I was in a very good mood yesterday and wrote this in response to someone's post. Figured some other people would also get some info out of it, so here you go:
Let's start at the beginning ... the very beginning.
There are 4 main "settings" when taking any photo with any camera. Some extremely simple cameras set most (or in some cases all) of them for you, but your Canon DSLR is a fully functioning SLR that allows you to go as automatic or as manual as you want.
The first setting is your Focal Length. This refers to the amount of "zoom" or "reach" your lens has. There are two types of lenses with regards to this. "Zoom" lenses ... do just that, they zoom. Your 18-55 is a zoom lens. It's focal length at its widest angle of zoom is 18mm. It's focal length at it's shallowest angle of zoom is 55mm. The other type of lens is a "Prime" lens. Prime lenses come in all different focal lengths but they do not zoom on their own ... you must move your body closer-to and farther-away from your subject to change what's in your picture.
The second setting is your ISO. ISO refers to the sensitivity your camera's digital sensor or film has to light. The higher the ISO, the more sensitive to light. What this means, is that for a specific amount of light, a higher ISO setting will be properly exposed quicker than a lower ISO setting. ISO on most Prosumer Canon DSLRs moves in 1 full stop incremets. What this means, is that for every notch of ISO you move up (100 -> 200, 200 -> 400, 400 -> 800, etc) ... the amount of time your sensor must be exposed will be cut in half. Conversely, if you move ISO down 1 notch and leave all other settings the same, you will double the amount of time the sensor must be exposed. Now, the simple solution is ... set the camera on the highest ISO setting possible and leave it. However, you must always keep in mind that the higher you set your ISO ... the more "grainy" your finished photo will be. Digial cameras increase the sensitivity of the sensor by pumping more power into it. This increase in power creates digital "noise" that can be visibly seen in pictures. On some of the older Canon DSLRs, there is a very noticible difference between ISO 200 and ISO 800. Ideally, you want to use the lowest ISO setting you can for any situation. This will produce the cleanest, smoothest photo.
The third setting is your Shutter Speed. Shutter speed is the amount of time you expose your sensor to light. You are human, you are not made of stone. Your body will shake as you take a picture. Your subject, whether it be leaves, or stars, or people, or birds will move while you take a picture. For this reason, we want to keep the amount of time the shutter is open as short as possible. The quicker the shutter opens and closes, the less time you have to shake the picture into blurryness.
Shutter speeds on your camera are represented in two ways. For shutter speeds represented in seconds ... it will read a number followed by a quote (ex. 1"). For speeds represented in fractions of a second, it will show you the denominator of the fraction. In other words, a shutter time of 1/500th of a second will be represented on the camera as "500" (no quotes). The higher the number (assuming there is no quote), the shorter the amount of time the shutter is open and the less likely you will be to "shake" the picture.
It is important to remember that as focal lengths increase, the amount of shaking is exponentially increased. If you've ever looked through a pair of binoculars or a rifle scope, you know that it takes a very steady hand to keep the view stable. The same will hold true for a 300mm focal length as compared to a 50mm focal length. There is a general rule in photography that says you should always be shooting with a shutter speed faster than 1 over the focal length. So, if you're shooting a 200mm focal length, your shutter speed should be at least 1/200th of a second or faster. That rule was accepted decades ago, back when all 35mm cameras shot film. Your Canon D30/D60/300D/350D/400D/10D/20D/30D has a "crop factor", you've probably seen that term around. The sensor on your camera is smaller than a 35mm film negative .... as such, your sensor is only exposed to a smaller portion of the image that comes through the lens than a 35mm film negative would be using the same lens. The cameras mentioned above have a crop factor of 1.6. For the record, a Canon 1D has a crop factor of 1.3 and the Canons 1DS and 5D have no crop factor (their sensor is the same size as a 35mm film negative).
You should always be shooting with a shutter speed faster than 1 over your focal length times your crop factor. So, if you were using a 200mm focal length as indicated on the lens you were using at the time ... you'd want to use a shutter speed of at least 1/320th of a second assuming a 1.6 crop factor (200 * 1.6 = 320).
In general, when handholding the camera (as opposed to it being supported by non-human means like a tripod or table), try not to go below 1/60th of a second if possible, regardless of focal length. Sometimes its not possible to use faster shutter speeds because there isn't enough light ... but don't expect a perfectly sharp picture if you handheld a shot for 1/10th of a second and you've had a 20oz Coca-Cola in the last hour. Also, know that there are many reasons to want a slower shutter speed ... "freezing" action isn't always desired.
The fourth setting is your Aperture. So far we've controlled how sensitive our camera is to light, we've controlled how long we expose our sensor to the light, the aperture controls the AMOUNT of light we expose to the sensor. The aperture itself is something like a door that sits inside the lens. Think of a space station door from a cheap sci-fi flick ... round, and it opens from the middle in a circular pattern outward. The amount of light an aperature allows through to the camera body is noted by a number called an "F stop". The smaller the number, the more "open" the door. The higher the number, the smaller the door is and the less light we allow through.
