What’s that f-stop stuff anyway?

Newcomers to photography are often intimidated by technical jargon. Walk into a room full of photographers, or read camera reviews, and you’ll quickly be overwhelmed by seeming-gibberish about megapixels, spot metering, chroma noise, low-pass filters, A/D converters, sync speeds, and a myriad of other not-quite-English terms. If only you hadn’t cut Swahili class in high school, maybe it would make more sense to you and your head wouldn’t be swimming.

Of all the technical terms, nothing is scarier for the beginner than f-stops. I’ll do my best to demystify them for you.

First off, a simple English lesson. You’ve almost certainly heard the word aperture before… if you’ve been reading this blog, you’ve eve heard me use it. What exactly is it? An aperture is an opening, and specifically an opening that lets light in. In photographic terms, the aperture is the hole in your lens that lets light into the camera.

That wasn’t so scary, was it? No? Good. let’s keep going.

So what’s an f-stop? In a nutshell, it’s how big that opening in the lens is. That’s it. Really. It’s not some magical, mystical thing that’s going to bite you if you get it wrong. F-stops don’t hurt. They’re not contagious. They won’t kick your dog, drink your beer, or max out your credit cards. OK, they might max out your credit cards someday, but only if you start chasing really big ones.

How do the numbers work? Basically, a small number means that the lens has a very wide opening, and big numbers mean that the lens has a small near-pinhole opening. Of course, we remember that big openings let lots of light in, and small ones only let a little bit of light in. The most common f-stops you will encounter will be in the range of about f/2 to f/22. f/2 is a low number, so it’s a really wide opening. f/22 is tiny. Sometimes you’ll see numbers as low as f/1.4 (and if you have deep pockets, f/1.2), or as high as f/32 or occasionally f/45.

Something important to know is that when you aren’t actually taking a picture, your lens is always open to its widest possible opening even if you have the camera set to use a much smaller f-stop. This lets you get as much light as possible for focusing and composing the image. When you press the shutter button to take a picture, one of the things that happens is that little metal blades in the lens come out to make the opening smaller while the shutter is open. This happens automatically, and you don’t need to think about much about the details.

By the way, photographers tend to use the terms aperture and f-stop interchangeably. That’s OK.

Every lens has a minimum and maximum aperture, but they’re usually only rated in terms of the maximum– that is, the widest opening. For example, Canon sells two 50mm lenses. One is the 50mm f/1.8, and the other is the 50mm f/1.4. The first one can open as wide as f/1.8, which is really rather wide. The other goes as wide as f/1.4, which is even wider. (But you knew that!) Other than the maximum aperture, what’s the difference between the lenses? The f/1.8 lens is about $80, while the f/1.4 is about four times that price. Lenses with with very large maximum apertures are called fast lenses; I think it’s because of how quickly they drain your wallet.

Did that all make sense? Was it not too scary?

Tomorrow I’ll show you some pictures of what it looks like inside a lens, and show you a neat trick for checking it out yourself.

Oh, yeah. If you’re confused, feel free to ask questions in the comments. I’ll do my best to answer them.

Next lesson:  What does f/8 look like?

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4 Comments

Filed under Aperture, Lesson

4 responses to “What’s that f-stop stuff anyway?

  1. KL

    Other than getting bigger and smaller, do the f-stop numbers actually mean anything specific? Is f/8 just wider than f/11, or is there some mathematical magic?

  2. stopshootingauto

    Good question!

    They actually mean something. In short, it’s the ratio of the focal length of the lens to the size of the opening. There’s a really good discussion of this in the Wikipedia entry on Aperture.

  3. Steven

    I was reading the Nature Photography mag. and one of the pages (as all of them seems to) shows the image, and next to the images it tells you what type of camera was used, the lense and other info about the seetings used. The photographer also said that he used +.07 stop exposure. Is he/she saying that they added a +.07 f/stop, aperture, to the exposure, or does this have to do with some other setting, and how do you get just a .0 anything setting?

    The issue is Winter 2008/2009 page 26.

    Thanks,

    Steven

  4. stopshootingauto

    A great question, Steven.

    +.07 didn’t make sense to me, so I walked across the street to a bookstore and took a look. It’s actually +.7, which makes a bit more sense. I’m not 100% certain of this answer, but it’s my best guess.

    Back in the days of film, it was not uncommon to “push process” film, which basically means increasing the exposure when it’s developed. Though it’s not exactly the same, the Photoshop equivalent would be to take the exposure slider and move it to the right.

    I believe that in this context, +.7 means that he increased the exposure by .7 stop in post-processing.

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