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06-02-2010, 01:34 PM   #1
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How is AF related to aperture?

I've read somewhere that the AF sensors are optimized for a certain aperture, is this true?

If so, what is the K-7 center AF point optimized for? Also, if the optimum aperture is f2.8-5.6 what exactly does this mean. Do lenses in that range of maximum aperture be better for AF?, is there anyway to make an f1.8 lens work better ?

Thanks!

06-02-2010, 01:57 PM   #2
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The af mechanism needs enough light and contrast to work optimally. When there's a lack of either or both, everything slows down. So below 5.6 or so, with lower ambient light, you don't get as good performance. So there are at least three main variables: ambient light, aperture of the lens, contrast of the lens + subject.

And aperture as you know impacts depth of field, which has something to do with any 'slop' in the af mechanism, or something like that. In other words, the AF has to be as good as the widest aperture / smallest DOF, in order to achieve the focus you want.

I don't know whether the 2.8 tale is true - if so, the 1.8 lens would have an advantage only when the ambient light starts to go, all else being equal.
06-02-2010, 02:00 PM   #3
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Wow. That is a really interesting question. I am going to guess that since the aperture, no matter it's size, is still opening at center, AF would be more optimized for focal length rather than f stop. I only say this since it would seem logical that any light drop off (and any ability the camera has to lock on to a subject) would be on the edges of the frame, not in the center, which would (or should?) be constant throughout any lenses aperture range. Since aperture affects the amount of light coming in throughout the plane of the photograph, not the focal point (at center weight) the only time it would matter would be when you are using AF at multi - segments. Which would defeat the purpose of using a wide aperture (and gaining a shallow depth of field). Does this make sense?
06-02-2010, 02:47 PM   #4
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The concept of autofocus "hunting" with a modern lens is comparable to focus confirmation on an old M42 Tak, and is what I experience:

It needs more light and contrast to find the focus point, which is why focus confirmation doesn't work at all at really small apertures on the old lenses.

06-02-2010, 03:46 PM   #5
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I think Nesster is exactly right. Optimized doesn't mean that AF will work better at F5.6, but that is the minimum light level its designed to work at.

I inadvertently did the test which lends some light on this subject, no pun intended :-)

A few months back, i tried to focus on some branches that were framing a moon shot. It was very dark of course. I couldn't manually focus the lens since there wasn't enough moonlight to see them clearly. The DA 50-135 would not lock focus with its F2.8 capability. On a hunch, i put on the FA50-f1.4 and it immediately locked focus on the dark branches. I presume that the FA50 used the light gathering ability of its 1.4 lens to get more AF light than available to the F2.8 lens.

So my strong hunch is, that if you are fortunate to have a wider lens than f2.8, e.g. 1.8 or 1.4, etc, then the AF circuit has an even stronger signal to work with than at F5.6. I think thats the only hypotheses that makes sense with my recent experience.

I've even read a post that the AF circuit is limited to f5.6 light on the wider end, so f2.8 and faster stops don't help. Where do these myths get started ??????????

Phil
06-02-2010, 03:55 PM   #6
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QuoteOriginally posted by Ari Quote
Which would defeat the purpose of using a wide aperture (and gaining a shallow depth of field). Does this make sense?
Focus confirmation can be done wide open, and then you use the F stop you want. Focus and exposure are actually two totally different aspects of the same image.
06-02-2010, 04:33 PM   #7
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QuoteOriginally posted by philbaum Quote
I think Nesster is exactly right. Optimized doesn't mean that AF will work better at F5.6, but that is the minimum light level its designed to work at.

I inadvertently did the test which lends some light on this subject, no pun intended :-)

A few months back, i tried to focus on some branches that were framing a moon shot. It was very dark of course. I couldn't manually focus the lens since there wasn't enough moonlight to see them clearly. The DA 50-135 would not lock focus with its F2.8 capability. On a hunch, i put on the FA50-f1.4 and it immediately locked focus on the dark branches. I presume that the FA50 used the light gathering ability of its 1.4 lens to get more AF light than available to the F2.8 lens.

So my strong hunch is, that if you are fortunate to have a wider lens than f2.8, e.g. 1.8 or 1.4, etc, then the AF circuit has an even stronger signal to work with than at F5.6. I think thats the only hypotheses that makes sense with my recent experience.

I've even read a post that the AF circuit is limited to f5.6 light on the wider end, so f2.8 and faster stops don't help. Where do these myths get started ??????????

Phil
Phil, this only considers the one aspect Nesster had mentioned which is the aperture. But, the other factor is dof which makes the AF less accurate, so where does the optimum fall?

