It’s often tempting to stop down your lens quite a long way when photographing very bright objects, like the Sun at sunset or a bright full Moon, or to maximise your depth of field.
However, it’s not always a good idea, as lens diffraction will start to make your images look soft with high f-numbers.
A very good tutorial on lens diffraction, along with some calculators for common cameras, can be found at this link:
This website is wonderful! Thanks Mike, loads to read on. Is it the weekend yet??
I think the info on diffraction limits is useful and interesting but at the same time over states its importance and over simplifies the factors that lead to less than optimum photographs. It does suggest that other factors are also going to influence results, but not enough in my view. Camera lenses are not simple creatures like astronomical optics. They have to provide a focused image at different distances and usually these days at different focal lengths.
The piece basically demonstrates mathematically that larger apertures are better. It does mention that large apertures may also be softer and that’s an important thing to understand. The camera lens is a work horse and the optimum performance and results optically will always be reached at between F5.6~8. A smaller Airy disc may be achieved at larger apertures but the combined effects of coma, astigmatism, spherical and chromatic aberration will far outweigh it. In fact the most likely sign of diffraction issues is likely to be the spikes visible at small f numbers with point sources of light.
Of course if we then throw astronomical imaging into the mix then the rules change again as it will be the earth’s atmosphere that will dictate the maximum performance.
I don’t wish to put a downer on the info provided as it is interesting. However optics are not something that respond to magic formulas. They are a compromise and good optics come from experience. I am reminded of the first optical test of the APM-LZOS 152/1200 Doublet 3 years ago. This was designed by Massimo Riccardi, the current poster child for optical design. The computer modelling of this lens suggested what could be achieved from the design and glass used. When tested the lens outperformed the computer model. Although delighted, APM did not understand how this was possible. They asked Riccardi. His answer was he did not understand himself. When asked LZOS replied, ‘this is our secret’.
Getting the most from optics is also much the same and these days a combination of everything from the conditions under which the image was taken, to understanding how your cameras sensor works at its best to using post processing to maximise the results are all in my view better things to learn.
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