Photography - Settings - Exposure
SHUTTER SPEED, APERTURE, ISO, & STROBE POWER
Taking a picture requires getting just the right amount of light from the subject to the film or image sensor in the case of digital cameras. Too little light and the picture is underexposed. Too much light and the picture is overexposed. But what factors effect the level of exposure and how are these factors related? And what can you do to better control exposure? For answers to these questions, read on.
The four parameters that you can control to get just the right amount of light are shutter speed, aperture, ISO and strobe power.
- Shutter Speed - How long the camera lens' iris stays open. The longer the iris stays open, the more light gets in to expose the image. I typically use 1/100 sec.
Using shutter speeds slower than 1/60 sec. may result in motion blurring if either the camera or the subject is moving. At 1/80 sec. motion blurring in the underworld environment is rarely a problem. And at 1/100 sec. I have only ever experienced motion blurring when rapidly panning the camera or when shooting speeding schools of fish.
For darker backgrounds, increase the shutter speed to between 1/200 sec. and 1/500 sec. or even higher depending on the effect you want to achieve.
- Aperture - How wide the lens' iris opens. The wider the iris opens, the more light gets in to expose the image.
The amount the iris is opened is measured in f stops where bigger numbers represent smaller openings. Settings used depend on the camera or lens in the case of DSLR cameras. Typical settings for digital cameras range from f 4 to f 12.
In addition to determining how much light gets in, the amount of the opening also determines the depth of field or how much depth into the picture is in focus. Low f stops like f 4 produce shallow depths of field whereas high f stops like f 12 produce deep depths of field.
- ISO - How sensitive the film or the image sensor in the case of digital cameras is to light. I typically use the lowest setting available.
Note: ISO settings for digital cameras range from 64 to 400 for the Olympus C-5050 and from 200 to 1600 for the Nikon D70. However, all but the lowest settings produce noise or graininess in the picture. Thus most underwater photographers prefer the lowest setting available.
- Strobe Power - How much light the strobe puts out. Light from the strobe is reflected off the subject and into the camera lens.
My Ikelite housing for my Olympus C-5050 has special circuitry that works with the camera to determine just the right amount of light needed given the other settings above. This feature, called TTL for metering Through The Lens, makes properly lighting pictures easy.
My Ikelite housing for my Nikon D70 does not currently have the TTL feature so I have to manually set the power settings on my Ikelite DS 125 strobes. The available settings are 1/8, 1/4, 1/2 and full power. I typically use 1/2 power.
Note 1: A strobe is required for underwater photography to provide light to properly expose the picture AND to bring out the colors lost at depth due to the filtering effect of water on the suns rays.
Note 2: Strobe light only reaches out about 3 to 5 feet maximum. Thus, it lights subjects in the foreground but has no effect on background objects beyond 3 to 5 feet away.
The basic measure of exposure change is one f stop. In simple terms it is the amount of change in shutter speed, aperture, ISO or strobe power needed to double or halve the amount of light in the final picture.
Here are some examples to help explain one f stop of exposure change and how it applies to shutter speed, aperture, ISO and strobe power.
- Shutter Speed - The longer the iris stays open, the more light gets in to expose the image. Thus, if the iris stays open twice as long, the exposure change will increase one f stop.
For example, if you change your shutter speed from 1/100 sec. to 1/50 sec. the iris will be open twice as long and thus the change in exposure will be an increase of one f stop.
On the other hand, if you change your shutter speed from 1/100 sec. to 1/200 sec. the iris will be open half as long and the change in exposure will be a decrease of one f stop.
Note: For strobe lit subjects, the strobe fires so much faster than the shutter speed that the shutter speed has no effect on strobe lit subjects i.e. for all practical purposes the foreground is only lit by the strobe. So, when using a strobe, assume that the shutter speed will only effect the exposure of the background.
- Aperture - The wider the iris opens, the more light gets in to expose the image. Thus, if the iris opens twice as wide, the exposure change will be an increase of one f stop.
