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Cropped Sensors and Equivalent Focal Length: the same as 35mm?
Old 04-16-2012   #1
Filson Back
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Cropped Sensors and Equivalent Focal Length: the same as 35mm?

I have a question about equivalent focal lengths with APS-C sized sensors. I have been shooting film rangefinders and digital everything for about a year. I am considering putting myself on a camera diet--picking one focal length and shooting only that camera as a means of improving my skills. In my musings I came upon a question. For a less-than-full-frame sensor to be equivalent to a 35mm focal length a shorter and more wide angle lens is required. This is why Fuji for example has the 23mm lens on the X100. So, because a wider-angle lens is used, is the perspective rendered differently, in terms of the relationship between near and far objects? It is widely known that wide angle lenses make more distant objects appear artificially far away, is this effect true for the equivalent of a given 35mm lens on an APS-C sensor? Does a 23mm lens on a cropped sensor give the same distorted near/far relationship, even though only a smaller portion of the image circle is being used? I hope the question makes sense--I am trying to see if, for all intents and purposes a cropped sensor/lens combo can be used like the 35mm equivalent, or if this is just another reason to shoot film. I guess the best way to compare would be to take a frame with a full-frame digital or film camera and a given focal length and use the equivalent on a cropped sensor and take the same picture and compare how the perspective is rendered--but I'm hoping this information already exists on the internet or collective wisdom of RFF.
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Old 04-17-2012   #2
Chris101
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A 1.5 crop factor camera with a 35mm lens would, for most intents and purposes, produce pictures indistinguishable from a "full frame" camera with a 50 mm lens. The perspective would be the same. Perspective is not a function of focal length, but of the distance to various objects in the scene.
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Old 04-17-2012   #3
kanzlr
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what Chris said.
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Old 04-17-2012   #4
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Yes and no. Take a look at wikipedia and the relationship between closer and more distant objects based on focal length: http://en.wikipedia.org/wiki/File:Focal_length.jpg

I guess I'm wondering if a wide-angle lens that produces a 35mm equivalent angle of view is still going to have the distortion of wide-angle. May have to take some test shots when I get my M2 back from Youxin.
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Old 04-17-2012   #5
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In order to make the pink container the same size in relation to the frame of the photograph the camera position had to be moved. It's not the focal length which changed the relationship, it was the distance from the camera to the object that changed.

If you repeated these series of photographs by leaving the camera stationary and cropping the images to show the same part of the scene there wouldn't be this difference in perspective.
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Old 04-17-2012   #6
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If you cut out the center of a 35mm negative to the size of your intended sensor you will have your answer.

Or cut out the center of a 4x6 print. You can see the only difference is the "crop factor".


The perspective stays the same but since the edges are cut off you may not experience the same distortion.

As a telephoto lens compresses the distance between near and far, a 150mm lens on a 35mm film or full-frame camera will give a different image than a 100mm lens on an APS-C sensor.
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Old 04-18-2012   #7
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Changes in perspective and depth of field are all due to camera position!

Think of it this way: you have a 80mm lens which projects an image circle on to a medium format back. Lets say you are shooting a portrait and the subjects head is pretty much filling the frame with some trees in the background. You swap out the medium format back for a 35mm back. The lens is still projecting the same image circle, however this time your back is taking a smaller portion of the subjects head. You swap to a APS-C back, it's the same picture, but a smaller portion yet again. You swap to a mobile phone sensor back. It's the same image circular landing on the sensor, only the sensor is recording a tiny portion of the subjects face.

Solution? As the backs image size gets smaller you have to move further and further back from the subject to fit them in. Result? Perspective changes and so does depth of field.

With the medium format back the ratio of the difference between the subjects head and the background is quite pronounced. Depth of field is shallower. As you move further and further back the relative distance between the subject and the background becomes less and less, depth of field becomes larger.
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