Over the past year the attention of photographers has been drawn to the problem of convection currents in the air affecting the definition in photographs taken at a long distance using long focal length lenses. The different densities and refractive indices of hot and cold air cause a shimmering effect as warm air rises through cold. The effect may be obvious or subtle. However, with a long distance between subject and lens that slight shimmer is sufficient to be picked up as a loss of definition.
All sorts of advice has been be given about avoiding less obvious pitfalls. 'Do not take photographs over a car’s hot bonnet'; 'be careful when using a lens hood since warm air may be trapped inside', etcetera, etcetera. Oh, and 'do not blame the lens'. Facebook Groups (the source though of a great deal of misinformation) and other photographic forums have seen much repetition.
Those of us in Britain and around photography since the 1950s will remember how cameras were tested for Amateur Photographer magazine. Somebody would do the short walk from the office to the Thames, point the camera at HMS Wellington, a sloop which escorted 103 Atlantic convoys in the Second World War, press the shutter, move the ship to the edge of the field and press the shutter again. Enlargements from centre and edge were published as part of each review. That went on for years until some bright spark pointed out that any differences were more likely due to changes in atmospheric conditions and at slow shutter speeds the rise and fall of the tide and the flow of the river would also be apparent.
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| A typical lens test from Amateur Photographer from Photographic Memorabilia website |
Incidentally, anybody wishing to test their own camera and lens 1954-style can still do so. HMS Wellington, the Thames and the Embankment are still there.
The information on Amateur Photographer can be found on the excellent website Photographic Memorabilia.
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