I've been playing around with my Samyang 85mm f/1.4 lens converted from Sony E to GF using a 3D printed adaptor.
The lens covers the 44x33 beautifully wide open. In 35mm terms, on a GFX body this gives you the field of view of a 70mm lens and a depth of field of f/1.0.
Can't resist jumping in
How blurred something a given distance away from the focus point is in the image depends on focal length squared over aperture (as f/ number). That sets how close to the focus point you need to be to still appear in focus with a given set of viewing conditions (a.k.a. depth of field).
TLDR as focal length increases, the typical reduction in maximum aperture doesn't cancel out the blurring effect of longer focal length.
Worked example. I have a 50mm f/1.4 for my DLSR and an 85 f/1.4 and I used to have a 135mm f/2.5
50² / 8 is 312 remember the old adage "f/8 and be there", to get the depth of a 50 at f/8 my 85 needs to be set to f/24, and the 135 to f/58
50² / 1.4 is 1785, the 85mm gets that at f/4; wide open 85² / 1.4 is 5160 to get that the 50mm would need be set to f/0.5.
The 135 at f/3.5 matches a wide open 85 f/1.4; when it is wide open 135² / 2.5 is 7290 - to get that the 85 would need be an f/1 and the 50 would need be something like f/0.3
If the lens is focused at ∞ the size of a circle a point at distance d makes is given by
c = f²/da ; if we are limited by the circle size that still appears in focus that distance (the hyper focal distance) is given by h = f²/ca
Traditionally focus tables use 0.03mm for 36x24; 0.02mm for APS-C and 0.04mm for 33x44 , if the test is viewing a 10x8 print at arms length the greater magnification for the smaller sensor means a small circle becomes the limit.
So 85 f/1.4 has more apparent d.o.f on a bigger sensor than a smaller one. It's like being set to f/1.8 on 35mm. Of course it has he view of about a 68mm lens and for a 68mm lens to have he d.o.f of and 85 at f/1.8 it would need to be about f/1.1
We'll struggle to find a lens with that angle of view and that shallow a d.o.f.
The answer to a question like "Is the d.o.f at 85 f/1.4 to shallow to be useful"... well I don't see people shunning say 200mm f/2.8 lenses ... Of course the combination of one wide open and minimum focusing distance on such a lens might be useful only very, very rarely.
I went for a walk around the block where I live and took some snaps wide open. Here's 3 examples:
Here, if the fence were sharp the picture wouldn't work nearly as well. The slight blur on the K and the N don't detract from the picture. This illustrates "find a way to make a small part of the frame stand out from the rest" composition using d.o.f to do it
D.o.F this shallow was definitely useful, shallower still would have worked.
Same again we have all of the post box sharp enough, the background going even further out of focus would have been better.
The depth of field on here is tiny, just a sliver. Frankly, what use is this narrow depth of field - 99% of each image is out of focus? Is there something this could be useful for?
This is where I quibble "with out of focus." Some of the post box in 3 isn't perfectly focused that's what you have to live with to isolate it from the background shooting from the distance you have Same with the branch in the last one, you can't isolate as you have and get every one of the berries sharp. But the impression rather than close examination is the pillar box was sharp, the berries were sharp, even a parts are not perfectly in focus. in / out of focus not a binary thing.
