Ask David: Why does aperture decrease when zoom increases? :: Digital Photo Secrets
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Ask David: Why does aperture decrease when zoom increases?

by David Peterson 4 comments

You're the proud owner of a new lens. It's a pretty awesome lens, if you do say so yourself. It zooms all the way out to 300mm, and down to 70mm, which means you can use it in a lot of situations - from photographing those birds in your backyard trees to capturing great candid shots of your kids playing on the lawn below them. But you just noticed something about that brand new lens - the aperture doesn't always stay where you want it to. Why?


[ Top image Birdy the Bird by Flickr user Théo La Photo]

What you’re experiencing with your brand new lens is actually typical for almost every consumer-grade zoom lens. It’s called variable aperture, and there’s a pretty simple reason why it happens—as your lens lengthens, the aperture itself actually stays the same, but because of the physical distance that light has to travel across that longer focal length, less light overall will reach your camera’s sensor. So what you get is an effective reduction in the size of the aperture. So while you can shoot at f/3.5 at 70mm, at 300mm you’re getting an effective aperture of f/6.3.

  • Canon EOS 500D
  • 100
  • f/14.0
  • 2.5
  • 70 mm

Canon EF 35-80mm f/4-5.6 USM by Flickr user sample-image.com

Now for most consumers this is going to be more or less OK. At that long focal length, you’re still going to get the background blur that is the usual reason why you like the larger apertures, just based on how optically close you’re going to be able to get to your subject. The only time this may actually be an issue is if you’re shooting at maximum zoom in low light—in that situation, you may need a larger aperture in order to allow you to capture the image without reducing the shutter speed so much that it adds motion blur or camera shake to the image. In this situation you have a couple of options—you can mount your camera on a tripod to eliminate that camera shake, or you can zoom out and take advantage of your lens’s maximum aperture.

Regardless of which choice you make, there’s no doubt that it’s an inconvenience. And you can actually get zoom lenses that aren’t variable aperture, but you’ll pay a premium for them. Nikon’s Nikkor 17mm to 55mm fixed aperture lens, for example, will run you roughly $1200 to $1500, depending on where you purchase it—while the Nikkor 55 to 200mm variable aperture lens, which gives you considerably more zoom, will only cost about $350.

The reason for the difference in cost lies strictly in the construction of the lens. The barrel of a fixed aperture lens does not lengthen or retract when you zoom, which means that the aperture remains the same regardless of focal length. To make this work, the lens ends up being a lot heavier than a variable aperture lens tends to be. The glass on a fixed aperture lens is typically better quality, too, and you’ll also get faster autofocus with a fixed aperture lens. All of that adds up to a much higher price-tag.

Do you need a fixed aperture lens?

That depends on a lot of different factors. Fixed aperture lenses do have advantages, the primary one being that you can shoot at that large aperture and not have to think about the fact that it’s going to change whenever you zoom in and out. You’ll be able to shoot at the long end of your focal length in lower light, too, which can be really handy, particularly if you’re shooting wildlife in the early or late hours of the day. But on the flipside, if you do happen to own a fixed aperture zoom lens it probably doesn’t have as much reach as a variable aperture version does—so you’re not going to get as optically close to your subject as you would with a longer, variable aperture lens.

Another reason why you might want a fixed aperture zoom lens is based on your shooting style. If you shoot exclusively in manual mode, you’re going to find that variable aperture a little bit awkward. Let’s say you’re shooting a person—you zoom out and you set your camera to 1/500 at f/4, which is the maximum aperture at the widest setting. Then you decide you want to zoom in, but you forget about that whole variable aperture thing. Your aperture is going to change to f/5.6, which is going to result in a shot that’s a full stop underexposed. If you want to avoid this problem, you have to remember to make an adjustment to your shutter speed every time you zoom in—or you have to switch to one of the priority modes so that your camera will do it for you.

  • Nikon D80
  • 800
  • f/5.6
  • 0.02 sec (1/50)
  • 200 mm

B by Flickr user Youssef Abdelaal

This is only a problem, of course, if you shoot in manual mode. In any of the auto or semi-auto modes, your camera will make those adjustments behind the scenes to compensate for the change in aperture as you zoom.

Another important consideration is the weight of the lens. Photographers who carry their DSLRs while traveling may find that the added weight of that fixed aperture lens does little for image quality, while simultaneously contributing to an aching back (that’s not a lot of fun when you’re supposed to be on vacation). If you do a lot of hiking or just walking around the places you visit, weight is an important consideration. And don’t just assume you’re going to get used to the extra weight—find out how much your camera is going to weigh with the lens-of-your-dreams attached to it, and spend a day carrying around something of equal weight. If you find it taxing or just inconvenient, you may want to consider a lighter variable aperture lens.

Conclusion

The bottom line is, most photographers don’t need fixed aperture lenses. The conveniences really don’t outweigh the inconveniences for the average person shooting photographs of most things. Having said that, if you like to shoot certain subjects (birds in flight, for example, which can benefit from a lens with fast autofocus and an aperture that stays put) and you have a comfortable budget, then you might want to consider a fixed-aperture zoom. But this is definitely not an investment you need to make just to keep up with the Joneses—I just don’t think you’re going to see a whole lot of differences in your work as a result of that considerable purchase.

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Comments

  1. Deirdre says:

    A very useful and objective article that makes immense sense. Thanks for not over promoting fixed focal lenses!

  2. George says:

    Therer is a "new kid on the block" for those that need low light, long telephoto capabilities, as I quite often do. The Panasonic Lumex DMC-FZ200 (and soon to released FZ300) bridge camera has a 25-600mm fixed aperture (2.8) Leica lens. It may not be a Canon DSLR (my old camera) but the price/performance of this camera is just fine and, as for carrying it around, it weighs in at just over a pound (empty) !

  3. Glenn says:

    Try Bracketing to solve the exposure problem, and always use RAW.

  4. Cal Daniels says:

    Article amazingly misses the worst drawback of the variable aperture lenses. They are aggravating and even useless when you shoot with studio strobes. And horrible at any time when shooting on manual. With studio strobes you are shooting on manual and the effective aperture changes every time you zoom. You could easily have a shoot where the images range from a stop too dark to a stop too light! Bad enough when you use only a few images from the shoot, but can you imagine doing a fashion or still life shoot and having to manually correct the widely-varying exposure on hundreds of photos!

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