Why use strobes? Aren't incandescent lights cheaper and easier?

The short answer: No, while the cheapest incandescent lights may be cheaper than entry-level strobes, the camera sees them as MUCH dimmer. For a comparable exposure of a moving subject like a person, strobes are much cheaper and more convenient.

If you want to be spared the detailed explanations, click here to go straight to example photos.

Strobes put out all their light in one very quick burst, and as long as the complete burst happens while the shutter is entirely open, shutter speed does not affect strobe exposure. But exposure using continuous lights is profoundly affected by shutter speed. So we can't compare continuous lights with strobe lights unless we take the shutter speed into account.

This is reflected in the way the lights are rated. Strobes are rated in Watt-Seconds, which is a Watt times one second (exactly equivalent to a Joule). Continuous lights are, of course, rated in Watts. Both of these are ratings of the electrical input, not the light output. Strobes are around two to three times more efficient at converting electricity to light, so if we work on the electrical ratings, we'll be unfair to the strobes by a bit, but we'll be within a stop or two of an accurate comparison. For simplicity, that's what we'll do for the rest of this page.

If you used a shutter speed of one second, then you could directly compare Watts of incandescent lights to Watt-seconds of strobes. A 500 Watt-second strobe would give an exposure approximately equal to a 500 Watt bulb, at the same distance, in the same fixture, using the same reflector/modifier, etc. But one second is an impractically slow shutter speed for most portraiture.

Continuous Watts at a shutter speed of one second are equivalent to Watt-seconds. But as you shorten the shutter speed, the continuous lights deliver less exposure to the film. So at typical moderate to high shutter speeds, 1000 Watts of continuous light delivers much less useful light than a 1000 Watt-second strobe. How many stops aperture difference will you see between a 1000 Watt-second strobe and 1000 Watts of continuous lighting? Or with a 500 Watt-second strobe versus 500 Watts continuous? The answer, of course, depends on the shutter speed which you need to use with the continuous lighting. This table gives values for several shutter speeds:

Stops Difference for Numerically Equivalent Power Ratings
Continuous Lighting Shutter Speed Advantage of Strobe in Stops
1 sec 0 stops
1/2 sec 1 stop
1/4 sec 2 stops
1/8 sec 3 stops
1/15 sec 4 stops
1/30 sec 5 stops
1/60 sec 6 stops
1/125 sec 7 stops
1/250 sec 8 stops

For example, if 1000 Watt incandescent gives an exposure of 1/60 @f/2 in a given setup, you could expect that a 1000 Watt-second strobe would give an exposure of around 6 stops better, or f/16, in the same setup, same modifiers, same reflectors, etc. Since lighting efficiencies vary, don't rely on this to be very accurate, but it should bring you into approximately the right ballpark.

Maybe you're not interested in comparing a 1000 Watt-second strobe to a 1000 Watt light. Maybe you would like to know "How many watt-seconds does my continuous light deliver?" Or "How much continuous wattage would I need to match a 500 Watt-second monolight?" Again, the answer depends on the shutter speed required of the continuous light. To compare exposures at shutter speeds other than one second, just multiply the incandescent light's Watt rating times the shutter speed in seconds to get the Watt-seconds of energy used by the light during the exposure. So a 1000 Watt light at a shutter speed of 1/125 uses 1000 * 1/125 = 8 Watt-seconds of electricity while the shutter is open. Here's a table with some more sample values for comparison:

Power Needed for Equivalent Exposure
Strobe Watt-seconds Watts @ 1/30 Watts @ 1/60 Watts @ 1/125 Watts @ 1/250
30 900 1,800 3,750 7,500
500 15,000 30,000 62,500 125,000
1000 30,000 60,000 125,000 250,000
2000 60,000 120,000 250,000 500,000

You can see that it's unrealistic to match the light of a 500 Watt-second or 1000 Watt-second powerpack using incandescents at 1/250. Most photo studios don't have electrical wiring required to support a quarter of a million watts of hot lights, and the air conditioning bills would be prohibitive, not to mention the fire safety considerations.

30 Watt-seconds is approximately the rating of a small battery operated on-camera flash with GN 120 (Vivitar 283, Sunpak 383, etc.). If you're willing to shoot at a slow shutter speed, you can approach this amount of light with high powered incandescents.

So the most powerful practical incandescent lights, used at fairly slow shutter speeds, can almost match the cheapest smallest strobes powered by AA batteries. There's only a small area of overlap where comparable exposures are realistic. If you need more light than this, you must go to strobes, while if you need less light than the smallest battery operated strobes, then ordinary household incandescent fixtures of few hundred Watts may be adequate.

Incandescents have other disadvantages compared to strobes.

You can somewhat make up for the lower quantity of light from incandescents by using high speed film, wide apertures, and long shutter speeds. You can place the lights very close to the subject, using efficient reflectors and no diffusion, but this affects the quality of the lighting significantly. For a similar amount of money, the more powerful strobes will give much more flexibility in the choice of slow film, small aperture, soft yet inefficient light modifiers, convenient light placement, etc.

Of course, strobes have their disadvantages, too. They need to be synced to fire when the camera shutter is open. It's hard to visualize the exact effect they'll have. They require a special meter capable of reading flash exposures. These are not huge problems, though. Learn to use slaves, modeling lights, and flash meters, and you can take advantage of the big power advantage that strobes can provide.

If you're taking photos of still life subjects using a tripod, then you can use as long an exposure as needed. Incandescents perform very well for this application. Candles can do a reasonable job for still life, too. But for portraiture, strobes are by far the cheapest way to get controlled soft light that will give an exposure near f/11 at ISO 100 at an exposure time quick enough to freeze an active four-year-old. Strobes do well in far less demanding portraiture, too

Now that you've read the detailed explanation, if you haven't already seen the sample photos, click here.

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