Photo Clinic, Part 2

Photo Basics:
      When taking a picture, light rays from your subject are bent by the lens to focus (upside down) on the film. There must be enough light to chemically alter some of the particles in the emulsion, but not so much light that all the particles are converted. And the light rays coming from any individual spot on your subject must be bent so they all meet at one precise point on the film. These are the two critical pieces for a picture: the right amount of light for the film you're using and the right focus.
      Get the exposure right. You have three variables to consider: film speed (measured by the ASA number), shutter speed (measured as a fraction of a second -- the "125" on your shutter speed control knob means 1/125th of a second), and the lens opening (called aperature or f-stop, and measured in numbers that get bigger as the opening gets smaller -- f-16 is a smaller opening than f-8). Which of these variables is most important? It depends on the type of picture you'll be taking.
     For example, to shoot moving things in low-level natural light, film speed is important, so you pick a 400 or even 800 ASA film. Otherwise, you'd pick a slower film, because the details and colors would be better with the lower ASA film.
     For rapid action photos, you want a fast shutter speed to freeze the action. (For underwater photos, shutter speed isn't usually important -- nothing moves that fast underwater.)
     Or, to keep both foreground and background in focus, you make the aperature as small as possible. When "depth of field" is important, the size of the lens opening may be the most important variable.
     Once you've decided which variable is critical, you set the others to get the right exposure. For example, if you narrow the lens opening so less light enters the camera, you reduce the shutter speed (so the film is exposed longer) to compensate.

The amount of light that enters the camera is controlled by the size of the lens opening, called the f-stop, and by the shutter speed, the length of time the lens is open. At left, the Nikonos V aperature knob. At right, the shutter dial.

So as you read the numbers on your camera, the principle to remember is:
           Bigger number = less light.
         Smaller number = more light.
The shutter speed number 250 means 1/250th of a second. This is half as much light as you'd get from 125, or 1/125th of a second. With aperature, each f-stop higher is half as much light: f-5.6 gives half as much light as f-4, and f-16 gives half as much light as f-ll. A higher number means the opening is smaller. Got that? The bigger the number, the less light your film sees.

Getting the exposure perfect determines whether the picture goes into the trash or onto your wall. At left, Dominic Bria with garibaldis, using a single YS50 strobe, not controlled for ambient light. At right, with the aperature set appropriately.

     There's a "see-saw" relationship between f-stop and shutter speed. For any given exposure, if you increase the f-stop number (which makes the opening smaller), you decrease the shutter speed number (making the exposure longer). And if you decrease the f-stop number, you increase the shutter speed number. All of the following are identical exposures:
             f-stop     shutter
               f-11          60
               f-8            125
               f-5.6         250

     The film. Your film is an emulsion of chemical particles. When the particles are hit by light of certain wavelengths, the chemical changes. The developing process converts this chemical change into colored dyes within the emulsion. Most scuba photography is done with 100 ASA film. The "slower" film has more particles in the emulsion, and can provide richer colors plus more subtle details. It's also more forgiving if you don't get the exposure absolutely perfect. And for any ASA rating, print film is MUCH more forgiving of imperfect exposure than slide film.
     But the camera doesn't care whether you use slide film or print film. Your choice depends on what you'll be doing with your pictures. If you want to sell pictures to a magazine, you need slides. And because you'll be throwing away most of your first few rolls, slides are cheaper. But print film has a better chance of making a beginner look good. You decide.

You need to tell the Nikonos V how "fast" the film exposes. You do this by setting the ASA dial to the appropriate setting. As you become expert, you can even "trick" the camera into taking a better picture by giving it the wrong ASA value.

The Sea&Sea Seamaster camera detects the ASA rating of your film. While this makes a screw-up less likely, it gives you less creative control. The MX-10 (left) has an internal manual switch for 100 or 400 ASA only.

The result of using some old 400 ASA slide film. The colors aren't crisp and vibrant, and there's little contrast. Invest in high-quality film, with the lowest ASA you can expose properly with your equipment. You're spending hundreds of bucks to go diving. Don't use the old film you found in the garage. Photo: Gary Argyle surrounded by Garibaldis at Catalina Island. Nikonos V w 35-mm lens, double YS-50 strobe.

     Shutter speed. If you're using a strobe, your choice of shutter speed is easy. On the Nikonos, set it to 1/60th of a second. There's almost nothing in the ocean that moves fast enough to require a faster shutter speed. 1/60 also happens to give you a good f-stop range when in shooting ambient light at mid-day. So I just leave my shutter at 1/60 -- except for above-water photos, for which the Nikonos V is excellent.

The Nikonos V shutter speed control dial is found beneath the film advance lever. This dial also has the film-rewind release ("R"). Speed varies from 1/1000 second to 1/30th, with a continuous open ("B") setting.

