Why choose a constant-aperture zoom lens over a variable-aperture zoom?
Asked 6/12/2011
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What practical advantages does a constant maximum aperture zoom lens have compared with a variable-aperture zoom? For example, why might someone prefer an f/4 zoom that stays at f/4 throughout the range over a zoom that changes from f/3.5 to f/4.5? Aside from price or build quality, does a fixed maximum aperture offer real benefits for still photography or video?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
15y ago
2 Answers
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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.
Originally by user1917. Source · Licensed CC BY-SA 4.0
user1917
15y ago
0
Generated from our catalog & community — verify before relying on it.
The main advantage is consistency: with a constant-aperture zoom, your exposure can stay the same as you zoom. If you are shooting at the lens’s maximum aperture, a variable-aperture zoom gets “slower” at longer focal lengths, which may force you to change shutter speed, ISO, or both.
That makes shooting simpler, especially when composition changes quickly. You can reframe without also having to rethink exposure. This is useful for action, low light, and any situation where you are already near your shutter-speed or ISO limits.
It is also especially helpful for video, where zooming during a shot with a variable-aperture lens can cause visible exposure changes.
If you do not need the widest available aperture, you can set a variable-aperture lens to an f-stop available across the whole zoom range and it will behave consistently there. So the benefit is mostly when you want to use the lens wide open at any focal length.
Constant-aperture lenses are often also associated with higher-end optical design, but the key practical benefit is predictable exposure throughout the zoom range.
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