Wildlife photography by John Devries, Kent UK. Inspirational images
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.
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
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.