For something that looks dead simple, metering is amazingly complicated. Fortunately a lot of approximations are possible because photography is very flexible and not anything like as critical as some people like to pretend.
First, there's the question of 'keying' exposure to the highlights (or brightest areas) or the shadows (or darkest areas).
With color slide or digital, exposure must be keyed to the highlights, because if it isn't, these will 'blow' to a featureless white. An incident light meter is ideal for slides and digital because the incident dome provides an artificial highlight. Indeed the old name for these meters was 'artificial highlight'.
But because the exposure is keyed to the highlighs, anything that is too dark will record as a featureless black. This doesn't matter with slides -- anything is better than large areas of blown highlight -- but with negatives it means you lose shadow detail if the brightness range of the subject is more than about 7 stops. This is unusual but it can happen, especially on a bright sunny day or when shooting interiors.
Now, it is difficult or impossible to 'blow' highlights in a negative (they can always be recovered via appropriate burning-in at the printing stage) so with negative (mono or colour), exposure should be keyed to the darkest tone in which you want texture. Ideally you therefore need a limited area reading of this area BUT, as already noted, because the meter is calibrated to deliver a mid-grey you need to modify your reading and give 2-1/3 stops LESS exposure (this seems counter-intuitive but it works if you think it through).
Limited-area metering is too complicated to go into here -- I once wrote an entire book on exposiure, called Perfect Exposure (reissued in paperback in 2004, details on www.rogerandfrances.com) -- so for negatives the best advice for a beginner is to favour the darker areas of the subject, i.e. point the meter cell towards the subject from the camera position, towards the darker areas rather than the lighter ones, usually angling it down slightly to avoid reading too much sky. This will usually give you enough shadow detail.
You may later revise your technique on the basis of experience (and, I hope, reading my book or some of the modules in the Photo School on the same website) but this is a good start.
Be wary of the '18% grey' myth (there's a free module about this in the Photo School on the website too). Overall reflectivity of 'typical' scenes as determined by the original Kodak research is 12-13 per cent, not 18 per cent.
Eighteen per cent is a Munsell mid-grey, that is to say, if faced with a series of cards from the brightest while available to the darkest black available, most people will pick 18 per cent as a mid-tone. No ISO speed criterion is based on 18 per cent grey and it is of much more limited use in metering than many people would have you believe.
The first commercially successful spot meter, the SEI Photometer which still enjoys a cult following today, didn't even have a 'mid point' index because such an index was substantially useless before people started carrying grey cards around with them: it had only indices for brightest highlight with texture (reversal metering) and shadow detail (negative metering).
The main reasons 18% grey cards work for many people are (a) the flexibility of silver halide photography as noted above and (b) the fact that many of them re-rate their films a lot lower than the true ISO speed. ISO speeds are a repeatable and usually accurate standard, but this is not the same as saying they are ideal for all types of photography or metering. On a sunny day, I will set a broad-area reflected-light meter 2/3 stop slower than the ISO speed to get good shadow detail -- but with a spot meter, with a shadow value 2-1/3 stops below the mid point, I'll use the full ISO speed.
Sorry for a long and possibly confusing post but it's as well at the start of your metering career, as it were, to realize three things:
1 There's a lot of myth and misunderstanding in metering
2 This doesn't necessarily matter much because the system is very flexible
3 With negatives, there is enormous latitude for overexposure, often several stops, but there is far less latitude for underexposure, especially on a sunny day where even 1/2 stop may result in loss of shadow detail depending on the metering technique.
Last edited by Roger Hicks; 05-25-2006 at 04:33 AM. Click to view previous post history.