Digital Exposure (Author: Mike Parmee ARPS CPAGB)
The Exposure Triangle
- ISO– the measure of a digital camera sensor’s sensitivity to light
- Aperture– the size of the opening in the lens when a picture is taken
- Shutter Speed– the amount of time that the shutter is open
What Is a Stop?
In photography, a ‘stop’ is a measurement of an exposure, depending on either the shutter speed, the ISO or the aperture.
For instance :-
If we look at apertures then every time that you double the size of the aperture then you double the amount of light that reaches the sensor; i.e. by ONE STOP. Conversely if you halve the size of the aperture you then halve the amount of light that reaches the sensor by ONE STOP.
The same is true if you halve or double the shutter speed.
The same is also true if you double or halve the ISO.
It is therefore a balancing act between the three actions to provide the effect that you want as a photographer.
As an example, a camera setting of 1/60th. at f8 at ISO 200 is exactly the same as if we increase the shutter speed by one stop to 1/125th. and increase the aperture to f5.6 and keep the ISO the same. In these cases the exposure will be the same
Another example is if we decrease the shutter speed to 1/30 keep the aperture the same but change the ISO to 100 then once again the exposure value remains the same .
One more example is if we increase the aperture to f5.6 (which will let more light in ) but increase the shutter speed to 1/125 keeping the ISO the same. This will result in the same exposure value.
So just a little clarification on what the stops are in the three factors:-
Most cameras will allow a setting of 30 seconds as a maximum after that you will have to use the BULB setting. (The term BULB was introduced because in the old days the shutter was manually kept open by the aid of a rubber bulb which used the air pressure to keep the shutter button depressed.)
Twice as fast as 30 secs. is 15 secs. (one stop). Twice as fast as 15 should be seven and a half secs but this was rounded up to 8 secs ( to make life less complicated) and so on – after 1/60 sec it should have been 120th but this was rounded up to 1/125 to make life easier!
f1.4 is probably the widest aperture that most of us will experience and this probably only on 50 mm prime lenses. Half as large an aperture as this is f2 ( one stop) and so on through f2.8, f 4, f5.6, f8, f11, f16, f22 with each stop making the aperture half as large in area as the previous f stop.
ISO stands for International Standards Organisation ( properly the International Organisation for Standards).
In effect ISO refers to the sensitivity of the camera sensor to light, the lower the number the LESS sensitive to light is the sensor.
Each camera has a BASE ISO which is usually 100 or 200 in the case of high end cameras this can be as low as 64 or less. The higher the ISO the more sensitive to light is the sensor and each increment of sensitivity is again one STOP.
i.e. An ISO of 100 is half as sensitive to light as an ISO of 200 and an ISO of 400 is twice as sensitive as an ISO of 200. Typically available settings are 100/200/400/800/1600/3200/6400. We would usually choose not to go above an ISO of 6400 unless prepared to use specialist noise reduction software.
The intended and unintended consequences of changing the settings.
Obviously the slower the shutter speed the more likely we are to record movement in our images this can be used to great artistic effect or can be a right nuisance.
So increase the aperture to compensate – Ah wel,l now we introduce a depth of field issue. The wider the aperture the less the depth of field and conversely the smaller the aperture the greater the depth of field. Once again this can be used to great artistic and compositional effect or once again it can be a right nuisance.
So let’s change the ISO to compensate – great except the higher the ISO the more digital “noise” that we introduce which may or may not be what you want or intend.
It is always a balance and in the end it is the photographer’s skill and vision that will decide these settings to produce the effect that is wanted.
So what is “correct exposure”?
How does the camera both help and hinder you ?
“In photography, DynamicRange is the amount of visible detail that can be seen or recovered between the lightest part of a photograph and the darkest. Having a good dynamic range in an image could make it look good, not only on screen, but when the image is printed as well.”
Dynamic Range is measured in STOPS. An increase or decrease of one stop will effectively double or halve the brightness level.
The human eye can cover approximately 20 stops whereas a camera will usually cover about 12 or 13 stops
So if the dynamic range of the scene is greater than the dynamic range of the sensor in your camera then you will have to choose either to expose for the highlights at the expense of underexposing the shadows or expose for the shadows at the expense of blowing the highlights.
Therefore if the scene you are photographing exceeds the dynamic range of your camera you will always have an image which is incorrectly exposed for some of the image.
The matter is further complicated by the way in which the clever boffins in Japan have written the software in your camera.
For example the camera has been made to see black as too dark and white as too light, it therefore will try to make a dark lighter and a light darker this is known as 18% grey.
The exposure modes can also complicate things as, for instance, with the MATRIX, CENTRE WEIGHTED AND SPOT METERING which most cameras have.
In MATRIX metering your camera will look at the overall lightness/darkness and create an “average” for the best exposure which will work well if the dynamic range is within the cameras capabilities. If not, it will either overexpose the highlights or underexpose the shadows depending upon which is most dominant in the image.
At the other end of the scale the SPOT metering will look at just where you have placed the spot ( which can be as little as 8% of the image) and totally ignore the rest of the image as far as light metering is concerned.
Other methods to create overall correct exposure.
- Exposure blending using specialist software. e.g. Greg Benz – Lumenzia, Jimmy Macintyre – Raya Pro and Create Luminosity masks in Photoshop. With these methods you will need to create at least two image files , one exposed for the highlights and one exposed for the shadows.
- Selectively dodge and burn in post production.
Remember that you will not be able to recover the detail in overexposed highlights as there is no detail there. However you may well be able to recover detail from moderately underexposed shadows but at the expense of introducing digital noise.
1. Use the Histogram
Your camera will normally show you a histogram of your image which is simply a graph of the dynamic range of the image.
To the extreme left the value is zero and pure black with no detail to the extreme right is the value of 255 which is pure white with no detail. In between are the peaks and troughs of the various tones.
Only as a guide, it is suggested that the histogram should tend towards the right side but not touch it. However the image is yours and should reflect what you feel it should look like.
i.e. Dark and moody or light high key or anything in between – you choose. The perfect exposure is the one that you artistically feel is correct to reflect what you want to convey through the image .
2. Experiment with exposure compensation.
3. Experiment with Bracketing
4. Decide beforehand what you want your image to say and convey through your image.
5. Decide how you want your image to “look” in terms of its dynamic range.
6. Prepare to be BOLD
The danger is that you decide to only take images that are within the dynamic range of your camera and this may lead to dull, grey, flat and uninteresting images.