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Obtaining Correct Exposure

Courtesy of Chris Groenhout

Exposure is quite possibly the most important factor to obtaining technical quality in a photograph, but also one of the most misunderstood aspects of photography. Some of this confusion is understandable as exposure methods vary greatly not only between types of film, but also the cameras in which the film is used. Added to this, there is a great deal of misinformation about such considerations as film ISO (speed), EI (exposure index) and the concept and implementation of the Zone System.

Firstly, before understanding how to obtain correct exposure in your image, it is important to understand what an 'ideal' negative or transparency looks like. Most people learn how to expose film through their mistakes, so a summary of potential problems follows...

B&W Troubleshooting
Negative is 'thin'. This is generally caused by underexposure though can also be due to underdevelopment. If underexposed, the negative will show little or no detail in the shadow areas (the less dense areas of the negative). If underdeveloped, the highlight areas in the negative will be thin. A combination of underexposure and underdevelopment will present a negative with neither shadow detail nor highlight density.

B&W exposure samples

Negative is 'dense'. This is caused by overexposure and/or overdevelopment. If overexposed, all of the negative will be denser than usual (there will be little or no 'thin' area). In conventional films (Kodak Tri-X, Ilford HP5, etc.), this is often not a major problem though 'T' grain emulsions (Kodak TMax) tend to lose detail in the highlight areas easily (especially if also overdeveloped). When the shadow areas look normal and the highlight areas of the negative are overly dense, the film has been overdeveloped and will appear high contrast and grainy. Again, this is not irrecoverable but by no means ideal. When a film is pushed, it is underexposed and overdeveloped - probably the worst case scenario!

Colour Transparency Troubleshooting
Transparency is 'thin'. This is a simple case of overexposure. It is unlikely that professional E6 processing would be so far out of control to produce a thin transparency from a correctly exposed film. More often than not, grossly overexposed film is a product of a malfunctioning camera, incorrect ISO setting or use of the metering system.

E6 film exposure samples


Transparency is 'dense'. To the casual observer, a dense transparency is simply underexposed but various factors including the subjects' contrast and film speed setting can also affect the final result. Fujichrome Velvia is a good example of this - rated at 50 ISO (as per Fuji recommendation), the results are often darker than you might expect. This is due more to the way in which the film deals with midtone values than the actual film speed or exposure method. Transparency film is a very contrasty medium and partly due to this, it's exposure latitude (or room for error) is extremely small. To further confuse matters, the ideal exposure for projection is different than that for publication and scanning. To deal with various situations, photographers determine their working film speeds and will often 'clip test' film before processing the entire roll to double check as when using transparency film, there is no second chance.

Colour Negative Troubleshooting
Negative is 'thin'. Underexposure is easiest seen in the final print when using colour negative film. Obvious characteristics include an overall lack of colour and contrast as well as an increase in graininess. Similar to E6 & B&W film, colour negative (C41) process will affect the density if the chemicals are exhausted, incorrectly mixed or at the wrong temperature. If the markings and numbers along the rebate are not very dark, or the overall negatives seem to lack any colour or density, it's quite likely the lab's fault. Colour negative is the easiest of the three emulsion types to expose as it can handle a degree of overexposure (often benefiting from this), allowing the photographer some exposure flexibility which can be compensated for at the time of printing. The downside of this is that minilabs are generally set up for normal to slightly underexposed negatives and often find it difficult to print overexposed negatives.

Colour Neg. exposure samples


Negative is 'dense'. More often than not, the negative is overexposed and for reasons outlined above, don't stress - it's generally not a problem. In situations of gross overexposure, the colour can be affected by 'cross curves' where it is impossible to accurately print all three colours, but this doesn't generally occur until several stops of overexposure.




So, now that we know what to look for, how do you actually get it right in the first place? Again, every emulsion type is different, so here we go again...

B&W Exposure Method
The old saying with B&W technique is 'expose for the shadows, develop for the highlights' and to some degree this is very good advice. The practicality of everyday photography is that most of us use roll film, and are unable to develop each and every negative separately to optimise contrast and density. Ansel Adams developed the Zone System to describe the tones in an image from 1 to 10 (5 being mid gray) and discussed the affect of over and under development on the final result. His teachings are ideal when used with sheet film (especially if it is to be contact printed) and high contrast subjects. For example if you are photographing a landscape scene with considerable shadow detail (maybe on a rock face) you would meter (using a spot meter or telephoto lens on your 35mm camera) this area and 'place' it on say, Zone III. This is done by underexposing by 2 stops, Zone V being 18% mid gray and the exposure value all light meters are calibrated to. Varying development will compensate for the negative's overall density and hopefully produce a negative that will print perfectly on a normal grade of paper.

Roll film is a whole other ball game. The luxury of development control is limited to the entire film so, unless you're shooting the same subject for the whole roll, you are stuck with exposure as a method of varying density. As mentioned before, all light meters will produce a result of mid gray if used normally. For the vast majority of purposes, this is fine, but there are some situations where a degree of compensation is necessary. If photographing a Caucasian face, overexpose from the meter reading by one stop. Very pale skin (or photographing the nude form) can benefit from up to two stops overexposure. Even lighter subjects such as snow or specular surfaces (eg. chrome objects), will require 3, sometimes even 4 stops of overexposure from the meter indicated exposure.

All of this compensation can be simplified by the use of a hand incident light meter which simply measures an average level of light from all directions regardless of the subjects' tonal range. Alternatively employing a gray card (18% gray piece of card) in the meter's field of view will make exposure a more predictable exercise. Personally if I'm photographing outside, I use green grass, asphalt or even brickwork as a midtone to set the meter and work from there. More often than not, the films' exposure latitude is wide enough to allow a little overexposure anyway and each of these surfaces are of mid gray density or greater.

