Panasonic 20mm f1.7 question: depth of focus/field, how to make entire image in focus?

Started Apr 22, 2013 | Questions thread
Anders W
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Re: No
In reply to richarddd, Apr 24, 2013

richarddd wrote:

Anders W wrote:

richarddd wrote:

I believe a reasonable argument can be made that people have an easier time judging relative sharpness (they can tell X is sharper than Y when viewing both) than absolute sharpness (looking at one image without a reference for comparison).

If the center of an image were much sharper than the edges, a viewer could easily notice the difference.  If sharpness were uniform although at a lower level than something else, because they are only seeing the one image, it would not be as noticeable.

While people can certainly make comparisons of the kind you outline, that's not the only kind of comparisons they can make. If you have looked at enough images from enough lenses, you can certainly judge how good sharpness is in a particular part of the frame without reference to how good it looks in other parts of the frame. Moreover, that's exactly what I do when evaluating lenses. I don't look at how much worse the corners are in comparison with the center. I look at whether the corners and the center are both good enough to meet my expectations.

I'd imagine you are better able to judge "absolute sharpness" than most people.

Well, at least, I have a fair amount of practice.

However, I don't see any evidence the difference between edge and corner sharpness on the 20/1.7 is greater than the difference on most other lenses.

Neither do I. But I think Sergey's point here is that it is slightly bigger, if calculated in lp/ih, than for lenses with generally inferior resolution. For example, using the figures from LensRentals for center versus average across the frame, the difference for the 20/1.7 at f/2.8 is 1050 - 875 = 175 as compared to 720 - 590 = 130 for the 17/2.8.

My point is that this is perfectly in line with what we would expect if evenness across the frame is roughly the same. While the difference of course increases in absolute terms as the overall resolution level increases, the ratio of center to average is about the same (although actually slightly smaller for the 20/1.7): 1.20 versus 1.22 (i.e., 20 percent better in the center for the 20/1.7 versus 22 percent better in the center for the 17/2.8) and that this, rather than the difference in absolute numbers, is what is perceptually relevant to the extent that one makes comparisons between different parts of the frame in the first place.

Your argument makes more sense to me than Sergey's, as it comports with perceptions in other contexts.  It's easier to see the difference between 1 cm and 2cm than 1001 cm and 1002 cm.

Going back to my point, we might be able to see the difference between 1001 and 1002 if they are next to each other and lined up.  If we saw 1001 and 5 minutes later saw 1002, we'd have a much harder time, unless we were very practiced in that sort of measurement.

Right. And there are of course a lot of photographically relevant examples that can be added here. In fact, pretty much all photographically relevant quantities that come to mind are, or are thought of, in ratio terms. Among those that are explicitly ratio-based, one might mention contrast ratio, signal-to-noise ratio, dynamic range, and everything having to do with exposure (f-stops, shutter speeds, EV). Among those that are pretty much thought of in ratio-based terms although not expressed those terms, one might mention focal length and field of view. Either way, it is always a matter of how many times smaller or larger something is, not a matter of how much larger or smaller it is in absolute terms. An difference in FL of 5 mm is a lot if it is MFT WAs we are talking about but next to nothing if it is long teles we are talking about. Conversely, a difference in FoV of 5 degrees is not a whole lot if it is UWAs we are talking about but a very significant difference already for intermediate tele lenses.

In any event, this seems to be the sort of question that would be advanced more by posting test images than going back and forth on general principles.

That might well be correct. The problem might be to find examples that fit the bill. Any suggestions?

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