Why buy a fast lens if you usually shoot it stopped down?
Asked 10/25/2014
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If two lenses have the same focal length but different maximum apertures, such as a 300mm f/2.8 and a 300mm f/4, and both are used at f/8, do they require the same shutter speed for the same exposure? If so, what are the practical advantages of owning the faster lens when you often stop it down anyway? For example, in bird photography you may prefer f/8 for more depth of field.
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
11y ago
2 Answers
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It's widely believed that a lens with a faster aperture will perform better stopped down compared to a slower lens wide open. Whilst this is often the case it's not a hard and fast rule.
With DSLRs the lens is wide open until you take the shot (at which point the iris quickly closes). Having a faster lens therefore means you get a brighter viewfinder image. It also means the AF system can use a wider baseline which in many bodies activates a set of more accurate AF sensors.
This is why Canon & Nikon make f/2.8 lenses all the way up to 400mm, in addition to providing more light for sports photography, they make autofocus more accurate as well. There is important number with regards to AF and that's f/5.6. This is the limit of most camera's AF systems and so you won't find very many AF lenses slower than f/5.6 at all.
Having a faster lens to begin with also means you can use teleconverters and still have a large enough aperture for AF.
Originally by user1375. Source · Licensed CC BY-SA 4.0
user1375
11y ago
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Generated from our catalog & community — verify before relying on it.
Yes—at the same focal length and the same working aperture (for example, both lenses set to f/8), exposure is essentially the same, so there is no inherent shutter-speed advantage just because one lens can open wider.
The benefits of the faster lens come from how cameras operate before the exposure and from lens design:
- the lens stays wide open until the shot is taken, so an f/2.8 lens gives a brighter viewfinder
- autofocus often works better with a faster maximum aperture; some cameras enable more accurate AF points or perform better with f/2.8 lenses
- faster lenses generally pair better with teleconverters and may retain AF more effectively
- many fast lenses are optically stronger when stopped down, though this is common rather than guaranteed
- they can offer different rendering, including smoother out-of-focus areas
So your understanding is correct: stopping both lenses to f/8 removes any exposure-speed advantage. You buy the faster lens for better viewing/focusing performance, more flexibility when you do need f/2.8, and often higher-end optical/mechanical design.
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