O-U (over-under) Indices

Exposure meters are calibrated to give a good exposure of an 'average' outdoor subject. An 'average' outdoor subject, as determined by Kodak research in the late 1930s (research which has never been bettered), reflects about 12-14% of the light falling upon it: not 18% as many believe.

If you look at a classic Weston exposure meter, however, you will find that in addition to the main index or arrow it also has four other indices, marked (from left to right, or clockwise) U, A, C and O. The 'U' index is four stops to the left of the main index; the 'A' (also marked '1/2') is one stop to the left; the 'C' (also marked '2x') is one stop to the right; and the 'O' is 3 stops to the right.

Even if you don't have a Weston exposure meter, and even though the forest of tiny numbers on a Weston Master can seem very confusing, it is well worth knowing and understanding what 'U' and 'O' are for, and 'A' and 'C'.


'U' and 'O'

'U' and' O' stand for 'Under' and 'Over' respectively and are are 7 stops apart (brightness range 128:1). They are in effect shadow and highlight indices for black and white. Take a direct close-up reading of the darkest tone that you want to differentiate from a solid black, and use the U index, and you will have the bare minimum exposure needed. Take a direct close-up reading of the lightest tone in which you want texture and detail, and use the 'O' index, and you will get the maximum convenient exposure. Of course you can give more with most negative materials.

weston master iv

Note that British and American Weston meters are similar but not identical. British Westons are dealt with here, but the same advice applies to American Westons as long as they have similar markings. The Weston IV is illustrated here as the easiest to read for this purpose, principally because it omits the 1/3 stop steps of some of its brethren. The glass is cracked, but this tends to happen with old Westons, at least after the II and III.

v dial

A subsidiary use for 'U' and 'O' is determining brightness range. Take two readings, one of the darkest shadow in which you want texture and detail. Use the 'U' index. Then take another, of the brightest highlight in which you want texture and detail. If it is more than the 'O' index indicates, you will have to sacrifice either shadows or highlights; or reduce your film development time (see below); or print on a softer grade of paper.

If (for example) the U reading on your Weston IV is 0.1, as above, the 'O' index is at 13. If the highlight is less than 13, all well and good. If it is more than 13, you will need to compensate as described at the end of the previous paragraph. Or on a Weston V, if your 'U' reading is 2, the 'O' index is at 9, as on the left.

'A' AND 'C'

These are supplementary indices for subjects requiring less exposure than 'normal' (and ideally more development - at least 30%) and those requiring more exposure than normal (and ideally less development - we suggest about 15%). In effect, this means scenes with a limited brightness range (overcast weather) and scenes with an unusually long brightness range (sunny days with deep shadows).

The original 1956 Weston II instruction book gives as examples 'distant views and landscapes on dull days', in which case you use the 'A' (1/2) index, and 'a sunlit street with dark shadows', in which case you use the 'C' (2x) index.

Spa, Spain

An example of a subject with a long brightness range and deep shadows. A good case for using the 'C' index (twice the exposure) or alternatively for taking a close-up reading of the door on the right - assuming, of course, that you actually want texture and detail in the door.

long brightness range


short brightness range

Abandoned spa, Czech Republic

Most of the interest in this picture is in the building itself, the overgrown foliage and thw muddy road. It is therefore a candidate for decreased exposure and increased development. Voigtländer Bessa-T, 50/2.5 Color-Skopar with x2 yellow filter, Kodak Tri-X, printed on Ilford Multigrade Warmtone.

A and C for Colour Photography

The instruction book suggests that you meter for colour as follows:

First, they specifically caution against reading blacks, whites or greys: they tell you to read only important coloured areas. Meter the darkest colour in the scene, then the lightest colour, and set the main index mid-way between the two.

If however the principal interest lies in the darker colours, set the 'A' (1/2) index against the reading for the darkest colour, or if the principal interest lies in the lighter colours, set the 'C' (2x) index against the reading for the lightest colour.


Mountains, Slovenia

Although the brightness range is quite large, from puffy clouds to deep shadows, the areas of deep shadow are small and unimportant, and can therefore safely be ignored - as can the brightest clouds. In other words, this is an 'average' subject: the brightness of ther day is cancelled out by the distance of most of the subject matter. Voigtländer Bessa-R2, 35/1.7 Ultron, yellow filter, Fomapan 200, printed on Ilford Multigrade Warmtone.



Reduced exposure and extra development for dull scenes, and extra exposure and reduced development for contrasty scenes, are fundamentals of sensitometry and are summarized (albeit at the price of a certain amount of jargon) in the Zone System.

If you take the main index as Zone V, then U is Zone I and O is Zone VIII. In between, on a Weston IV, there are two blobs after U (Zones II and III); A is Zone IV; C is Zone VI; there's another blob (Zone VII); then 'O' (VIII). On the Weston III, there are 1/3 stop blocks; use the filled-in white bits. The Weston V is nothing like as convenient and you will need stick-on numbers.

There are plenty of theoretical objections to this, but it is about as accurate as most Zone System theory and admirably easy to use. The sequence is:

1 Broad area reading. Normal development.

2 Shadow reading. If this is below 'U', move the dial until 'U' is against the reading and stick with normal development.

3 (Optional) Highlight reading. If this is at or close to 'O', stick with normal development. Or:

If the highlight reading is a stop or more below 'O', increase development, either using the N+1, N+2 etc. sequence (read a tract on the Zone System if you want to know what this means) or just adding 50% to your usual time.

If the highlight reading is above 'O', decrease development, either with the N-1, N-2 sequence or by knocking 15% off your normal development time.


The 15/50 sequence will give negatives that are easy to accommodate using different paper grades, and will print better than if you do everything at a standard, normal development time. In the light of experience you may choose a different pair of figures, such as 10/40 or 20/30, but this is mere refinement.

If you find you aren't getting enough shadow detail, reduce the film speed you set on the meter. Us 1/3 stop increments until you're happy. If you're getting plenty of shadow detail, you may even care to increase the film speed in 1/3 stop increments.

Water spigot, Spain

On average, Roger wanted the sunlit stone on Zone VI in the print (light mid tone), so the obvious way to read it would be with the 'C' reading on a Weston, then to check the dark area on the lower right against Zone III (darkest area with texture and detail, the blob next to A); the only way to get a highlight reading would have been via an incident light reading. Hasselblad 500-series, 150/5.6 Zeiss.



The (Japanese) Institute of Radio Engineers scale is another 'Under-Over' scale and is therefore worth mentioning in passing in this brief module. It runs from 0.1 to 1 but is typically represented as x10 at 1 to 10; this is the form in which it is used on Pentax spot meters, and the form in which it is quoted here.

The whole range covers just 5 stops, brightness range 32:1, instead of the 7 stops, brightness range 128:1, of the Westons. See the Subject Brightness Range module for more information on this topic.

I.R.E. 1 is 2-1/3 stops below the mid-point, which is about I.R.E. 3.5. This can be taken as Zone III, where you are certain to get texture and detail with a shadow reading..

I.R.E. 10 is 2-2/3 stops above the mid point. This can be taken as Zone VII, where you are certain to get texture and detail with a highlight reading.


Pentax spotmeter dial


pentax spotmeter dial

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© 2010 Roger W. Hicks