When we refer to an image being taken wide open, we're talking about the aperture and referencing the fact that the aperture was at it's largest opening (lowest F number) when the photo was taken. Obviously, it's always nice to have a lens that allows as much light to get to the sensor as possible. This lets us use a faster shutter speed since we won't have to expose the sensor for as much time. It might also let us use a lower ISO setting since with more light, the sensor won't need to be as sensitive in order for us to get an acceptable shutter speed. For this reason, the maximum amount of light a lens will pass through itself is represented on the lens itself.
There are two types of lenses with regards to maximum aperture. There are Constant Aperture lenses ... which is, as you change the focal length (zoom) of the lens ... the maximum aperture remains constant. The Canon 70-200mm F4 lens is a zoom lens that is able to achieve F/4 regardless of whether you are at the 70mm focal length, the 200mm focal length, or anywhere in between.
The other type of lens (regarding apertures) changes its maximum aperture setting as the focal length changes. The standard kit lens, the Canon 18-55mm F/3.5-5.6 is one of these lenses. At 18mm, the lowest F/value that lens is able to achieve is F/3.5 ... at 55mm, the lowest F/value that lens is able to achieve is F/5.6 (less light). The reasons behind the change in the amount of available light has to do with the internal contruction of the lens, the size of the lens, and the ultimately, the cost of the lens.
So of course, the ideal situation here is that you leave your lens "wide open" all of the time to allow the maximum amount of light through to keep the shutter speeds as fast as possible (reduce likelyhood of shake) and to keep the ISO setting as low as possible. There are two problems with that.
The first problem is simple. Lenses do not always perform at their best when wide open. Many aspects of image quality are affected, but each individual lens is different in how well it performs at it's different aperture settings. It is highly recommended that after you have some time behind the camera, after you understand exposure and the many different ways with which to set it ... you spend a little bit of time evaluating each of your lenses at different F/stops to determine the image quality difference between each stop.
The second problem is the Depth of field. Now, this is a big one. If you're head's already hurting from everything you've read so far. Stop now, take a breather, bookmark this page, come back later.
You still here?
Okay. Depth of Field refers the distance in front of and behind your focus point (the exact point of focus you locked onto) that is also in focus. There are 4 things that affect your depth of field.
The first is the size of your sensor or film. This is easy because unless you buy a different camera, you can't physically change this. With all other things being EXACTLY the same, a 35mm film camera and a large format view camera will have very different depth of fields.
The second factor is your distance from subject. This is easy to see. Go look through a photo album. Find a picture you took of some sort of scenery. Maybe a mountain that was far away, or a city from atop a high perch ... whatever. Assuming it's a good picture (not blurry all over and incorrectly exposed) I'll wager a good amount of money that there are hundreds of yards worth of trees/grass/buildings that are in focus and can be recognized. Now find a really close-up picture you've taken of someone's face .... no, I mean really close-up .... you'll notice that objects even a few feet behind your subject are out of focus to the point where you probably can't even identify them. Here's an example:
Notice how although her eye is in focus, her ear is not. Now, this is an extreme example and in this particular shot I was looking to achieve that affect ... but the point is that if she had been standing 10 feet away from me as opposed to the 12 inches she actually was .... getting a depth of field that shallow using that lens would not have been possible.
The third aspect of a lens that affects the depth of field is focal length. With all other things being an equal ... a longer focal length will have a shallower depth of field. So if you were shooting a picture of a person standing 10 feet away from you and you were using an 18mm focal length ... more of what is in front of and behind that person will be in focus than if you zoomed the lens out to 55mm and took shot.
And of course, the fourth aspect that affects depth of field is the aperture. The lower the F/number (the more light) ... the shallower your depth of field. In the picture I posted above, I used my "fastest" lens (fast refers to the amount of light the lens is capable of passing through) and I used the lens ALMOST wide open {wide-open = lowest F/value (1.8 for this particular lens), in this case however, I was using F/2.2}. The lens in question is the Canon 50mm F/1.8 (AKA the Nifty Fifty). To recap, that means that it has a constant 50mm focal length (it doesn't zoom) and its maximum aperture is F/1.8.
Here are some other examples of very shallow depth of field:
Some misc ramblings:
In general, prime lenses are faster (allow more light in) than similarly priced zoom lenses covering the same focal length because there are less mechanical components inside the lens (less parts = more space = more light).
As with ISO, with each stop of a lens, the amount of light either doubles as the aperture opens (lower number), or it gets cut in half as aperture closes (higher number). The amount of light your lens passes through at F/4 is exactly half of the amount of light it passes through at F/2.8.
