These pages give an introduction to some of the basics of photography. They help me answer questions people sometimes ask me.
There are many, many resources on the Web that give the same information I give here, and probably do it better. But I have found that sometimes people ask me for answers, and here I can give those answers in my own voice. So here it goes...
What is a camera?
In order to understand photography, let’s start by asking the question, what is a camera? In spite of all the fancy stuff that camera makers keep throwing at us, in the end, the camera is a tool to capture light. In that sense, not much really has changed in the past 150 or so years.
Now, some cameras are better at capturing light than others; my $1,500 Nikon D300 semi-professional has some definite advantages over a cellphone when it comes to capturing light (although cellphone cameras also have some advantages over my expensive rig). But structurally, all cameras do the same thing: capture light.
What is a lens?
Almost every camera needs a lens (there is an exception, a “pinhole camera”, but let’s talk about that one later). Why does a camera need a lens? Because you don’t want to just capture random light, you want to capture light in an organized way.
Inside the camera, there is what is called a “sensor” which receives the light that falls on it and translates that light into electronic information that in the end will make up the picture. Sensors are a relatively new thing; prior to the digital camera revolution, cameras used to have a “film” in exactly the same spot that would use a chemical proces, rather than an electronic one, to record the light falling on it.
Imagine holding up a sheet of paper, and let’s say that sheet of paper is the light capturing sensor. The paper captures a lot of light, but that is no good. It is just all uniformly white. You may title it “Montana landscape in a blizard” but it’s not really a very interesting image at all. This is because the light falls everywhere on the sheet.
A lens takes care of this problem. A lens will literally bend the light, so that all the light that came from one point will end up on a single point on the camera sensor. The illustration shows how individual light rays that come out from a light source hit everywhere on the lens, but the lens then bends those light rays so that they all come together in a single point on the camera sensor.
Unfortunately, only the light from those points at exactly the focus distance will end up as a point on the sensor. Light coming from points a little bit close by, or a little bit further away, will show up as tiny fuzzy circles on the sensor; the further away from the focus distance, the bigger the fuzzy circle. Fortunately, modern cameras provide an auto-focus function to select the correct focus distance.
It is worthwhile to remember here that a light source, in this context, is more than just the sun or a lamp or a flash. For instance, when the sunlight hits your skin, your skin will reflect part of the light back; it is this reflected light, when it hits our eyes, that we “see.” In this sense, every point of your subject becomes a new light source that sends out rays of light which the lens bends to come together on your camera’s sensor to form the image.
The digital camera’s sensor is very sensitive to light. Really, it is VERY sensitive to light. If you let it look at a regular scene for a minute, it gets completely overwhelmed. In fact, if you let it look at a regular scene for only a second it gets overwhelmed and records a purely white picture.
Except in special circumstances, you want to expose the camera sensor to the light for only a fraction of a second. Typical shutter speeds are 1/125th or 1/250th of a second, which is already pretty short. Many cameras can take pictures as fast as 1/1000th of a second, 1/4000th of a second, or faster.
Why would you want a shorter shutter speed? A short shutter speed can “freeze” the action; it can also minimize the effects of camera shake. A shorter shutter speed can mean you end up with less blurry photos.
Sometimes however you want a longer shutter speed, since that can allow more light to fall on the sensor, which in turn helps when taking photos when it is dark out.
There is a more-or-less standard series of shutter speeds that would be useful to remember. Each item in the series is about half the time of the previous one, that means that with each next one, half of the light falls on the sensor than it did with the previous one. The series typically starts at 1/60th, which in many scenarios is the longest time one can use and still get a reasonably sharp hand-help photo, and goes on to 1/125, 1/250, 1/500 and 1/1000th of a second.
Shutter speed and aperture often go hand-in-hand. We’ll discuss this some more after introducing the concept of aperture.
Inside the lens, the camera has a screen with a hole in it. The hole in this screen, wich is called the aperture, can be made bigger and smaller. When the hole is big, a lot of light comes through the lens onto the sensor. When the hole is smaller, more of the light falls on the screen and less of it reaches the sensor.
