Monday, June 25. 2007
Photo Exposure explained - histogram, ISO, shoot speed, aperture
Introduction
There are three things that influence on your photo exposure: duration of shot, how open is aperture, and ISO.
In this article I will explain all of them.
I suggest to have your camera with you, while you read the article, and check and test things as you read them. Choose manual mode (often marked as M), so that you will have full control over camera settings.
Exposure and histogram
Photo exposure is the term that we use to describe how bright is the photo. Simply said: photo has good exposure when it is not neither too dark or too bright. However, there are ways to scientifically describe when a photo is in good exposure. Open your favorite photo editing software. Most of them will have option to show you the photo histogram. It graphically shows the dark and bright
parts of your photo. So, by looking at this graph, you can say if the photo is overexposured or underexposured and make the corrections. Most of newer digital cameras show this histogram when you preview the photo on its display. Some even show it during the framing, so that you can make the changes in settings even before the shot. The more the graph is gathered in left part - the photo is darker, and vice versa.
Here is a histogram of underexposured photo:
Overexposured:
Correct exposure:
Of course, specific conditions or wanted feel and look can define different wanted histograms. For example, the overexposed histogram above is from this photo:
The photo is just the way I want it to be. So, the fact that the histogram shows that the photo is not correctly exposured does not mean that it is not good. However, histogram is very useful and I suggest you to use it. Learn how to read it - it can tell you things about photo exposure, contrast and other things.
Shooting speed
The time for which the photo sensor (matrix) is exposured to the light is called shooting speed. Obviously, the more time the light falls on the sensor, the brighter the photo will be. If we set the three settings to some values, and then change the shooting speed to be twice longer. Technically speaking the photo made with second settings will have one stop of exposure more light than the first one. Exposure stops are important and if you want to master the manual mode of your camera, you have to understand it. So, if you have speed setting at 1/100th of a second and you want to adjust the speed so that you will receive photo which will be two steps underexposured, we would have to use speed of 1/25th of a second. For one stop, the value is 1/50th, and from that value one stop more is twice shorter than that.
Aperture
Aperture is a measure for how much is the lens opened. The more you open it - the more light comes in for the same time. One of most important feature of each lens is the one which shows how much the lens can be opened. The values are measured in f-stops and written as: f/value. Since the value is a denominator, the smaller the value - the lens is more open (and more light is passing through the lens). Lenses are always mentioned together with their maximum f-stop value. For example: Canon 70-200mm f/2.8 IS L is the full name of one of the best zoom lenses by Canon. It means that the biggest aperture you can set this lens is f/2.8. Some lenses even have aperture values lower than 1.0 - the 'lightest' lens is made for film cameras and its aperture is 0.7.
I already mentioned f-stops. Here are the values of apertures that are full f-stops:
0.5, 0.7, 1.0, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 90, 128 ...
Modern lenses often have one-third stops values for finer settings.
Now, you might wonder why those strange numbers. The F number itself is calculated by formula:
f# = f/D
where f# is the f/number, f is the focal length, and D is the diameter of the entrance pupil. The one f-step difference means that the light that passes through lens at f/2.8 is twice more than when we set the lens at f/4. Now, remember that we already had that 'twice light' in the definition of exposure.
This knowledge gives us the power to guess what speed and aperture to use in situations where we can not measure the light.
For example, one of the most popular exposure rules is the Sunny 16 rule: "An approximately correct exposure will be obtained on a sunny day by using an aperture of f/16 and a shutter speed close to the reciprocal of the ISO speed of the film; for example, using ISO 100, an aperture of f/16 and a shutter speed of 1/100 second"
If your camera (in Auto mode) says that in order to get good exposure it will use f/5.6 and 1/40, but the photo is blurred because of fast moving objects, you can make the needed calculations so that you will have correct exposure and faster shooting speed. Lets say that you want to double your shooting speed. Set camera to manual mode and the speed to 1/80 (twice faster). This means that if we don't change anything else the photo will be 1 step under correct exposure. In order to achieve correct exposure, we can change the aperture. take a look at above list of full stop f-numbers. The number with one stop bigger aperture than 5.6 is f/4. So, we will set the aperture to f/4. The photo taken with 1/40 and f/5.6 will be equally exposured as the one taken with 1/80 and f/4.
ISO
In short, ISO is sensitivity to light which measures the matrix's speed.
Sometimes you want to use very high shooting speeds, like 1/4000, but your lens does not not have aperture so big that the photo will be correctly exposed.
The part of your digital camera that catches the light is called matrix.
It has one property, called ISO and it can be changed. It also influences exposure and it also has full stops values. Here is a list of ISO values with one exposure stop difference between them:
25, 50, 100, 200, 400, 800, 1600, 3200, 6400, ..
Lets say that you set your ISO to 100, and your aperture is at f/2.8, and your shooting speed is 1/250 of a second and with these settings you manage to get photo which is correctly exposured, but you need even higher shooting speed. Professional sport photographers often use speed no slower than 1/2000. Lets imagine that we want to achieve speed of 1/1000th of a second and our lens is already at its widest aperture - we cannot open it wider. So, from 1/250 to 1/1000 are two stops (we calculate this in this way: from 1/250 to 1/500 is one stop, and from 1/500 to 1/1000 another stop). Now, in order to use same aperture of f/2.8, speed of 1/1000 and achieve good exposure we will have to use ISO 400.
Notes
All this might seem to mathematical to those of you who just want to take snapshots of friends. Actually, most modern cameras have modes which will do needed calculations for you. However, there are situations where the camera is wrong. Actually it does not make mistake, it just doesn't know what exactly you want to picture. In those situations, it is handy to know about speed. f-stops and ISO.
And for the end - don't forget to test and experiment.
In one of next articles I will focus on ISO, aperture and speed and how they influence the final result more closely.
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