Paul C. Buff, Inc. Technical Forum

Hi Everyone,

I am new to photography and have three AlienBee B800's. My question is regarding metering. I have a Minolta incident light meter and know how to take the flash readings, but what I have difficulty grasping is what to do with the reading. More over, how do I know what arperture and f/stop to set my Nikon D300 in manual mode at based on the light meter flash reading?

I'm sorry if this questions sounds elementary, but up to this point, I've really been relying on the camera's histogram, but want to learn how to do this correctly with the meter.

First, you must have a meter that is not only incident, but also one for flash (a sub-type of incident). You would determine what ISO you want to use, typically as low as possible, so 100 or 200. Set your camera and meter to the same ISO. Shutter speed also needs to be predetermined, and matched on the meter and camera. Your shutter speed should not exceede your x-xync speed. This is 1/250-1/320 on your camera. You can set anything slower, but anything higher will lead to black bands going across the frame. The slower the shutter speed, the more likely ambient light will contribute to the image. This may or may not be desirable.

Once these paramters are set, you would meter the flash pulse from the subject's position. The meter will give you an aperture reading, which you will set in your camera. This is typically very accurate. If you do not like that aperture, adjust your flash output to compensate.

You can also leave the flash output the same and adjust your aperture.

Think of it this way: There is a range of levels of illumination in your composition - you have highlights, midtones, and shadows - the more contrasty your light, the more dramatic the differences in brightness may be in these areas.

Let's say you have a single light on one side of your subject. If you meter the side of their face that is facing the light, it might be very bright - maybe f/22 (pointing meter toward the light). If you meter the other side, it might be much darker - perhaps f/5.6 (pointing meter away from the light). If you meter right under their chin, maybe it will be f/11 (pointing meter toward camera).

So which aperture do you use? It depends on how you want the picture to look. Given the above, if you use f/22, the lit side of the face will be perfectly exposed and the dark side will be completely black. Overall it'll be a fairly dark picture. If you use f/5.6, the unlit side will be well-exposed but the lit side will be completely blown out. Overall it'll be a very bright picture. If you use f/11, it'll be somewhere in-between.

So the trick is to adjust your various lights and modifiers until you have a reasonable dynamic range in your composition for what you want to achieve. You can experiment with having a key light that is one stop brighter than the fill, two stops brighter, etc. "Key" and "fill" are generic terms implying that the key is creating your highlights and the fill is bringing up your shadows. But this would apply to whatever regions of your image are going to have different levels of light. If the brightness of the different regions gets too far apart, you won't be able to expose for all areas - you'll have to choose. For a start, try to get a dynamic range of 2-3 stops between your highlight and shadow areas. then meter an area that you'd like to be a midtone (middle amount of brightness) and try that aperture to take your picture.

You can then check your histogram for spikes on the left or right border - which would signify lost shadow or lost highlight detail. Either may be acceptable to you depending on the picture you're trying to take.

One more thing that can ensnare new users of flash meters is the 1/10 stop increments and translation to your camera.

Lets start with a typical range whole f/stops:
...f/1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22....

Now lets take a look at the prgression of setting from f/5.6 to f/8 in 1/10 stop increments
...f/5.6' 0, 5.6' 1, 5.6' 2, 5.6' 3, 5.6' 4, 5.6' 5, 5.6' 6, 5.6' 7, 5.6' 8, 5.6' 9, 8' 0....

This progression is easy until the next number after 5.6' 9 is not 5.7' 0, but is f/8. The "' 9" part means it is 9/10 of a stop above f/5.6' 0 and 1/10 of a stop below f/8' 0 (which is one whole stop above f/5.6). Like whole apertures, the tenth stops are exponential, not linear.

The practical problem is your camera does not have f/5.6' 3 or f/5.6' 7 as a setting option. So what do you do? Most cameras have 1/2 and 1/3 stop settings options (the option you choose is set in the menu settings of the camera). 1/3 stop will give you finer adjustment, so that is typically used, but neither is "wrong". Typical 1/3 stop increments would look like this:

...4, 4.5, 5, 5.6, 6.3, 7.1, 8, 9, 10, 11, 13, 14, 16, 18, 20, 22...

So,we know 6.3 is 1/3 stop higher than 5.6, since that is the 1/3 stop increment given by your camera. We also know 5.6' 3 is also 1/3 stop higher than your camera based on the above paragraph. From this, we can conclude f/5.6' 3 and f/6.3 are the same as each are 1/3 stop more than f/5.6. Similarly, f/ 5.6' 7 and f/7.1 are the same as they are each 2/3 stops more than f/5.6.

But what if your meter says f/5.6' 1? You should round down to f/5.6' 0. Similarly, you should round up a reading of f/5.6' 2 to f/5.6' 3. This will make your exposures accurate to 1/6 of a stop, which is so close, it would likely not make a difference. In the cases that it is a problem, the flash would have to be adjusted to compensate.


***Note to the analytical types***: Because of the practical limitations of camera hardware, the above description has some math that is "slightly" off in how it is described due to some necessary rounding. Any differences are inconsequential in real world application. This also assumes you are matching the camera to the meter and not offsetting adjustments to intentionally over or under expose an image for a preferred workflow. This type of compensation would need to be accounted for in the metering process, but is outside the scope of this intro discussion.