The Sea & Sea camera has a "motion blurr" problem with 1/60. I don't know why, because I never see this with the Nikonos. Perhaps the timing is less accurate. So I use the Motor Marine at 1/125th for all photos except macro (where I can be sure neither I nor the subject are moving). Unfortunately, this gives you only half the light to work with.

     The aperature. Once the film is in the camera, set the ASA dial, and put the shutter speed at 1/60th (1/125th for the Sea & Sea), the only remaining variable is the aperature, or f-stop. In practice, I meter the ambient light as soon as I get on the bottom, then set the aperature. (Most cameras come with a built-in light meter.) Then the only thing I have to do to take a picture is set the focus.
     The Nikonos uses shutter speed to indicate the proper exposure. You partially depress the shutter and a number such as 250 flashes. That's OK. Just use the camera's suggested shutter speed to indicate which direction to change the f-stop. If 250 (1/250th of a second) is flashing, the camera thinks there's too much light at 1/60th. So close the aperature by clicking to a higher f-stop number, for example from 5.6 to 11. If the camera flashes 30 or the "lightning bolt" flash alert, open the aperature by moving to a lower f-stop number. The point is, move the aperature setting until the camera lights up the "60," meaning the exposure is perfect (assuming you previously set the shutter speed to 1/60th).
     Getting the background exposure right with the Sea & Sea cameras is a bit harder. (1) Turn off the strobe. (2) Turn the aperature dial all the way to f22. (3) Depress the shutter half-way (carefully). If the "underexposure" light turns on, move the aperature dial one click. (4) Repeat the exposure test. When the underexposure light no longer turns on, move the aperature dial one more click. (5) Turn the strobe back on and take your picture. (This assumes you want a fairly light, neutral-appearing background. If you want the background darker, leave the apperature at the position where the underexposure warning no longer appeared.)
     Setting the aperature properly is important with flash pictures of divers and large fish. You want the background to appear colorful and interesting. So you set the camera to expose the "non-subject" area properly, and use your flash to add color and highlights to your foreground subject. In the "Flash" section, I'll talk about balancing the natural light with your strobe.

The aperature setting of the Nikonos V can be read on the front of the lens. As the aperature control knob is turned, it also changes pointers on the upper part of the lens (in the focus indicator) to show the depth of field -- how much area will be in good focus. This is handy!

The aperature control on the MX-10 is in the same location next to the lens. Like the Seamaster, you partially depress the shutter, then dial the f-stop lower until the "underexposure warning" light goes off. Because you can't set the focus distance on this camera, the larger aperatures will give "fuzzy" pictures when your subject is closer than 4 feet.

     The focus. Of course, all is lost if your image is blurry in the final photo. The light needs to be bent precisely so every ray that leaves a particular spot on your subject strikes at one tiny place on your film. If your subject is far away, the light rays arrive at the lens traveling nearly parallel. A bit of bending puts them all on the same spot of your film. If the object you're photographing is closer, the rays of light are spreading out more. These light rays must be bent back inward to a greater degree so they can meet at the same spot on the film. The camera's focus adjustment moves elements within the lens so the light rays are bent precisely for whatever distance you set.
     (With simpler cameras such as the MX-10, the focus distance is set. This means that, at larger aperatures -- low f-stop numbers -- these cameras may give you a blurry image when shooting at the usual "arm's length" distance for diver photos. With these cameras, I recommend a faster film -- such as 400 Max -- so you can use a higher f-stop number. The smaller lens opening gives you a crisper focus and greater depth of field.)
     If the opening behind the lens is smaller, there's less light, but the light rays that do get in are traveling more parallel. Therefore, for light waves originating from different distances away, the amount of bending required is more alike. So the focus is less critical, and more objects will appear in clear focus. And conversely, when the aperature is wide, the focus must be more exact to be sure the object you want to photograph is crisp in detail.

The Seamaster Pro EX camera by Sea&Sea has a focusing knob on the side of the lens. The last position is a closeup adjustment that changes the focus from 3 feet to 18 inches. There is no depth of field indicator.

     Focus (and the depth of field) is also affected by the type of lens you're using. Telephoto lenses and macro lenses are the most sensitive -- they must be focused very precisely, because the depth of field is very narrow. On the other hand, wide-angle lenses give much better depth of field.

     Learning more: Your dive shop can give you an excellent underwater photo course, including basic science and hands-on camera upkeep and get-wet picture-taking.

Click on the garibaldi for the next lesson.

About the author: Bruce Argyle is a computer programmer whose "Cardiac Arrest!" simulator remains a best-seller after 12 years in publication. He has no particular qualifications to write a scuba photography article, other than he happens to have a web site and needed something to do.