The limits of optical zoom

How image quality is affected by optical zoom

To record videos, images need to be projected on the cameras image sensor. Earlier models used electron tubes, todays cameras use semiconductor based sensors, CMOS for example.

The optical zoom is basically a telescope in front of the camera. Typical magnification rates are 25x for TV cameras, 50x for sports objectives and 100x for telescopes. Though, there is no limit for optical zoom.

But, the image quality decreases with increasing zoom level. Two problems occur when zooming in: chromatic aberration and blur.

Chromatic aberration causes the image to be divided into all its different colors, comparable to the colors in a rainbow. Because each color has a different index of refraction, each color channel of the image appears to be at another position. For video cameras only red, green and blue are of importance. So the chromatic aberration in video cameras causes the color channels to appear at different positions on screen.

There are three different ways to reduce the chromatic aberration: moving the channels with software to the right position, using higher focal lengths, which makes the cameras bigger and using so called “optical glass” as lens, which is lighter than normal glass and therefore has a reduced refraction. Pure quartz glass is a very good optical glass. Non-optical glasses are also made of quartz glass but with additives, that lower the production cost.

Blur is another problem that occurs when zooming in. This has to do with the wave nature of light and the probability of sharp light getting through the lens. When the diameter of the aperture is too small, not enough light gets trough to make the image appear sharp. Photographers might experience the opposite effect, which is due to enhanced depth of field. When the aperture size is too big, there is too much light, which doesn’t appear as a sharp image.

Smaller exposure times can compensate too much light. Though long exposure time cannot compensate too small aperture sizes and may cause noise in the image. For RGB cameras orange objects cause the most image noise and white objects like walls and buildings cause the least noise.

Finally, it’s hard to tell which camera zoom objective to use for a certain purpose. It is highly recommended to try before you buy, because there are other quality aspects and design issues to consider.