Bong S. Eliab
School of Arts and Sciences
Ateneo de Davao University
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Flash units have traditionally been used indoors or after dark when light levels are low. But there's a lot more to flash photography. Flash units can be used for many wonderful creative effects and can technically enhance image quality in several ways. You can buy a basic flash unit for $20 or $30, and prices go up to several hundred dollars for sophisticated dedicated units. Every photographer should have a flash unit, even so-called outdoor photographers. There's no excuse NOT to have a flash unit.
The duration of a flash is very brief, from 1/1000 of a second to 1/50,000 of a second. Because of this you can get razor-sharp images--no camera movement problems here! Fill flash (adding a little bit of flash so that it doesn't appear that you're using a flash) is great for daytime shooting, adding details to shadow areas and catch lights (that sparkle in the eye) in people/animal photography. More on that later. At night, off-camera flash units can be used during a time exposure to light up large subjects such as a statue or building. There are many kinds of flash units. Built-in flash units are common on point-and-shoots and many cameras designed for the "amateur" photographer market. Advantages are that the flash is always ready to use and on some models the flash turns on automatically when light levels are low. Be sure to check your manual for flash operating range. If, for example, it's 4 to 12 feet, that means subjects closer than 4 feet will be over-exposed and anything beyond 12 feet will be under-exposed. Another limitation is that the position of the flash can't be changed, resulting in flat, harsh light. Also, built-in flash often causes "red-eye," caused by the flash reflecting off the subject's retina.
Shoe-mount flashes are typically used with 35mm SLR cameras. Such flashes attach to the top of the camera or by means of a cord that attaches to an "X" plug-in on the front of your camera body. Handle-mount flash units are larger still and typically are used with medium- and large-format cameras. Studio lights are large, not very portable but produce large quantities of light and recharge almost instantly.
Light Output/Flash Power
Greater light output means your flash will reach longer distances and smaller f-stops can be used (particularly useful in close-up photography where depth of field is so critical). Check your camera manual for the flash unit's Flash Guide Number. Numbers typically range from 40 to 160; the higher the number the more powerful the flash. In general, the larger the flash, the more powerful it is. Flash guide numbers vary according to film ISO and according to zoom flash head, found on some models. Manufacturers assign guide numbers in "average" conditions, and of course, conditions are not always average. You'll have to compensate slightly for light or dark backgrounds, etc. To determine the performance of your flash, you should make careful tests and notes before photographing anything important. Some simple flash units have charts printed on the backs of the units to help you compute your settings. Fortunately, few flash units nowadays are completely manual, so this chore doesn't always need to be done. Professional photographers often use a flash meter when computing exposures with manual or multiple units, eliminating the need for math and measurement.
Remember, when using a manual flash, flash-to-subject distance is critical. Your manual or back-of-the-flash chart will give you f/stop guidelines.
Camera and flash must be synchronized so that the shutter is fully opened to expose the film when the flash goes off. Otherwise, part of your picture will be black. This is a serious problem in SLR cameras, which have focal-plane shutters, meaning that one shutter curtain opens and then a second curtain follows behind it to control the time of an exposure (shutter speed). Since both curtains must be completely out of the light path for the entire film fame to be exposed by the brief light of the flash, you can't use a shutter speed that is too fast. On manual cameras, you'll find the fastest shutter speed that can be used with a flash marked with an "X" or marked in a different color. Typically this is 1/125 of a second or 1/60. Some more expensive cameras permit flash synchronization speeds of 1/250. You can also get correct synchronization at speeds SLOWER than your fastest flash sync. Be aware that your film may then record ambient (available) light, which can be good or bad. On the negative side, you may get "ghosting." On the plus side, you can mix ambient and flash for great creative effects. For example, at night, use the "B" (bulb) setting on your camera together with a locking cable release and walk round with an off-camera flash unit, flashing it in several locations for unusual photos. This technique is used in cave photography.
Dedicated Flash Units
These computerized flash units, which tend to be expensive, have revolutionized flash photography. These units mix many creative options with automated flash, so that you don't have to worry about computation. They are worth having. These units will only work with certain models of cameras. For example, the Nikon SB26 only works with certain models of Nikon camera bodies. These "smart" flashes are probably most widely used for easy fill flash. These sophisticated units also have a great feature called "rear-curtain" flash, useful for lengthy time exposures of moving subjects. Normally a flash fires at the end of the camera's shutter sequence; with rear-curtain flash, the flash fires at the beginning, so that subject movement seems to follow the flash exposure. Some dedicated flash units also offer such options as repeating flash, multiple exposure, high-speed flash sync (to 1/4000 second) and more. Generally, you get what you pay for. Study the manual carefully, as there's a long learning curve with these units.