However, unlike shutter speeds, aperture openings on your camera are not marked in easily understood and mathematically linear terms. Instead, the openings are marked in f stops which are seemingly arbitrary. To make matters worse, the larger the f stop number the smaller the iris opening and thus the less light that gets in. But, rather than diverting to a course on physics and optics, let's work with a camera specific example.
From the table below, if you have an Olympus C-5050 and you increase the f stop setting from 4.0 to 5.6, the change in exposure will be a decrease of one f stop. While this is not at all obvious when looking at the numbers, that's the way it is. Your choice is to accept it or delve deeply into a study of physics and optics. Click here if you must know more.
Available f Stops in 1/3 Stop Increments
- ISO - Higher ISO settings result in a greater film or the image sensor sensitivity to light. And here, unlike aperture settings, logic seems to prevail.
For example, from the table below, if you have a Nikon D70 and you increase the ISO setting from 200 to 400, the sensitivity of the sensor to light will be twice as great and thus the change in exposure will be an increase of one f stop.
Note: In the case of the Olympus C-5050, the lowest ISO setting is 64 which is not 1/2 of 100 and therefore, going from 64 to 100 will not increase result in an increase of one f stop but rather something less than one f stop ... I'd guesstimate the change is an increase of about 2/3 f stop.
Available ISO Settings in One Stop Increments
- Strobe Power - Full power delivers the most light. Duh. Reducing the power 1/2 will result in a decrease of one f stop. And reducing the power from 1/2 to 1/4 will result in an additional decrease of one f stop. Now all you have to do is understand what the markings on your strobe or controller mean.
In the case of the Ikelite Ds 125, the settings on the strobe are marked Full, 1/2, 1/4, and 1/8. Since each setting is 1/2 the value of the next higher setting, clicking down from Full will result in a decrease of one f stop for each click down.
In the case of the Ikelite Manual Controller, the settings are confidently marked as f stops. Thus clicking down from Full will result in a decrease of 1/2 f stop for each click down. In this context it's interesting to note that while there are 10 possible settings, only the first 5 are really useful i.e. I have never found a need for a setting of less than 2 stops down from full. Have you?
Available Power Settings
Note: My Ikelite housing for my Olympus C-5050 has special circuitry that works with the camera to determine just the right amount of light needed given the other settings above. This feature, called TTL for metering Through The Lens, makes properly lighting pictures easy. Some however argue that, for artistic purposes, they prefer the additional control possible with manual settings.
PUTTING IT ALL TOGETHER
Ok, now let's put this all together with a real life example. If I shoot my D70 with a Nikkor 12 - 24 lens as show below I get good results.
- Speed - 1/200 Sec.
- F Stop - 8.0
- ISO - 400
- Strobe - 1/2 power
However, I'm concerned about the noise aspects of shooting at ISO 400. Assuming I decrease my ISO from 400 to 200 (a decrease of one f stop that effects both the background and the foreground) what change(s) can I make to increase my exposure one f stop to keep my overall level of exposure unchanged for both the background and the foreground? Using the material covered on this page my alternatives are;
- Speed - Decrease the speed from 1/200 sec to 1/100 sec. This will get me the one f stop increase in exposure that I want for the background but it will have no effect on the foreground which is being lit by the strobe.
- F Stop - Decrease the f stop from 8.0 to 5.6. This will get me the one f stop increase in exposure that I want in both the background and the foreground. But the price I will pay is a loss in depth of field.
- Strobe - Increase the strobe power from 1/2 to full. This will get me the one f stop increase in exposure that I want in the foreground that is being lit by the strobe. But it will have no effect on the background.
However, if I also decrease the speed from 1/200 sec. to 1/100 sec. I will get the the overall one f stop increase that I want for both the foreground (lit by the strobe - the strobe only effects the foreground) and the background (lit by speed - the speed only effects the background).
What to do? While all of this knowledge does not answer my question, it does allow me to quantify my choices. And to me this sure beats the alternative of underwater trial and error.