With camera prices plummeting, a lot of hobbyist photographers are keeping aside their point and shoot(P&S) cameras and investing in an entry-level or a mid-range DSLR. But most tend to keep the camera on Automatic mode or on one of the many default modes built-in. Sure one ends up with prettier and cleaner photographs than with a P&S purely due to the fact that the DSLR is a better specced camera; but this approach defeats the purpose of a DSLR, which is to give the photographer as much control as possible over all the factors which affect exposure. This post is my attempt at trying to explain Exposure, what parameters influence it and why the photographer should control it instead of the camera controlling it for the photographer. Exposure is the simplest of ideas to spell out - it is the amount of light that hits the sensor. When a photograph is overexposed, it means too much light has been let in onto the sensor and the photograph will be too bright with a lot of fine details lost in the whites and conversely in an underexposed photograph, most detail will be lost in the blacks. There are no hard and fast rules that define a perfect exposure - one will generally set an exposure so that the subject of interest in the photograph stands out with no details in it lost, which sometimes means that other parts of the image will be severely under- or overexposed.
Three parameters determine the exposure - the aperture(how wide the lens is open), the shutter speed and the camera sensors sensitivity to light. Getting the right exposure is a balancing act that needs to be managed by the photographer. Thinking about how a camera works from a 20 mile high view helps understand why these three affect the exposure. When one presses the shutter release button on the camera, the shutter opens for a certain amount of time allowing light to come through the lens, get focussed and hit the sensor. All three parameters are directly proportional to the exposure - the wider the opening, the greater the light that comes through; the longer the shutter is open for(slow shutter speed) the more the light that goes through; the more sensitive the sensor, the camera can ‘see’ more with less actual light. Easy! Lets talk about each, one at a time.
Aperture is denoted by a so-called f-number such as f/3.5 or f/4 or f/8 and so on. The ‘standard’ apertures are - f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16 and so on. Smaller f-numbers indicate wider apertures, so f/1.4 is wider than f/2 and so on. In fact, I refer to these as ‘standard’ because when one moves one step along this sequence, the amount of light that goes through the lens is halved. So an aperture of f/2 lets in 16 times more light than f/8. Most lens support in-between apertures too to give finer control. As an interesting aside, f refers to the focal length and it turns out that the aperture is really the focal length divided by the f-number indicated. So on a 80mm lens at f/4, then the aperture is physically 20mm across while the same aperture on a 32mm lens would be 8mm across. This ultimately makes the f-number a convenient way of specifying the aperture in terms of light allowed into the camera independent of the focal length. For the more mathematically inclined, the denominators are so chosen because if you square them all - 2, 4, 8, 16… - you get a geometric series with each successive term the double of the previous. This makes sense because the amount of light depends on the area(which goes as radius squared) of the opening.
Shutter Speed is much easier to understand since it is physically independent of any other parameter(the way the aperture is physically tied to the focal length). What you se is what you get. The shutter speed set on the camera simply specifies how long the shutter remains open for. Most DSLR cameras support shutter speeds from 1/4000 - 30 seconds. The faster the shutter speed, the less light is let through and lower the exposure.
Sensitivity of the camera sensor is measured by the so-called ISO rating, which usually ranges from as low as 50 and goes up as high as 6400 or more. This is due entirely to the camera sensor design; as a general rule as one goes towards the higher end cameras, the range of ISOs that it supports also increases. The ISO rating is basically a throwback from the film days, when the chemicals used on faster film(higher ISO) was more sensitive to light. Nowadays with digital sensors, a similar effect is achieved by electrically keying up individual pixels so that even very little light triggers a response which gets recorded in the image. In most cameras, ISO numbers simply keep doubling - 50, 100, 200, 400, 800 ... - indicating a doubling of sensitivity. So at a given aperture, a photograph taken at 100 ISO with a shutter speed of 1/100s is equivalent to one taken at 200 ISO with a shutter speed of 1/200s. The ISO rating is like a cheat code, it allows the camera to 'see' more with less light.
However, the fun part is that along with exposure, each parameter affects the characteristics of the photograph differently which gives you creative control over how your photograph should look.
Arguably the most important control is Aperture. When the aperture is opened(i.e decrease the f-number), the subject is isolated and objects in front of and behind it get blurred out. Conversely, when the aperture is closed, objects in front of and behind the subject seem sharper. Look at the two photos below which are shots of the same scene with vastly different apertures.
In the image to the left, the plant is focus with all of the background beautifully blurred out and in the one to the right, the background building and cars are quite clear. It's actually a bit messy with too many things going on because a lot more is now sharper. For a given focal length, the distance from the focal plane(i.e the sensor since you want the subject to be focused here) and an aperture there is a mathematical relationship which defines the so called Depth of Field(DOF). The DOF is the range of distances in front of and behind the subject which too will be in focus along with the subject. In other words, a wide aperture gives a shallower DOF and a narrower aperture gives a large DOF.
