FZ200 Diffraction Limit - Panasonic Tech Service

Started 8 months ago | Discussions thread
Ron Tolmie
Regular MemberPosts: 364
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Re: Resolution at long focal length settings
In reply to Ianperegian, 8 months ago

Ian:

The angular resolution (in arc sec) is inversely proportional to the aperture diameter:

sin (angle) = 1.22 x wavelength/aperture diameter

The focal length is not a factor in determining the angular resolution. If the lenses in a view camera and a miniature camera have the same angular angular resolution they will appear to be equally sharp. If the aperture diameter is 3mm and the lens has no optical aberrations then the image will be acceptably sharp, and it doesn't matter if the focal length of the lens is 4mm or 4000mm. If the aperture diameter is 1mm the image will be soft, irrespective of the focal length.

In comparing lenses that have the same focal length (such as the 50mm lenses used in FF cameras) it is a common practice to resort to a linear measurement (lines per mm) because it is easier to make such measurements. Unfortunately that leads to confusion if you try to compare lenses that have different focal lengths. If you have two lenses of equal quality (i.e. the same angular resolution) but different focal lengths the short FL lens will deliver more lines per mm even though it is not any better than the long FL lens. To make a useful comparison of the lenses you would need to divide the l/mm number by the focal length.

My statement about diffraction as a function of the aperture diameter was correct. If you want to make comparisons between measurements made at different focal lengths then you need to measure the angular resolution, not the l/mm. Otherwise you end up in a state of confusion.

Ron

I'm disappointed that you didn't address the quote I posted in my previous reply. Here is a shorter quote from the same source:

"Diffraction thus sets a fundamental resolution limit that is independent of the number of megapixels, or the size of the film format. It depends only on the f-number of your lens, and on the wavelength of light being imaged. " (My emphasis).

So, just to reiterate, it is NOT the case, as you claimed (quoting your first post):
"Since the diffraction is inversely proportional to the diameter the diffraction effects are much less significant at long focal lengths."

In fact, the diffraction effects are very much just as significant at long focal lengths, and the diffraction effects are the same as for short focal lengths for a given f number.

I think you became sidetracked in talking about the way resolution can differ, and certainly does differ for the FZ and ZS cams, at different focal lengths with the same f number setting, quite apart from diffraction effects.

To take an example where diffraction is not a limiting factor on resolution, for the FZ35/38 I carried out detailed resolution testing at different f numbers and FLs as shown here .

The results show that at max zoom (486mm equiv.) and the widest aperture of f/4.4 (which is not diffraction limited) the resolution is lower than at full WA (27mm equiv.) and the same f number. However, and this is the crucial point, the difference in resolution is not due to diffraction, it is due to the lens itself. I think that is the source of your confusion, in addition to your incorrect statement quoted above.

BTW, for my testing I used Jimmy's (JC Brown) coloured Es test chart and I did, of course, use a tripod and the self timer, and with the OIS switched off. I also took replicate shots at each setting to ensure that my results were valid.

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Ron Tolmie

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