Estimating Exposure

by Ed Buffaloe

Even if you have a light meter, there are times when you forget it or when the batteries are dead and you need to estimate exposure.  In addition, there are a great many factors that affect light and film and might require you to choose a different exposure from that suggested by your meter.  Almost every book on photography has something to say about metering and exposure--the serious practitioner would be well-advised to read everything available.  Here is what I have been able to garner:

Light Conditions Outdoors

The first rule everyone learns is the so-called “Sunny Sixteen” rule that says the base exposure for any film is one over the ISO speed rating of the film at f/16 in full sun. Hence, for a film rated at ISO 125, the exposure in full sunlight would be 1/125 second at f/16.  A film speed of 100 would have its exposure rounded off to 1/125 second--one could also open the aperture by 1/3 stop to be absolutely precise. A film rated at 80 would typically have the exposure rounded off to 1/60 second--one could stop down 1/3 stop (if that were possible with a given lens) for greater precision. Various other outdoor lighting situations are described, each one requiring an additional stop of exposure.  Typically these situations are described as hazy sunlight, cloudy bright, cloudy dull, and cloudy dark, though everyone has variations on the descriptions.  There is usually one situation added at the other end of the scale for scenes that are brighter than a typical day with bright sun: sunny brilliant (or sun on light sand or snow), wherein one gives one stop less than the base exposure.

Kodak’s standard descriptions for these situations are quite useful, so I will enumerate them:

1. -1 Stop. Bright or Hazy Sun on Light Sand or Snow.
2. Basic. Bright or Hazy Sun with Distinct Shadows.
3. +1 Stop. Weak, Hazy Sun with Soft Shadows.
4. +2 Stops.  Cloudy Bright with No Shadows.  (Also use this exposure for back-lit closeups.)
5. +3 Stops.  Heavy Overcast.
6. +4 Stops.  Open Shade (subject shaded from the sun but lighted by a large area of clear, unobstructed sky).

Sunny Brilliant. This is a brighter than normal situation, usually only encountered in sunlit snow or white sand.  If you were at the beach, photographing the ocean with white sand, the ocean would average out the scene to the extent you would not use this exposure. This is the most intense light possible, with very hard-edged black detail-less shadows.

Bright Sunlight.  Sometimes with light haze.  This is the Basic exposure, representing a typical sunlit scene with hard-edged shadows that contain some detail, but not much. When appropriately directional (i.e., at nearly right angles to the plane of the subject), this is what Mortensen calls texture light, suitable for revealing local textures rather than tonal values (see my article Mortensen Revisited for further details).

Hazy Sunlight. The sunlight is weak and the scene is somewhat diffuse.  Shadows are evident, but their edges are soft.  Contrast is reduced because haze is scattering light into the shadows, which contain considerable detail.

Cloudy Bright. The sun is mostly obscured by clouds, but you can tell where it is in the sky. The light is diffuse and shadows are faint. There is sufficient light for photography, but everything looks flat and uninteresting. The photographer would likely increase development (N+1) to boost negative contrast when photographing in such light. This is great light for maximum detail--what Mortensen calls tone light, suitable for distinguishing local tonal values (subtle shades of grey) rather than textures.  This is probably the light Mortensen preferred for his “7-Derivitave negative”, for which he prescribed slight underexposure and “gamma-infinity” development.

Cloudy Dull. Heavy overcast sky.  The clouds are thick enough that the sun cannot be seen and there is little evidence of where it is in the sky. The light is so soft and diffuse there are no shadows. Such a condition may call for N+2 development, depending upon the subject.

Cloudy Dark. Such conditions prevail early in the morning or late in the evening, or right before a storm.  One normally does not take photographs when the light is this flat.  The exposure required is 16 times the basic. This is also the exposure recommended for Caucasian people shot in open shade. Such a condition might require N+3 development to give a normal rendering (though not for people photographed in open shade).

Color of Illumination

Sunlight is less intense early or late in the day. Additionally, sunlight has much more blue in it at midday than it does early in the morning or late in the evening--and since most film is more sensitive to blue than to red, one must take this factor into consideration.  Within two hours of sunrise and sunset, exposures must be increased 1-1/2 to 2 stops over the basic exposure.  Closer to noon, say around 9 AM or 3 PM, exposures might only need to be increased 1/2 to 1 stop.

