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Photoshop Tutorial - Lens diffraction


Photographers need to be aware of the phenomenon of diffraction in lenses as it affects image sharpness. All but the very best lenses work better when "stopped down" a little from their widest aperture, but as you continue to reduce the aperture beyond a certain point, diffraction starts to creep in and reduces sharpness significantly.

As you stop down a lens aperture, the light passing through tends to diffract, reducing sharpness, while Depth of field increases. The reason for this is that the edges of the diaphragm blades in the lens can disperse the light. At larger apertures the diffracted light is a small percentage of the total amount of light hitting the camera sensor, but as the aperture is stopped down (made smaller) the amount of diffracted light becomes a larger percentage of the total amount of light being recorded by the sensor.

It is useful to test each of your lenses to determine the point at which they are visibly affected by diffraction and work inside the zone that you consider acceptable.You are most likey to encounter diffraction when taking landscape shots or when working at high magnification in macro photography, as you will probably be using a small aperture in order to maximise the depth of field available. In the example below I have used a Canon 100mm f2.8 macro to examine the effects of diffraction.

Starting image - watch


Starting image

Camera - 1Ds mk II,
Lens - Canon 100mm macro f2.8. ISO 100. Gitzo Tripod, cable release, mirror lock up.
All images shot in RAW and converted in ACR in a single batch conversion.Unsharp mask applied identically -Amount: 300, Radius, 0.3, Threshold 0.

Lighting- two desk lamps outside a Maplins Portable Studio (£10 from Maplins order code A03BL)

 

Shots taken at various apertures -
100% crops follow

diffraction - Lens Canon 100mm macro f2.8

diffraction - Lens Canon 100mm macro f 4

diffraction - Lens Canon 100mm macro f 5.6

diffraction - Lens Canon 100mm macro f 8

diffraction - Lens Canon 100mm macro f 11

diffraction - Lens Canon 100mm macro f 16

diffraction - Lens Canon 100mm macro f 22

diffraction - Lens Canon 100mm macro f 32

From the above test you can clearly see diffraction at work. Lens sharpness improves on reducing the lens aperture from f2.8 to f4 and then holds a pretty constant image quality from f4 to f11. By f16 there is a small loss of sharpness (albeit still very useable) but by f22, sharpness has reduced significantly and by f32, I would consider the aperture unusable.

Although I now tend to try to use this lens at f16 minimum aperture, I still frequently use f22 when photographing sbjects such as single orchid florets - to maximise depth of field. The small loss of quality can be largely compensated for by applying slightly firmer sharpening. In the example below I have sharpened the previous f22 image for a second time with settings of Amount 57, Radius 1.6. Threshold 1.
I was careful to maximise sharpness while avoiding edge-halos while doing this.

diffraction - Lens Canon 100mm macro f 22

diffraction - Lens Canon 100mm macro f 22 sharpened twice