This week I want to talk a little bit about lenses, or glass as it is often referred to. Glass is probably just as important as the body in my opinion. I took a Canon Rebel XT, my first DSLR, and shot with L glass and saw a huge improvement of what I was able to capture versus the cheaper third party lenses. L glass is Canon's line of professional lenses that are the high dollar lenses that are built to last for years and can take moderate abuse.
I will talk a little about prime and zoom lenses and the differences, but the biggest point I want to cover is aperture and how it affects your images. This is probably one of the most misunderstood things by most beginners and some professionals alike.
What is a prime lens? Simple answer is a it is a fixed focal length lens; you can't zoom. An example would be the "nifty fifty" (50mm). There are different lengths and they are used for different purposes but they all have the fact that you become the mode of zoom by moving towards or away from your subject
A zoom lens is one that has multiple focal lengths built into the lens, the lens will zoom by turning the zoom ring on the lens. A pretty common zoom lens is the 18-55mm kit lens that is often offered when purchasing a camera. There are many types of zoom lenses that are produced and are used for different purposes. Here is a pretty good article that give a little more detailed explanation of the types of zooms.
If you read my first TT blog you may remember me briefly touching on something called crop factor. Crop factor is a multiplication by a camera specific number to equal a full frame or 35mm slide. That comes into play with all lenses on a crop camera because that number also changes the focal length of lenses. If you are using a 50mm lens on a crop camera then the focal length of the lens is actually 50 x crop= actual MM. So on a Canon crop camera, the crop factor is 1.6; your 50mm becomes 80mm (50x1.6). Why is this important? This could potentially change the look of your photo. I have a 24-70mm and 70-200mm lenses but because of their build at 70mm (112mm actual) if I were to take a headshot with both cameras from the same distance the faces would look totally different. This is from barrel distortion and compression. I know it is confusing, and I probably didn't explain it too well but this is very important to remember. If you aren't shooting on a full frame camera you need to know what your camera's crop factor is so you know what lens to use for what situation.
Now to the "good" stuff. One thing that I can count on people asking me when purchasing a new camera is what lens to get. I've struggled with trying to explain why I would go with a particular lens because in most cases they aren't sure on what exactly they will be shooting and technically speaking it can be very daunting. As I said in my first TT blog, I'm extremely technical so I've done a lot of research and trial and error as to know what I need and get for my use(s). Aperture is most often the reason as to why I will or won't get a lens.
So what about aperture is so important to me? There are two different types of aperture systems on lenses, variable and fixed. Variable is what seems to give everyone headaches. Variable aperture lens is just that, the aperture will vary at different focal lengths. So on my 18-55mm lens at 18mm the maximum aperture will be f/3.5 but at 55mm it becomes f/5.6. Confusing, right? If you don't know, the smaller the f-stop number the more light it lets in and the larger the number the less light the lens will let reach the sensor. A fixed aperture lens will maintain a constant f-stop throughout the focal range. An example would be my 70-200mm that has a maximum aperture of f/2.8. So I can set my f-stop to f/2.8 at 70mm and it will be the same at 200mm.
|Fixed Aperture Lens|
|Variable Aperture Lens|
Why is this important? I'll use real world example; dreaded low light shooting. I typically shoot in very poorly lit environments and I really need to let the most light as possible no matter what focal length I am shooting at. So I can throw my 24-70mm on and shoot wide or zoomed and my f-stop won't change.
Now that confusing part has been discussed, I also need to mention the other confusing part that aperture has two primary functions. I've briefly discussed how it affects how much light is let in but it also controls the depth of field. Depth of Field (DoF) is defined as the distance between the nearest and the furthest objects that give an image judged to be in focus in a camera. This is what gives blurry backgrounds to portrait images. The larger the aperture, the more shallow the DoF will be and the smaller the aperture the deeper the DoF is. So at f/2.8 there will be less in focus versus shooting at f/22. More detailed information can be found here and it is in pretty plain terms.
The best way to understand this is to go and shoot a static subject with your camera on a tripod and in Aperture priority mode, where you select your f-stop and the camera sets the rest. If you're more advanced shoot in manual mode and see how the exposure triangle comes into play. I truly hope that this is of some help to you, even as basic as the information is.