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Selecting the right reading point for spot readings

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by Carl Koch

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Addressed to experts, this article as­sumes familiarity with the principles of full-area reflected-light metering (meter pointing at subject) and of in­cident-light readings (meter pointing from subject at light source). Familiar should also be the fact that spot readings yield more exact re­sults, especially in the film plane (see page 4). However, one problem with spot readings, especially in the be­ginning, is the selection of the cor­rect reading point and evaluation of the readings for optimum exposures. Spot readings have a similar purpose to close-up readings of a subject with a hand-held exposure meter. Both types of measurement involve read­ing one or more subject brightness values and using these readings to establish a mean exposure. A hand­held meter used with a spot attach­ment does the same but measures subject luminances even more ex­actly for subsequent calculation of a mean exposure.

This way of metering becomes still more accurate with spot readings of the image in the film plane of a view camera. For this allows automatically for exposure factors, possible aper­ture tolerances, flare, filter factors, vignetting factors, lens transmission efficiency, etc. And as such precise readings are always taken at the cam­era, operation is simpler still – for in­stance you are then unlikely to cast shadows on the subject while taking a reading.
In all such cases of reading specific luminances the question is what ex­actly to read. This can be learned and with spot readings exposure mea­surement becomes very systematic rather than more or less empirical with various sources of error.

We shall deal here in detail with spot readings. The explanations and illus­trations are in part based on the ‘Pro­fessional exposure technique’ chap­ter of the handbook on the SINAR System ‘The large format’ (1982 edi­tion).
Basically we have to deal with three types of reading:

  • single-point readings
  • multipoint readings
  • contrast readings

For professional purposes the single-point and multipoint readings described below are only suitable for subjects of limited to medium con­trast. See page 4 (fact no. 2).

Single-point readings

Single-point readings may be used in three ways:

Single midpoint reading on a grey card (Fig. 1 and 2)

Fig. 1
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Fig. 2
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In the subject position place a neutral grey card of 0.7 nominal reflection density. Precisely read this area with the meter. That yields an exposure at that point to give the same image density (0.7) in a correctly processed print.
Every darker and every lighter tone in the original is rendered correspondingly darker or lighter respectively in the print. This is thus the simplest spot reading method. Note however the following points:

  • Locate the grey card as closely as possible to the subject.
  • Avoid glare reflections from the grey card towards the camera.
  • For flat objects locate the grey card parallel to the subject (prefer­ably laid on top).
  • With solid objects and directional lighting locate the grey card at right angles to the bisector be­tween the lens axis and the direc­tion of the main lighting. Incline the card slightly forward to avoid glare reflections towards the camera
  • For diffused lighting, backlighting and close-ups locate the card at right angles to the camera axis.
  • In low light read from the white side of the grey card (90 O/o reflec­tivity). In this case open the aper­ture by 21/3 steps more than dis­played on the meter or allow for this by setting the meter to a little under one-quarter the normal ASA film speed (7 DIN steps lower).
  • With subjects of predominantly light tones reduce the exposure reading by 2/3 lens stop.
  • With subjects of predominantly dark tones increase the exposure reading by 2/3 lens stop.

Single-point readings of a midtone in the subject (Fig. 3)

With distant views it is not always practical to place a grey card in the subject field. In that case select a midtone brightness in the subject it­self and read that. When searching for a midtone, look through half­closed eyes – that gives a better impression of actual tone values. Another way is to look through a deep neutral density filter, as often used by motion picture cameramen.

Single-point readings on a brightest Image point (Fig. 4)

If the subject has no suitable midtone (for instance snow, distant views, line originals on white background) or if the light is exceptionally weak, read a brightest image point that should still show detail (but not a catchlight). Then reduce the setting by 21/3 EV steps or set the ASA film speed to one step below one-quarter the origi­nal speed (or reduce the DIN value by 7).
A single-point reading on a brightest image spot can also check an uncer­tain midpoint reading.
As a single-point reading generally only covers a single luminance value, it is of course subject to tolerances and may often need individual correc­tion. For that reason a multi-point reading is often preferable – and not just with difficult subjects, either.

Multipoint spot readings

For multipoint spot readings the fol­lowing procedure is suggested:
For greater accuracy read several im­age points that can count as mid­tones (Fig. 5) and use a mean of the readings for the exposure.
Such multipoint readings at first sight appear more elaborate than a single­point reading. But the electronic con­trol of the SINARSIX-digital exposure meter with continuously calculated, displayed and set exposures makes this method more reliable and still sufficiently fast and simple.