Semi-Dedicated Flash Units
These high-end flash units have many of the features of the dedicated units, but will fit different models of cameras. Again, study your manual carefully.
If you have a flash with an adjustable head (top part) or you're able to plug the flash into your "x" socket with a sync cord, you can sometimes use this beautiful lighting technique. Aim the flash at a ceiling or wall and the light is diffused, resulting in beautiful "soft" light. When computing your settings with a non-automatic unit, add up the distance from camera to the ceiling and from the ceiling to the subject. Add another half-stop to stop of light to allow for the diffusion.
Fill flash is used in outdoor situations where there is plenty of ambient (existing) light for a photograph, but you want to fill in shadow details, such as a person's face under a hat. For a photograph that looks like it was made with natural light, you don't want your flash power to be the same as or greater than the ambient light. Ideally, fill flash should be 1.5 to 2 stops less than the ambient light exposure. Dedicated and many automatic flashes will take care of this for you, but here are a few tricks to make sure you get correct fill flash with inexpensive units.
Fill flash has two distinct steps, as follows:
(1) Forget for a moment that you have a flash. You must first set your ambient exposure, based on your reading of highlight areas of the important part of your photo. Say, for example, you're taking a photo at noon on a sunny day and you are using your Sunny 16 rule with ISO 100 film. That will give you an exposure of 1/100 (or 1/125, which is the closest shutter speed) at f/16.
So far, so good. However, because of the problems with slow flash synchronization on some cameras, things can get tricky. Say that your fastest flash synchronization is 1/60. That means you'll have to set your shutter speed at 1/60 and thus you must adjust your f/stop to f/22 to get a correct exposure. If you try to shoot at 1/250 at f/11, for example, part of your picture will have no flash and you'll have a kind of line down the middle.
(One way to think of this as a two-step process is to tell yourself, "First, I'm going to figure out my exposure just like I always do. Now, I'm going to go get my flash and work with that part of the process.")
(2) Now you must trick your flash not to make a full flash exposure. There are several ways to do this, depending on your equipment.
Method 1: Cheap, low-tech all-manual flash. After taking your ambient light reading, do the following: Look at the chart on the back of your flash or one that comes with your flash instruction manual. Let's say that your ambient light reading is, for example, f/8 and 1/125. But don't forget-you have an old camera with a maximum flash synchronization speed of 1/60, so you have to reset your camera to f/11 and 1/60. Your subject is five feet from your camera. Your flash chart indicates that you can get a correctly exposed flash photo at f/8. Putting all this together, your flash is already under-exposing by one stop. If you want two stops of under-exposure, set your camera to f/16 and 1/30.
Method 2: The ISO Trick. Some semi-dedicated flash units measure their exposure through the camera circuitry and have no allowances for fill-flash (this fortunately is becoming more and more rare). Since automatic flashes always want to give you full exposure, you must "trick" your flash. Read your correct ambient exposure as noted above. Now set your film speed two stops higher, directly on the camera with a dedicated flash unit or on the back o the flash unit with an automatic but non-dedicated unit. By doing this you've gained more effective power and distance because you've asked your flash to fill in less light than normal. Your ambient exposure has already been set using the proper film speed, so don't change any camera settings. For example, you're photographing a sunset with a shorebird in the foreground. Put your camera on a tripod, make a manual exposure setting-and hold it there-for for the clouds at f/22 at 1/4 second to hold depth of field. Then switch film speed from ISO 100 to ISO 200 to fool your flash into delivering two stops less light.
Method 3: Partial Power. On some automatic flash units, you can set power from full to one half or one-quarter. Simply set the correct ISO on the flash dial and set the power to one-quarter for a two-stop under-exposure for flash.
Batteries often cause flash problems. If you're having battery problems, first check contact points on your batteries. Try cleaning with a pencil eraser. Avoid touching battery contact points with your fingers, which contain oils. IMPORTANT: don't store your camera or flash for lengthy periods with batteries in them; acid can leak and destroy the equipment's insides.
