Intro to photography part 1: Focal Length

[engstrom]

***************** Disclaimer *********************
For those of you that have been into photography for a while you probably don't need to read this but I've been noticing quite a few posts from some newcomers into the photography hobby. I figured I would try to put together a series of information without getting into too many technical details that would help beginners to this wonderful hobby. It took me many years to piece together my knowledge of how photography works so I thought I would try to write some of it down and see if it helps others. This is going to be written from the perspective of 35mm SLRs but the concepts will apply to all cameras. I'm going to start slow and build slowly.
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OK, so you got your nifty new camera and a lens to go on your camera. The lens has writing on it that says "50mm 1:1.8" or maybe "24-70mm 1:2.8" or maybe even "100-400mm 1:4.5-5.6". So what does all this mean? Well, the number before the "mm" is the focal length of the lens. "mm" stands for millimeters and is a metric unit of length. 25.4 millimeters is equal to one inch. The part of the lens that says "1:xxxxxxx" is the aperture which I'll get into in the next essay.

"So what good is knowing the focal length?" Well, some people say that the higher the focal length the higher the magnification of the image. This is not strictly true. You also have to take into account how closely the lens will focus on a subject to determine the magnification. For example if I have a 400mm lens but it can only focus on subjects 4 feet away I won't get as high a magnification as a 100mm lens that can focus on subjects 4 inches away. However, when you're at the zoo taking pictures of lions it would not be possible to get 4 inches away from the lion. And even if it were possible this author would not recommend it. So we can see that there are situations where we can't be close to the subject but we want to fill the frame up as much as possible with the lion, or little Jimmy scoring his first touchdown in Pop Warner. That's where lenses with a high focal length come in handy.

"So I'll ask it again since you didn't answer it in the previous paragraph, what good is knowing the focal length?" The short answer is not much good at all. Really, apart from the rule of thumb which states your shutter speed must be faster than the inverse of the focal length (that's fancy talk for 1 divided by the focal length) to get a sharp picture, it doesn't serve much purpose. One purpose it does server, however, is as a comparison to other lenses you own to get an idea of which lens to put on your camera. For example if you have a 50mm lens but you want the subject to be twice as big in the viewfinder you need to put on a 100mm lens (or zoom to 100mm if you're using a zoom lens).

"Great, so since it really doesn't do me much good to know the focal length of a lens I hesitate to even ask but...what exactly does that number mean?" Phew, I thought you were never going to ask. Well, all lenses have an "optical center". That means if the lens was just a single piece of glass it's where that piece of glass would be located. I'm not going to get into design of lenses because, quite honestly, it's way over my head. Those optics guys that design lenses are true propeller heads and I salute them! So, back to the point. The focal length is the distance from the film/sensor on your camera to the optical center of the lens when the lens is focused on infinity. I've included a picture that illustrates this. In my example below the tree is a long way away (trust me, it is, the picture is not exactly to scale) and the lens, focused on infinity, brings this tree into focus on the film/sensor. Notice how the angle of view (the amount of stuff you can see in the viewfinder) is much smaller at high focal lengths. In my example below the tree takes up the entire film/sensor with a 400mm lens but only takes up 1/4 of the film/sensor with a 100mm lens. The optical center of the 400mm lens is, well, 400mm (15.75 inches) away from the film/sensor. Lenses move closer or farther away from the film/sensor when they focus. I'll get into that in a later essay.

I hope this was helpful even if it was a fairly light lesson. I'll get into aperture, what do the numbers mean, and how it effects depth of field in the next part.

[PhotoByMark]

Nice little discussion.

This and other articles should be stickied or perhaps another "Article" forum for future reference and so it does not fade away........

[Ben]

Great tree!

One question though:

"For example if I have a 400mm lens but it can only focus on subjects 4 feet away I won't get as high a magnification as a 100mm lens that can focus on subjects 4 inches away."

How do you know how far it WILL focus to?

[engstrom]

Originally Posted by Ben

Great tree!

One question though:

"For example if I have a 400mm lens but it can only focus on subjects 4 feet away I won't get as high a magnification as a 100mm lens that can focus on subjects 4 inches away."

How do you know how far it WILL focus to?

It varies depending on lens design. The only way to tell is to look at the manufacturer's material for the lens. For example, Canon has a pamphlet called "EF Lenses for EOS Camera" with a table in the back. Every lens is listed on its own row in the table and one of the columns is "Closest Focusing Distance". As to what exactly limits each lens I'm sure it has to do with limits on how far lens elements can move without hitting other things in the lens and causing degradation to the image. You'll need to ask one of those propeller heads .

[JohnRushing]

I'm nowhere near an expert but I do understand the design of the lens has a lot to do with how close it can focus, consider a macro. Also the type of glass has a lot to do with how well a lens can focus up close, compare a cheap lens to an "L".

[coreymcl]

Thanks, this type of information will help to set this site apart from the others photo sites out their. As a newbe I look forward to reading the next few articles.