This post is first in a series of posts about the science of photography.
For most of my life I have been both a computer engineer and a photographer.
Here are some ways that you can use science to make better photographs. I have friend who is a terrific computer engineer, but is new to serious photography. This is for him and anyone in a similar situation.
Making fine photographs is an art, and in art there are few rules. However, there are two qualities of a good photograph that benefit from science. A good photograph is usually sharp (it shows sufficient detail), and displays a good range of colors or blacks and whites. Of course there are other artistic considerations but I don’t know how to write about them.
It’s useful to know some physics so you can make sharp photos with a good range of colors or tones.
It’s comforting to know that physical laws are never going to be changed or violated by technology. Ignore any claims to the contrary.
Photographic technology has evolved enormously in the roughly 200 years since its invention. I won’t spend any time on the details. Where we are now is what counts. Modern digital cameras, (especially Digital Single Lens Reflex Cameras – DSLRs) with modern lenses and computer technology can create amazing images; much better than just a few years ago.
Knowing the laws of physics will help you avoid wasting time trying to make impossible pictures. The laws of physics are much kinder to a good DSLR than to a tiny point-and-shoot or a phone camera. So using a DSLR is the first step. Compact digital cameras with large sensors are almost as good.
It All Starts With Light
Light comes from the subject through the lens and exposes the sensor or film. Getting the right amount of light is the key to recording a good range of colors or tones. Here is a simple formula that governs the exposure of an image in a camera. It relates four variables. Multiplied together, they give a constant.
(Exposure Time) x (Sensitivity of Sensor) x (Relative Area of Lens) x (Scene Brightness) = 1
Exposure time is easy to understand. It is just the time the shutter is open. This is nearly always given as a fraction of a second. Like 1/250, 1/1000, etc. These are usually abbreviated as just 250 or 1000, which can be confusing because larger numbers are a shorter time. This is also called shutter speed. This camera dial (on a Nikon F3) shows speeds of 1/2000 all the way to 8 seconds.
Sensitivity of Sensor (or Film):
Modern cameras can adjust their sensitivity to light, automatically or on demand. When film was king, film sensitivity was set at the film factory, and to an extent, also during developing. The principle is the same on a digital sensor, but it’s set just before taking the picture. Sensitivity is given as ISO numbers, like 100, 200, 400, 1000, 2000. The lower the ISO number, the less sensitive the film (or sensor) is to light.
Relative Area of Lens:
The job of the lens is to take light from the subject and project an image onto the sensor.
A bigger lens can gather more light and deliver it to the sensor. That much is obvious. But how this is expressed can be very confusing. Lenses are marked in f-stops, or f-ratios. The f-ratio for a simple lens (like a magnifying glass) is just the distance of the lens from the film or sensor divided by the diameter of the lens. Real camera lenses are not simple at all, but the principle is the same.
Camera lenses have an iris or aperture stop. It is a clever mechanical device that works like the iris of your eye to shrink down the effective diameter of the lens. This can cut out a large fraction of the light.
The standard f-ratios are 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22,… (Sometimes written 1:1.4, 1:2, etc.) Why this strange set of numbers? Notice the square root of two and its multiples? Each number represents half the light gathering area of the previous number. For example f 2.8 is half the light gathering area of f 2. f 2 gathers four times the light of f 4.
These numbers are the reciprocal of the relative lens diameter. The light-gathering area is the diameter squared. Again, larger numbers mean less exposure. (Seems the old time photographers thought backwards.)
Direct sunlight is the brightest light we normally find. Indoor light is much dimmer, and night scenes are even dimmer. Photographers don’t usually assign numbers to scene brightness. Instead we usually say the exposure time, the sensitivity, and the f-ratio appropriate to a scene. Or we just describe it as bright sunlight, open shade, etc.
It’s easy to see that reducing the exposure time by half, and doubling the lens area, will give the same exposure. If the clouds roll in and a scene becomes 10 times dimmer, you can make the sensitivity 10 times greater and still get a good exposure. Likewise we can juggle any of the 4 variables to get a good exposure.
So how do you choose? What are the effects of adjusting the four variables? That’s the subject for next time.