Thank you very much for your recommendations about diffraction aspects. Some of the posts point out, that not only mathematical approach is sufficient, because there are many other reasons (defocusing, shutter vibration, etc.) for decrease of image sharpness. And sometimes higher f-stops might not influence the image quality very badly. Additional, few post processing software programs are able to reduce diffraction.

That is surely right, but when I’m in the field with my camera I like to know which maximum f-stop I generally have to respect to be on the safe side. So, for myself I like to summarize the information’s for my personally better understanding and for that I like to follow only the mathematical approach. My camera is a Hasselblad H6D-100c with a pixel stitch of 4,6 µm.

I noticed that diffraction first becomes visible for 100% crop if the relation between Airy disk and pixel stitch is not balanced. The Airy disk A is calculated by the formula

A = 2,44 * lambda * N

N is the lens f-stop and lambda the average light wavelength (yellow-green lambda = 0,55 µm). The diffraction calculator e.g. of the website photopills (

https://www.photopills.com/calculators/diffraction) points out, that diffraction becomes visible, when the product of 2,5 and sensor stitch p is larger than the Airy disk

2,5 * p > A = 2,44 * lambda * N

Due to this relation the maximal N (f-stop) for a lens before diffraction may become visible must be approximately (2,44 ~ 2,5)

N < p / lambda

Bill is calculating for the diffraction limit with the factor 2,0 instead of 2,5 for the maximum N (f-stop), which leads to smaller N (f-stop) values

N < 2/2,44 * p / lambda = 0,82 * p / lambda

Jim points out in his blog about Q (

https://blog.kasson.com/the-last-word/whats-your-q/) that the factor Q relating diffraction is defined as

Q = 2 * Fcsensor / Fclens = N * lambda / p => N = Q * p / lambda

For the maximum N (f-stop) this leads with Q = 1 to the same relation as above mentioned

N = p / lambda

For a poor amateur Hasselblad photographer with a 100 MP H6D camera (pixel stitch 4,6 µm) this means that the maximal N (f-stop) due to diffraction is

N = 4,6 µm / 0,55 µm = 8,4

The Airy disk A is

A = 2,44 * lambda * N = 2,44 * p = 2,44 * 4,6 µm = 11,2 µm

So, I am on the safe side with my H6D camera with the f-stop of N = 8. For the higher sophisticated Phase One 150 MP XF camera with a smaller pixel stitch of 3,76 µm the maximal N (f-stop) is corresponding to the above formula

N = 3,76 µm / 0,55 µm = 6,8

How in reality diffraction is visible when we use higher f-stops than the maximal calculated N can be discussed as mentioned before. This is only the mathematical approach. For prints the circle of confusion (CoC) with higher possible f-stops may be more important.

But generally, more and more pixels on a same sensor area size leads to smaller pixel stitches and lower maximum N (f-stop). This means at my point of view that newer lenses must deliver the best performance at smaller N values (f-stop). Due to smaller N (f-stop) the Depth of Field (DoF) is also decreasing.

On the other hand, the sensor quality increased in the last years (e.g. BSI sensors), so the image quality today is excellent. Also, we have with higher amounts of sensor pixels a better oversampling, that results in reduction of artefacts and in that way in a better image quality.

But what is the limit for the amount of sensor pixels on the big 54x40 full medium format sensor area in the future? Do we need more and more pixels or do we need pixels with more or other features?

Thanks for any response, George