In the photographic world, it is often convenient to compare camera lenses in terms of 35mm “equivalence”. Of course, for someone just picking up a camera in the last 10 years this might be quite a strange concept, but similar to many technological advancements the terminology and concepts are rooted in history.
The term “35mm” technically refers to the total width of 135 film. In the digital era we now refer to this as “full frame” (FF). Ironically, the historical size of 35mm film (and thus full frame digital sensors) is referred to as “small format”, as opposed to “medium format” and “large format” (your full frame camera sensor doesn’t seems so large now, does it?). Anyway, 35mm format has been, by and large, the most popular consumer film format for the past 40 years. With such a broad familiarity of cameras that use 35mm film, it has been (and continues to be) quite convenient to compare, among other things, the focal length of camera lenses between new image formats and FF.
Why do you need to compare to full frame in the first place?
The idea here is if you are familiar with the mapping of focal length to angle of view on FF then, given the focal length of a lens and image sensor size (crop factor), you can figure out what the equivalent 35mm focal length would be (and hence, the FF angle of view, which is what we’re mostly interested in). Or in other words, what does it look like when I take a picture at a certain focal length with camera B, compared to my FF camera A. This helps you answer the question “What lens should I buy for my
Of course, all of this assumes you know what lenses look like on full frame cameras. If you don’t, well, you’re kinda out of luck :) (But no, seriously I would suggest to get familiar with the mapping of focal length to angle of view on FF so you can have a baseline of comparison in the future…even if you don’t have a FF camera try to think in terms of FF equivalence. Often lenses for cropped sensor cameras will be advertised in terms of FF equivalence).
How to calculate focal length equivalence
35mm equivalent focal length = lens focal length * crop factor
For APS-C image format (Nikon DX, Fujifilm X, and some Sony) the crop factor is 1.5. For APS-C on Canon the crop factor is 1.6. For Micro Four Thirds (MFT) image format (Olympus, Panasonic) the crop factor is 2.
This means that a 25mm lens on a MFT camera will have the same angle of view as a 50mm lens on a full frame camera. A 33mm lens on APS-C will have the same angle of view as a 50mm lens on a full frame camera.
Aperture size
Of course, angle of view isn’t the only thing to consider when thinking about FF equivalence. There is also the concept of the equivalent aperture size. It is calculated much in the same way as equivalent focal length:
35mm equivalent aperture size = lens aperture size (f-number) * crop factor.
What this means is, say you have a 12-35mm f/2.8 MFT lens. The FF equivalent is actually 24-70mm f/5.6 ! It is most certainly not 24-70mm f/2.8 as some camera manufacturers would have you believe according to their marketing materials. Be careful what you buy!