Colour Negative Exposure Method
If you use the roll-film methods discussed above, you should have no problems obtaining usable negatives as colour negative film has an even greater exposure latitude and will even benefit from slight overexposure in terms of colour and contrast.

Colour Transparency Exposure Method
Colour transparency or slide film has the least exposure latitude of the three types of film but in its favour, is least affected by inconsistent processing and printing. To seriously pinpoint exposure technique, pick one emulsion and work with it for some time and you'll find the transition back to negative material easy and far more predictable. Unlike negative film though, transparency exposure is not based on shadow detail, rather an ideal overall density often assuming some loss of detail in both shadow and highlight areas of the image.

Purists state that exposure must be determined from the highlight area but if that was to be the case, in high contrast situations, the overall transparency would be dense and possibly unusable. High quality drum scanners are capable of working with such transparencies, but the cost and skill level of the operator are both critical factors in this.

MeteringProbably the most consistent way to expose transparency emulsions is by the use of an incident hand light meter as it will average the light independent of the subjects' tonal and colour range. Even with such a light meter, it is possible to obtain incorrect and misleading readings, so point the meters' dome toward the camera from the subject, parallel to the film plane to get the average light on the subject from the camera's point of view. You can also get a feel for the overall contrast range of the lighting by moving the meter around the subject, pointing the sensor toward the light source to approximate the highlight reading and so on.

With important subjects, bracketing your exposures is a safe and useful habit to get yourself in to. Again, purists would laugh at such a suggestion, instead insisting that there is only one correct exposure, but I beg to differ having had a multitude of photographs 'saved' by the adherence to this regime. To the uninitiated, 'bracketing' is when you make a number of exposures of the same subject, maybe one underexposed, another overexposed and finally one at the meter indicated exposure. In the case of transparency film, I tend to meter for the highest possible reading and overexpose in half-stops from there. For example, when shooting an interior, I'll meter the bright area around a window (using an incident attachment on the meter) which might be 1 second at f22 and becomes my first exposure. From there I'll overexpose in half-stops for a further five frames (you can tell I don't pay for my film).

Frame two1 sec @ f16.5
Frame three2 sec @ f22
Frame four2 sec @ f16.5
Frame five4 sec @ f22
Frame six4 sec @ f16.5

In less critical situations and where contrast is less of a problem, a simple bracket of two or three frames is generally enough. Try to bracket overexposing with darker subjects and underexposing with lighter subjects to maintain highlight detail.

Modern camera meters using such techniques as 3D Matrix Metering certainly make the exposure of colour transparency more consistent and reliable, but an understanding of tonal values and contrast help in generating more usable frames of film.

Personally, I prefer to rely on the old 'rule of thumb' - the f16 rule for sunlight exposures... f16 at the film speed (as a shutter speed that is). For example, in bright sunlight, you would set the camera to 1/125 sec on 100 ISO film at f16. From there I would still recommend a bracket of overexposure by a couple of half-stop intervals. The method is remarkably reliable as is the advice inside every packet of film for a range of different lighting conditions. The best bit is that after a while, you'll depend less on your electronic exposure meter and more on your experience so that when the battery dies or you're using a medium or large format camera, you'll spend less time stressing and more time making great images!

Film Speed Determination
No article on exposure would be complete without at least a mention of film speed and its affect on your final results. It may seem obvious that the ISO on the packet of film is what you set on your camera for ideal exposure. It should however be considered as a 'starting point' which can be adjusted to suit lighting and subject conditions, your camera meter's calibration and processing situation. The ISO that the manufacturer is in fact determined using highly scientific and non-photographic means. It is primarily meant as a comparison between different emulsions from different manufacturers and just that. Using a combination of the recommended ISO, and the factors mentioned above, it is important to find yourself an exposure index (EI) which is like your custom film speed.

This is by no means a compromise, rather a practicality of photography and essential if consistent results are what you're after. Returning once again to Fuji Velvia - here is a film that virtually every professional photographer rates at 32 or 40 ISO rather than the 50 ISO that Fuji state on the packet. According to Fuji, the 50 ISO is accurate, but the film is unusual in the way it records mid tones and exposed 'correctly' will appear denser than you might expect. They then go on to suggest a higher film speed (eg 64 ISO) when photographing high key subjects such as snow.

So, how do I find MY film speed? The best way is to take photos - it's as simple as that. Be careful about your exposure method, check your light meter is accurate (most camera shops can help you with this) and/or use a hand meter. Expose your film in 'known' conditions such as bright sunlight and process in a good lab (don't swap about labs when you're testing - slight inconsistencies will affect your results). Bracket the film in 1/3 or 1/2 stop intervals both sides of the manufacturer's recommendations and keep notes of the settings. Then, use what you consider the best exposures to go out again and confirm your findings.

B&W film is a little more difficult as evaluating exposure requires more experience than with transparency film. One method is to proof the roll of film so as to make the density of the film rebate just discernible above the density of the pure black between the film strips. This is referred to as 'minimum density, maximum black' or a 'proper proof'. Make sure you're using your normal grade or filter and keep notes as to your exposure and development times. The 'best' exposed frame will be obvious and may be 1/2 or 1 stop lower film speed than the manufacturer states. This tends to be the case with Kodak TMax 400, AgfaPan 400 though Ilford HP5 seems to hold its film speed well. The results with B&W film are not as critical as colour transparency, but can help in your overall photography enormously.


Summary
Film exposure is a very personal thing and is dependent on a number of controllable and uncontrollable factors including subject matter, film speed, meter accuracy and usage as well as processing.

In 2500 words or so, I have only touched on a fundamentally controversial but extremely important subject. To fully understand film exposure, there is nothing better than taking photos, processing them and carefully analysing the results so that next time they'll be better.

Good luck!


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