Direction of Illumination

The direction from which light hits the subject relative to the camera-subject axis is very important in determining proper exposure. The greater the angle of illumination in relation to the direction the camera is pointed, the greater the exposure compensation must be.  Side lighting generally requires an additional stop of exposure, whereas back lighting requires at least two stops.

Subject Reflectance and Color

Exposure calculations are based on the idea that subjects are average in their reflectance--that some parts will reflect white, some black, and some various shades of grey, and that these reflectances will average out to a middle grey value, which is usually defined as having a reflectance of 18%.  However, Phil Davis points out in his Beyond the Zone System that the middle grey of a typical 7-stop black and white scene really has a reflectance of about 9%. He states, “...the standard gray card is a full stop too light in value to represent middle gray for normal photography.” Bottom line is, grey is a relative term for tonal values somewhere between black and white, and when we seek to identify a base exposure, we are really averaging all the various shades of black, white and grey in a scene to come up with an exposure that will reproduce the portion of the visible scene more or less as we visualize it.  So we have to take into account what reflectance values predominate in a scene.

Some objects reflect more or less light because of their surface texture or color. A light object reflects more than a dark object.  White cotton reflects as much as 85% of white light, whereas black velvet reflects only about 2%.  An object with a smooth shiny surface reflects more light than an object with a rough grainy surface. Even a black object reflects specular highlights if its surface is sufficiently shiny. If the predominant subject in a scene reflects more light than middle grey, it will require more exposure than the base to be rendered with its appropriate tonal value, because the base exposure we are learning to calculate here is designed to provide the correct exposure for an average scene, which is to say, middle grey. Similarly, if the predominant subject in a scene reflects less light than middle grey, it will require less exposure in order to appear darker than middle grey in the final photograph.

Colored objects typically reflect only certain wavelengths and absorb others.  White reflects all wavelengths evenly, but a blue surface reflects only blue and absorbs red and green. If the light striking a blue surface is predominantly blue, the blue object will appear almost white in a black and white photograph.  However, if the light striking a blue surface has a higher component of red, the blue surface will appear to be a shade of grey.  If the light is entirely red, a blue object will appear nearly black. Similarly, red objects will appear quite dark if the light striking them is predominantly blue. For objects of the same apparent visual brightness, the density that will be recorded on the negative will depend upon the color of the light reflected from each object and the relative sensitivity of the film to that color of light. Since most panchromatic films are considerably more sensitive to blue light than to red, blue objects will almost always appear lighter and brighter than red objects in a black and white photograph taken in white light.  That is why for portraiture and outdoor photography in general, a yellow filter is often utilized to give a slightly darker rendering to blue values.  Without the yellow filter, blue skies appear washed out.

Spectral Sensitivity

Films vary somewhat in their sensitivity to colors of light, though most modern panchromatic black and white films are quite similar. Some exceptions are the 25 and 50 ISO emulsions from Efke, which are somewhat more sensitive to green light than other films and render foliage lighter. Kodak T-Max 400 film is said to have slightly less blue sensitivity than other contemporary films, so that less yellow filtration is needed to correct its blue rendering.  Kodak Technical Pan film, which was originally developed for astronomical photography, is considerably more sensitive to red than other films. Orthochromatic films are not sensitive to red light at all, and may be developed under a red safelight.  Orthochromatic films can sometimes render skin tones too dark, emphasizing lips, freckles and dark or ruddy complexions.  On the other hand, they render green foliage and blue shadows beautifully.

Effective Lens Aperture and Bellows Extension

The f-numbers marked on a lens are only truly accurate when the lens is focused at infinity. As the lens is focused on objects closer than infinity, it is extended further from the film plane and the effective aperture is reduced so that there is less light reaching the film and exposure must be increased to compensate. The inverse-square law states that light falls off in proportion to the square of the distance.  So if the lens to film distance is doubled the amount of light reaching the film is reduced four times--effectively, the light is spread out over four times the area and is consequently 1/4 as bright.

