Today’s Question: A 105mm lens on a full frame camera in FX mode has the same field of view as a 70mm lens on the same camera in DX mode. However, images won’t be the same. The 105mm lens image would have more apparent compression of distance than the 70mm lens because of the real difference in focal length, and at the same aperture, the 105mm lens would have a shallower depth of field than the 70mm lens.
Tim’s Quick Answer: The compression of the scene is only a factor if the photographer changes position. From the same position the perspective of the full-frame versus cropped-sensor image will be the same. Only if you back away when using the cropped sensor will the perspective of the image change. And there will be a reduction of depth of field when a longer focal length lens is used, regardless of in-camera cropping.
More Detail: Today’s question is a follow-up to one about the option to crop in the camera on some full-frame digital SLR cameras. These cameras essentially let you choose between capturing the full frame, or cropping to only capture a portion of the image circle, matching what you would have achieved with a smaller image sensor using the same lens.
When your camera provides an option to crop the image in-camera to produce a smaller field of view, or you are using a camera that has a sensor smaller than a full-frame sensor, the images you capture will reflect a narrower field of view than you would achieve with the same lens on a full frame camera.
If you do not change your position or adjust the camera settings, you are literally just cropping the image. So with a full-frame camera compared to in-camera cropping or a smaller sensor, if you are using the same lens everything is the same except that the image is cropped.
Note that cropping the image circle doesn’t alter depth of field in the same way that changing lens focal length would. There are factors such as sensor resolution and focal length that affect depth of field. But if the same focal length lens is used on the same camera, and you are merely cropping the image in the camera (such as with the FX versus DX option on some Nikon cameras) the depth of field would not be altered.
The perspective change referred to in today’s question is not really caused by the focal length of the lens, but rather by the change in position by the photographer. If you stay in the same position, the perspective will not change. But, of course, if you are getting the result of a longer focal length in terms of field of view, you would need to back up if you wanted to maintain the same framing of the scene. That would result in a change of perspective.
You can see an example of the effect of your position versus the focal length of the lens in an episode of Tim Grey TV on my YouTube channel here: