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Author Topic: HC 80mm becomes less sharp with apertures above 8.0... Any ideas why?  (Read 5986 times)

ErikKaffehr

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Re: HC 80mm becomes less sharp with apertures above 8.0... Any ideas why?
« Reply #20 on: December 04, 2011, 03:42:26 am »

Hi,

Actually, I would say that David is correct on some of the issues.

1) The sensor that both the original poster and David is using has relative large pixels. For diffraction to come into play the Airy ring needs to be at least as wide as the pixel. I have made measurement 6 and 7.8 micron sensor while Dave's is at 9 microns.

The experiment I made with my first two APS-C cameras having 7.8 micron and 6 micron pixels gave the following results:




The large pixel sensor produced about 1400 lw/ph at f/8 but only 1100 at f/16, that is 78.6%. Now the image has two dimensions squaring 0.786 we get 0.617. So the 6 MP sensor is in effect converted into a 3.7 MP sensor.

The same calculation for the 6 micron sensor:

1200 / 2000 -> 0.6, 0.6^2 = 0.36, 10 * 0.36 -> 3.6 MP. So stopping down the 10 MP sensor to f/22 would make it into a 3.6 MP sensor.

The sensor on the H3D-22 has 4.6 times larger surface, so my guess is that the same experiment I have made would come up with 16.8 MP, a minor loss. A P45 with 39 MP would also have 16.8 MP at f/22 (I think it has the same sensor surface as the Hasselblad H3D-22.)

2) Stopping down to achieve decent sharpness across the whole image might be a good idea, especially if only a little resolution is lost.

3) Obviously with diffraction the more you have, the more you have to loose. So if anyone is buying the best lenses and highest resolution backs he/she should probably be careful at stopping down beyond f/16.


Best regards
Erik



That sensor is a bit smaller than 6x4.5, it is 48x36mm, so the diffraction limited f-stop by that calculator would be about f/17 ... putting f/22 somewhat info the diffraction limited regime.

Perhaps you are both right but saying things differently. Here are a couple of different ways that people might want to compare. The key is the distance the light gets smeared in the focal plane is proportional to the aperture ratio (f-stop), and the wavelength of the light, and nothing else except some minor variations in the effect due to lens design. Thus the threshold F-stop for adequately low diffraction effects scales in proportion to the pixel size, or perhaps it is better to say it scales inversely with the lp/mm of resolution that you require in the image formed at the focal plane.

A. Aiming for the same image resolution, like with equal pixel count (or in the old currency, equally many line pairs per pictur height of resolution) which is what that Cambridge in color calculator is set up for:
The "diffraction limited threshold" f-stop varies in direct proportion to linear format  size (because it scales with pixel size, and the larger sensor has proportionately larger pixels in this scenario). By the way, DOF scales the same way if you judge by equal sized prints, so you get the same maximum DOF when aperture is limited to get the same resolution on same sized prints.

B. With equal pixel size and the larger format being used to get a more detailed image, suitable for larger prints intended to be viewed from equally close:
Then getting the full resolution offered by the pixel count requires keeping the diffraction spot below the same size limit, and that sets the same f-stop limit. By the way, this means that the maximum DOF attainable decreases as the "total image detail" desired increases.
« Last Edit: December 04, 2011, 03:49:29 am by ErikKaffehr »
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