Metering 101

Light meter at “zero” means a good picture. Right?


Shoot something black, filling the entire viewfinder with that object, and make sure the light meter points at “zero” as you are pointing at the subject (use the viewfinder!). Take the picture.

You get this:

The histogram shows why this is bad:

A histogram of a black object should peak on the left (the dark side).

Now do it again, with the light meter pointing at –2 (minus 2):

Perfect. Look, the histogram is right for this type of scene:

The moral of this post:

  • “The meter displays zero” does not equal “this will be a good picture”. It merely means “this will be a mid-grey picture, neither dark nor light”.
  • “The meter points to minus” does not equal “this will be a bad picture”. It merely means “this will be a dark picture”.
  • “The meter points to minus” does not equal “this will be a bad picture”. It merely means “this will be a dark picture”.

And there you have it. Now you understand the camera’s built-in light meter.






No Meter? No Problem

In studio shoots, you use a flash meter.

But if you do not have one, can you do it? Sure you can. Here’s a trick:

  1. Set up your lights. Guess the light’s power setting.
  2. Get a grey card, and hold it in the exact spot where your subject will be, aimed half way between the light and the camera, as your model may be.
  3. Set focus to manual (we are worried here about exposure, not focus!)
  4. Fill the viewfinder entirely with the gray card (be sure not to block the light)
  5. Click.

Now review the pictures. Press INFO or DISP, or hit UP/Down, until you see the view that includes the histogram.

Now here’s the trick. A good picture has the histogram peak (or peaks) in the centre. So if you see this, you are ok:

What if you see this, a histogram on the left side:

That means you are underexposing. You need to turn up the flash power and try again:

And if you see this, the histogram on the right side:

The histogram is on the right; you are overexposing: turn down the flash power, wait a few seconds so it can dump its excess charge, and try again.

As soon as you are in the centre, take a real shot and check – you should be OK. And you metered it – and all without a light meter!


Flash Meter

If you want to do “studio type” shooting, set your flash power manually. On strobes you have to do it that way; on speedlights you can. Then use a flash meter.

How? Here’s how.

  1. Set your camera to the desired settings. For instance, 100 ISO, 1/125th second, and f/8. These are pretty typical studio settings.
  2. Verify that a shot taken like this without the flash is all black. That means ambient light will play no role. If not, go to 1/200th second.
  3. Now set up your flash or flashes. Set the power to, say, quarter power for a start – or whatever you think might be roughly right. With experience, you will get this just about right.
  4. Holding down the the MODE button, set your meter to flash metering mode (the lightning symbol; not the sun symbol, which is ambient metering). Your meter now reacts only to flash.
  5. Set the meter to 100 ISO and to 1/125th second (if those are your desired values).
  6. Hold the meter, with the white dome extended, where the subject will be.
  7. Reset the meter with the side button – it now reads “0” for aperture.
  8. Fire the flashes.
  9. Read the value. If the value is higher than f/8 (eg f/11), reduce the flash power or move the flashes away. If the meter reads lower (eg f/4), then increase the power or move the flashes closer.
  10. Repeat steps 7-9 until the meter says f/8.

That is how you meter a studio, type shot like the one above. I usually meter each light separately and allow for that (e.g. two lights that both say f/5.6 will give you a total of f/8, if light from both hits the subject.).


Light Meters Are Old Hat. Not.

Not! A light meter is an indispensable tool if you want to ace your exposures first time.

Take this scene (taken, incidentally, amidst a whole bunch of naked people):

That meter is well exposed. Perfectly, in fact. Values were 100 ISO, f/5.6 at 1/50th second.

How? By reading the values off the incident light meter (a meter you hold where the subject will be):

  1. Set the meter to ambient (not flash) metering
  2. Move the ball out
  3. Select the camera’s ISO and the aperture you want
  4. Hold the meter where the subject will be.
  5. Click and read the value for shutter.
  6. Set those values on your camera
  7. Click.

With the camera’s built-in light meter, however, the exposure came out like this, since the light background was also read by the meter:

That’s nice for the background, but if the meter is the subject, this exposure is all wrong – 2 stops too dark (the camera thought 1/200th was the correct shutter speed).  You would now have to adjust the exposure manually, or instead aim your camera, set to spot metering, at a gray card held there. Which is less convenient.

And that is why light meters are far from old hat. Pros use them all the time, even as ambient light meters as here.


Your light meter is not perfect

Your camera’s light meter is a reflected light meter.

Here’s how it works. And you need to simply accept and remember the following:

The in-camera light meter is designed to give a good reading when aimed at a mid-gray (“18% grey”) subject.

By implication, this means that when you aim at a non-midtone subject (like a dark subject or a light subject) the image will be incorrectly exposed.

In other words, because the camera “thinks” that it is looking at mid-grey it will try to render the subject as mid-grey.

One solution is to set your exposure manually while looking at a grey card; then using that exposure for your subsequent pictures taken in that light. That way I get pictures that are right regardless of the subject’s brightness.

Like these two taken at yesterday’s Sheridan College class, of two of my students:

[1] Darker subject, coat, camera:

[2] NBow a lighter subject, dress, wall:

Both were correct at the metered settings of 1/125th second, f/2.8, at 800 ISO. Which I measured off a gray card!