AF, fast vs slow glass

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Archive 2008 · AF, fast vs slow glass

Makten
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p.2 #1 · AF, fast vs slow glass

AF speed has nothing to do with aperture. The fact that fast lenses often has good and fast AF is because they are more expensive (only applies to AF-S lenses) or that the optical design is free from spherical aberration that can confuse the AF module.

Kerry Pierce wrote:
no comments? Nobody telling me I'm FOS?

You are right. The AF module does not care for light at angles greater than ~f/5.6 or so (probably a bit slower to be on the safe side).

Nov 20, 2008 at 06:02 AM

Alan321
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p.2 #2 · AF, fast vs slow glass

90 5.0 wrote:
Alan321 wrote:
AF system is happy with whatever lens it has but for technical reasons the aperture must be f/5.6 or bigger no matter how much or how little light is available. If the aperture was allowed to be too small then the AF system would see everything sufficiently well focused and not be able to determine the correct focus.

- Alan

Not true exactly. my 70-300 is 5.6 at 300 and it will af with a tamron pro 1.4 tc in decent light and even indoors with good light. This combo would be greater than 5.6 at 300/450 equivalent...

OK, perhaps not true exactly. Still there is a limit and Nikon have chosen to make f/5.6 the notional cut-off. Some Canon cameras work at f/8 quite ok. In general, and as you seem to have noticed judging by your references to "good light", there is a noticeable deterioration in AF speed and/or ability at smaller apertures especially when non-reporting TCs are used or when small lenses are used that lie about the maximum aperture to trick the AF into working (e.g f/6.3 reporting as f/5.6).

- Alan

Nov 20, 2008 at 06:05 AM

R. Francois
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p.2 #3 · AF, fast vs slow glass

I have no idea really. My fast lenses (2.8 or faster) seem to focus really well (AF-s or not). But they better be! sheesh they are expensive

Nov 20, 2008 at 06:27 AM

Alan321
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p.2 #4 · AF, fast vs slow glass

I just thought of something else that may seem a bit contrary to what I've been saying. A lens that has a large aperture presents a narrow DOF and so increases the chance that what you want to focus on cannot be detected at all the camera with the lens at its present focus position. More DOF would give the camera a better chance of measuring focus in cases of extreme defocus. Unless the camera has a focus sensor set up for that role (as some Canons do, but I'm too new to Nikon to whether or not they do) then the camera must initiate a focus search. That will be way slower than if it could determine focus without such a search.

- Alan

Nov 20, 2008 at 11:48 AM

Kerry Pierce
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p.2 #5 · AF, fast vs slow glass

Alan321 wrote:
ok, you're FOS

No, not really. I'd have expected the AF to be slower in poor light / poor contrast situations and that would be made worse by dark lenses. Did you not notice that ?

Had no objective way to discern that, really. My only short f/5.6 lens is 24-120vr AF-S. All of my faster glass, other than 28-70, is screwdriver. My inclination though, is to believe that there is no difference with respect to max aperture. The only difference, IMO, would be the actual focus speed of the lens, which would be there in daylight too.

I also had no way to objectively judge "critical" focus. I was shooting hand held with ISO maxed out on all 3 cameras and still getting some very long shutter speeds. Noise was pretty nasty too.... But, again, my inclination is to believe that the AF wouldn't care about max aperture for that either. When the AF hunted, it hunted about the same for f/1.4, f/2, f/2.8, as it did for f/5.6. On targets with "good" contrast in those test conditions, AF just worked pretty much the same on all 3 bodies.

Nov 20, 2008 at 02:55 PM

Kerry Pierce
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p.2 #6 · AF, fast vs slow glass

Alan321 wrote:
I just thought of something else that may seem a bit contrary to what I've been saying. A lens that has a large aperture presents a narrow DOF and so increases the chance that what you want to focus on cannot be detected at all the camera with the lens at its present focus position. More DOF would give the camera a better chance of measuring focus in cases of extreme defocus. Unless the camera has a focus sensor set up for that role (as some Canons do, but I'm too new to Nikon to whether or not they do) then the camera must initiate a focus search. That will be way slower than if it could determine focus without such a search.

- Alan

Nikon doesn't have the special f/2.8 center cross sensor. IMO, it doesn't seem to need it. The current AF modules in use, like the modules used in my test cameras, worked very well in extremely dark conditions. AF tracking is different story and has much more to do with the base camera AF support electronics than the AF module. For example, the d3 and d300 have the same AF module, but by most accounts, the d3 has more processing power, faster electronics all around, thus will track slightly better than the d300.

Nov 20, 2008 at 03:00 PM

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