If shooting at f/22, is a fast 1.8 lens still considered fast?

Started Apr 17, 2011 | Discussions thread
noirdesir Forum Pro • Posts: 13,580
Re: tell me Steve

Ronj2 wrote:

It would appear that modern Af sensors will benifit from faster glass. Even if the Af has an aperture of 2.8 or F5.6, a brighter lens will allow more light to the AF sensors aperture. The AF sensor may be limited by it's own aperture but the light it sees is influenced by the light the lens lets in, so it makes sense that a faster lens will allow the AF sensor to see it's maximum potential of light more often than a slow lens in certian situations.

That's the way I see it anyway, maybe I'm wrong.

Two things to understand:

  • Assume you sit directly at the sensor and look out towards the lens, a cone of light will hit you. When the lens is stopped down from, eg, f/2.8 to f/5.6, what changes is the opening angle of the cone, ie, it gets narrower for smaller apertures.

  • The PD-AF sensor never sees the full cone, it only sees a small ring (ie, subtracting a narrower cone from wider cone gives you a ring impacting the AF sensor at a certain angle).

Going from a f/5.6 to a f/2.8 lens gives you a wider cone but if that extra light is coming at an angle that is wider than the 'acceptance' ring angle of the AF sensor, the sensor will not see it.

I hope this is clear enough. If you do not believe the two bullet points I made above, look at the links Steve and myself posted earlier in this thread, they describe this in more detail.

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