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01/23/2006 01:09:23 AM · #1 |
Ok. So I think I read somewhere that many DSLR's are not able to use autofocus when the native max aperture of a lens is narrower than F5.6.
I recently read an article about how Phase Comparison autofocus works and I see now why that could happen.
Now, my question is about lenses that when zoomed right to max telephoto have a max aperture of say 6.3. I currently believe that this is due to the physical opening at the end of the lens and the dynamics of zooming.
When your lens passes the point where the max aperture is 5.6, do you lose autofocus?
I think the Sigma 18-200mm F3.5-6.3 is a possible candidate for this type of issue, as is the 'Bigma' 50-500 with a similar aperture range.
Can anyone help me understand this please?
Thanks!
Message edited by author 2006-01-23 01:12:26. |
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01/23/2006 01:20:42 AM · #2 |
It is true that lenses with a small maximum aperture (such as you have described) may "seek" incessantly and fail to find focus. A lot depends on how bright it is, and how contrasty the scene is. Even an f/2.8 lens may have trouble finding focus on a blank black wall, for example. Even a white one, for that matter.
R. |
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01/23/2006 09:58:56 AM · #3 |
This much I gathered from that article I read (which is the most exciting thing I read all week really - I must be a certifiable geek) regarding the mechanics of Split Screen focusing and Phase Comparison focusing. It helps me understand how my digital camera works much better.
However, I understood from the context of that article that part of the reason the autofocus is impaired at narrow apertures isn't just light falloff, but actually having the portions of the lens which are used for the autofocus system blocked by the physical parts of the lens... This wouldn't necessarily be dependent on available light.
This seemed to make sense, as when the physical blockages that reduce aperture (not referring to blades, as they are used in taking the picture and unless you use the aperture preview function on a lens, don't come into play for autofocus) moved in far enough they would block direct light passing through those parts of the lens, and any scant amount of light entering and bouncing off at oblique angles would gently illuminate the autofocus, fooling it into thinking it could do something (hunting).
Now having said that, I am trying to understand a few different peices of information. Or at least the theory behind them.
1. Using teleconverters can increase the effective aperture and cause autofocus to actually stop working (as in the case of putting a 2x TC on a 70-200 F4.0. This would put the max aperture down two stops, passing the F5.6 aperture cutoff for autofocus and apparently shutting the autofocus system right off, not even trying in some cases where it might actually be able to do something.
2. I've heard that there is a trick where you put tape over the contacts in some cases and it fools the camera into trying (and sometimes succeeding in good light) where the camera would ordinarily not try. I think it's prof_fate who has used stacked TC's with native apertures around F/11 and still got some autofocus ability. This tells me that the camera makes use of more than just the edge portions of lenses to create something of a gradual dropoff in autofocus ability rather than a simple on/off
3. Floating aperture lenses could not shut the autofocus down because obviously there are parts of the zoom range where it would be fine. Therefore, there must be a physical aperture limit within the lens where the camera simply cannot see to autofocus. After this point, even a properly lit scene with tonal and color variatons would be effectively blind-folded to the camera and it would not be able to autofocus.
I am wondering if the last part of 3 is the case. If a lens which has an MAX aperture of say, 6.3 at max telephoto, even on a bright day, does it suddenly shut down autofocus? Or does it keep autofocus on but experience sudden loss of abilities, still trying it's best? |
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01/23/2006 11:26:01 AM · #4 |
I read similar info about focusing before I bought my 20D. More than losing focus at a narrow aperture, I would say it focuses faster and more accurately with a wide aperture lens. My Tamron 200-500 at max zoom can open no wider than 6.3 but autofocus works fine if there is enough light. In conditions where it will not focus it hunts as opposed to shuting down completely. But there comes a point where it is useless to make it keep on hunting without ever achieving focus. This comes in pretty dark conditions where you would not expect to get good results anyway.
What it does when I say it is hunting is to go from one end of it's range to the other trying to find focus, then it will stop at the end but the viewfinder will show that it has not "locked on" by the absense of the green light that signifies that focus has been achieved. Sometimes a slight adjustment to composition will get it to focus. Sometimes changing from evaluative to partial metering and/or selecting only the center focus point will help. But usually it is correct in telling you that there just isn't enough light to get a decent shot.
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01/23/2006 11:46:40 AM · #5 |
Thanks for these details Coolhar. This suggests to my limited perspective that there is more than one area of the lens used in critical autofocusing decisions. Probably several points, just to get around this very problem, which no doubt rears its ugly head for a lot of top end pro-shooters who actually do a lot more than the average guy with big honkin lenses with necessarily narrow apertures.
Those boys at Canon (or any other company that makes DSLRs or film bodies with Autofocus for that matter) really do some fine work.
PS. How do you like that 200-500? Is it the next magical step after walking the 70-200 road? |
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01/23/2006 12:07:35 PM · #6 |
I like the 200-500 a lot. I don't know of any other lens with the reach available at a lower price. It suffers from zoom creep, and can be less than it's sharpest at exteme settings. But given good light and stopped down to f8 or narrower, it will get great images.
Here are a couple of my shots with it:
 
jbsmithana has a Nikon version that he uses to get even better images than mine:
 
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