Some point last year I wrote about aperture and its effect on depth of focus and exposure.
Talking to some friends recently, one of the things that came up was that exposure, despite the millions spend by camera manufacturers on their metering systems is a bit of a closed book to many people.
There is no one correct exosure, you can exposure for the midtones, high lights or the darker tones. It all depends upon the picture you want to produce. However, getting that exposure means achieving the correct the amount of light that you need to hit a piece of film of a set reactivity to light to produce a picture that looks ‘right’.
This means that there are three factors involved: your shutter speed (how long the film sensor is exposed for); your aperture (how wide the ‘hose pipe’ is); and your ISO setting (the reactivity of the film or sensor to light.
Think of it like a see-saw. Once the plank is resting across the pivot at a given point, you need to move the weights at each end to specific places to get the plank to balance.
For example, if you really want depth of field but are at the slowest shutter speed that you can hold the camera steady at, you can increase the ISO sensitivity. If you want a slow shutter speed to show movement but also large depth of focus, you can drop the ISO.
But is it not that simple. For a start not all photographs produce their best effects when the exposure is ‘perfect’ from the light meter’s view point. If you are pointing the camera into the sun, a ‘contre jour’ effect may be more dramatic. Also, the light meter has a couple of effects too.
The first factor is that light meters are calibrated to provide a correct exposure of a tone equivalent to c.18% grey (I have seen values quoted ranging from 14% to 18%). This means that a tone brighter than 18% grey is viewed by the camera as over exposed 18% grey and the meter correspondingly ends up under exposing the scene. This is why that white horse came out looking a dull white or grey and why snow pictures are often under exposed. Conversely, blacks are seen by the meter as 18% in serious need of some extra light so left to its own devices the camera will probably produce a dark grey. To test this, take pieces of white, black and 18% grey card and take a picture of each, filling the viewfinder each time. You will probably get something like this:
To rectify this, try adding in around +1.5 to +2.0 EV (exposure value) for the white card and -1.5 to -2.0 EV for the black card. You should get something like this:
Once you know this, you can get a feel for what EV compensation you need in order to accurately reproduce reds, blues, yellows etc The example values below are from Chris Weston's 'Exposure Hndbook'.
Pale yellow: +1.5EV
Talking to some friends recently, one of the things that came up was that exposure, despite the millions spend by camera manufacturers on their metering systems is a bit of a closed book to many people.
There is no one correct exosure, you can exposure for the midtones, high lights or the darker tones. It all depends upon the picture you want to produce. However, getting that exposure means achieving the correct the amount of light that you need to hit a piece of film of a set reactivity to light to produce a picture that looks ‘right’.
This means that there are three factors involved: your shutter speed (how long the film sensor is exposed for); your aperture (how wide the ‘hose pipe’ is); and your ISO setting (the reactivity of the film or sensor to light.
Think of it like a see-saw. Once the plank is resting across the pivot at a given point, you need to move the weights at each end to specific places to get the plank to balance.
For example, if you really want depth of field but are at the slowest shutter speed that you can hold the camera steady at, you can increase the ISO sensitivity. If you want a slow shutter speed to show movement but also large depth of focus, you can drop the ISO.
But is it not that simple. For a start not all photographs produce their best effects when the exposure is ‘perfect’ from the light meter’s view point. If you are pointing the camera into the sun, a ‘contre jour’ effect may be more dramatic. Also, the light meter has a couple of effects too.
The first factor is that light meters are calibrated to provide a correct exposure of a tone equivalent to c.18% grey (I have seen values quoted ranging from 14% to 18%). This means that a tone brighter than 18% grey is viewed by the camera as over exposed 18% grey and the meter correspondingly ends up under exposing the scene. This is why that white horse came out looking a dull white or grey and why snow pictures are often under exposed. Conversely, blacks are seen by the meter as 18% in serious need of some extra light so left to its own devices the camera will probably produce a dark grey. To test this, take pieces of white, black and 18% grey card and take a picture of each, filling the viewfinder each time. You will probably get something like this:
To rectify this, try adding in around +1.5 to +2.0 EV (exposure value) for the white card and -1.5 to -2.0 EV for the black card. You should get something like this:
Once you know this, you can get a feel for what EV compensation you need in order to accurately reproduce reds, blues, yellows etc The example values below are from Chris Weston's 'Exposure Hndbook'.
