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Article 4
Exposure Control
'Taking control of your camera's light meter'

By Rob L. Suisted

(Originally published in New Zealand Outdoor magazine)

EXPOSURE CONTROL. Hmmmmmmm. What the heck does that mean? For me, varying definitions might have popped to mind a few years ago. Such a term seemed unlikely to be connected with photography....... 

Far more plausible for the outdoors person I’d have guessed, ‘Exposure control’ might have referred nicely to the desperate attempts to protect your bits the swanny doesn’t hide, from the icy blasts of wet wind, with a piece of damp tissue, while crouching and grimly hanging onto a wad of sopping wet tussock with the only free hand you’ve got left; while it’s turning a nice orange/purple colour, such is the temperature! Oh that brings back memories of many a rain soaked Tahr hunting fly camps.  

Seriously though, exposure control is a term that every keen photographer has to meet head on at some stage. What we’re talking about here is understanding how to measure the amount of light in a scene, and correctly use it to create a correctly exposed photo. You might wonder why we have to concern ourselves with this when we might own a super duper expensive camera that’s got enough bells and whistles for Africa, so to speak. Surely it contains a fairly complex system for taking light readings and getting exposures right, right? Well, the truth is for much of the time it can do fine...but, the rest of the time you’ve got to provide it with a little advice to get the exposure right. While this is not quite as exciting as the host of more creative definitions that may have popped into your mind, my promise is that what I’ll endeavour to explain will be a simple and enlightening experience. Hopefully nothing like the horrible tussle I recall having while learning photography.


A tussle caused by an archaic ‘How to, Manual of Photography’ that seemed to take a 'maximilist' approach when it came to writing unnecessarily complicated dribble. Oh how I now look back and think of the trouble I could of saved myself if someone had ‘boned out’ the important bits. So if you can bear a few thousand of my words I’ll try to do just that for you. Otherwise, you can just look at the pictures.

Let’s just think for a minute how simple photography really is. Forget the bells and whistles you get on most new cameras. At the end of the day it simply comes down to projecting an image onto film which is then ‘captured’ by the changes the light makes to the chemistry in that film. Of key concern to us is knowing how much light we should allow onto the film to get the optimum image results. No different really from knowing how long to subject a camp oven to glowing embers in order to get an ‘optimal’ loaf of camp bread (notwithstanding the fact that some of my hunting mates have never actually managed to produce a camp bread of ‘optimal exposure’, opting normally for the underexposed look - rather like the inside of a cows’ tummy, in crust colour......and texture!). Fortunately though overcooking your photos is somewhat harder than overexposing your dough due to the precise way we can measure light compared to highly unsophisticated method my mate ‘Crispy’ uses for assessing whether his embers are giving off enough horsepower for the baking job.


PROBLEMS - When your subject is much brighter, or darker, than the 18% grey mid tone you’re in trouble.

These photos show the difference between letting the camera’s lightmeter do it’s own thing (top) when the subject is much brighter than the mind tone, and you providing a bit of compensation (bottom photo). The first is underexposed because, as explained in detail in the article, the meter is calibrated to measure an 18% grey mid tone and render that on the film. Hence the snow becomes an underexposed light grey instead of pure white. If you averaged out all the tones in this image you’d get pretty close to a constant mid grey. For the second image, a light reading was taken from the back of my hand, and then overexposed 1 stop (explained in the text), giving an acceptably exposed image in the trying conditions. Above the Whataroa River, Westland. Kodachrome 64



So, what are we dealing with? Basically, in every photo taken during the day, we’ve got the sun pumping down heaps of light into the scene we’re about to photograph. The horsepower rating of the sun doesn’t really change, but the level of light reaching our picture is highly variable due to many things such as cloud cover, shadows, angle of light, filters etc. This is where problems start when trying to get a correctly exposed photo. Therefore, if the level of light was to remain exactly the same, all day and everyday, we would not need light meters in cameras, and cameras could be made with fixed exposure settings. This is why we have a rule that you may of heard of called the ‘Sunny f16 Rule’? That’s because on a bright sunny day (when the light level is always the same due to no cloud cover), we know we’re dealing with the full output of the sun and can set the cameras exposure controls with this rule, rather than having to measure the light every time. I’ll explain this rule shortly.

