FZ200 Diffraction Limit - Panasonic Tech Service

Started Aug 27, 2013 | Discussions thread
J C Brown Senior Member • Posts: 1,595
Re: Diffraction Limit - A bit of revision.

photoreddi wrote:

J C Brown wrote:


As you have correctly stated:

The equation for the diffraction limit is

sin(angular resolution) = 1.22x(wavelength/aperture diameter)

However it is important to recognise that the above equation defines an “angular” limit

To relate that to the corresponding linear limit at the focal plane it is necessary to take account of the focal length. What follows is based on my Napier University lecture notes.


Have you taken this into account?

Yes I have. If you read my post again carefully you should find that it is entirely consistent with the Cambridge in colour Technical Note which you quote.

In particular if you check the final equation for the diameter of the Airy disc "d" you will see that d = 2.44 x N x lambda defends only on the F/No N and the wavelength lambda and is therefore independent of focal length.

Perhaps I should have emphasised that fact and stated that the analysis I presented was entirely consistent with the results of ianperegians resolution tests and with the resolution tests that I've done with my FZ50 and TZ30. See for example: Resolution measurements - TZ30 (ZS20) - Many images

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).



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J C Brown

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