Tutorial on Exposure Metering

8/11/2019 Tutorial on Exposure Metering

http://slidepdf.com/reader/full/tutorial-on-exposure-metering 1/10

Correct exposure is essential to successful photography, and an understanding of exposure and light metering is a

core skill for anyone wishing to explore the art of photography.

Most photographers rely almost exclusively on the automatic lightmeter in their camera to produce an accurate

exposure, and under most circumstances this is not a problem. The through-the-lens (TT! multi-segment evaluative

lightmeters found on most modern good "uality cameras are very sophisticated, and can cope with most situations

"uite ade"uately. #owever even the best meter can be fooled by unusual lighting conditions or very high contrastscenes into producing under- or over-exposed images.

$or example, if you%ve ever take a photograph of someone standing in front of a brightly lit window, you%ll probably

have found that you can see the pattern on the curtains but that the sub&ect is far too dark. This is because the

lightmeter was fooled by the bright light from the window and the shot was under-exposed.

'imilarly if you%ve ever taken a photo of something light against a dark background you%ve probably found that the

background looked lighter than it should and the main sub&ect was pale and burned out. gain the lightmeter has

been confused by the unusual background, but this time the shot is over-exposed.



)n order to understand how this happens and accurately correct it, it is necessary to know how lightmeters operate,

and the rules by which exposure is calculated. *on%t worry, there%s no maths involved, and it%s a lot simpler than you

might think.

Take a look at this picture+



hat you see there is a nice scene of some fishing boats, with a good tonal range, plenty of colour and some nice

late-evening sunlight. hat your camera%s lightmeter sees is this+



Try it out for yourself+ copy the fishing boat scene onto your hard drive. 'tart up your image editing software and

open both pictures. ightmeters only see in black and white, so reduce the saturation of the fishing boat shot to ero.

ext, add a /aussian blur set at its maximum level, so that the whole picture is reduced to a field of grey. 0se the

eyedropper tool to measure the 1/2 colour value of the resulting tone. 3ou should find that it is a mid-tone grey with

an 1/2 value of around 456,456,456.

)t%s an interesting and curious fact that any average scene reflects 47 per cent of the light falling on it. ook out ofyour window, and unless you live in ntarctica the scene you see is reflecting exactly the same amount of light as the

scene out of my window. That 47 per cent reflection is exactly the same as a mid-tone grey, mid-way between black

and white.

ight meters are calibrated with this fact in mind. hen your camera takes a light reading, the meter averages the

scene and ad&usts the exposure to produce that mid-tone grey (or 45 per cent luminance, but that%s another

discussion altogether!. )f you point the camera at a black stage curtain, it will try to make the black into a mid-tone

grey, so it will over-expose. )f you point it at snow it will try to make the white into grey, so it will under-expose.

)n 4898-:; the pioneering photographers nsel dams and $red rcher developed (no pun intended! an exposuresystem based on this fact, a system that is still used today. )t is called the <one system, and is "uite possibly the

most useful piece of photographic knowledge you%ll ever learn. There are several variations on the original system,

but )%ll go with the one that is easiest to understand.

'tarting with 47 per cent grey as the mid-point, the system divides all the tones between black and white into 44

ones, numbered ;-4;. <one ; is featureless black with no details visible, which in your image editor would have an

1/2 value of ;,;,;. <one 4; is pure white with no details visible, and an 12/ value of 5==,5==,5==. The mid-tone 47

per cent grey is one =, and should have an 1/2 value of about 456,456,456.



Theses ones represent exposure values, or >?. The difference between one one and the next is e"uivalent to the

difference between one exposure setting and another one exactly one stop higher or lower.

This is probably a good time to take a slight detour and explain how exposure is controlled.

>xposure is set by ad&usting two settings@ aperture and shutter speed. 2etween them they control the amount of light

that hits the sensor (or film! when the shot is taken.

'hutter speed is fairly self explanatory, it is simply the amount of time that the sensor is exposed to light. This is

usually controlled by an electrically operated mechanical shutter in front of the sensor that opens and closes very

"uickly for a precisely measured period of time, usually in the order of a few hundredths of a second. Abviously a

shutter speed twice as long lets in twice as much light, one half as long half as much.

The aperture is also literally &ust that@ a hole through which light passes on its way to the sensor. The diameter of that

hole can be ad&usted to precisely calibrated sies. Abviously a smaller hole lets in less light, and a larger hole lets in

more.



These calibrated aperture sies, for largely historical reasons, are called stops, or f-stops. n aperture setting one

stop larger lets in twice as much light. $or reasons that are both historical and horribly mathematical, the standard

full-stop aperture settings that you are most likely to encounter go f4.7, f5, f5.7, f:, f=.B f7, f44, f4B and f55.