We’ve just spoken on the amount of light entering the scene we’re about to photograph, and this can be highly variable. The next problem we have is that the things we’re about to photograph are all highly variable in the way they reflect light. White objects are extremely good at reflecting most of the light that strikes them (about 90%), while something that is dark coloured is very good at absorbing the light that strikes it, therefore reflecting very little (black around 4%, that’s why it looks dark, right).

You can start to see that getting the right exposure reading is not necessarily an easy undertaking when not only can there be a huge variation in light getting to the subject, but also the huge variation in light levels that are reflected from the subject, say a snowfield on a sunny day, and a black object on an overcast day.




How do cameras attempt to measure this light (and why do they often get it really wrong)? The lightmeter in your camera can only measure the amount of light that is reflecting off the subject you’re about to photograph, and instructs on what settings to give to correctly expose the image. The problem is that your camera is measuring the amount of light that is being reflected from the objects, but it can’t tell how reflective the objects are, and as we’ve seen this is highly variable due to the colour (and texture) of the objects. I guess you could say your camera is tone blind.

Just think for a moment how much easier it would be if we could measure the amount of light landing on an object before it was reflected off at your camera. We would remove most of the variability caused by the different ways things reflect light. Well it is possible to measure this way - it’s called an incident light reading, instead of the reflected light reading your camera measures. However you need a hand held incident lightmeter, and you also need to measure the light actually in the scene; a bit difficult if you’re taking a photo of Mt Cook! Nevertheless, incident light readings are very accurate, and this is essentially what the ‘Sunny f16 Rule’ deals with. It says that on a bright sunny day, what ever you’re photographing, whether it’s a bright snow field, or a piece of burnt toast, you simply set your aperture on f16 and your shutter speed as close as possible to the ASA rating of the film you’re using. So if you’re using 100ASA film, you’d set f16 and 1/125th shutterspeed (or f11 @ 1/250th, or f8 @ 1/500th etc). Why does this rule work? As shown, the sun has a highly predictable light output and we know that on every sunny day the amount of incident light entering the scene will be the same, regardless of whether you’re photographing light or dark objects. The problem with your camera, as you’ll soon see, is that it confuses the reflectance of objects with the brightness of the day.

Even though there is a large area of high reflectance (snow) in the foreground and background, the dark centre balances out a mid tone average for the whole image and the results are spot on with a centre weighted camera meter. No compensation is needed. Looking up the Perth River, Westland. Kodachrome 64, f11 @ 1/125th sec.


So, as your camera uses reflected light readings, how does it attempt to get the most accurate reading? Basically all camera lightmeters is set to assume everything they ever see reflects exactly 18% of the light that hits it. 18% reflectance is about half way between white and black, i.e. an object would be that same tone as middle grey. From this assumption it can then work back and figure out exactly how much light is landing on the scene from the sun, and expose that photo appropriately. Well that’s great, if you go around photographing objects and scenes that only reflect about 18% of light at the camera. Luckily, most photos do actually average out to about this level. There will be bits with higher reflectance (say whiter) and bits that are darker, but they’ll tend to average out to around 18% and your camera will cope like it’s supposed too (like the Perth River photo).

What happens though when we take photos that are not all mid toned, say predominantly lighter, or darker? Ever got your skiing photos back and found the snow is not white, but a sickly grey colour?? Or taken a photo of something black and it’s come back a pukey grey tone instead of jet black? Here’s the reason. Your camera is set to expect those scenes to be of objects that reflect 18% of light, not 90% like snow does, or 4% like charcoal might, and you’ve done nothing to tell the lightmeter to expect any different. What’s happening in the case of the snow photo is that the lightmeter thinks its dealing with a scene made of 18% reflectance (not 90% as the snow is) and is trying desperately to turn it into a 18% tone on the film by letting much less light in (about five times less because 90/18 is about 5), thereby chronically underexposing your photo, hence the sickly grey snow photos you get back (like the photos of me on the snow). The opposite happens with the black subject. The lightmeter thinks that it’s dealing with an 18% grey subject, but thinks there must be sod all light from the sun because the thing is only reflecting 4%, so it will let about 5 times more light to compensate (4% x 5 is about 18%), and hey presto a pukey grey instead of black because it has just chronically overexposed your photo.