Corrected f-stop values can be calculated by multiplying the stop marked on the lens by the actual lens-to-film distance and dividing the result by the focal length of the lens:

indicated f-stop X  actual lens-to-film distance
                       focal length

so if the indicated f-stop is f/4, the actual lens-to-film distance is 100mm, and the focal length of the lens is 50mm, then the actual f-stop is 4 X 100 divided by 50 = f/8. The focal length has doubled, but the exposure increase is two stops, or 4X basic exposure.

Or, to calculate a bellows extension factor you divide the square of the bellows extension by the square of the focal length:

     (bellows extension)²
         (focal length)²

You multiply your normal exposure for infinity focus by the bellows extension factor to obtain the correct exposure.  For instance, if you have a 100 mm lens extended to 200 mm, the bellows extension factor is calculated as follows:

200²  = 40000
100²  =  10000

so the bellows extension factor equals 4.  If your normal exposure were 1 second, you would multiply by 4 to obtain 4 seconds.  If your normal exposure were 1/250 second, you would multiply by four to obtain 1/60 second.  Again, the focal length doubled and the basic exposure was multiplied by 4. Such calculations work out very neatly if you always double your focal length, but get a bit more complicated for odd bellows or lens extensions.  For precison in the field, the photographer may find it useful to carry a portable calculator.

Let us examine a particular case.  I have several old Pentax screw mount cameras that either do not have meters, or the meters don’t work.  I also have a 50mm f/1.8 Pentacon lens, originally made for a Practica screw mount camera, that fits them.  I like this Pentacon lens because it will focus as close as 12 inches.  At its greatest extension, it has a lens-to-film distance of 62mm.  So if I want to calculate the lens extention factor for this lens I will divide 62 squared (3844) by 50 squared (2500) to obtain a factor of 1.52, equivalent to 2/3 stop.

Using the other method to calculate an f-stop value, let’s say my lens is set on f/8.  I multiply 62 X 8 (=496) and divide by 50, to obtain a stop value of f/9.92, essentially about f/10.  The results are the same.

Estimating Exposures Indoors or at Night

My experience is that estimating exposures indoors is very difficult.  There is a guide for hard-to-meter scenes in London & Upton’s Photography that I find useful. It says that for “stage scenes, sports arenas, and circus events”, shooting an ISO 400 film, the typical exposure would be 1/60 second at f/2.8.  Interestingly, when I turn on all the lights in my livingroom (8 four-foot flourescent tubes bounced off the white ceiling and two incandescent lamps) and take an incident reading standing up with the meter pointed at the ceiling, this is precisely the exposure I come up with.  But in many situations there isn’t that much light blazing in a room, and many is the time I have shot at 1/30 second, f/2 with my Leica or Pentax and found my negatives underexposed. I have a Harris Photoguide for Xisting Light (copyright 1982, revised 1989) that suggests 1/30 second, f/2.8 for “bright home interiors”, but in most circumstances I would be inclined to give one or even two stops more exposure. When in doubt, bracket your exposures.

At this point, it’s time to get a light meter and learn how to estimate exposures with it. I mean just that--even with a light meter, you must know what you are doing in order to get good results. An exposure meter only allows you to estimate with greater accuracy.  Perhaps the most important thing to know about exposure meters is that they are designed to read an average scene--one where the light and dark values average out to a middle grey value.  But if you take a light reading directly from a value that is not middle grey you must interpret the reading and adjust your exposure accordingly.  Here, the study of the Zone System is invaluable. The best explanation I have found on how to use a light meter (both reflected and incident) is in Phil Davis’ book Beyond the Zone System.  I highly recommend it--there is no way I could explain it better than Phil, so I won’t try.

References:

Ansel Adams. The Negative.  Boston: Little, Brown & Co., 1981.
Bureau of Naval Personnel. Photographer’s Mate 3 & 2. Navpers 10355-A, 1971.
Phil Davis.  Beyond the Zone System (Second Edition). Boston, London: Focal Press, 1988.
Barbara London Upton and John Upton. Photography (Fourth Edition). Boston, London: Scott, Foresman & Co., 1989.

An interesting system for making exposures by Fred Parker can be found in his article
The Ultimate Exposure Computer.

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