A large aperture of f1.4 will indeed bring more light and make it easier for the AF mechanism, however due to the narrow dof field the allowable error diminishes as well, so it has to be very very accurate. Where as say f2 might allow enough light and have the latitude to be off a bit, and thus work even better......I don't know.

But I think this balance between between 'light allowed' and 'dof' resulting in the optimum aperture will depend on subject distance, ambient light etc. If this is correct, then I cannot say the K-7 center AF is optimized for a particular f-stop.

I think it's generally accepted that even in bright light a lens with an aperture narrower that F8 will not AF (is this a myth as well...at least it seems to hold true most of the times with TC's I've used....but, not always....)

But, can we say f1.4 lens will have an easier time focusing than an f2.8 in non-dim light situation? If AF circuit is limited to f5.6 then it's not true?????????

P.S: never mind....a fast lens is always better than a slow lens. An f1.4 lens set to f2.8 would be ideal. It allows f1.4 amount of light for AF and has the dof for f2.8, which is ALWAYS better than an f2.8 lens set to f2.8. So, this sort of clears what I was wanting to clarify...........

Last edited by pcarfan; 06-02-2010 at 04:49 PM.
06-03-2010, 05:54 AM   #8
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As long as the lens aperture ring is set to "A" or the lens doesn't have an aperture ring (aperture is controlled from the camera body):

The AF will always be done at the lens' maximum aperture. So a f1.4 lens will enable AF in lower light than a f4 lens. This will be true wichever aperture is set from the camera body. Aperture will close to the set value only after AF is performed, and when shutter is released.

In accordance to this, you can control the DOF to your liking. This will not have any effect on AF as long as the lens in question is set to "A" or is whitout Aperture ring.

06-03-2010, 05:54 AM   #9
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QuoteQuote:
P.S: never mind....a fast lens is always better than a slow lens. An f1.4 lens set to f2.8 would be ideal. It allows f1.4 amount of light for AF and has the dof for f2.8, which is ALWAYS better than an f2.8 lens set to f2.8. So, this sort of clears what I was wanting to clarify...........
So a lens that is labeled as f4-5.6 would be ideal with an f-stop of 6 or higher?
06-03-2010, 06:09 AM   #10
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Yes - going by the generalization that every lens is better when stopped down one or two stops.

The other f/2.8 thing that has been bandied about: that focusing screens themselves are optimized at f/2.8. A wider aperture lets in more light but doesn't change what you see on the screen. I don't know if that's an urban legend as well.

Perhaps this generalization isn't too far off the truth: in regular daylight, a f/1.4 lens has no advantage over a f/2.8 one (or a f/5.6), all other things being equal, when it comes to the efficiency of autofocus. However, as soon as light level falls off, so does AF performance - though a f/5.6 will experience this sooner than f/2.8, and the f/1.4 will be able to focus at the lowest light.

The other thing about f/5.6 or smaller max aperture - these are usually at the long end of low priced lenses, and lens contrast is often also compromised at the long end for these lenses. Therefore, there may be AF problems more often shooting with such a lens at the long end.
06-03-2010, 06:25 AM   #11
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QuoteOriginally posted by Nesster Quote
Yes - going by the generalization that every lens is better when stopped down one or two stops.
Regarding lens performance (sharpness etc.) Yes

Regarding AF: Doesn't matter.
06-03-2010, 12:38 PM   #12
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Here's my experience with the K20D: I just rough-tested my DA18-250 (the nearest AF lens I had at hard) and found no significant difference in focus-hunting times between f/3.5 and f/16. I also found that AF works MUCH faster with AF-C than AF-S. Test conditions: ISO=200, FL=18mm, matrix metering, aim at not-very-light corners of my home office. Retest with my FA100-300, FL=100mm: AF is a little slower, but otherwise similar results.

With manual lenses and focus confirmation, it's a different story. I shoot old manuals a great deal, usually with Catch-In-Focus (and AF-S, of course), and I find there's a cut-off around f/9.5. At f/8 I get focus confirmation and auto-snap; at f/11, I don't, and that's pretty much regardless of lens and lighting. Some variation: pointing at the same well-lit contrasty target, I'll get confirmation with a 24mm at f/11, but with a 50mm only at f/8. That's with center-weighted metering, of course.

Hmmm, I wonder if the metering makes a difference? I retry the FA100-300, with both AF-S and AF-C and switching from matrix to center-weighted -- and center-weighted might be very slightly faster, it's hard to tell. Has anyone done AF timing tests on various lenses and bodies?
06-03-2010, 12:53 PM   #13
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Are you stopping down the lens when you do the focus test? If not, the lens is wide open for the focusing, regardless of what f/stop you put it in. The only variable will be the variable max aperture as you zoom.

With old manuals, are you stopping down to do the focus test as well?
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