What can we do then? If you can hang on, I’m going to give you a few tips to stop this problem whenever you’re taking photos that aren’t of mid toned subjects.



You may have heard of Grey Cards? They are a piece of cardboard that is printed to reflect exactly 18% of the light that strikes it. Sound familiar? The idea is that if you want to take a photo with a very exact exposure, you simply pop this piece of cardboard in front of the camera so that it is enjoying the same level of light as the scene you’re about to photograph. You then let your camera’s lightmeter take a reading from this rather than the scene. Because the card is 18% grey and your lightmeter is calibrated to 18% grey, you’ve now got a benchmark to work from. Therefore the camera will be able to get a precise handle on how much light is actually falling onto the card and advise the exact exposure setting you’re after. Easy, all you have to do next is use this exposure setting instead of the original the meter gave you, and whip the card out of the picture before taking it. You will end up with a photo that is exactly as you saw it, not under or overexposed. Think for a minute, if you’d left the grey card in the scene and taken the photo you should be able to hold the final picture next to the card and they would be exactly the same tone. All very well I hear you say, I’m not carrying around a blimin’ piece of grey cardboard with my hunting or fishing gear, I’ve got enough to carry. It’s also bloody hard to sneak up to a wild animal with a bit of grey cardboard to get a good light reading before taking it’s piccy. You’re right, it’s a hassle. For this reason I use a few of Nature’s own 18% cards in the same way, although you can buy Kodak Grey cards through most camera shops if you wish.

Firstly, it would be good to fix in your mind what 18% reflectance looks like. Try and think of a mid tone half way between black and white. It can be any colour, we’re talking about tones only. Wet cement would be very close. The key is to start looking at everything you photograph in tones, and continually compare them to the average tone that your camera is set for. When you spot a photo that looks like it would average out into a tonal range that is a lot different from 18% then warning bells should sound and you will then set about helping your lightmeter by compensating the reading that it is given. That’s why we call it Exposure Control, or Exposure Compensation. For example, next time you go to photograph a predominantly white scene, such as a ski field, you’ll think to yourself...”Shoot, this is reflecting a heck of a lot more light than wet cement would, because its mostly white. I remember reading ages ago in NZ Outdoor that white reflects about 5 times more light than the 18% grey my camera meter works from - boy will it be confused. Therefore I have to let in about 2 stops extra light (2 stops equals 4 times - see issue Feb/March 97 if you’re lost) than what the meter will come up with to compensate because it is trying to turn the white snow into the really sickly looking 18% grey snow on the film. Not it’s fault though, it’s programmed to see everything as mid toned which is good enough for most of the time.....”


The expanse of light toned white water in the centre causes the camera’s lightmeter to underexpose the image (as explained in text). Normally you would consider compensating for the meter, but in this case underexposure gives the image more atmosphere and was left without compensation. Enderby Island, Sub-Antarctic Auckland Islands. Kodachrome 64.


Well what are a few handy tricks to use to get you through if your subject is not mid toned. Conveniently green grass gives almost exactly 18% reflectance. So if some is handy, take a light reading off it and use this reading for your photo. Make sure though that the grass is receiving the same level of light as the scene you’re going to photograph, and that the light is coming in from roughly the same direction. Also, any blue sky in the southern sky is about 18% toned, but don’t get any white clouds in your camera’s metering zone. Note that you should probably use the Sunny f16 Rule if there is a clear sky anyway.

Lush green forest, such as broadleaf/coprosma green is about smack on the 18% mid tone. Darker green forest such as beech forest is about 1 stop darker than mid tone, so you can meter off this and underexpose 1 stop.

I know that in the mountains if there’s a lot of snow about that I can take a light reading off my raincoat, rather than the bright white landscape. Even though it’s blue it still reflects about 18% so is perfect for the light meter to use as a bench mark to get the light reading perfect.

