Dear Georg,
I'll try to answer your question and explain this as simply and shortly as possible.
The diffraction of a certain lens, respectively what is the maximum stopping down before diffration becomes visible, is depending on different factor:
- the film size used or the digital medium size
- the reproduction scale of the subject on this capture medium (analog or digital)
- the f-stop/aperture used
- the viewing distance of the final image (screen, print, photo print for exhibition, newspaper print, etc ..)
What happens during diffraction is that the light rays entering the lens are deviated from their linear course and broken at the edge of the aperture blades. This will induce, roughly said, interferences with the "normal" light rays entering the lens opening. These broken/diffracted light rays from the edge of the aperture do create reproduction points on the capture plane, the more the aperture is closed, the bigger those reproduction points will become. When the size of these points reaches or overcomes the size of the "normal" reproduction points of the subject, created by the normal light rays entering the lens, this will show up with an additional "unsharpness" or fuzziness. Please refer to the attached image which gives you a good visual idea of what happens (image 1). It can be basically and to understand better what happens, be compared to some wawes entering a sea port and hitting or being broken at the jetty of a port (see image 2). The smaller the opening of the jetty, the bigger the interference of those broken wawes with the rest.
Now, when does this diffraction have an influence on the overall image? As said, the amount of diffraction depends on the opening of the lens. But it also highly depends on the reproduction scale of your subject: in other words, how big it is reproduced on the capture medium compared with its original size. Or put differently, how long is the distance between your lens and your film plane: the smaller the reproduction scale the smaller the distance between lens and film/sensor plane. The shorter the distance the less dramatic the effect of the broken light rays and their produced fuzziness in the image (or the longer the distance = bigger reproduction scale, the more effect those broken rays will have. In other words, depending on your reproduction scale you can stop down more or less before the diffraction becomes visible in your image.
The next factor having an influence on diffraction is the size of your capture medium, coupled with the viewing distance: the bigger the image, the longer should be the viewing distance of this image. there is a general rule which says that the viewing distance should be twice the diagonal of the image size. So this influences also the diffraction and when it becomes visible. But let us leave this on the side and take in account only the 2 previous influencing factors.
I have put you another image/table, showing the different film sizes, in relation with the normal viewing distance, and the resulting maximum "allowed" size of the reproduction points created by the broken light rays before creating a for our eyes visible fuzziness in the overall image. Unfortunately I don't have the values for digital sensors, only for the different film sizes existing, and I would have to calculate them in relation to the size of the pixels. But it gives you an idea what is in play and how it works and the maximum "allowed" size of the reproduction points from the broken light rays (image 3) to not add fuzziness to the overall image.
Then I have included a table (image 4), which gives you the maximum stopping down/aperture at the different possible reproduction scales and/or at the different lens to film/sensor plane distance. This table is for 4x5" film, but one can calculate easily the maximum apertures based on other capture formats. this maximum aperture is also called "critical aperture". Forget about the 4th part in this image, called "Corrections Factors": that is something different and not in relation with diffraction (I just can't take it out).
I hope this makes some sense and is understandable.
Kind regards,
Thierry
Greetings,
I am trying to understand something about diffraction with reference to lenses such as the Sinaron Digital AF Planar T* 2.8/80
The future in optics appears to look like this here:
-
Charles S. Johnson, Jr. Professor of Chemistry Emeritus at the University of North Carolina at Chapel Hill. -
However, if I look at the datasheet here :
Sinaron Digital AF Planar T* 2.8/80
I have trouble understanding how to read that in respect of the optimum aperture on such a high end lense. May be it is not possible to read that from the given information, at least I can not conclude above what aperture diffraction comes into play, but this is just my own lack of education in optics I suppose.
See, in the DSLR world, I know if I go over f11 on my zuiko digital 14-54mm, diffraction will come into play. I know this from experience and from talking to people who use the same lense.
But what I wonder is, can such information be learned about just by looking at the datasheet?
Sorry if this is a very basic question, but I find it always interesting to know in what range a lense performs on it's very optimum, and if there would be an easy way to know this for any given lense, that would be helpful in deed to minimise the learning curve on new optics.
