Most modern cameras offer Through-The-Lens (TTL) flash metering capability. If you want to read up on the virtues of TTL flash metering, consult the marketing department of your favorite camera maker. I'm not a fan of it, at least not for off-camera setups with more than one light. I'll grant that the idea has a lot of appeal, but the devil's in the details.
First, TTL's strengths: It's simple to use, at least if you keep the flash on camera. It automatically meters precisely the area of coverage that your lens sees. It's usually well integrated with the camera settings, to make it "goof proof". It automatically compensates for filters on the lens, for bellows extension factor, for any diffusers placed over the flash, and it works at any aperture where the flash has adequate power. If you want to change aperture, you merely change the aperture setting on the lens, and the flash will automatically compensate, where with non-TTL flash, you must change both the flash power and the aperture to make them match.
Now the weaknesses:
TTL is a form of reflected light metering, and suffers from the same problem as all reflected light metering: namely that large areas of very dark subject material tend to make it overexpose, while large areas of very light subject material tend to make it underexpose.
There is no industry-wide standardization for connecting TTL flash units to cameras, so TTL flash equipment is dedicated to a particular camera brand.
Cords to allow you to take a TTL flash unit off the camera are obscenely expensive. Wireless slaves that support TTL are even more obscenely expensive. Both are only available from your camera manufacturer.
Some TTL systems use a "pre-flash" for exposure determination. This interferes with the use of additional slave lights.
TTL is extremely limited in its ability to control more than one light. Those systems that CAN control more than one light are expensive, offer limited power, and limited range.
TTL is only available in fairly low-power flash units. Studio powerpacks and monolights do not support TTL.
TTL flash units are often three or more times as expensive as non-TTL flash units with equivalent power.
Many TTL flash units don't offer the same manual controllability as a similarly priced non-TTL flash.
It can be less convenient to do exposure compensation with TTL, since you have to turn an exposure compensation dial somewhere on the camera or on the flash. How this affects ambient exposure, flash exposure, or a combination of both depends on your camera brand. With non-TTL, you can simply set the lens aperture to a different setting than the flash is set for, and it's obvious how this affects flash and ambient exposure.
TTL is probably the most convenient way to control a single light, particularly if that light is on-camera. But too many amateur photographers use TTL as a crutch, thinking they can't meter flash photos without it. This can cause them to keep the flash on-camera when the situation calls for off-camera flash, and it can cause them to stick to a single light when multiple lights would work better. If you can get over the notion that TTL is essential, you'll open the door to a variety of flash techniques that produce beautiful results economically.
In a simple non-dedicated automatic flash, there's a window on the front of the flash unit that stops the flash output the instant enough light has hit the subject. The window's field of view will not be the same as the field of view of your camera lens, but otherwise, this is very similar to TTL flash at a fraction of the cost.
Like TTL flash, auto flash units are sensitive to unusual subject reflectance.
Auto flash units don't integrate as well with the camera, forcing the photographer to take a few seconds to set them up. Typically, you must select the power setting, which determines the aperture you'll use with a given film speed. You must set the lens to this aperture and set the shutter speed to the camera's sync speed (or slower). Once this is set, shoot away...
As an example, here's the control panel of a Sunpak Auto 383 on one of its three automatic settings. You set the film speed in the top window, and then the aperture is displayed in the next window, and the usable distance range is displayed below that. The switch to the left lets you select either of the other two auto settings, where a different aperture and effective distance range would be shown. You can see that at this film speed, you should set the lens to f8, and the flash will automatically expose anything between 4 and 30 feet.
Auto flash is for single-light setups only. Two lights, both on auto, will produce unpredictable results at best. Depending on the setup, each sensor will see light from both flashes, but it can only control one flash. The sensor on flash "A" may see lots of light from flash "B", and shut down flash "A" when flash "B" is the one that really ought to be shut down.
Auto flash CAN be used off camera and in combination with umbrellas or other diffusers. Just be sure the sensor is pointed toward the subject. You'll get no measurement of light on the shadow side of your subject (the side away from the auto sensor), but this may not be a big problem if you use a reflector for fill.
Here's an example showing the same Sunpak 383 on manual, at 1/8 power. You set the switch to the left to "M" for manual, and select the power on the sliding switch at the bottom. Then the F/stops match up with the appropriate distances. This type of control panel frees you from having to do the (relatively simple) math of guide number computations. But still, you must focus the lens, read the distance to subject from the focusing scale, check the flash to find the corresponding aperture, and set the lens aperture accordingly. Not good for action photos.
Although there are reflected flash meters and spot flash meters, by far the most popular type is an incident meter, usually with a white "ping pong ball" hemispherical dome covering the light sensor.
If you just want to measure exposure, the simple technique is to stand at the subject location, point the dome toward the camera, fire the flash, read the result, and set your lens aperture accordingly. This will very precisely determine your exposure, regardless of subject reflectance.
But you can do so much more with the meter than just measure overall exposure. When you have multiple lights set up, you can turn them on one at a time, pointing the dome toward each light in turn. By measuring each light separately, you can set the ratio between them. This lighting ratio helps determine the amount of contrast between the highlights and shadows. For starters, put a main light at about a 45 degree angle off the camera's axis, and put a fill light very close to the camera axis. Try setting the fill light one to two stops dimmer than the main light, as measured at the subject location. For example, the main light might indicate an exposure of f8 while the fill would indicate between f4 and f5.6. There are many, many variations on lighting setups, and there are plenty of books describing them. I won't get into them here, but I'll just say that this is one of the most fun things to experiment with in the studio. To make your experiments repeatable, use a flash meter to measure each light separately, and record the indicated f-stop for each light. The difference between f-stops is what's critically important to the "look" of the lighting setup.
