James Dec 07, 2011
Welcome to the second part of this photography class. After getting an overview of what a camera is and how focal length works, let’s now go on to what is probably the most important and scariest parameter of any photograph: exposure.
In order to keep things (relatively) short, we will split this vast subject into many small digestible pieces. In this lesson, we will see what exactly exposure is, and how we can use three camera controls to modify it. In the next lesson, we will talk about a very important tool for reviewing exposure: the histogram. In the subsequent three lessons, we will talk about each of the three controls (shutter speed, aperture and ISO) in more detail. Finally, we will discuss the slightly arcane topic of metering modes.
A photograph, as the name suggests, is a record of light. Exposure, quite simply, is the amount of light to which the sensor is exposed. We are all familiar with photos taken indoor without a flash and which appear too dark: they are underexposed, not enough light was allowed on the sensor. Conversely (though perhaps more rarely), we have also seen images too bright, with pure white in large areas: they are overexposed.
There is not one correct exposure of a given scene – depending on what you are trying to say with your image, you might actually over or underexpose on purpose. For instance, a backlit scene could be underexposed to create silhouettes against the sky. Or a portrait might be carefully overexposed to create a high-key feeling. However, what we will generally consider a good exposure is one with an even (but not necessarily linear, as we will see tomorrow) distribution of tones, from pure black to pure white, with no details lost to either shadows or highlights.
With the exception of some very manual film cameras, all modern camera bodies include one or several light meters, whose role is to measure the quantity of light and give a guess of what the correct exposure should be. What you will do with this information will depend on the shooting mode you are using: in auto, the camera will simply set all the required parameters so that you can shoot without questions asked. Alternatively, it can let you set one or more parameters and fill in the remaining ones (aperture or speed priority modes), or it can let you do the whole thing yourself, mentioning how your settings compare to what it thinks you should do, but not acting on it (manual mode).
Three, and only three, parameters control the quantity of light to reach the sensor. They are the usual suspects: aperture, shutter speed and ISO. Let’s see briefly how they work with an analogy.
Imagine that your sensor is a bucket. Light is water coming from a pipe (your lens) into the bucket. What you want to achieve is a good exposure – just the right quantity of water, to the rim but without spilling any on the floor. You can achieve that by doing three things:
You can decide to modify any of these parameters as you wish to achieve your perfect bucket, with some limitations of course: for instance, you can’t have a pipe of infinite diameter, there is a maximum size. Likewise, your filter can’t be too coarse or you might get dead rats in the bucket and it would be unusable.
Something that is extremely important to realize is that all three parameters are bound together. If you modify one and want to keep the same exposure, you need to modify another in the opposite direction. For instance, if you want to use a pipe with twice as much area (doubling the flow), you need to either cut the flow duration by half or use a twice as fine filter. Modifying a single parameter will result in a modification of the bucket content.
As you probably guessed already, the diameter of the pipe corresponds to the aperture, the duration to the shutter speed and the filter to the ISO. Things get even more interesting because each of these parameters has another consequence beside modifying exposure: aperture changes depth of field, shutter speed can introduce motion blur and ISO modifies the noise levels.
Let’s be a little more concrete. When you put your camera in a non-automatic mode (if it has one, if not, you can look at the metadata of old photos to find this information), you should see three numbers in the display, for instance f/8, 1/50, ISO 400 (the ISO is often hidden, you may have to hunt it down in the menus). What this is telling you is that the aperture is f/8, the speed 1/50th of a second and the ISO is 400. What you want these numbers to be will be covered in the next lessons. For now, let’s take a look at how modifying them changes exposure.
Put your camera in A mode. What this does is let you control the aperture and set the shutter speed accordingly. Turn the control wheel in one direction to modify the aperture. You should now see instead f/5.6 (if you turned in the correct direction). What this is telling you is that you are now using a wider pipe diametre and have doubled the flow. What you should notice is that the speed changed as well: now it is showing 1/100, and the ISO hasn’t changed. To compensate for the modification of one parameter, the camera changed another one, and kept the same overall exposure.
If you do want to modify the overall exposure while in a mode other than manual, you should use the aptly named button called “exposure compensation”. What this will do will depend on the mode you are using, for instance if you are in Aperture Priority, it will change the shutter speed to fill the bucket to a different level, while leaving you in control of the aperture.
In manual mode, the camera lets you modify all three parameters yourself without attempting to compensate and keep the same global exposure. It will usually let you know how far away you are from what it considers the correct exposure, but whether you want to follow its recommendation is up to you. In this mode, since we have full control anyway, the exposure compensation button is useless.
This should hopefully give you a good idea of what is going on in a camera brain, and what the A, S and M modes are for, but we have left a lot of things out, to be covered in the next lessons. For now, make sure you have really understood all the concepts here, as they are absolutely crucial for the rest of this course (and of your photographic career).
The goal today is to get a bit more familiar with exposure and how it is affected by the main three parameters of shutter speed, ISO and aperture. I am afraid the assignment will require control of these elements. If your camera has no ASM modes or manual controls via menus, you won’t be able to complete the assignment, sorry.
Keeping a single scene for the whole session, the assignment is basically to play with your camera in semi and full manual modes. Make sure to turn “ISO Auto” to off. What we will call “correct exposure” in the assignment is simply what your camera think is correct. Obtain a correct exposure in full auto, aperture priority, speed priority and full manual mode. Now do the same but with a big underexposure (2 stops, or 2 eV). Same with a big overexposure (2 stops/2 eV again). Get a correct exposure with an aperture of f/8 in aperture priority (easy), full manual (easy-ish) and speed priority (a bit harder). Do the same with a speed of 1/50. Now get a correct exposure with both f/8 and ISO 400 (you can use any mode). Finally, try to get a correct exposure with ISO 200 and a speed of 1/4000.
Also remember that there are many pieces of software, some free, which allow you to review which parameters were used for the capture. It is always stored in the metadata of the image.