Underwater characteristics of light for underwater photography

The characteristics of light in water are different from its characteristics in the atmosphere. Underwater photography first has the problem of selective absorption of light by water.

Long wavelength light above 600 nanometers is highly absorbent in water. Below a water depth of 10 meters, red light that is effective for photography does not exist. Therefore, only natural light is used for underwater photography. The color is single blue, while all colors in the red series are expressed. is black. This phenomenon also affects shallow water, so underwater photography almost always uses auxiliary light (mainly flash) to compensate for the lack of red light. Using flash to assist photography, underwater photography has better color expression, detail expression and image contrast than natural light photography.

The second problem with underwater photography is that light is scattered by plankton. When taking pictures underwater, the contrast of the image decreases greatly due to the presence of scattered light in the water. Naturally, the longer the shooting distance, the greater the halo and contrast reduction rate caused by scattered light

in the water. Generally, when the shooting distance is more than 10 meters, the image contrast is extremely low, and practical photos cannot be obtained.

To fully express the scenery photographed in the water, it is necessary to limit the shooting distance to 2 meters

to less than 2 meters. For close-up photography and when the shooting distance is short, choosing to use a short focal length lens that can obtain a wide-angle image

is a way to obtain realistically expressed photos. When using flash photography, the picture quality is also affected by scattered light. The intensity of scattered light is greatest when the direction of the scattered light and the incident light of the flash are consistent, and is at its maximum when the two are at an angle of ±90°. Based on this characteristic, you should try your best to focus on the subject when shooting. Lighting at a certain angle

. Facing the subject from the front, the light reflected from the plankton in the water is strong, and the photo quality

is poor.

The third problem in underwater photography is the decrease in light quantity caused by light scattering and light absorption characteristics in water. Since the sun, a natural light source, is very high from the water surface, when the incident angle of sunlight relative to the water surface is in the range of 0 to 60°, the amount of light lost by reflection from the water surface is less than 10%. There is not much difference in the amount of light between photography in shallow water and photography on land. However, in seawater with a transparency of 30 meters and a water depth of 10 meters, the amount of light is only 50% of that in the atmosphere. The decrease rate of light intensity in water with low transparency will be greater, so it is necessary to choose a high-sensitivity film according to the shooting location.

The above is the case of natural light. This reduction in light intensity is even more severe when photographing with artificial light sources. The exposure determined based on the flash index (in the air) of artificial light sources such as flash lamps cannot be used directly. It must be based on the transparency, perspective and photography of water. It can be used only after the distance has been corrected.

Generally, when the photography distance is within 1 meter, the flash index in water is 1#2 in the air; when the photography distance is 1 to 3 meters, the flash index in water is 1#2 in the air. #3; When the photography distance is more than 3 meters, the water is 1 in the air

#4; When the photography distance is more than 5 meters, the effect of the flash with a large guide index is also very poor, and the use is lost

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When taking pictures in shallow water, the incident angle of natural light changes locally due to the fluctuation of the water surface. The result is that the amount of incident light is uneven and is directly reflected on the screen.

In addition, due to different wavelengths and refraction angles of light, decomposed monochromatic light also appears on the screen

. Another point in underwater photography is the change in the focal length of the lens caused by the refractive index of water (1.33). The size of objects seen in the water using diving glasses is larger than the actual size, and the viewing distance also feels closer than the actual distance

.

Photography through a lens is exactly the same as wearing underwater glasses, so when setting the photography distance, set it to 1.33 times the actual measured value.

In the case of visual inspection or using a single-lens reflex camera to observe directly through the viewfinder and adjust the focus, the distance between the camera and the subject is the photographic distance that has taken the refractive index into consideration.

At this time, just use the shooting distance to take pictures. In addition, when using a land photography lens to take pictures in the water, the focal length of the lens changes. At this time, 1.33 times the focal length of the land photography lens used should be used as the focal length for underwater use.

focal length.