In one of the previous articles on this website I discussed in detail the many different metering modes and their advantages. Metering modes are what tells the camera what exposure value to use for a right exposure of a given scene. As you are already aware, there are any different types of metering modes. None of these are suitable for all situations though. Some photographers are particularly biased about certain metering modes. I in particular like the spot metering mode and normally my camera is always set to shoot on spot metering. The main reason is that being I can accurately select a spot that is middle-grey in the scene and set my exposure based on that.
The problem of reflected light metering method
I mentioned ‘accurate’ in the previous paragraph when the problem is that with built-in meters you are never sure that you have the right exposure. This is inherent of how they work. All built-in light meters measure light on the basis of reflectance. Plus,all camera systems are hard-wired to look at everything as if it’s middle-grey. That means a black surface which does not reflect any light is going to be metered differently from something like a white surface, which reflects all of the light that falls on it. The camera will look at a predominantly black scene and increase the exposure. It will do the opposite when it sees a predominantly bright scene. This is inaccurate, as you can understand.
The solution is in using a technique / tool which assesses the incident light falling on a surface and not the light that reflects off of it.That basically eliminates the problems of metering for the whole scene and just takes into account the incident light falling on the subject. But how do you meter for incident light? Built-in light meters on DSLR camera don’t have any option to do it.
The hand-held light meter
Enter the hand-held / external light meter. The hand-held light meter is a very useful tool that allows you to meter the incident light and not the reflected light. This is something that your built-in light meter does not. Additionally, it is far more accurate because it only takes into account about 1 ° of the frame. Usually spot metering is the preferred mode when it comes to a selective (and thereby accurate) metering of a scene. But built-in light meters sample a much larger area, depending on the lens that you are using, which makes them less precise.
Advantage of hand-held light meters in a studio environment
In a studio environment,knowing the right exposure value often makes all the difference between a set of good images and a day wasted. The hand-held light meter basically gives you the edge you need to perform in such a demanding shooting environment. A light meter can be programmed to read the light emanated from a flash. Once you dial in the shutter speed on to the light meter (the one at which you are going to shoot) and test fire the strobe / flash, the light meter will tell you what the right aperture value should be for a proper exposure.
In order for the light meter to correctly read the incident light value you will need to hold it correctly. Your hand-held light meter should have a white dome like structure. That should be facing the camera, very much like the same way the subject is. Test fire the flash and meter will tell you what the desired aperture / shutter speed is.
A word on metering for incident light outdoors
I have already explained that the hand-held light meter is much more accurate when it comes to doing an accurate read out of a scene. I have also mentioned the reason thereof. This makes it ideal when you are shooting landscape or other scenes (basically outdoor photography). It allows you to accurately measure a point in the scene that is middle-grey and basically make your exposure on that basis.
Just where sheer performance matter a hand-held light meter has no equal.