FZ200 Diffraction Limit - Panasonic Tech Service

Started Aug 27, 2013 | Discussions thread
Ianperegian Veteran Member • Posts: 3,596
Re: FZ200 Diffraction Limit - Panasonic Tech Service

Ron Tolmie wrote:

Since the diffraction is inversely proportional to the diameter the diffraction effects are much less significant at long focal lengths.

Hi Ron,

I agree that the Panasonic "tech" staff response is misleading because what does "begin" mean in practical terms?  Also, it depends on the criteria used for calculating the Airy disk and the acceptable circle of confusion.

I don't agree with what you said above though.

For a given lens/sensor system the Airy disk diameter depends on the f number, and is independent of the focal length. As explained on the Cambridge site (here ), quote:

"Technical Note: Independence of Focal Length
Since the physical size of an aperture is larger for telephoto lenses (f/4 has a 50 mm diameter at 200 mm, but only a 25 mm diameter at 100 mm), why doesn't the airy disk become smaller? This is because longer focal lengths also cause light to travel further before hitting the camera sensor -- thus increasing the distance over which the airy disk can continue to diverge. The competing effects of larger aperture and longer focal length therefore cancel, leaving only the f-number as being important (which describes focal length relative to aperture size)."

I found that to be the case when I tested the variation of resolution at different apertures and focal lengths on my ZS20 (and earlier on my FZ38).  At f/8, where diffraction was decidedly the limiting factor, the resolution was similar at all focal lengths.


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