Light Metering

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Different films have different sensitivities to light (the ISO speed rating) and different characteristics about the range of light intensity they can reproduce. Slide films require an accurate exposure to get a good result, print film (negatives) are not so fussy and can handle some degree of underexposure and a large degree of overexposure. That can be fixed at printing time.

In order to arrive at the correct exposure for the film in use we need a means of measuring the light.

With no Meter.
With nearly every box of film comes a “light meter”. It was on the outside but now is often only printed on the inside of the box. Some brands don’t bother.

It shows what apertures and shutter speeds to use under various daylight conditions. Follow those settings and you will get good exposures for average scenes and subjects.

The “sunny sixteen” rule can achieve the same effect. It states that set the camera to f/16 and set the shutter speed to the ISO rating of the film for average sunny scenes. The combination of the printed information and the sunny sixteen rule can be used to check if your camera is doing strange things with its exposure readings. Aim at an average sunny scene or directly at a patch of sunlit grass and you should get those readings or reasonably close to them.

With a Hand Held Meter.
There are two types of hand held meter to consider, the reflected light and the incident light type. Sometimes they are dedicated types but usually the two functions are combined. Better meters will also have a flash meter function.

The incident type measures the light falling on the subject, the reading only varying with the brightness of the light. The reflected type measures the light reflected off the subject so effectively reads the brightness of the light and takes into account the subject’s reflectivity. The flash meter will measure the flash test burst and give a reading of the aperture to use.

The incident light meter has to be held at the position of the subject and its receptor hemisphere aimed at the camera lens. Sometimes getting to hold the meter next to the subject is difficult so holding the meter in the same relative position and lighting, but closer to the camera will work. This works for sunlight but not for studio lights due to the light falloff with distance. From this meter you have a reading to set the aperture and shutter speed. You then have to make adjustments based on the perceived reflectivity of the subject to get a good exposure.

All meters are designed to give a reading for an “average” scene, usually you are told that this average works out to be 18% grey colour all over. Subjects that are more reflective than 18% grey may overexpose the film, subjects less reflective than 18% grey may underexpose the film. You have to judge this and make the adjustments, working from experience with the type of film you are using and the result you want to achieve.

The 18% grey thing is really a bit of a myth, it is a "standard mid-point" of exposure that I think was first mooted by Ansel Adams (wiser heads will put me right on that). In fact camera manufacturers set their exposure meters up to assume the scene they read is 13% grey, because 13% is closer to the real "average"  scene reflectivity (whatever average is). If you set a camera based on metering off an 18% grey card then your exposure will be out a whisker for the real situation of the 13% average scene, the difference is not much and will never be seen by print workers, and slide workers should adjust the ISO setting of their cameras until they get the results they like for that particular film, so that adjustment makes things right anyway. The 18% grey card is 18% reflectivity, it's just that cameras assume the scene is 13% reflectivity. Just a bit of technical fluff to clear the air about the sacred 18% grey issue. We'll just carry on with calling it all 18% grey to avoid confusing the innocent.

Reflected light meters are held near the camera position and aimed at the scene to be shot. They measure the light bouncing back into the photocell. They also provide a reading based on the scene being an average 18% grey all over. 18% grey is normally touted as being roughly equal to an average landscape scene or to European skin tones or the back of your hand.

Reflected light meters will give a higher reading (need smaller stop and/or faster shutter speed) if they are aimed at a scene containing large bright areas like sand or snow or a white painted building. When you use the exposure information for the exposure, the film is exposed as though it was all 18% grey so the bright white snow will be underexposed by the meter to try and make it the equivalent of 18% grey. Likewise a very dark subject will lead to overexposure because of the attempt to equate the reading to an 18% grey scene.

With both incident and reflected light meters you have to judge what’s important in the scene and estimate whether it will tend to overexpose or underexpose and make the appropriate compensation to the selected aperture and shutter settings. This is not so important with colour negative film due to its wide latitude but colour slide film will show the result of this effect.

The main problem with reflected light meters is that they usually have only one angle of acceptance, maybe about 30 degrees and that angle of acceptance may not match the lens you are using on the camera at the time.

With A Meter in the Camera.
When cameras started to get electronic the meter was built into the body peering through its own hole in compact cameras and inside the body effectively peering through the lens for Single Lens Reflex (SLR) cameras. With the compact cameras the angle of acceptance problem is usually the same, but the SLR through the lens (TTL) type meter solves that problem.

So the first meters in SLRs are incident light meters with the same basic problem, that is, they sum up the whole scene and make an exposure based on the assumption the whole scene is an average 18% grey. Later meters tried to overcome this by selectively narrowing the angle of view and by dividing the whole scene into many segments to measure individually.