While we're here ... the "standard" stops of light are:
F/1.0 - F/1.4 - F/2.0 - F/2.8 - F/4.0 - F/5.6 - F/8 - F/11 - F/16 - F/22
Most Canon DSLRs are able to adjust the aperture of any lens mounted to it in 1/3 stop increments. So for instance, with my Canon 50mm 1.8 lens on my Digital Rebel .... if I were to click up the aperture values one click at a time, I would see F/2.0 -> F/2.2 -> F/2.5 -> F/2.8.
Moving onto the dial on the top of your camera. For anyone using a DSLR who wants to have total control over their picture, you must use the camera in one of the 4 modes: P, Av, Tv, or M
P = Program mode. Essentially, it tells the camera that you're serious about wanting control and the camera now allows you to set your own ISO setting (the camera doesn't let you do this in any of the automatic modes), it will allow you to change the shutter speed and aperture settings, and it will not popup the flash unless you tell it to do so. When you select P mode, the camera will select an aperture for you based on some sort of internal logic and then select the correct shutter speed to accompany that aperture. P mode will not adjust the ISO setting, if you wish to change it, you must do it yourself. When you half-depress the shutter button while in P mode, the camera will show you the aperture and shutter speed settings it is going to use for the picture ... if you don't like these settings, rotating the dial next to the shutter button will move the exposure settings (aperture value and shutter speed) around until you've reached something you do like.
Av mode = Aperture priority mode. In this mode, you are telling the camera exactly what aperture setting to use and the camera is finding the correct shutter speed that will expose this picture properly for you. Aperture priority mode is great for situations where, for instance, you have little light and your highest priority at all times is achieving the fastest possible shutter speed to avoid motion blur in your pictures. You set your aperture value at the lowest possible number and from that point on, your camera will be using the fastest possible shutter speed (because it is now receiving the largest amount of light possible) for whatever your are shooting. Aperture priority is also good for situations where there is more than enough light, but you'd like to be able to control depth of field very quickly. Just turn the dial a few clicks in either direction and you will change your depth of field.
Tv mode = Shutter priority mode. In this mode, you are telling the camera exactly what shutter speed to use and the camera is finding the correct aperture value that will expose this picture properly for you. Tv mode is good for situations where you need a very specific shutter speed. Maybe you're shooting a baseball game and you know from experience that if you want to get a shot showing a nice blur of the baseball bat as the hitter swings ... you have to shoot at around 1/60th of a second. Just set the shutter speed to 1/60th of a second in Tv mode and the camera will pick the aperture value for you that will properly expose that photo. Perhaps you're at a car race and you want to set a shutter speed that isn't too slow, but isn't so fast that the wheels don't look like they're moving: Experiment for a few shots until you find a shutter speed that works for you, and then lock that shutter speed in and let the camera pick the correct aperture value for you.
M mode = fully manual mode. In this mode, the camera sets nothing. We've had a recent discussion about M mode vs. Av/Tv mode that had very good arguments for both side of the equation, so I'll stay out of it here. It's a very good discussion in my opinion and you can find it here:
https://photography-on-the.net/forum/showthread.php?t=239183
So going very simple here. Let's say that for a particular subject, you set your camera to Av mode, you are using the Canon 85mm F/1.8 lens. You want as little noise as possible because you plan on making a big print of this photo so you set your ISO to 100. You want a very sharp picture so you set your aperture to F/4. You half-depress the shutter button and the camera tells you that with the amount of light hitting your subject right now, you will get a shutter speed of 1/60th of a second.
Now, you're using an 85mm lens on a 1.6 crop body. Following the rule of thumb for handholding, you know that you want to be using a shutter speed of at least 1/136th of a second (1 over 85 * 1.6).
So you know that this lens is still sharp if you open it up 1 stop. So you make 3 clicks of the dial and now you've set the aperture at F/2.8. You've doubled the amount of light which means that the amount of time the shutter must stay open has been cut in half. So now your camera tells you that the shutter speed will be 1/125th of a second. Now, you may determine that you are a steady enough person that this is a fast enough shutter speed and you're going to take the shot. But maybe you REALLY need this shot to be sharp and you dont' want to chance it.
Since your maximum aperture on this lens is larger (lower number) than F/2.8 ... you can open the lens up another stop. But at F/2.0, the depth of field is going to be really shallow and this particular lens might not produce the most crisp image possible at that aperture. You can bump the ISO up 1 stop from 100 to 200. This doubles your camera's sensitivity to light. Because you are in Av mode and the camera will not change the aperture value by itself (remember, in Av mode, only you can do that) ... the camera will adjust the exposure by cutting the shutter speed in half for you automatically and now you're exposure will be 1/250th of a second @ F/2.8 using ISO 200.
That's a very basic, fairly general introduction to photography but the information contained here is universally transferable to all cameras regardless of size.
Bill
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And when they are first learning, getting things precisely right is not as important as imparting the basics. (A key difference is that with photography, you hope the student immediately absorbs and puts into practice the new knowledge; but with pre-teen daughters you hope the practicing is put off many many years!)
