baselining your strobe for quick exposures

Digital cameras have been a great boon to photographers working with off-camera lights.  The ability to review an image instantly on an LCD (with histogram!) has obviated the need for tedious polaroiding and exhaustive metering of every inch of a scene to ensure correct light ratios and eliminate unwanted shadows.    So much so, in fact, that many photographers have begun to eschew the use of a flash meter entirely – relying on the LCD and histogram via trial and error to set their lights correctly.   Now while quick and easy, this method has it’s drawbacks, particularly for young photographers.   The question that frequently arises is:

“well, I’m essentially just guessing what power to set my flash on then chimping the exposure and adjusting accordingly but how do I know *where* to start with my flash”

Essentially this comes down to a combination of making an educated guess about the exposure and *knowing how much light your flash will put out*.

The beauty of light is that it is predictable.  Whether from the sun, a lightbulb, or a flash, given the same source and conditions, you will always get the same light.    We can use this to our advantage!

When working with flash, we have fundamentally 2 variable that we control to determine how much light falls on our subject – power and distance.   Power meaning how much actual light our flash is outputting and distance meaning how far away it is from the subject (remember that light falls off predictably according the inverse square law).    Since we know that light always behaves the same, we can be certain that at a given power and a given distance from our subject our flash will give the same result every time.

Now also remember that light from a flash behaves linearly – going from 1/4 power to 1/2 power is doubling the amount of light that it puts out and vica versa.  Thus if our flash exposes properly at f/8 at 1/8 power, it will give us f/11 at 1/4 power, f/5.6 at 1/16 and so forth (given the same distance to the subject).

Armed with this knowledge, we can quickly and easily estimate a “starting point” for exposure in almost any situation.  We do this by establishing a “reference point” at which we *know* the exposure of our flash, and can calculate from there.  I like to call this “baselining” the flash.  To do this:

  1. start with the flash on a medium power, which gives room to adjust up or down.  1/8 power is a good starting place
  2. now we need to ensure that we can replicate a consistent flash->subject distance.  You could carry around a tape measure but a fantastic trick I learned from the inimitable Don Giannatti is to measure using your outstretched arms.  Given that the average (male) photographer is probably between 5’5″ and 6’something, your outstretched armspan or “wingspan” is somewhere around 6′, which is a comfortable working distance for lights.   This also has the advantage of being quickly and easily reproducable “on set” – you simply stretch our your arms from subject and place the light at the end.
  3. Meter your light at your set power, at “wingspan”  – if you don’t have a flash meter, you can approximate by photographing an 18% greycard till the histogram spikes dead center and recording the appropriate aperture.
  4. adjust your light till you get a “comfortable” baseline.   Let’s assume that at “wingspan” we find our flash gives us f/8 at 1/8 power.   This is our baseline – we write it down (or just remember it).

Now lets put this info to use!

Let’s say we’re in the studio.   We want to do a shot with a key and fill light in a 2:1 ratio.   We want to shoot at f/11 to give  good depth of field for our subject.  What do we do?  We place our lights in the desired position, both at “wingspan”.   We know that each of them gives f/8 at 1/8 power at that distance.  f/11 is one stop up from f/8, so we set our main light to 1/4 power (one stop more power).  Our main light is now already double the light of our second, so we have our ratio right there – the second light stays at 1/8.   We shoot at f/11,  and our main light should be spot on with the second 1 stop under.   now if we want to “blow out the background” we simply add another light on the background at 1/2 power, giving us an exposure of f/16 – one stop over main. Of course this is not as “exact” as using a meter, but this gives us our starting place and we can adjust the lights from there based on the histogram.

It’s that easy!

This technique becomes particularly powerful when balancing ambient and flash outside.   Combined with the sunny/16 rule, we can use our baseline to roughly estimate the combined exposure of flash and ambient without chimping a single frame!

consider the common situation:  We are shooting outside and want to drop the ambient by 1 stop.  We see that it is mildly overcast – the sunny/16 rule says that our exposure should be approximately f/11@1/100 sec.   Again, we know our flash give f/8 at 1/8 power, so we set it at arms length from our subject.  In order to drop our ambient by a stop we increase our shutter speed to 1/200 (still at f/11) and adjust our flash up one stop to 1/4 power to give us f/11.    Done and Done.  Chimp, and adjust as needed.  If we are in situation where we can’t drop the ambient by shutter speed (already at sync limit), we can simply adjust the flash power to compensate.   Assume the same situation (ambient is f/11).   to keep the shutter speed the same and still drop the ambient by 1 stop, we need to shoot at f/16.   Again, knowing our flash gives f/8 at 1/8 we simply bump it up 2 stops to 1/2 power (f/8->f/11->f/16 = 1/8->1/4->1/2)

This may sound complicated, but once you  are comfortable estimating these exposure, it becomes almost second nature.  by using your baseline you will find yourself able to get exposure dead on within 1 or 2 “chimping” shots.

5 thoughts on “baselining your strobe for quick exposures”

  1. Very sound advise. My only addition is, if you wanted to change your exposure by, flash to subject distance, use the aperture settings as a guide. For example, If your flash is 8 feet from your subject and you want to double output by moving the flash closer, you would need to go to 5.6 feet. Double again would be 4 feet, then 2.8 feet. etc. A factor of 1.4 every time.

    1. Trevor – Indeed, that is true but when I start getting into careful light placement I usually just bust out the meter 🙂

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