There are lots of flash meters available. I got this Wein WP-500B, the cheapest flash meter I could find, about $65.00 new. It works simply and fairly accurately. It reads in 1/3 stop increments, though it's hard to read below f4, and it doesn't read above f22. To use, just turn the meter on, place it at your subject's location, pointed toward the camera, and fire the flash. The meter needle then points toward the aperture setting for ISO 50 film. If you're using a different film speed, you can either compensate the reading in your head, or you can use the funky little (non-electronic) dial. Point the arrow at the meter's reading, find your film's speed on the inner ring, and read the aperture on the outer ring. Extremely crude, but it works.
More expensive meters boast greater range, much more precise digital readouts, ability to measure both ambient (non-flash) and flash light, ability to integrate both ambient and flash light together at any desired sync speed, computerized ability to store readings, measure lighting ratios, etc. They also look less like a home-built project out of the back pages of Popular Electronics. But the simple and very basic Wein meter gets the job done.
If you use a digital camera with manual controls, you may be able to get by without using a meter at all. To do this, learn to read the camera's histogram. The little LCD display on a typical digital camera is a very crude judge of exposure, but the histogram can give you much more accurate feedback. Basically, you do trial and error exposures, adjusting flash power and/or aperture until the histogram "looks right". What looks right depends on the subject matter, but generally speaking, you want to avoid sharp peaks at the very far left or very far right of the histogram. The very far edges of the histogram correspond to pure black with no detail and pure white with no detail; these are usually undesirable.
Most flashes come with a sync cord that's not long enough to do much useful with. Paramount Cords will custom-manufacture a proper cord for your camera and flash combination, in any length, either coiled or straight. But sync cords are never long enough, and they always present something to trip over.
If you use sync cords, run them from the flash down to the bottom of the light stand, and wrap them around the bottom of your light stands before extending them to the camera. That way, when (not if) you trip over the cord, you'll tug on the bottom of the light stand instead of the top, and there's less chance of the light stand coming crashing down.
Cords may not work so well when using more than one light at a time. There's no standardization of the polarity of trigger voltage for various flash units, so you can't expect to wire two flashes to a single camera by just wiring sync cords together. There are adapters that will let you connect multiple sync cords to a single camera.
I'm not a fan of sync cords in the studio, though I do use them for macro lighting where the distances are small. They're also great for use with a flash bracket that holds the flash above your camera lens but doesn't get the flash very far off camera.
Simple optical slaves are electric eyes that fire a flash anytime they see another flash go off. You still have to figure out a way to get one flash triggered, but the slaves take care of triggering all the other lights in your studio. Most monolights and studio powerpacks have built-in slaves that work well. Wein peanut slaves are very cheap, and have a PC socket to accept any standard sync cord. Wein also makes a hotshoe slave that holds a hotshoe flash and triggers it. Slaves generally don't require batteries. Slaves typically cost something like $10.00 to $40.00 apiece.
Slaves are sensitive to infrared light. My favorite way of triggering flashes in a studio setting is to put a cheap low-power flash on my camera's hotshoe, and cover the flash with a #87 polyester infrared filter. This filter is opaque to visible light, but it lets enough infrared through that slaves will respond to it.
A slightly more sophisticated infrared filter for Nikon Cameras with a built-in flash is Nikon's part number SG-3IR. This mounts in the hot shoe (without using any electrical connections) and holds an infrared filter in front of the pop-up flash. Below are some pictures of it in use:
These are literally one-minute test photos, self portraits in the bathroom mirror (note the reversed brand name of the camera). They were unartistically lit with a slaved Sunpak 383 on the counter in front of the mirror. In both cases, the slaved flash was triggered using the built-in flash of my D200 in manual mode at 1/32 power. In the first photo, the SG-3IR is flipped down, blocking almost all visible light from the pop-up flash. In the second photo, I flipped the SG-3IR's infrared filter up out of the way, so you can see what the IR filter is stopping.
The SG-3IR is cheap (under $15), well made, and fully up to Nikon standards. Functionally, it's no better than taping exposed color negative film in front of your flash, but it's a lot more professional looking, and it leaves no tape residue. If you're using a camera with a built-in pop-up flash, it's a lightweight and cheap way to trigger studio lights cordlessly.
The disadvantage of all simple optical slaves is that they'll respond to everyone else's flash as well as yours. This isn't a problem in a controlled studio environment, but it can be a big problem if you're shooting at a wedding or sporting event where other cameras will be present. For these situations, there are modulated infrared slaves or radio slaves. The modulated infrared slaves respond to a unit that fits on your camera's hotshoe that sends out coded pulsed infrared light, like a TV remote control. Radio slaves, like the name suggests, are similar but use radio waves. Though I have no experience with them, I understand that Pocket Wizards are the "gold standard" of radio slaves, and they are well loved by wedding photographers. They work well at enormous distances, are very dependable, and don't depend on a line of sight between your camera and the light.
The two leading makers of generic flash suitable for off-camera use are Vivitar and Sunpak. I like the Sunpak 383 Super as a good general purpose battery powered flash unit. Vivitar makes the 283 and 285 flash units, and these are great flashes with a proven track record, as well. The advantages of the Sunpak 383 over the Vivitar 283 are that the Sunpak has built-in manual power control, its head swivels as well as tilts (allowing it to be backwards into an umbrella while the sensor points forward toward the subject), and it has a more understandable exposure calculator on the back of the flash unit. But the Vivitars are old reliable standards. Both makes are competitive, and offer similar light output and good value.
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