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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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  • \$\begingroup\$ 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. \$\endgroup\$
    – dpollitt
    Jun 14, 2011 at 2:40
  • \$\begingroup\$ @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. \$\endgroup\$
    – fmark
    Jun 14, 2011 at 4:46
  • \$\begingroup\$ 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). \$\endgroup\$
    – dpollitt
    Jun 14, 2011 at 13:16
  • \$\begingroup\$ Related: Why doesn't the picture become darker the more you zoom in? and How do constant aperture zoom lenses work? and Does the diameter of the aperture increase with increases in the focal length with constant F-number(F-stops)? and Why don't all zoom lenses have variable f-numbers? \$\endgroup\$
    – Michael C
    Jan 5, 2019 at 22:51

3 Answers 3

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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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    \$\begingroup\$ "your exposure stays the same regardless of your zoom"--this is not always true (you'll need to adjust exposure if zooming into the light at the end of a tunnel, for example). \$\endgroup\$
    – mjs
    Feb 8, 2015 at 13:26
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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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    \$\begingroup\$ 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). \$\endgroup\$
    – user2719
    Jun 14, 2011 at 9:53
  • \$\begingroup\$ @Stan: Wouldn't this be better on the question, instead of another answer? \$\endgroup\$
    – jrista
    Jun 14, 2011 at 15:45
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Fixed maximum aperture lenses don't really have any advantage, and they never did.

If you want an aperture that you can maintain throughout the entire zoom range you can just select one that is available at all zoom settings. An f/3.5-f/5.6 lens will stay at 5.6 no matter how much you zoom. Fixed maximum aperture basically come in two variations; High image quality with a very large aperture (f/2.8), and extreme image quality with a pretty good aperture (f/4). They could give you a bigger aperture at the wide end of an f/4 lens, but the image quality would drop from extremely good to just (very) good. People who buy such a lens don't want that, they want extreme quality every single shot. Likewise they could probably give you f/1.4 on the wide end of a lens that is now f/2.8 all the way, but the image quality would be very poor at f/1.4.

If lens makers would be worried about their customers having every aperture they choose available at all zoom settings they would have just made an aperture limiting switch that will limit the choice to what is available at the maximum zoom. That way the larger aperture at the wide end would still be available to the majority of the people that do want to use it sometimes.

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    \$\begingroup\$ -1: there is a fundamental difference in construction between fixed aperture lenses and variable aperture lenses. Manufacturers don't "choose" to have their fixed aperture zooms "sandbag" at the wide end - the lens really does have the same aperture at all focal lengths. \$\endgroup\$
    – Philip Kendall ♦
    Jun 29, 2015 at 21:51
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    \$\begingroup\$ This answer shows a fundamental lack of knowledge about how constant aperture zoom lenses work. The actual physical diaphragm size does not change as the lens is zoomed. The entrance pupil (effective aperture) is enlarged because the increased magnification is all done between the diaphragm and the front of the lens. \$\endgroup\$
    – Michael C
    Jan 5, 2019 at 21:10
  • \$\begingroup\$ @MichaelC The question and answer are not about how they work, but about what the advantages are. After all these years I still believe that there is no significant advantage. \$\endgroup\$
    – Orbit
    Jan 5, 2019 at 21:21
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    \$\begingroup\$ @orbit Your entire answer is based on an assumption that is demonstrably incorrect. The aperture diaphragms of constant aperture zoom lenses do not close down as the lens is zoomed out to wider focal lengths. The magnification of the diaphragm as seen through the front of the lens (the effective aperture or entrance pupil) is reduced at the same rate as the lens' overall magnification, thus a constant f-ratio is maintained. Think about it. Even an 18-55mm f/3.5-5.6 does not follow what you are claiming. The e.p. at 18mm/f3.5 is only 5mm. The e.p. at 55mm/f5.6 is 9.8mm. 55/5 would be f/11! \$\endgroup\$
    – Michael C
    Jan 5, 2019 at 22:07
  • \$\begingroup\$ @MichaelC I don't agree that my answer is based on that. What you say was already clear after Philip Kendals comment. I do agree that the answer is not very clear though, i'll update it if I have time. \$\endgroup\$
    – Orbit
    Jan 5, 2019 at 22:14

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