Pale yellow: +1.5EV
Light blue: +0.5EV
Mid blue or green: OEV
Red, dark blue: -1 EV
Black: -2EV
The second thing is that if your object of interest is in lighting conditions that differ greatly from its surroundings, the camera’s meter will be influenced by the overall scene and not provide accurate exposure for what you want to show. To avoid this you can several things, the most basic is to get close to the subject, fill the frame with it, take a reading, keep it e.g. use the camera’s ‘exposure lock’ function and then go back to where you are going to take the picture from.
Or you can change how the camera’s meter looks at the world. Most DSLRs / compacts today let you alter the metering settings. Nikon’s offer Matrix (everything in the view finder: the camera looks at different portions of the scene, does some clever thinking and then decides what the best overall exposure is), Centre Weighted (the middle 65% odd of the scene influences the overall reading), and Spot metering (the camera only reads from a narrow point, in some camera this point is the middle of the scene/viewfinder image and in others it is the focus point that you are using).
Canon provide something similar although I believe that the simpler models e.g. 350D skip Spot metering and instead provide ‘Partial’ metering which is a halfway house from Centre Weighted to Spot metering. By selecting the relevant mode (and positioning the camera accordingly) you can control what the meter is looking at and thus gain more control over the exposure.
Histograms in brief.
You should also check the histogram. Expose to the right (i.e. exposure as far to the right (highlights) as possible before you start blowing i.e. (massively over exposing) important parts of the picture because it is easier to regain data from shadow (not clipped shadow) than it is from light areas. When checking the histogram it is worth noting that the standard histogram is based on a jpeg, even if you are shooting raw, which you ought to be. If your camera provides them, the histograms for the red, blue and green channels are however likely to be based on the raw data, and are therefore more accurate.
Some cameras can be set up so that blown highlights / low lights ‘flash’, another useful aid for the photographer in managing their exposure.
The second thing is that if your object of interest is in lighting conditions that differ greatly from its surroundings, the camera’s meter will be influenced by the overall scene and not provide accurate exposure for what you want to show. To avoid this you can several things, the most basic is to get close to the subject, fill the frame with it, take a reading, keep it e.g. use the camera’s ‘exposure lock’ function and then go back to where you are going to take the picture from.
Or you can change how the camera’s meter looks at the world. Most DSLRs / compacts today let you alter the metering settings. Nikon’s offer Matrix (everything in the view finder: the camera looks at different portions of the scene, does some clever thinking and then decides what the best overall exposure is), Centre Weighted (the middle 65% odd of the scene influences the overall reading), and Spot metering (the camera only reads from a narrow point, in some camera this point is the middle of the scene/viewfinder image and in others it is the focus point that you are using).
Canon provide something similar although I believe that the simpler models e.g. 350D skip Spot metering and instead provide ‘Partial’ metering which is a halfway house from Centre Weighted to Spot metering. By selecting the relevant mode (and positioning the camera accordingly) you can control what the meter is looking at and thus gain more control over the exposure.
Histograms in brief.
You should also check the histogram. Expose to the right (i.e. exposure as far to the right (highlights) as possible before you start blowing i.e. (massively over exposing) important parts of the picture because it is easier to regain data from shadow (not clipped shadow) than it is from light areas. When checking the histogram it is worth noting that the standard histogram is based on a jpeg, even if you are shooting raw, which you ought to be. If your camera provides them, the histograms for the red, blue and green channels are however likely to be based on the raw data, and are therefore more accurate.
Some cameras can be set up so that blown highlights / low lights ‘flash’, another useful aid for the photographer in managing their exposure.
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