Confusingly the smaller numbers refer to larger apertures, and the larger numbers to smaller ones. Many cameras

can set apertures in increments of 49rd of a stop, but these nine numbers are the ones to remember.

et%s take an example. 3ou set your camera to automatic, point it at a scene and take a light reading. $or the sake ofargument, say your camera%s lightmeter sets an aperture of f7 and 45;;th of a second. 3ou can produce the same

exposure by increasing the aperture by one stop to f=.B and halving the shutter speed to 4:;;th of a second,

because this lets the same amount of light through to the sensor. 'imilarly, reducing the aperture to f44 and setting

the shutter speed to 44;;th of a second will also produce the same exposure.

#owever if you alter one setting without altering the other you will change the exposure. )n our example, changing the

aperture to f=.B but leaving the shutter speed at 45;;th of a second will increase the exposure by one stop, or one

>?, making the picture brighter. *ecreasing the aperture to f44 will reduce the exposure by one >?, making the

picture darker.

'imilarly, changing the shutter speed while leaving the aperture alone will also change the exposure. *ouble the

shutter speed to 4:;;th at f7 and you reduce the exposure by one stop, halve the speed to 44;;th and youincrease the exposure by one stop. This is also how exposure (>?! compensation works.

et%s go back to the one system we were discussing earlier. 1emember that each step on the chart represents one

stop or >?. ltering the camera%s exposure by one stop moves the overall brightness of the scene by one one. This

allows us to set the exposure precisely to produce the correct tones in an image.

An this chart, which was originally developed for black and white photography and printing, the ones are roughly

e"uivalent to the following scene elements (adapted from dams% descriptions!+

Zone 0 - Dure black, no details or texture visible.

Zone 1 - 2lack tone but no texture. This is normally as black as you want to get in a picture.

Zone 2 - $irst hint of texture and detail, very deep shadow.

Zone 3 - *ark materials, details visible.

Zone 4 - *ark foliage. *ark stone. andscape shadow. 'hadow on portraits in sunlight.

Zone 5 - Clear north sky. *ark skin. /rey stone. eathered wood. 47 per cent mid grey.

Zone 6 - verage Caucasian skin value. ight stone. 'hadows in sunlit snow.

Zone 7 - ?ery light skin. ight grey ob&ects. 'now with side lighting.

Zone 8 - hite with texture. 'now in shade. #ighlights on Caucasian skin.



Zone 9 - /laring white surfaces. 'now in flat sunlight. hite without texture.

Zone 10 - ight sources, reflections of sunlight on metal. Dure white.

To use this system effectively, it really helps to have a spot meter. This is a lightmeter that measures light from a very

narrow angle, typically &ust two or three degrees in the centre of the frame. #and-held spot meters are best, but they

are very expensive. #owever many digital cameras these days have spot metering as a built-in option. )f your camera

has a setting that looks like either of these, you%re in luck.

)f you don%t have a spot meter you can get around this to a certain extent by ooming in on the area to be metered, or

holding your camera closer to it, and noting down the exposure reading. )t%s not ideal, but it can work.

$or our troublesome backlit portrait shot, we can use the spot meter to take a reading from the sub&ect%s face. e

know that the lightmeter will give a reading that would make the face mid-tone grey, which is one five. #owever from

the one chart we know that average Caucasian skin should be one six, so we need to increase the spot metered

exposure by one stop, in this case from 49;th at f=.B to 49;th at f:. )n the resulting shot the background details are

all burned out, but the sub&ect is correctly exposed.



$or the over-exposed photo of the car on the black background, we can use a similar but different approach. 'pot

metering the background gives an exposure setting of 49rd sec at f=.B to render it as one five mid-grey. 2y reducing

that exposure by four stops to 49rd at f55 we can make the background come out as it should be, one one black.



The one exposure system can help you with all difficult exposures, but it can also vastly improve even your general

photography. earning to think of images in terms of the one system will encourage you to take more care with your

shots, and also to become more creative. Control over exposure is the primary creative tool of the photographer, and

learning how to use it will make the difference between mere snapshots and artistic photographs. This system has

been used by professional photographers for over B; years. 0sed properly, it can help to improve your photography

immensely, probably more than any other single techni"ue.

Note: )%ve made a card that can be printed out at 4;x4=cm which has the one system chart on it. 3ou can download

this reference card by clicking here.

http://staff.bit-tech.net/trustedreviews/Zonecard.jpg

http://staff.bit-tech.net/trustedreviews/Zonecard.jpg

Tutorial on Exposure Metering

Documents

Transcript of Tutorial on Exposure Metering