF isn't changing. The relative DOF does change, however, as you take smaller and smaller crops of the image. The relevant term here is "circle of confusion," and the smaller your format is, the bigger the circle of confusion is.

So yes, a specific lens at a specific aperture will exhibit both a smaller FOV and a greater DOF when switched from a full-frame format to an APS sized sensor.

This is why getting a sufficient background blur is a pain on small-sensor digicams, and why large format cameras have movements to help you maximize the relatively shallow DOF.

Here's the information you're probably looking for:
http://www.vanwalree.com/optics/dof.html

This guy also has a DOF calculator, which would probably be right up your alley!
http://www.vanwalree.com/optics/vwdof.html

Will · 11-09-2005

Thanks a lot, will give it a read and see what I can digest...

Solinar · 11-09-2005

A 180mm lens is 180mm lens. I think that the confusion occurs when comparing the same field of view between formats. Medium format and large format always use a longer focal length to get the same field of view that one would use for 35mm film.

By the way, my 6x9 medium format camera has a 105mm lens and has a field of view that is just a tad wider than a 35mm camera with a 50mm lens.

Obviously there is an easliy perceived difference in the depth of field if one views images from both cameras using their repsective normal focal length, if the pics were printed full frame.

Switch lenses on the 35mm camera to 105mm focal length and you get a cropped image of the 6x9 origninal and will have the same depth of field as the original 6x9 image.

StuartR · 11-09-2005

Though I don't have the brain power at this time of night to go through the answers above, I can tell you the practical answer -- the depth of field is the same for any given focal length, regardless of format. Look at it this way -- a standard lens for 4x5 inch large format is a 210mm lens. Wide open (usually f/5.6 or f/8), the plane of focus is very small. Just as small as it would be at f/5.6 or f/8 using that 210mm lens on a 35mm camera. You just don't have much depth of field to work with. For LF, that is why in a lot of shots you have to stop down to f/32 or f/64 and/or employ the Schleordslfhslkdfplug principle (not even pretending to be able to spell it) to get more depth of field.

So. Your 180mm lens will still have small depth of field, but it will have a different angle of view as you change formats. You are basically changing the crop size. So if you have your 4x5 inch shot taken with a 210mm lens and crop out a 35mm sized chunk of that 4x5 inch negative it would be exactly the same as if you used a 210mm lens on a 35mm camera.

Or, even more basically, since it is the exact same lens, it could not change! You are not changing the lens at all by putting it on different cameras, you are only changing the way that you record the image that it projects.

jamiewakeham · 11-09-2005

What they all said.

Damn, it's a nice piece of glass, though, isn't it? Mine spends a lot of time on my EOS rig, though I keep thinking I should get a Kiev 6x6 setup to make the most of it...

Jamie

Will · 11-09-2005

Quote:

Originally Posted by StuartR

So. Your 180mm lens will still have small depth of field, but it will have a different angle of view as you change formats. You are basically changing the crop size. So if you have your 4x5 inch shot taken with a 210mm lens and crop out a 35mm sized chunk of that 4x5 inch negative it would be exactly the same as if you used a 210mm lens on a 35mm camera.

This I understands......

Quote:

Originally Posted by jamiewakeham

Damn, it's a nice piece of glass, though, isn't it? Mine spends a lot of time on my EOS rig, though I keep thinking I should get a Kiev 6x6 setup to make the most of it...

Yes, they are good! I used to have a 300mm f/4, just as good, but I am sick of the weight!

Those modified Kiev have MLU, useful...

Quoting from the tread She shoots with her Grandfather's IIIa and 50/1.5 Sonnar

Quote:

Originally Posted by Mike Kovacs

Consider the 50/1.5 Sonnar is a 7 element, 3 group design. No more air/glass interfaces than a Tessar or Cooke triplet. They are also very contrasty as a result.

Same goes for the 180...

Then there is the bokeh...

...can you find anything that came close to this CZJ Sonnar?

jaapv · 11-09-2005

The focal length does not change, that is true, so the standarized DOF does not change, but for practical purposes DOF is not an absolute value. It depends on the perceived sharpness of the end print.It depends on the circle of confusion chosen, for standarisation reasons this is 0.03 mm on 35 mm film. However depending on the use of the negative, be it a huge enlargment OR A CROP one needs to choose a smaller circle of confusion. That goes for a 135 that has been sensor-cropped to the FOV of a 180 as well.
The crop is the surface, the diameter of the COC is linear, thus the effective FOV will be about the same as a 150 mm lens on full frame. Just remember that depth of field is an optical illusion based on the defects of our eyes and the photographic process. The only sharp element in your photo is the plane of focus, which is infinitely thin.......

Quote:

Originally Posted by StuartR

Though I don't have the brain power at this time of night to go through the answers above, I can tell you the practical answer -- the depth of field is the same for any given focal length, regardless of format. Look at it this way -- a standard lens for 4x5 inch large format is a 210mm lens. Wide open (usually f/5.6 or f/8), the plane of focus is very small. Just as small as it would be at f/5.6 or f/8 using that 210mm lens on a 35mm camera. You just don't have much depth of field to work with. For LF, that is why in a lot of shots you have to stop down to f/32 or f/64 and/or employ the Schleordslfhslkdfplug principle (not even pretending to be able to spell it) to get more depth of field.

So. Your 180mm lens will still have small depth of field, but it will have a different angle of view as you change formats. You are basically changing the crop size. So if you have your 4x5 inch shot taken with a 210mm lens and crop out a 35mm sized chunk of that 4x5 inch negative it would be exactly the same as if you used a 210mm lens on a 35mm camera.

Or, even more basically, since it is the exact same lens, it could not change! You are not changing the lens at all by putting it on different cameras, you are only changing the way that you record the image that it projects.

StuartR · 11-09-2005

That may be true, but how does it manifest itself in the real world? For example, my 100mm slr lens has the same apparent depth of field on film as a 100mm lens and on the DMR as a 137mm lens. What are the situations where the apparent depth of field would be different?

jaapv · 11-09-2005

For practical purposes the DOF is about (don't shoot me with exact fractions please!) halfway between the real focal length on full frame and the perceived focal length on crop. So for your 100 mm maybe 110 or 115. I don't think you'd notice in your photo's. But with a 1.5 crop sensor, or even more for instance something like a Digilux 2 the difference would be far more noticeable.

JohnL · 11-09-2005

On the DMR, it might be reasonable to consider 0.025mm as an appropriate diameter for the circle of confusion, assuming you plan to make prints the same size as you would with 35mm. On a 1.5x or 1.6x crop camera, 0.02mm would be about right.