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Particles of water vapour suspended in the atmosphere. These particles tend to reflect the rays at the blue end of the spectrum, but they are more transparent to the rays at the red (and particularly the infra-red) end. The result is that light shining on to mist looks bluish-white, and light shining through mist looks yellow or orange.
When an object is seen through mist, the mist reflects the blue and violet rays and only allows the rays towards the red and infra-red end of the spectrum to pass through. At the same time, light falls on the mist from the observer's side and the opposite thing happens: the redder rays pass through the mist and away from the observer, while the violet and blue rays are reflected back towards the observer. So the object appears veiled with a bluish-white haze.
To the camera, matters are even worse, because the normal photographic emulsion is more sensitive to the blue end of the spectrum than to the red. So that in the ordinary photograph of a misty scene, the effect of the mist is much more noticeable than it is to the eye.
The effect of mist can be minimized by using a filter which holds back the violet and blue rays. Yellow, orange, and red filters are suitable; the deeper the tint of the filter, the less the mist will show in the photograph. If the mist is to be removed as completely as possible, as in the telephotography of distant objects, a special infra-red sensitive emulsion is used in conjunction with an infra-red filter.
Mist suppresses the details of the scene and leaves only the main masses visible.
It divides the whole scene into clearly defined planes, with near foreground objects appearing as very dark shapes, while those farther away become rapidly lighter in tone until the more distant simply merge into the general whiteness of the mist itself.
So mist photography is usually concerned with broad, simple ideas.
Because the mist scatters light into the shadow areas of the subject, photographs taken in mist should be given a shorter exposure than indicated by any of the normal types of exposure meter or calculator (these guides to exposure always assume normally-lighted shadows).
The beauty of a mist photograph is in its softness; any tendency to over... or even full, exposure produces a negative with too much contrast for this class of picture. For the same reasons, development should never be continued beyond the recommended time, and a soft-working developer should be chosen.