Where should you point your light meter?

Well, this blog entry was prompted by  this post on a photography forum. It started off with a perfectly simple question but I made the mistake of mentioning that the meter needs to be pointed, not at the light, but at the camera.

And that started some discussion, with different people expressing different views, so I thought it might be a good idea to demonstrate why the laws of physics proves that the meter needs to be pointed towards the camera (which sees the actual amount of light reflected towards it, not the amount of light that the meter sees when it’s pointed directly at the light.

Lighting is really just simple applied physics, but it’s a pretty corrupt form of physics because there are so many variables in play. Most of these variables don’t affect the reality but they can and do affect our perception of reality,  for example:
Bounced light adding light where it isn’t expected or wanted
Meters, digital camera screens or computer monitors may not be calibrated correctly, giving false results
Some camera sensors may be more or less sensitive to light than the ISO setting indicates
Individual tastes and preferences


So, here we go.

All of the images shown here are of course straight out of camera, with no adjustments of any kind except re-sizing. The setup shots are  just quick snaps, using the built in camera flash. You can click on the pictures to see a larger version.

I started off by setting up a very simple shot. A beauty dish in a typical position, high and on-axis to the direction of the face.

I could also have added an on-axis fill because that’s a pretty typical lighting arrangement, but although it would have demonstrated the need to meter to the camera pretty well, it would also have made the whole thing a bit more complicated than it actually is.

The shot on the left was metered to the light and the meter said that the correct exposure was f/8 d6 (f/8 + 0.6 of a stop) so that’s what I set on the camera. It’s underexposed of course. The  shot on the right was metered back to the camera, and the meter therefore registered only the light that the camera would see. Because of this it saw less light and indicated that the correct exposure is f/8, so that’s what I set on the camera.

And here is the setup shot, showing the position of the camera, subject and light. You can see that the beauty dish is at a pretty different angle to the camera. If the light had been closer to  the camera then the result would have been very similar whether I had pointed the meter towards the light or the camera, because the 180 degree acceptance angle of the meter would have guaranteed a very similar result – but not in this case.


What about 2 lights? Well, in this shot I put a softbox either side of the subject,to light each side independently.Each softbox was set to exactly the same meter reading of f/8 d6.  The shot on the left is again metered straight to the light and is underexposed. The shot on the right is again metered back to the camera, to take account of the loss of reflected light, and the reading was again f/8, which was set on the camera.

The differences in the meter readings are due to the fact that the angle of reflectance always equals the angle of incidence – so if the light hits the subject from say 45 degrees then it will bounce off at 45 degrees too, and if the camera is at 90 degrees to the subject  then of course it won’t receive anywhere near as much light as a meter reading to the light source indicates. But this mannequin consists of complex shapes and the light reflects off at all sorts of different angles, which is the main reason , apart from shadows, why the tone always  varies across the image, and the main reason why only a meter reading towards the camera position can take any account of these varying angles.

Some subjects absorb far more light than they reflect, and with this type of subject it doesn’t always make a big difference whether we meter to the camera or to the light – but conversely, if we want to photography a highly reflective subject that’s all on one plane it becomes even more important to get it right. I once had to photography a stainless steel machine with a truly mirror finish, and it was a complete nightmare to get both the light and camera angles right  because, except at the perfect angle, all of the light just bounced away into the ether and it was impossible to show it as anything other than black! Metering to the camera was a dramatic demonstration of the importance of correct metering, because there were 8 stops of difference between metering to camera and metering to the key light.

You’ll be glad to know that I don’t happen to have a sheet of stainless steel handy, but here are shots of a 5 in 1 reflector, with a crumpled shiny surface. I’ve lit it from the side and of course most of the light has just shot off  in the opposite direction. Taking a meter reading from the reflector centre to the light indicated f/22, which as you can see is hopelessly wrong. Metering to the camera instead indicated f/11, which is correct.

I think that this demonstrates the importance of pointing the flash meter towards the camera, using this very simple lighting.

If the lighting was slightly less simple, for example if an on-axis fill was included, and especially if the fill was set to a fairly high level, the fill light would contribute to the exposure and so the error created by taking a reading towards the light would be less obvious – but it would still be there.

So, do I always meter to camera? Sometimes I don’t meter at all, for example rimlighting, backlighting and hairlights are really just effect lights and if they look right on the camera screen or on the polaroid then that’s really all that matters, they don’t need to be technically correct and in fact often look better when they’re not.