It's quite a valuable tool, is aperture. This is why when shooting portraits, one would want to choose a wider aperture and while shooting landscapes one would opt for a narrower aperture. On the wider front, say one is shooting a portrait shot and ends up choosing too wide an aperture. It is then quite possible to get the nose of the subject tack in focus but the ears off focus. Click on the next pic to see it in full size - it has such a shallow DOF that even though the doll face is quite small, only a part of it came in focus - the left eye is in focus and almost everything else is slightly off focus. I would have done better to choose an aperture of f/2.8 or even f/3.5 to ensure that all of the subject came into focus and still keep the reflections on the glass behind it out of focus.
On the other end of the scale, for most landscape shots an aperture of between f/9 - f/16 usually suffices. Besides a sharp focus, narrow apertures also tend to give cool effects like starbursts shown below. These are caused because of diffraction of light through the narrow aperture. But if they are to be avoided, opening up the aperture will help.
However, if one goes too far the narrow end then diffraction, which gave starbursts earlier, causes clarity issues. Simply selecting the narrowest aperture supported by the lens, around f/22 or f/36, will almost always give an image that has a slightly hazy look, poor contrast and low details. For instance look at this one here, click on it to open it up in full resolution, which I shot at f/22 - too soft.
Shutter Speed is the next parameter I'll take up. Lets say that the subject is moving, then the shutter speed literally determines how big a smear its image makes on the sensor. With a slower shutter speed, in the duration that the shutter is open for, the subject moves a relatively large distance and hence is recorded as a blur on the sensor. With the shutter speed set to a fast value, then the camera catches the subject moving very little and hence gets a sharper image. These are both artistic considerations. Lets say one is photographing traffic speeding down a highway; choosing a fast shutter speed will freeze the motion of the vehicles and the image will lose a certain dynamism - it wont convey motion. Other examples where one intentionally slows down shutter speed is to blur out the motion of water(like in the hazy waterfall shot above) or of clouds in a landscape shot. But while photographing sports or any other explosive event, it makes sense to freeze the motion to show the moment of a slam dunk in baseball or the neck-to-neck competitiveness of a sprint or the crashing of a wave onto rocks on a beach. The two photographs below illustrate both.
Depending upon what the subject is and how its being photographed, the shutter speed is either the single most important parameter or the most unimportant. If the subject is stationary then the minimum shutter speed for hand held photography is usually 1/f i.e if one is shooting at a focal length of 80mm, then 1/80s is recommended as a minimum. In such cases, the photographer would decide the aperture first and then choose a shutter speed which gives him the correct exposure at that chosen aperture. If the subject is moving, then one would start by choosing the right shutter speed and then see which range of apertures gives the correct exposure.
ISO sensitivity is the last parameter and like I said earlier kind of a cheat code. To understand why, look at this pic below. Its a photograph of a pair of decorative glass bowls on a table in a room with low ambient light and drawn curtains for background. Its shot at f/2, 1/30s and ISO 400.
I like the exposure, the photograph has a nice moody feel. But the DOF is too shallow - only the inner bowl is in focus. To fix it I'll need to step down the aperture(increase the f-number, remember!) to around f/8. This means a reduction of exposure by 3 stops(f/2 --> f/2.8 --> f/4 --> f/8) or in other words a reduction of light reaching the sensor by 1/8th. So to ensure that I get the original exposure with the new aperture, I must reduce my shutter speed by an equivalent factor i.e to 1/4s(1/30 --> 1/15 --> 1/8 --> 1/4). But with me hand holding the camera, this is no good - I'll end up with a blurry image. Here I use the ace up my sleeve, ISO. Instead of changing the shutter speed, I ll increase the sensitivity by 3 stops(or by a factor of 8). Setting the ISO to 3200, lets me work with less light but yet get the same exposure as earlier. Here is the same photograph at f/8.
Perfect! But there has got be a catch right? Yep, with higher ISOs you pay penalty in noise. Open both the images full size and compare the dark areas in both. The one at 3200 ISO is slightly blotchy with noise. In general, unless you are going for a deliberate artistic effect, you would want to stay as close to the base ISO of your sensor as possible. Most entry level DSLRs give you good image quality till up to 800-1600 ISO, which gives you a leeway of 4-5 stops from the base ISO(of 100).
Now that I've talked about how one can influence exposure and shown how its a fine balance of different parameters, a hobbyist photographer probably would ask why go through knowing all this complexity? Simply point and shoot, let the camera do its stuff! While its true that all cameras in the market now have Automatic Scene Modes, they tend to work well only in the most forgiving of conditions. Take for example this image.
It chose an exposure which although correct did not give me the effect I was after. For starters, the camera didn't even get focus correctly - this is most probably because I'm shooting through a window and the slight reflections off it threw it off. I wanted deep focus, but the camera chose a very wide aperture of f/2. Since it is a night shot, the camera then tried to maximise exposure by spiking the ISO and reducing the shutter speed. All in all a pretty bad image. Taking manual control of exposure let me shoot the photograph as I had envisioned it.
Like this instance there are several others where the Auto mode falters - low light, turning on the flash in the wrong situation etc. Any given exposure can be arrived at by a myriad combinations of aperture, shutter speed and ISO. In Auto mode, the camera choses one such combination which it calculates to be the best for the most general of ambient conditions. Taking manual control of exposure is essential for the photographer to faithfully convert his vision into a photograph.
These last 2000 odd words explain only part of the beautiful puzzle that is photography. Composition is the other half, which I've not touched at all here. Maybe that will be the subject of another post.