Are automatic apertures really necessary?

Started Jul 18, 2012 | Discussions thread
Tom2572
Contributing MemberPosts: 922
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Re: Are automatic apertures really necessary?
In reply to chlamchowder, Jul 18, 2012

chlamchowder wrote:

Autofocus lenses focus much faster at larger apertures. It isn't just the minimum amount of light required to focus, its focus speed and accuracy, the less light the more the AF system will struggle.

Technically a lens can focus at F6.3 on the alpha system, and sometimes a little slower with good light, but I would never want to restrict a f1.4 lens to 6.3, as the AF performance would be much, MUCH worse.

No, an AF sensor designed to work at f/6.3 will not see any additional light if you mount a faster lens. The AF sensor only uses the light rays coming in at the edge of the aperture at f/6.3. Extra light rays coming in at different angles won't hit the AF sensor. There might be another f/2.8 AF sensor that catches the set of light rays at the edge of the aperture at f/2.8 (like on Canon models and some earlier Sony models), but the total amount of light that an AF sensor receives does not depend on the aperture setting unless the light rays it needs to collect are actually cut off by the aperture.

In other words, AF performance is dependent on subject brightness, not aperture setting.

If you don't believe me, compare the low light AF performance of your f/1.4 lens to that of a f/5.6 or f/6.3 lens at the same focal length. You might be surprised at how similar the AF performance really is.

I mean, I completely understand your question, but I honestly think it's like asking why modern cars have steering wheels instead of yokes, and the answer for both is that at one time they were needed, and while they are no longer needed in the same way as before they still exist because people expect them to exist.

Put it this way, if someone built a DSLR that focused stopped down, it wouldn't need a DoF preview button, and while people can argue the finer points of what constitutes a pro-level DSLR, most would agree that a DSLR without a DoF preview button would never be considered a pro-end DSLR whether it needs one or not, so ipso facto high-end DSLRs will continue to have DoF preview buttons and cars will continue to have steering wheels.

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