I previously wrote about the true focal length modifier for Micro Four Thirds when compared to a “full frame” camera, that is a camera that mimics the size of “35mm” film (which is actually 24 x 36 mm).
But with square photos being most important for Instagram, we should be comparing Micro Four Thirds to square cameras. The most famous square cameras were the Rolleiflexes, which used 6x6cm film.
The correct Micro Four Thirds to 6×6 focal length modifier, when cropping to a square, is 4.61.
The standard lens on 6×6 cameras was 80mm, which translates to 17mm on Micro Four Thirds. Good news, Olympus makes three lenses in that exact focal length!
55mm was considered wide angle, and that translates to 12mm. Once again, we are in luck, Olympus also makes a 12mm lens! And so does Panasonic!
135mm was considered telephoto, and that translates to a 29mm lens. We are out of luck if we want a 28mm prime lens. However, the closest common Micro Four Thirds prime-lens focal length is 25mm. So this is where things get weird. Everyone thinks that the 25mm is a “normal” lens, but actually when you crop to a square, and then compare that square to how photographers used to think about things, 25mm lens is actually a telephoto focal length.
The same modifier is used for comparing f-stops. So that f/2.0 12mm prime lens, which seems like a “fast” lens, is actually equivalent to a quite “slow” 55mm f/9.3 lens on a 6×6 camera.
Even if you can afford that seemingly “super-fast” 17mm f/1.2 lens, it behaves like an 80mm 6×6 lens with the very unimpressive maximum aperture of f/5.5.
To summarize again, for square photos:
12mm = wide angle
17mm = standard
25mm = telephoto