Let's Talk About Focal Length
Fujifilm's X-Trans sensor is a size called "APS-C" for reasons that don't matter, but what does matter is that this sensor is 23.6mm x 15.6mm in size. The various Fujifilm lenses create an image circle just large enough to cover this sensor size, just as lenses for 35mm film have image circles just large enough to cover 36mm x 24mm. Those rectangles have the same 2:3 aspect ratio, so comparing lenses for the two formats, a simple multiplier can be used to find the APS-C lens's equivalent field-of-view 35mm-format lens.Medium format 6x6 square photo
The math to do this involves comparing the diagonal across the two frames. For the Fujifilm X, this is 28.3mm, and for 35mm film, this is 43.3mm, or 1.53x the size of the Fujifilm sensor. So we can take that 1.53x multiplier, apply it to, say, the XF 35mm lens, and determine it has the same field of view as a 53.5mm lens would on 35mm film. That's what we'd expect, since Fujifilm markets the XF 35mm lens as a "normal", and normal lenses in 35mm-film are anywhere between 45 and 55mm in focal length.Medium format 6x6 square photo
Wait, what does "normal" mean here?
As a quick aside, what does "normal" mean in regards to lenses? Well, the longer the focal length of a lens, the more "zoomed in" the picture gets, and the narrower the field-of-view. This results in objects which are far apart looking close together, in a visual trick called "compression". On the other size of the spectrum, wide lenses have short focal lengths, resulting in very wide field-of-views. This means distance away from the lens is exaggerated, and objects in the extremes of the image circle can look distorted as their shape is stretched in one direction and not the other. A "normal" lens is simply a lens at the mid-point between those two effects. This typically works out to a focal length nearly the same size as (or just slightly larger than) the diagonal of the sensor size.
Back to Square One
Get it? "Square" One? Ha. Ha.
Enter medium format cameras, which do not necessarily shoot at the 3:2 aspect ratio of 35mm film and most digital sensors. Medium format cameras are much more varied – but my particular love is for those which shoot 6x6cm squares. Since everything in photography is a lie, the 6x6cm square is actually a 56mm by 56mm square, but that doesn't sound as sexy. To cover this 79.2mm diagonal, a new "normal" is needed, and we need to adjust our fields of view. The normal lens in 6x6 medium format is typically between 80mm and 85mm in focal length. You might think that would mean a 1.83x multiplier could then be used to determine equivalent fields of view from 35mm lenses, but this would be a mistake, since it's not just a larger frame size, but also a different aspect ratio. The 1:1 aspect ratio of 6x6 medium format is a significantly different composition experience from 3:2 photography, such that determining an equivalent field-of-view is not straightforward.Fujifilm X square photo
Luckily, we don't have to worry about that, since shooting square 35mm film frames is extremely rare. But with Fujifilm X cameras, shooting square is a reality, and so we want to know with our Fujifilm XF lenses, what is their equivalent focal length not just to a 35mm-film lens, but also to a 6x6 medium format lens. To determine that, we need to return to that APS-C sensor size of 23.6mm x 15.6mm, and realize that in shooting square, we're using only 15.6mm x 15.6mm of it. Therefore, the diagonal is 22.1mm, which works out to a 3.6x multiplier factor to reach 6x6cm's 79.2mm diagonal. This means, for example, that our XF 35mm lens shot square has the equivalent field of view as a 6x6 lens of 125mm focal length.
Using this knowledge, we can make a chart.
|XF focal length||35mm film equivalent||6x6 equivalent|
We've now mathed the shit out of this. Using this chart, we can see that a XF 23mm lens, shot square, covers the "normal" field-of-view, and sets a new point in determining whether a lens is wide or long. But how does it look in real life?
I shot test images using my X-Pro2 and my four favorite XF primes, and compared them to the viewfinder view of the Hasselblad 500C/M with those four lenses attached. The 500C/M has about 95% viewfinder coverage, so the actual photo will have slightly more in the frame than what we see here. I also bumped the tripod while switching cameras. I also shot the test photos of my kitchen at night. This is a sloppy, shitty test, and I should be ashamed, but I'm currently the first-time-parent of an infant and that's my excuse. Hopefully I can improve this test sometime soon.Hasselblad on the top row, Fujifilm X on the bottom
As the numbers say, the XF 23mm lens is similar in view to the Hasselblad 80mm lens. And the XF 35mm lens's view is between the Hasselblad 80mm and 150mm lenses, and the XF 50mm is between the 150mm and 250mm lenses. Hooray, the math works.Fujifilm X square photo
Hopefully, you find this information helpful. It certainly does help me understand why I rely so heavily on my XF 16mm lens – and it makes me wonder if I should look into trying that Zeiss Touit 12mm. But that's for a different article.