This discussion will start by describing general concepts using non-digital cameras on some pictures, but do not fear, the same concepts apply for digital cameras. (In fact the examples I used to illustrate the topics were taken with my Olympus 3040 Digital. Please note that the pictures may not be the optimal examples, and I may reshoot them at some time, but they get the point across) But instead of turning rings and knobs, you are pressing buttons to get the same functionality. In the digital section below and in the review, these concepts will tie in.
When you adjust various items on your camera, the amount of light that reaches the film changes. When the final picture is does not look too light or dark you have the proper "exposure". If the photograph is too light (the photograph appears washed-out) it is "overexposed". If it is too dark (you see more of a black or colored blob, or less detail) the photograph is "underexposed". Three main things affect exposure, the f-stop, shutter speed and film speed. They all work together to provide the proper exposure. Sometimes you will here people speak about things called "stops" and that a photograph was 1-stop over or underexposed.
The lenses on you camera have a ring that have a series of numbers that usually range from 2.5 to 16 or 22. These are f/stops.
F-stop Settings on a traditional U/W lens. This one is set at f/2.5 |
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In a nutshell, there is a hole at the back of the lens that changes in size as you change the f/stop. As the number of the f/stop gets larger (from 5.6 to 8 for instance) the hole becomes smaller, and vice versa. Now, the larger the hole is (the smaller the f/stop number) the more light is let in to the film.
There is also something called "depth of field" which basically is how much of the image other than the point you are focusing on is also "in focus". The greater the depth of field (obtained with a higher f/stop number) the greater the portion of the photo is in focus. (Both in front and behind the area that you are foccusing on.)
| f/stop |
Aperture or "Hole" Size |
Light In |
Stop |
Depth Of Field |
| f/4 |
|
|
|
|
| f/5.6 |
less than f/4 |
less than f/4 |
1 Stop Down From f/4 |
greater than f/4 |
| f/8 |
less than f/5.6 |
less than f/5.6 |
1 Stop Down From f/5.6 |
greater than f/5.6 |
| f/11 |
less than f/8 |
less than f/8 |
1 Stop Down From f/8 |
greater than f/8 |
b.) Shutter Speed
Your camera will have a dial or some other mechanism to set your shutter speed. You will see numbers such as 1/30, 1/60, 1/90, 1/125 and so on. These are shutter speeds and are based on seconds. So 1/30 means 1/30 of a second. There is a "eyelid" over the camera hole described above that opens, remains open for the time set in the shutter speed, and then closes. Sort of like blinking your eye in reverse :). The longer the shutter remains open, the more light reaches the film. A thing to keep in mind is that faster shutter speeds are better for capturing action and also helps reduce camera blur, since there is less of a chance for you to move the camera while the shutter is open. So for shooting that shark that is cruising by you you are going to want a higher shutter speed if possible, while you can use a slower shutter speed for shooting the coral or starfish, since they move a bit slower than sharks. Except for nurse sharks of course. They usually just lie there. (Finally, a reference to something underwater :) )
| Shutter Speed |
Light In |
Stop |
| 1/30 |
|
|
| 1/60 |
less than 1/30 |
One Stop Down From 1/30 |
| 1/90 |
less than 1/60 |
One Stop Down From 1/60 |
| 1/125 |
less than 1/90 |
One Stop Down From 1/90 |
c.) Film Speed
When you buy film, you see those numbers with the film speed. 100, 200, 400, 800 and so on. The film with the higher numbers are referred to as "faster" films. The advantage of faster films is that they need less light to be able to obtain a properly exposed photograph. The downside of a faster film is that they have a tendency to produce more "grain" (those small dot patterns that you see.) Another thing about film is that different films have different color characteristics. You can take the exact same picture with the exact same settings on you camera, and depending on your film, a picture may be a little more red or blue. For instance Ektachrome Slide Film has a tendency to make blues underwater look "bluer"..
| Film Speed |
Light Required |
Grain |
Stop |
| 100 |
|
|
|
| 200 |
Less than 100 |
More than 100 |
One Stop Down From 100 |
| 400 |
Less than 200 |
More than 200 |
One Stop Down From 200 |
d.) Exposure
As mentioned earlier f/stop, shutter speed and film speed all work together to get your exposure - when your f/stop, shutter speed and film speed all are at the proper settings for a particular lighting situation then you get the "correct" exposure. If you have a proper exposure for a picture set up, and change any setting item, you then HAVE to change one of the other settings to achieve the proper exposure. I think an example will make it clearer.
Assume for second that you are outside taking a picture of a tree. You set you camera to f/16, shutter speed 125, a film setting of 100 and click. You look at the photograph you just took (either you are using a digital camera or have real fast processing :) and the exposure is perfect.
Then a bird flies out of the tree and you want to take a picture but want to make sure the bird is not a "blur". (Of course the bird caws loudly before taking off so you have time to change your settings, and the lighting conditions are not changing.) Remembering that faster shutter speeds are better for quick moving things, you increase the shutter speed from 125 to 250.
If you do not change any other setting on you camera and take the picture of the bird, the photograph will be underexposed, since the fatser shutter speed of 250 lets less light in than the shutter speed of 125. So there has to be an adjustment to let more light in.
If you are shooting with a film camera you only have one option -- to change the f/stop because you cannot practically change the film speed at that point. A shutter speed of 250 is one stop away from a shutter speed 125, so you have to change the f/stop from f/16 to f/11 to have the same exposure as in your first picture. If you had a digital camera you could have increased the film speed from 100 to 200. The following table shows a series of exposure settings. They will all result in the same exposure.
| Shutter Speed |
F/stop |
Film Speed |
| 1/250 |
F/4 |
100 |
| 1/250 |
F/5.6 |
200 |
| 1/125 |
F/8 |
200 |
| 1/125 |
F/11 |
400 |
In general you would like to use the slowest Film Speed possible to reduce grain, though film (and digital cameras) keep on improving so higher speed film settings do not have as much grain as they did a few years back. Then you think about how quick your subject is moving and how much depth of field you want.