The first attempt (and is still common on low end SLRs) was to make the centre of the scene measurement more important than the edges. This assumes your subject of interest is always in the central area. You have to read your camera manual to understand exactly what areas your camera measures under what conditions.

Another approach is to have a selectable spot metering measurement which only measures a small area of the scene, maybe 2% of the image and this small spot can be aimed at the important parts of the scene and the settings frozen before realigning the scene for the actual shot. This approach will give a more accurate exposure for the point of interest. It is equivalent to walking right up to the subject with a hand held reflected light meter.

Variations on the spot metering method with more modern cameras include the spot metering following the point at which the autofocus is working.

The next improvement was to break up the scene into various segments and measure each segment for brightness and for contrast with other segments and decide a possibly better exposure than just the simple average-the-whole-scene method. This segment count can vary from two to 35 in the camera brochures I’ve seen. The camera handbook will show the layout of the segments and explain the method of exposure evaluation involved. The Nikon system in various models has from six to ten segments and also checks the maximum aperture of the lens being used. This way the camera knows from the readings from the segments the absolute brightness from any segment and also the variation (contrast) between them all. The camera logic then looks up a table made up from 30,000 actual photos taken under all lighting and contrast situations and analysed for best exposure and comes up with a better exposure than a simple scene average.

Be aware that you really have to know what sort of result you want so may need to vary what the camera has decided to do. That is why the smaller area metering and spot metering options are provided so you can make more intelligent decisions about what exposure to use for what part of the scene and hope the rest of the scene won’t be too over or underexposed. Again, knowledge about what your film will do is necessary.

The very expensive Nikon F5 professional camera has gone a major step further in having a meter consisting of 1,005 pixels that sense red-green-blue so evaluating colour as well as brightness and contrast. Things like deciding where the sky is located in the scene and avoiding possible exposure problems are now possible.

The way to see for yourself how your camera meter behaves is to aim at a general sunlit scene and include a small amount of sky, maybe less than 10% of the scene. Note the exposure settings and then aim the camera higher so maybe 75% of the scene is now sky, see how the exposure settings change. A simple meter may show a two or three stop change, a clever meter will only show maybe a one stop change or less. But you have to decide what is important in the scene and aim at that so as to fill the viewfinder, set or freeze the exposure and re-aim the camera and take the photo.

To make life easy you can zoom the lens to include only the area of interest, note the exposure reading then zoom back to the whole scene and use the original reading. That is a simple way to achieve more or less the same reading as a spot or small area meter may achieve. Either use manual mode or the exposure lock, it depends on your camera and the features it has.
When in doubt about an exposure the usual advice is to bracket, that is take maybe three shots, one at the indicated exposure, one at one stop under and one at one stop overexposed. That way one may look right.

That very general bracketing advice needs to be modified for both slide film and print film. For slide film bracketing by one stop each side will yield two out of the three shots being bad, it is better to get closer to the final correct exposure by using your head and then bracketing by ½ stop (or maybe 1/3 stop) under and over. That way all three will be viewable and one may have exactly the effect you want. Slide film is very sensitive to exposure so be as correct as possible in the first place.

For print film it is unnecessary to bracket as the printing process can do that for you. The most important part is to capture the information on the negative. Underexpose and the shadow details will be lost forever, over expose and the highlight details may be lost in extreme cases but the shadow details will be fine. Usually you can overexpose any print film by two stops and still get a great print from it, and you will have all those shadow details intact. At printing time the decision has to be made to print for the shadows or the highlights or by expensive hand printing to get correct exposure on all parts of the print. With digital scanning of the negative and manipulation, both bright and dark areas can be made to show detail on the final print.

Flash meters are usually used in studio setups where the large flashes always fire at the same full flash output level or some switchable level below maximum. In contrast the modern TTL flash metering cameras use dedicated circuitry to measure and control the built-in or plug-in flash output to get the correct exposure.

The studio flashes have to have their light output measured by the meter when the setup of maybe multiple flashes are test fired. The flash meter is used like an incident light meter at the position of the subject and aimed at the camera, the test firing of the flashes is done and the f/stop read directly from the meter. That reading is transferred to the camera and the real exposure made with the same flash setup. If the flashes are moved around to achieve different lighting effects then another test firing and measurement is required.

So the summary is to know and understand the meter you have in your camera and when to swap to another metering mode if you think it may provide a better result. On top of that you have to know the characteristics of the film you are using and the effect you are trying to create and make adjustments to suit.

The above is a copy of the sheet handed out at my beginners class talk at the camera club on light metering.

There are some more metering details as Nikon sees fit to do things at......   Link to Nikon metering page.
 
 

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