Although it is not immediately apparent, the means of determining optimum exposures for reversal transparencies (slides) exposed in camera, and for negatives, are not always identical.
The exposure required for a transparency is critical for two reasons. First, it is a camera original, so anything that is lost in either shadow or highlight detail is lost for ever: it cannot be recovered by dodging and burning, as it can from a generously exposed negative. Second, highlight detail determines maximum exposure: ‘blown’ or washed-out highlights in a transparency generally look far worse than inky black shadows. Logically, therefore, the best way to determine exposure for a transparency is by reading the brightest highlight in which texture and detail are desired. The easiest way to do this is by reading an artificial, constant highlight: and this is, in effect, what an incident light dome is.
Alternatively, a spot meter can be used to read an actual highlight, though it is important to use the highlight index, not the mid-tone index. The most widely used highlight index is probably 1 on the IRE (Institute of Radio Engineers) scale, which is 21/3 stops above the (somewhat notional) ‘mid-tone index’. Most meters actually show the IRE scale × 10, so 1 is shown as 10 and 0.1 is shown as 1. The highlight index needs to be above the mid-tone index in order to ensure extra exposure as compared with a mid-tone: otherwise, a bright white in the subject would be rendered as a mid-tone grey in the slide.
All other reflected light readings require interpretation and modification, whether by the photographer or, via empirically derived exposure algorithms, by the software in a multi-sector exposure meter.
For exposing negative films, on the other hand, all that normally matters is adequate shadow detail. If this is present, then a fortiori the highlights will not be underexposed. The logical approach is therefore to meter the darkest areas in which shadow detail is required, preferably with a spot meter, using a shadow index. Again, the IRE scale is widely used: 0.1 (normally shown × 10 as 1) on this scale is 22/3 stops below the ‘mid-tone index’. In this case, it needs to be below the mid-tone index in order to ensure reduced exposure as compared with a mid-tone: otherwise, a deep shadow in the subject would be recorded on the film as if it were a mid-tone grey.
If the subject brightness range between the brightest highlights in which texture is required and the darkest shadows in which texture is required is about five stops, 32 : 1, the optimum exposure for both transparencies and negatives is identical; the IRE scale reflects this, with 1 (10) being five stops above 0.1 (1).
If the brightness range is more than a stop or so greater than this, the negative will require more exposure than the transparency, in order not to lose shadow detail. If the brightness range is less, the negative may safely be given less exposure than the transparency, though most photographers would not bother, given the modest penalties for overexposure (see below).
Through-the-lens (TTL) meters are generally designed for transparency exposure. They may therefore fail to give optimum exposures for black-and-white for the reasons described above. Spot metering of the shadows is much more reliable, or rule-of-thumb corrections may be made to incident or matrix through-lens readings: 1/3 to 2/3 stop under ‘cloudy bright’ daylight, around one stop extra on bright, sunny days with deep shadows.
Three important points remain to be made. The first and most significant is that there is always room to modify exposure in the light of personal preference, or to take account of variations in subject, equipment, or technique. Many photographers establish personal exposure indices (EIs) for their favourite films: that is, film speeds that they know will give them the results they like best, whether or not they match the ISO speeds. The second is that, within reason, overexposure will have far less adverse effect on the image quality of negative films than underexposure. Contrary to popular belief, even quite drastic overexposure (as much as three stops) cannot ‘blow’ highlights to an unprintable featureless black in the negative (white in the print), unless the characteristic curve of the film is an unusual shape (and some are). The third is that excessive overexposure is best avoided as it means reduced sharpness, increased grain (except with
— Roger W. Hicks
Bibliography
- Langford, M. J., Advanced Photography (6th edn. 1998).
- Hicks, R. W., and Schultz, F., Perfect Exposure: From Theory to Practice (1999)
