Jman13
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 p.28 #6 · Pre-order: Fujifilm GFX 50S Medium Format body ($6,499) | |
People need to stop conflating softening due to diffraction and softening due to lens design at smaller apertures. They may have the same effect on the image, but they do not have the same cause. A lens with an aperture at f/11 causes the same amount of diffraction as any other lens at f/11. The amount of diffraction is based on the focal length, physical aperture size and wavelength of light. In the equation to describe the airy disk, the relationship between focal length and physical aperture size is f/d, which is also the definition for f-stop. So the amount of diffraction in a projected image is SOLELY based on f-stop and wavelength of light.
Other image degradation, that may happen due to lens design, may also happen at less than optimal apetures. (Most lens designs aren't as good at f/16 as they are at f/8, just like they aren't quite as good in most cases at f/1.4 as they are at f/5.6). You will see additional softening due to this as well, but it isn't due to diffraction. Diffraction limiting is something that is very easily calculated by simple equations about the known properties of light.
If you want to know the math behind it, look here: https://en.wikipedia.org/wiki/Airy_disk
Specifically, the part in Examples about Cameras has the equations as it relates to diffraction limitations in relation to f-stop.
Using this calculation, you can determine the resolution where a sensor becomes diffraction limited for each f-stop. For instance, if we look at a 24 megapixel Full frame sensor, it becomes diffraction limited (from a pure sampling standpoint) at f/11. That's where 24 MP can't be resolved, and you start softening the further down you go.
However, for a 24MP sensor in this mini-MF sensor (7.15 micron pixels), you get limited at f/14. So f/14 is where 24 megapixels becomes limited...a 2/3 stop advantage, and exactly what we'd expect to see. This Fuji will hit diffraction limitation at f/11 as well, but that's diffraction limitation on a 50 megapixel sensor, so that's where it stops resolving 50 MP, and starts going down (the further down you stop). With oversampling, the resulting resolution will also be better than the pure diffraction limitation suggests.
FWIW, the A7R Mark II hits the diffraction limit at f/9...that's where it stops resolving 42 MP of detail. However, with both shot at f/11, the A7R II will still yield a more detailed image than the A7 II due to oversampling.
Going back to format, a quick way to think of why larger formats show less diffraction at the same f-stop is to think in terms of enlargement. The airy disks are going to be the same size projected on the sensor at the same f-stop, regardless of what's behind the lens...if you take a certain image formed with that level of diffraction and enlarge it to say, 8x10, you have to enlarge the airy disks more on the smaller sensor than you do on the larger sensor, so the resulting final image will be softer on the smaller sensor when both have the same physical level of diffraction from the same aperture lens.
Edited on Sep 21, 2016 at 07:35 AM · View previous versions
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