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Lens diffraction in photography8/30/2023 ![]() ![]() My immediate impression was that f22 is essentially unusable for landscape work unless I have a pressing need to keep the aperture small. Detail drops away rapidly after f8 with f22 being the softest of all the apertures. It became rapidly apparent that the sharpest aperture on this particular copy of the lens is F5.6. Below are the results of a 100% crop from a section just to the right of centre. The above image is the very bland shot that I chose to perform the test. There wasn’t any wind, so the images should to all intents and purposes be shake free and in focus. I set the tripod up and released the images via a cable release on Mirror Lock-up after having focused and checked that focus in Live-View zoomed in. Then I realised that I don’t shoot bank notes and test-charts for a profession, so instead took a series of images while on my last workshop in the Drakensberg. I initially tested the camera and a 24-120mm f4 lens (recommended by Nikon and DXOMark for use with the D800) on a bank note and test chart in studio. Which finally brings me to my own test on my D800 camera. For most people though, we can try to use sharpening techniques in post-production to hide any loss of resolution as a result of the diffraction inherent in our lenses. Some landscape photographers get around this by using tilt-shift lenses so that they can get increased depth of field via something called the Schiemflug effect. Photographers ultimately hav e to make compromises between depth of field and sharpness when they are creating images. Most other lenses tend to be at their sharpest and have the least amount of vignetting (another phenomenon in the way that light passes through the lens) 2 stops from wide open (so if you have a f5.6 lens then theoretically it will be at it’s sharpest at f11)for some more details on this you can also read my post on confusing circles and airy disks on this link. Long exotic telephotos are usually designed to be at their optical sharpest wide-open. Lenses are designed for particular purposes to be at their sharpest at a certain aperture. Diffraction in the glass is also apparent in the way that light spreads as it hits a transparent medium. Sadly this doesn’t mean that opening up the lens to wide open will result in tack-sharp details in the image capture. Small apertures will consequently result in slightly softer images in comparison to larger apertures where there is less interference with the edge of the aperture blade in relation to the size of the aperture itself. If the aperture is closed down to a very small hole, there is effectively more interference at the edge of the aperture with the light in relation to how much light is passing through the aperture. Now take this visual demonstration and apply it to the light travelling through a lens and past the aperture blades onto the sensor or film plane. if the cardboard with it’s hole is brought closer to the wall the spot will appear more solid, the further the hole gets from the wall, the smaller it is in relation to the circle of light on the wall and the less distinct the edge of light on the wall is (closer the hole, the more ‘focussed’ the light, further the hole, the less ‘focused’ the light). ![]() The light that hits the wall will have a bright central spot that will gradually fade to its outside circumference. Imagine a small hole in a sheet of black cardboard that we shine light through onto a wall. In optics diffraction is the limiter on the resolution of any optical device (camera, telescope, microscope etc.) For photographers diffraction is both inherent in the way that light passes through the glass medium of the lens and is refracted, and in the way that it must travel through a small aperture before it finally lands on the sensor or film. In physics diffraction it refers to and explains the apparent bending of waves around small objects or the dispersion or spreading of a wave past a small opening. This is what causes chromatic aberration in our images where the red and violet wavelengths of light become increasingly separated towards the edges of the frame where the light has has had to ‘bend’ the most. Passing through a transparent medium the light refracts and can diffract due to the different wavelengths of light travelling at slightly different speeds. Light travels as a wave and when it meets an obstacle in its path it is affected by that obstacle.
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