Join Tim Grey for an in-depth discussion in this video Cropped sensor and focal length, oh my, part of Photo Gear Weekly.
- One of the issues that photographers have been talking about for a very long time relates to crop sensor cameras, or what was often referred to, especially in the early days of digital photography, as a focal length multiplier. The idea is, depending on which camera you're using, a given lens might behave as, for example, a 100 millimeter lens, or with a different camera, as a 160 millimeter lens. But am I really getting free focal length out of my lenses? Sort of, but not exactly.
What's really happening is that there are different sensor sizes in different cameras. And of course, for many photographers, we got our start with 35 millimeter film photography. Many digital SLRs relate to that format, and so we have, for example, I have the 5D Mark IV, which is a full frame sensor camera. And that means that the frame size, the image sensor that's actually capturing the photo, is the same size of a single frame of 35 millimeter film. But, I have other cameras that have a smaller sensor. Here, the 7D Mark II for example, with what we refer to as essentially a cropping factor of 1.6x.
It really means, that with that smaller sensor, we're seeing a smaller field of view. We're seeing a smaller view of the image circle that's being projected by the lens. That means that we're seeing a tighter area of the overall frame, and I have a sample set up here, actually, to help illustrate the concept. I've zoomed in and focused in onto a set of billiard balls here, and with my current set-up, with a full frame camera, I can see all of the rack of billiard balls there. I'll go ahead and capture an image of those billiard balls.
I'll then turn off the camera, and then switch to my crop sensor camera. And once again, this hase that factor, that cropping factor if you will, of 1.6x. So a 100 millimeter lens will behave like a 160 millimeter lens, but again, what that really means is that I'm seeing less of the overall frame, so a smaller field of view. And so, with the exact same lens, with the exact same setup from the same distance, now when I take a look at my shot, I'm not seeing all of the billiard balls.
I'm seeing, well, maybe about half of those. I'm getting a much tighter view. I'm still using the entirety of that image sensor, the full resolution of that camera, to capture the image. So in both cases, I can expect an image of excellent quality. I'm just seeing a different field of view. The idea, really, is that you need to get familiar with how your lens behaves with your specific camera, under given circumstances. Really, I think what's most helpful there is to just have a sense, an anticipation, of which lens you might need under given circumstances.
So, when you approach a scene you can anticipate which lens to take out of the bag to get set up. But otherwise, just keep in mind that of course there's different behaviors among different sensor types. It's not free focal length, per se, it's just changing what field of view we're actually capturing.