Fig. 3
Fig. 4
Fig. 5
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Two-point brightness range (contrast) spot readings

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As indicated on page 4 (fact no. 2), the contrast reading described below is suitable wherever the subject con­trast (including lighting contrast) ap­pears to reach the limiting exposure range of the film used. (That happens fairly soon with direct positive pro­cesses.) But as it is easy to make mistakes here, contrast spot read­ings should be the preferred method. In principle and application such readings are also simple, reliable and very informative. Proceed as follows: If a subject has no obvious midtones but both bright and dark areas, you can establish a precise exposure va­lue also from successive readings of brightest and darkest points (Fig. 6) that should still show detail.

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For such a measurement note the ex­posure values indicated at each read­ing and use a mean of these values for the exposure. The electronic SINARSIX-digital meter computes from these individual readings a mean value and automatically sets it on the SINAR DIGITAL-shutter.

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Fig. 7

Two-point readings at the same time provide brightness range or lumi­nance ratios (contrast measure­ment). This brightness range is also indicated by the SINARSIX-digital meter.

A practical hint: Many photographers aim for brilliant (i. e. high-contrast) transparencies. That impresses the customer but this brightness range has to be compressed for reproduction in print. Such reproductions then look disappointing. You get better re­sults in print by controlling the bright­ness range of the transparency from the outset.

Arriving at a correct exposure is how­ever not the end of the line for spot readings. This technique can also de­liberately control tone values in the print. More about that later.

Fig. 8

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Problems of Accurate Exposure

Even with ideal metering methods every professional now and again comes across mysterious failures. So it is worth look­ing more closely at their causes.

Fact no. 1

Equipment, materials, reading meth­ods and also processing are subject to tolerances. It is illuminating to list all these critical sources of error.

1. The material:Film speed
Meter readingExposure meter variations
Meter technique (including reciprocity characteristics)
The camera:Lens diaphragm tolerance
Exposure time tolerance (precision of shutters used)
Bellows extension, filters, flare etc.
7.Processing:Development time and conditions
8.Illumination:Illumination variance, flash guide number variance

If we assume that each of these eight items is subject tea ± 1/6 EV error, that is quite a tight tolerance. How­ever, by unlucky chance most or all of these eight tolerances could deviate in the same + (or -) direction, yielding an overall error of a whole EV step. So this problem needs specific counter­action.

A closer analysis of the tolerances that the photographer can influence (items 2, 3, 4 and 6) shows that spot readings in the film plane are vital. This metering method eliminates two of the eight sources of error (lens diaphragm toler­ance and exposure factors) and a fur­ther two (exposure meter and mete­ring technique) are reduced to a mini­mum.

Fact no. 2

Photographic subjects often have a greater brightness range than the film material can record. (Often the case with direct positive processes.) If the subject brightness range at the time of exposure exceeds the exposure range of the film material, you have virtually programmed for failure, even if the technical tolerances are zero and the exposure measurements exact.
The table above illustrates these fun­damental limiting ranges:

EV differenceContrast
(Brightness or luminance range in film plane)
Density range of
film(assuming gamma = 1.0)*
Reproduction limits for various materials in terms of EV difference or contrast measured in the film plane (gamma = 1 .0) *
Printing on low-quality paper (newsprint)
5**1:32**1.5**Colour slides, colour prints, printing on art paper
71:1252.1Colour negative film (approx. 1: 150)
81:2502.4Black and white film (approx. 1 :200)

* The density range indicated in the table Is that of a film Image processed to a gamma of 1.0. However negative materials are usualty devel­oped to gamma 0.65 and colour transparencies normally have a gamma of around 1.7. The den­sity range actually measured on the processed film therefore differs from these tabulated va­lues. To obtain likely real values, multiply the densities in the table by the average gamma of the film used.

** Maximum permissible range is 5.1/3 EV – 1 : 40 or density 1.6.

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Exposure latitude and contrast:

The nearer the luminance range of the subject approaches the limiting den­sity range on the film, the smaller the exposure latitude. Subjects of such limiting range therefore need particutended brightness range. Failure to base exposures on such spot read­ings is the most frequent cause of the exposure errors indicated above.

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How to avoid exposure errors

  • Reduce sources of error 2 and 3
  • Eliminate sources of error 4 and 6 = fact. no.1
  • Carefully observe the limiting den­sity range of the film (fact no. 2)


  • Use contrast spot readings (in the film plane)
  • Reduce the lighting con­trast. Where this is not pos­sible:
  • If the subject brightness range exceeds 1 : 32 make extra exposures on a softer working negative material which can cope with a con­trast up to 1 : 150 (in addi­tion to a usual shot on transparency material).

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Telex: 76740

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