Here’s my attempt at explaining something that’s been explained a million different times by a million different people. If you take a photo with the same length lens on different digital sensor sizes, or on a film camera compared to a digital camera, then the image looks different. But the term “focal length equivalency” is bullshit. It’s actually a “crop factor”. An 80mm lens is a 80mm lens. Sticking it on a 1.6 crop factor sensor doesn’t magically transform it into a 50mm lens. So the PERSPECTIVE on a lens is identical whether it’s on a full frame (35mm, film camera) sensor or on a 1.6x APS-C sensor or on a micro 4/3rds sensor. What does change is how much of the image you can see. Full frame is our reference point, so an 80mm lens on a full frame (35mm camera) you see a wider portion of the image. That same lens on an APS-C sensor, you’ll only see 1/1.6th of the image. That’s the crop factor.
Ok, let’s see if I can actually demonstrate this. Here’s a picture taken on film with a 50mm lens.
Now here’s the same scene taken on a digital APS-C 1.6x crop factor sensor (also on a 50mm lens).
Looks kinda the same, huh? Well look at the edges. The film (full frame) picture has a wider field of view. There’s more image to the left, more to the right, more on top and bottom. You can’t even see that tree on the left in the APS-C shot. So now here’s the same scene taken on the APS-C sensor at 30mm (approximately 1/1.6th of 50mm).
|30mm lens, APS-C sensor||50mm lens, film|
Now we have about the same frame, you can see everything you can see in the 50mm film shot, but doesn’t it look “zoomed out” compared to that 50mm film shot? The telephone pole in the distance looks farther away, the angle of the brick wall is much more dramatic, etc.
Now finally, here’s the 50mm film shot cropped so that it shows the same frame as the 50mm APS-C shot. Compare the two (ignore the depth of field), and don’t they look pretty much … identical?
|50mm lens, film, 1.6 crop||50mm lens, APS-C sensor, uncropped|