This is a continuation of a report on new ways to look at depth of field. The series starts here:
I’ve heard photographers say that sharper lenses — and cameras, for that matter — have less depth of field (DOF) than lesser ones. Is that true?
Like so many things, it all depends on the details of the question.
Let’s say you focus a lens at infinity, and measure the sharpness there as MTF50 measured in cycles per picture height (cy/ph). Call that MTF50Inf. Then you pick an acceptable MTF50 as a certain percentage (call that the tolerance ratio) of MTF50Inf, say 80%, call that MTF50Tol, and ask what distance you should focus to have the MTF50 for an object at infinity to equal MTF50Tol. That would give you a new kind of hyperfocal distance, similar in concept to the old, reliable, circle-of-confusion (CoC) based hyperfocal distance, but one that takes into consideration diffraction and lens aberrations.
With our simulated Otus and Nikkor lenses, what would these hyperfocal distances look like? Here’s a set at a tolerance ration of 80%:
You can see that the hyperfocal distance using MTF50 ratios decreases as you stop down the lens, just like CoC hyperfocal distance. You can see that, from f/8 through f/16, it doesn’t make any difference which lens you use; that’s because in that range of apertures, in-focus sharpness is limited by diffraction, not lens aberrations.
You can also see that, at the wider apertures where aberrations play a bigger role than diffraction, that, yes indeedy, the sharper Otus does have less DOF, as indicated by the longer hyperfocal distances.
At a tolerance ration of 60%:
And finally, 20%, which is getting downright fuzzy:
At 20% tolerance ratio, the quality of the lens doesn’t make much difference a any f-stop, but at 40% and up it does.
Now let’s say that the boundaries of our acceptable DOF are determined by an absolute MTF50, say 1400 cy/ph.
Then we get:
By that measure, the Otus has more DOF than the Nikon, which doesn’t even get to the bar at any distance at three f-stops. BTW, neither lens gets there at f/11 and f/16, because of diffraction.
If we lower the bar to 1200 cy/ph:
Yup, the sharper lens has more DOF by that measure.
At 1000 cy/ph:
And finally at 800 cy/ph, where we finally get all f-stops represented: