Dr. Strangemeter - Or How I learned To Love The Diffusion Disc
If you are carrying a digital camera, you are carrying a light meter. Depending upon the make and model of camera, it might share the knowledge it has of the world with you or not - but it most certainly will impose its will upon your images - generally for the better.
Set your camera to Auto, Aperture Priority, Shutter Speed Priority, or Program, and you will be helped by the meter. Set the camera to Manual and you are on your own, though the meter may try to prompt you to make a sensible decision in some way. Some have little bar graphs - some have little warning lights that flash. The Flapoflex D has a plastic bar that comes out and pokes you in the eye if you are more than 5 stops out...
If you've got an older film camera - or a camera that is meant to be used by people who know what they are doing - it probably won't have a meter. You are expected to buy a separate meter and take exposure measurements yourself - or not, if you prefer to be an iconic art treasure of the 21st century.
Let's start off right now by saying that the meter built into your DSLR, mirror-less, or compact camera is smarter than your brother-in-law. Those of you who point out that a jug of milk is smarter than the brother-in-law have my sympathy. The meter may be capable of seeing multiple things about the scene in front of it - looking at dark and light with a clinical precision. It has the benefit of programming that has seen many such scenes before and can work out the best compromise for the shot. This works under a wide range of illumination. It is still possible to present the meter with a scene that is too dark, too bright, too contrasty, too flat, or too weirdly-lit and if you continue to press the shutter button after the " Danger Will Robinson! " announcement the meter will just let you go ahead anyway.
The meters will work effectively with their sister circuits built into dedicated flash guns - they will decide between themselves how much of the exposure can be safely taken from the ambient light and how much must be paid for in electricity. The wise photographer might ask them to bias the thing one way or the other, but after asking should let them get on with it and be grateful for the help. Anything is better than those little aluminium dials that sat on the back of older flash guns.
Do you need a separate meter anyway? Well, apart from the film people with big cameras, the other use of the hand-held meter is to pre-determine the range of exposure you might want, to look at the evenness of illumination on a flat art work, or to minutely analyse a scene in case there are any anomalies in it.
What you do with the anomalies after you find them is your affair, but I tend to expose for the highlights, shadows, and mid-tones and then skew the D-max to the over-temperature zone VII before dropping the gradient mask into the toe of the K-curve. You'll see the effect in the image of the Mississippi traffic patrolman at the top of the page.