Why would anyone in their right mind want to restrict the amount of light coming to the sensor (other than for special artistic effects)? After all, we have seen before that the more light goes through, the shorter the shutter speed you use, and the sharper the photo is likely to become.
As it turns out, sharpness, in the form of depth of field (often refered to as “DOF”) is one of the most important qualities of a photo that you may want to control. We will talk more about DOF later.
Aperture is measured in fractions (one-over-something), so the larger the number, the smaller the opening. In fact, every time the aperture number doubles, the amount of light going through the lens is reduced to a quarter of what it was. For those curious about such things: this is because the aperture value is a linear measure, whereas the light going through a hole is related to the area of the hole, and we know that if you double the diameter of a circle, you quadruple the area.
Just as with shutter speeds, there is a “standard” series of aperture values. Just like with the shutter speed series, in the standard series of aperture values, the next one has half the amount of light as the previous one: f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32. You can see the pattern in the series: if you only look at the bolded values, they are double the value of the previous bolded value. But because doubling the aperture gives only a quarter of the light, the standard aperture series has additional intermediate values.
Relationship between aperture and shutter speed
Let’s do this thought experiment. Imagine we are taking a photo with a shutter speed of 1/125th of a second and an aperture of f/8. But it is cold outside, and we are shaking, so we really want to use a shorter shutter speed. Say, 1/250th of a second. How can we adjust the aperture to end up with the same amount of light on the sensor?
Easy: when the shutter speed changes from 1/125th to 1/250th of a second, only half the light falls on the sensor. So we need the next stop in aperture setting to compensate. The next stop from f/8 would be f/5.6.
Shutter speed and aperture affect the amount of light that reaches the sensor that “takes” the picture. There is a third element that goes into the equation of exposing a photo, and that is how sensitive the sensor is to light that falls on it. Sensitivity is controlled by electronically amplifying the signal from the sensor.
With film cameras, sensitivity was a chemical property of the film that was used. With digital cameras, sensitivity is an electronic property, and it can be changed (within limits). The sensitivity is expressed as an “ISO” value. Many cameras have a starting point (“base ISO”) of either 100 or 200, and then provide settings to increase from there. With the ISO values, doubling the number also doubles the sensitivity. The “standard” series of ISO values is 100, 200, 400, 800, 1600 and up.
As always in photography, there is a trade-off. Higher ISO values make the sensor more sensitive, allowing you to take the same picture with less light, so you can use a shorter shutter speed, but at a cost: by increasing the sensitivity of the sensor, the susceptability of the sensor to noise is also increased. At what point this makes the resulting photo unacceptable depends on circumstances and personal preferences...
Depth of field (DOF)
The final item in the series of basic concepts for photography is the depth of field concept. This concept is related to how a lens actually works.
Remember that when we discussed a lens, we said that light coming from points closer or further than the focus distance end up as little fuzzy circles rather than a sharp point? As it turns out, we don’t notice the fuzzy circles if they are small enough. So some points in front of and beyond the focus distance will still look as they are sharp; only points further away start looking fuzzy.
The DOF is the area where points still look sharp. How far the DOF extends in front of and beyond the focus distance depends partly on what is still “acceptable” and what is considered fuzzy, which in turn depends on how big the sensor is, how many megapixels it has, and also how closely you look at the photo. The other major contributor to the DOF, though, is the aperture size used.
The smaller the aperture (opening) used to take the photo, the further the DOF extends in front of and beyond the focusing distance. In fact, the effect of aperture on DOF is quite dramatic.
The example photos were taken with a very short focus distance to emphasize the effect of aperture on the DOF. The first image on the left was taken at f/16, the second image was taken at f/5.6. Even at the small size in which the images are displayed here, the dramatic difference in DOF is visible. In the left-most photo, the house and tree in the background are clearly recognizable; in the second image, they are just blurs.
Many common lenses for DSLR cameras are able to at least provide apertures from f/5.6 to f/16 and above. Larger apertures (lower aperture values) can give even more DOF, which can help in emphasizing the subject. The photo on the right was taken with f/2, and shows how much blurrier the background can still become. In this photo, the one twig really stands out!