All, I've asked here before for help in understanding perspective distortion and equivalent focal lengths for lenses used on APS-C and full frame, and I got some very helpful responses.
I thought I understood it, and I think I still do, but last night I got myself a bit muddled up (probably one glass of wine too many!
), so I'd like to confirm my understanding. Can you tell me if the following is correct, or if not, where I've gone wrong?
So... Let's say I have two cameras - one full frame DSLR, and one APS-C DSLR with a 1.5x crop factor. They are fitted with the same 85mm lens set at f/5.6, and the cameras are set to ISO 200 and 1/100 second shutter speed. They are set up on tripods the same distance away from the subject, in order to take a head and shoulders portrait shot, with both lenses focused on the eyes. A little further behind the subject is a brick wall acting as a background for the shot.
Looking through the full frame camera's viewfinder, the head and shoulders are perfectly framed, the subject is in focus and the brick wall is just out of focus enough to give a smooth, pleasing background. I take my shot and I'm happy with it.
I move over to the APS-C camera. I look through the viewfinder and the view is cropped all around compared to the full frame - no good at all. I have two choices: (a) move back until the field of view is the same as with the full frame camera (ie. zoom out with my feet
), or (b) leave the camera where it is, and swap out the lens for a 55mm model which will give close to the same field of view as the 85mm on full frame.
Looking at both approaches individually:
(a) Moving back
I move the APS-C camera back until I get the same framing of the subject as I see through the full frame camera's viewfinder, I re-focus on the eyes, and take my shot. It's OK, but what I've captured doesn't look the same. Since my subject is now further away from the lens, perspective distortion has caused the subject's features to look a little flatter (more "compressed") and the background appears to be closer. The background is also less out-of-focus (thus, more distracting) because the depth of field has increased with the subject distance. There's nothing I can do about the perspective distortion, but I can improve the background by opening up the aperture one stop to f/4, hence reducing the depth of field. To compensate for the extra light coming in, I use a faster shutter speed of 1/200 second or drop my ISO to 100.
(b) Fit a 55mm lens in place of the 85mm
I fit the 55mm lens to the APS-C camera, set it to f/5.6, focus on the subject's eyes, and take the shot. It looks good, but - again - not exactly the same as I got from the full frame setup. The framing, field of view and perspective look about right, and there is no difference in compression of the subject and background. But, the background is less out-of-focus, since the shorter focal length of the lens has resulted in a greater depth of field at the same aperture. I can address this by opening up the aperture to f/4, and using a shutter speed of 1/200 second or setting ISO to 100 to compensate.
Drawing on option (b), if I wanted to emulate a full frame street photographer who is using a 35mm lens at f/8.0 with a shutter speed of 1/60 second and ISO of 400, I could use my K3 with a 24mm lens at f/5.6 and a shutter speed of 1/125 second at the same ISO, or keep the shutter speed the same, and drop the ISO sensitivity to 200.
Have I got this right, or am I badly mixed up?!?
Thanks in advance!
Last edited by BigMackCam; 01-07-2016 at 12:48 PM.