Orquid "Phoenix"

Orquid "Phoenix"

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What are the advantages of a lens having a Fixed Maximum Aperture? Previous answers have indicated that "pros tend to want constant apertures in zoom lenses".

While in general lenses with a fixed maximum aperture tend to be more expensive and thus have a larger aperture, this is not always the case. To take a random example, the Canon 10-22 has a variable maximum aperture from f/3.5 - 4.5, while the Tokina 12-24 has a constant maximum aperture of f/4. In cases such as these, is there a reason to prefer a fixed maximum aperture?

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Ultra wide angle lenses such as the examples given are probably not the BEST examples, I would compare two zoom lenses such as 70-200 or 70-300 for the best example. Typically in ultra wide angle lenses, the maximum aperture matters less then other factors. –  dpollitt Jun 14 '11 at 2:40
    
@dpollit I used that example because the constant max. aperature fell in the middle of the variable max. aperture range, and because the two lenses are of comparable quality. Can you suggest two lenses with the same range of focal lengths to compare? The best I could come up with is the Canon EF 100-300mm f/4.5-5.6 USM and Canon EF 100-300mm f/5.6 L in which case the more expensive fixed maximum aperture lens has a higher aperature and than the variable max. aperture lens. –  fmark Jun 14 '11 at 4:46
    
I think your problem will be finding ones of comparable quality as you suggested. Canon doesn't have comparable quality lenses that overlap in focal ranges in both the fixed maximum aperture and variable aperture, at least that I can think of. The closest might be something like - Canon EF-S 17-55mm f/2.8 IS USM Lens as compared to a Canon EF-S 15-85mm f/3.5-5.6 IS USM Lens. Or something like the Canon EF 70-300mm f/4.0-5.6 IS USM Lens as compared to the Canon EF 70-200mm f/4.0 L USM Lens(this might be the best example). –  dpollitt Jun 14 '11 at 13:16
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2 Answers

up vote 8 down vote accepted

One big reason, apart from the fact that these are usually better built, higher quality, lower aperture cameras - your exposure stays the same regardless of your zoom. You don't have to worry if you need to take the shot a little wider, just to keep the shutter speed down or if you'll drop too low in shutter speed by zooming in. It's one less thing to worry about.

Think of it like this, you should be able to treat the composition and exposure as two separate creative forces to be controlled. With a non-constant zoom, now your composition drives your exposure creativity within a certain limit - you may no longer have the ability to do the shot you thought you could. You wanted to zoom in and freeze the action - oh wait, your aperture stopped down, you can't raise ISO more, so you're forced into a slower shutter speed and its a whole different photograph than you thought you had.

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The advantage of a fixed maximum aperture is that you can maintain your current exposure throughout the focal range. That may not seem like much, and in the case of still shots, it may not be particularly earth shattering. It does generally make life a bit easier, when you can use any focal length supported by a zoom lens, and not have to fiddle with your exposure settings to ensure you always get a proper exposure for the given lighting conditions.

Another benefit of fixed maximum apertures comes into play with video. The ability to shoot video with still photography cameras is still a very new field, however the benefits of a fixed aperture are probably greater there. You can achieve those beautiful cinematic effects, such as a telescopic zoom on a subject, with relative ease when you do not have to account for a changing aperture. If you did have to deal with a changing aperture, you might end up with either changing exposure as you zoom, or stuttering exposure if you have automatic ISO enabled that adjusts in stages as your aperture changes.

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Not worth an answer, so I'll make it a comment: It becomes a Big Deal(TM) when you are using an external meter. That's much more common with studio-type flashes or strobes lights these days than it is with continuous lighting or TTL-controlled flash. It's much more likely to be an issue with location or environmental work that with classic studio use, though, since studio shots are usually stopped down to where almost all zooms would keep a consistent aperture across the zoom range (that is, nowhere near wide-open). –  user2719 Jun 14 '11 at 9:53
    
@Stan: Wouldn't this be better on the question, instead of another answer? –  jrista Jun 14 '11 at 15:45
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