Lens Diffraction

May 8, 2014
Lens Diffraction

Photographers are generally obsessed with sharpness. Put your hand up if you’ve ever scutinised a print just a few inches from your nose to check its sharpness?

Hmmm, perhaps it’s just me then.

Ultimately it’s the lens, the aperture you choose, the focus point and how steady your camera is that determines the sharpness of an image.

Keeping your camera steady is fairly straightforward. If you handhold your camera and wobble like a jelly during the exposure you’re more likely to get unsharp shots (the longer the shutter speed, the greater the likelihood). A good rough-and-ready rule is to set a faster shutter speed than the focal length of the lens. As an example, if the lens is 50mm the shutter speed should be faster than 1/50 (if the lens or the camera has image stabilisation you can bend this rule to a certain degree). However, if the desired shutter speed can be measured in whole seconds then it really is time for a tripod.

Next let’s consider the lens. There’s no getting away from the fact that prime lenses are usually optically superior to zooms. However, modern zoom lenses are generally perfectly acceptable. Less-than-perfect technique is usually more to blame for imperfection than defects in a lens. Used well a good zoom lens is more than capable of creating exhibition-worthy prints.

The aperture you choose and where you focus probably has the greatest bearing on the sharpness of an image. The size of the aperture determines depth of field – essentially the extent of the image that is sharp. Depth of field extends out from the point of focus, which is why where you focus is so important. The smaller the aperture you use, the further depth of field extends (extending further away from the camera than towards it).

The temptation therefore is to use the smallest aperture possible to maximise depth of field. However – and didn’t you just know there would be a however? – the smaller the aperture you use, the greater an effect known as diffraction starts to be apparent. Optically a lens is usually at its peak two or three stops from the maximum aperture. Unfortunately the edges of the aperture blades in the lens cause light to be scattered randomly and it is this effect that reduces overall sharpness across the image. The smaller you make the aperture, the greater the light scattering – or diffraction – and so the softer the image.

This when a concept known as hyperfocal distance comes in useful. For every aperture setting and lens’ focal length there is a specific distance that the lens can be focused where everything from half that distance to infinity will be acceptably sharp (it’s slightly more complicated than that because the physical size of the camera sensor or film area also has to be taken into account but that’s the general principle). The upshot is that it’s possible to calculate the exact aperture needed to keep everything that needs to be sharp sharp and so avoid using smaller, more diffraction-prone apertures.

Once photographers who cared about such things used to carry around hyperfocal distance charts for their various lenses. Now, it’s possible to use Smartphone apps to do all the hard work for you. I use an app called PhotoCalc for my landscape photography, though there are many other apps for both Android and iOS that do a similar job.

Arguably the most influential American landscape photographer of the 20th century was Ansel Adams. He once said that “There’s nothing worse than a sharp image of a fuzzy concent”. It’s true, just because an image is sharp doesn’t mean that it’s interesting. However, you don’t want a good shot to be ruined because it wasn’t sharp where it should have been. Even if it’s only me who peers too closely to see.

Originally posted on MyPhotoSchool

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