Batteries fall into two major categories: chargeables and non-rechargeables. Don't try to recharge non-rechargeables; they may explode. Categories of non-rechargeables include alkaline (the most common), lithium (extremely long shelf life), mercury and silver oxide cells (those little button cells that power some cameras).
There are two main kinds of rechargeables: NiCd and lead-acid (also known as "dry fit" or "gel cell.") NiCds are most common and come in standard sizes. NiCds can discharge their power a lot faster than alkaline batteries, which allows for faster recharging. NiCds, however, might burn out circuits that are not designed for the additional load. If your flash manual (most often inexpensive flashes) says "no NiCds" that also means don't use lithium AAs.
Lead-acid batteries are the other major category of rechargeable batteries. They can last longer between charges and live longer than NiCds. They are more durable as well, but they have a higher initial cost and are heavy. In photo gear, lead-acid packs are external to the equipment.
NiCds are very sensitive to charging abuse. NiCds have a problem called "memory." Let's say a particular NiCd battery requires five hours to be completely recharged. You've used up about half of the voltage in your flash pack, but decide to go ahead and give the NiCd a full charge instead of waiting until it's completely dead. Bad idea, and here's why: Technically, it still has half its charge, equivalent to another 2 1/2 hours of charging time. So when you stick it in the charger and give it a full five hours, it confuses the battery. It "learns" that it is now no longer a five-hour recharge battery, but a 7 1/2 hour recharge battery (the initial five hours plus the 2 1/2 hours that still remained). That is what is meant by memory. Much of the loss attributed to memory effect is really premature cell failure due to overcharging including continuous charging). Once the battery is fully charge, additional charging energy is dissipated as damaging heat. Avoid overcharging and dropping NiCds as some can be damaged by physical shock.
There is also a rechargeable alkaline battery called Rayovac Renewal. It's supposed to last up to three times longer than NiCds, retains its power in storage up to five years, retains no memory, and uses no cadmium (making it environmentally responsible). Unfortunately the stored energy decreases with each charge/discharge cycle. It compensates for this by having greater energy storage when new.
Battery shelf life varies according to the battery type. An alkaline cell loses five percent of its capacity per year just sitting on a shelf. Silver oxide batteries will lose six percent of capacity and a Lithium cell will lose one-half to one percent of capacity each year they sit unused. A rechargeable lead-acid cell will lose 20 percent of its capacity in seven months sitting on the shelf, while a NiCd will lose 20 percent in three months.
Nickel-metal hydride (NiMH) offer increased capacity and power density over NiCds and are better for the environment when you dispose of them.
Lead-acid batteries (commonly used in cars) come in camera versions, too. The only place you'll usually find these are in high-capacity flash power packs and external camera packs. Also called "dry-fit," some of these can be charged at temperatures as low as -49 degrees F. Unlike NiCds and NiMH cells, storing lead acid cells in a discharged state will damage them.
If you're going to be in the field for a long time away from AC power for recharging batteries, you can buy a power converter that uses the 12-volt battery power from your vehicle.
Keep in mind that if you use an external battery pack, you are immediately voiding your camera's warranty. Battery packs can fry a camera's insides when a short occurs. The battery pack will be protected with a fuse, but the camera won't. Check your camera manual.
Don't mix batteries and don't let them get hot. There is acid inside most batteries. Mixing types could release that acid into your equipment. Also, don't mix old and new batteries of the same type. This is especially true for NiCds; don't mix NiCds of different capacities (capacity is the AH or mAH number on the side of the cell). The major safety concern with most batteries is heat. Excessive heat can cause acid to leak from batteries.
Batteries and Cold Weather
As the temperature drops below 10 degrees F (-12 degrees Celsius) alkaline and mercury batteries become useless and silver batteries begin losing their capacity. Voltage of lithium batteries will drop and peak current capacity will suffer. Below approximately 0 degrees F. (-18 degrees C.), there are few options. NiCds do pretty well. Below -40 degrees only lead acid batteries are an option some of these can operate at -85 degrees F. (-65 C.).
Keep several sets of batteries warm inside your clothing and rotate them as soon as the set in use shows some signs of losing capacity. An external battery pack can be placed under your clothes and kept warm by your body heat. Nikon, Vivitar and Pentax companies all make remote battery packs.
Syllabus | Notes | Papers | Projects
All Rights Reserved 2001
Ateneo de Davao University
10 June 2002