Are there any "rules" for using ISO?

Started Apr 3, 2013 | Discussions thread
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Barrie Davis
Barrie Davis Forum Pro • Posts: 21,460
Re: Are there any "rules" for using ISO?

GreenMountainGirl wrote:

Barrie Davis wrote:

Spot metering only works if the subject represents a good target for metering. Just BEING the "subject" doesn't do that....

.... (statistically the chances of a subject by itself integrating tonally to the ideal target of an 18% grey are rather worse than a whole scene doing it with an averaging type reading.

The suggestion of spot metering has been put forth by OP, but although it sounds like a good possibility, I wondered about how it was more appropriate than evaluative metering.  Especially since it is often difficult to get that center spot to stay on a moving subject.

Leaving difficulties of aiming aside, the assumption is too often made that metering tightly around a limited subject area somehow increases accuracy of reading. Well, it doesn't do that UNLESS the subject also happens to be an ideal TONE for metering purposes... (either 18% grey or integrating to 18%grey).

As stated, it happens more often that a WIDE field integrates (mixes together) multiple differing tonal areas to sucessfully simulate a single tone of 18% grey equivalency, than does a NARROW spot.

Evaluative metering endeavours to identify the nature of the subject being photographed. It does this by taking dozens of separated different area readings, instead of just one. Then, while also taking into account the general brightness level across those areas, it calls up an actual shutter speed and aperture from its database of known correct exposures for hundreds (thousands?) of subject types

Although more sophisticated, isn't this also where we run into problems with trying to reach that "average" exposure?

No. Evaluative is anything but averaging.

It is the only metering system with built in "experience" using a database of known correct exposures...using actual shutter speed and aperture settings for differing subject types....that is, setting the shutter speed/aperture that WOULD have been used after appropriate compensations had been applied by a photographer who knew how to do so properly.

Obviously, like any smart systems, it isn't utterly infallible. But evaluative does better than almost anything else (see below line for exceptions), and its hit-rate is improving all the time as manufacturers seek to out-do each other..

For your future assistance, make a note of these important facts about metering....

The lightness or darkness of the tones in the subject are of little importance to what shutter speed/aperture combinations will yield correct exposure.

At first this may seem counter-intuitive, but, in fact, the Lightness or Darkness of tones in a subject tends to mislead ordinary averaging reflected light meters -(non-evaluative kind)- such that we have to apply an Exposure Compensation Factor to cancel out the errors....

.... UNLESS the tones metered happen to (conveniently!) integrate to a middling tone of 18% reflectance, in which case compensation is not necessary.

So what IS of importance in determining the shutter-speed/aperture combinations that will yield correct exposure?

That's simple, there are only two factors which determine correct exposure...

(1) The strength of the light falling on the subject,


(2) The ISO value currently in use on the camera.

That is all. The lightness/darkness of the subject can be safely ignored on the vast majority of occasions.

That is why methods of determining exposure WITHOUT pointing a light meter at the subject are more reliable and much more consistent. I'm talking about separate Incident Type light meters with a white plastic light receptor dome (flash meters are of this kind)...

... and even simple "incident light-strength rules" derived from scientific tables can and do beat a camera meter for accuracy a lot of the time... the most famous of all, the Sunny 16 Rule, which states....

When shutter speed is set to reciprocal of ISO in use (thus 200-ISO = 1/200th second)
correct exposure is as follows..

Clear Sun (sharp shadows) f/16
Hazy Sun (soft shadows)..........f/11
Cloudy Bright (no shadows)......f/8
Cloudy Dull (as when raining)...f/5.6

There is more to it than that, like shooting in snow or on the beach, but basically the whole thing can be written on one side of an index card...

... or be printed on the inside of a film box, which is exactly what they used to do.

Hey! That's enough to be going on with...

I hope it was interesting, and not too long.

Best Wishes

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"Ahh... But the thing is, these guys were no ORDINARY time travellers!"

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