The back of my hand reflects about 1 stop more light than 18% grey (but about +1/2 if I’ve got a suntan). This means that if I’m photographing a light or dark toned subject I can reach around in front of the camera and take a light reading off my hand (taking care to angle my hand to catch the same level of light that the subject is getting). I then know if I overexpose the reading the camera gives me by one stop (because my hand is one stop whiter than what the meter thinks) I will get a perfect exposure no matter what I’m photographing, light or dark.




Grey river rocks, such as in the braided East Coast rivers tend to give a reflectance of about 1 stop brighter than mid grey, so you can meter off them and set your camera to over expose 1 stop. You can meter directly off snow and overexpose the meter reading by +2 1/2 stops for an accurate reading.

The key message of this column is to get you thinking about taking light readings from subjects or objects that are not going to baffle your lightmeter. Therefore, use objects as close to mid toned whenever possible. When you can’t you need to use objects with a known reflectance, and compensate for the difference, such as the back of the hand technique.

Remember that it works against your natural expectations.....if you’re photographing a BRIGHT toned subject, say snow, you have to let MORE light in than the meter recommends, and if the subject you’ve metered off is DARK toned you need to let in LESS light. Your compensation will be up to about + or - 2 1/2 stops each way depending how far away from mid toned the subject you’ve measured is.

How do you do this with a point and shoot camera? Well most of these will lock all their exposure and focusing mechanisms when the button is half pressed down. The trick here is to point the camera at a mid tone (and roughly the same focusing distance away as your subject because this often locks the focus also), press the button half way down until the green light comes on (if that’s what it has), hold it there, reframe the photo you want, and press it fully down. You’ve hopefully just forced the camera to take a reading off a mid tone rather than the background you thought was too bright, or too dark.


Same as for the ocean shot. Much light tone in the centre, forces you to compensate the camera. I took a light reading off the mid toned cliffs behind, as they are almost equivalent to 18% grey. Upper Mahitahi River, Westland. Kodachrome 64.

A few other points are that if you’re using slide film it is a lot more critical to get your exposure readings spot on. There are two reasons for this. Slide film has less exposure latitude, that is it can only register a range of about 5 stops of light from white to black, while print film (negative film) can register about 7 stops. But most importantly, when you shoot slide film the exposures that you make are exactly as you’ll get them back. The film in your camera is developed and then cut up and stuck into the plastic frames. What you take is what you get. However, with print film, the film in your camera is the negative strip you get back, and the prints in the packet have been exposed from the negatives you took. Therefore, if you’ve made stuff ups with the exposure of your negatives then the one hour lab will correct these to a certain extent when they print your film. That means if you’ve chronically underexposed your negatives, then the lab will be able to chronically overexpose the prints to retrieve reasonable results for you.

There is another problem to this though and that is the machine used to automatically print your prints has a lightmeter very similar to your camera, and it too has problems with negatives that differ from mid tone. So if you do get snow prints back that are a sickly grey colour, or a photo that should be dark and comes back a sickly mid tone, then take them back and ask for them to be reprinted correctly.


Lastly, get to know your camera’s lightmeter. It normally doesn’t cover the whole of the picture you’re framing. Most only use the inner 3/4 and this is weighted towards the centre. Some have spot metering which only uses the inner 2-5% of the focusing screen, which is very useful for precise readings off smaller objects. Your camera manual will provide information about this.

Well, I know on the face of it this article probably seems rather complex. That’s because I’ve gone into the reasons why problems occur, rather than just giving a simple rule of thumb, say open your camera up if the scene is lighter than mid tone. Believe me that while you might be a little baffled now, I know that if you work the principles into your photo taking, these concepts will become second nature to you (it might be useful to carry some notes with you till you get the hang of it?). Soon you’ll be second guessing the reaction of your lightmeter as though it was a sixth sense and when you achieve this you’ve already lifted your photography onto another level.

Go for it!

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This article and images are copyright to Rob L. Suisted - Nature's Pic Images.  All rights reserved.