Exposure control of microphotography

Although all cameras now have automatic metering or automatic exposure function, if there are more than half of the bright areas in a field of vision, it is easy to be underexposed. Therefore, if the specimen is small and the background is bright, it is necessary to increase the exposure time appropriately.

When controlling exposure, we should pay attention to the following factors:

Influence of light source intensity: due to the change of voltage, the brightness of light source may change by 5- 10 times. Distance of light source and irradiation area of sample. The wavelength and color temperature of the light source. The light source has a long wavelength and a low color temperature. When the color light is red, the exposure time is longer. The light wave is short, the color temperature is high, and the exposure time is short when the color light is blue. The fluctuation of voltage will cause the change of exposure, so the voltage should be kept relatively stable when shooting. After experimental fixation, the position of condenser should not be changed. When using color filters, opal glass, polarizers, etc. , the exposure time should be increased according to its exposure index (manual exposure). Magnification of eyepiece. When the exposure benchmark is 1, the x 10 eyepiece is exposed 4 times and the x 15 eyepiece is exposed 9 times. Magnitude and N A value of objective lens: If the eyepiece× 1 0 is taken as1,the eyepiece× 40 needs 2.5 times exposure, and the objective lens×100 needs 4 times exposure. The distance between the eyepiece exit point and the photosensitive plate will also affect the exposure. The medium between the specimen and the eyepiece has a large refractive index and a short exposure time. On the contrary, the exposure time is long. The exposure time is inversely proportional to the refractive index. Changing the aperture of the condenser can change the exposure by 2 ~ 20 times.