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Old Mar 4, 2006, 5:56 AM   #21
NHL
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Just a comment here: :idea:

1. At f/2.8 or larger the camera uses a higher precision AF sensor than normal so faster lenses can be centered better in their shallower DOF

2. In low-light any lens will have an issue, this is why the camera comes with the built-in flash illuminator which is terrible!!!
-> The next option is the IR AF illuminator on the flash which I really recommend, instead of a separate ST-E2 which costs as much as another flash...




I'm talking from experience here with a 50mm f/1.4 with USM. A lens does not focus by itself (it's dumb remember?). The camera does all the focusing based on it's internal phase detector (and AF algorithm), a lens just obey to what it's been told by the camera... and that's all. Ditto with front/back focus <- it's the camera fault! (... and why the AF adjustment screw is in the body)

This has nothing to do with USM/HSM! With short focal lenght a non USM lens can be just as fast as USM - My 50mm f/1.4 USM is quite slow, but my 85mm f/1.2L with ring USM is even slower still... in low light :-)
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Old Mar 4, 2006, 6:17 AM   #22
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harana wrote:
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I am going to using this Sigma 18-50mm f/2.8 lens for walk around pictures and for wedding photography. How is this for this kind of usage?
Another bright wide zoom to keep an eye on is the new Tamron 17-50mm f/2.8 XR Di-II LD (DSLR models with smaller sensors only, since it's not a full frame lens).

http://www.tamron.co.jp/en/news/rele...s0215_a16.html

I haven't seen any user reports yet, and Adorama is showing mid April Delivery for it in Canon Mount (see the note they include)

Tamron 17-50mm f/2.8 XR Di-II at Adorama

I don't see this note for it in Minolta or Nikon mount (so maybe they started shipping other mounts first, unless Adorama neglected to note that they're not available in those mounts yet).

Search for Tamron 17-50mm f/2.8 at Adorama

But, it's probably not long enough for some uses (you may be better off with something a bit longer, depending on what you're shooting). Of course, a brighter prime is preferrable in some lighting, too.

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Old Mar 4, 2006, 6:33 AM   #23
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NHL wrote:
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I'm talking from experience here with a 50mm f/1.4 with USM. A lens does not focus by itself (it's dumb remember?). The camera does all the focusing based on it's internal phase detector (and AF algorithm), a lens just obey to what it's been told by the camera... and that's all. Ditto with front/back focus <- it's the camera fault! (... and why the AF adjustment screw is in the body)
Well, I don't know about Canon, but I'd tend to disagree. I've seen too many problems with lenses reported (one lens front focus consistently in controlled conditions, with another focusing spot on). Sure, most of the time is the camera. But, lenses can be an issue.

I was participating in another thread recently, where someone had consistent front focus testing a Tamron 28-75mm f/2.8 on a KM Maxxum 5D. Yet, a Minolta 28-75mm f/2.8 focused perfectly on the same body in the same condtions (and these lenses are optically identical, except for the coatings and eletronics). He was using a tripod to test with, too.

My guess, is that it was probably the way the Tamron was chipped (my guess is that they had some early production problems with the way it was intrpreting camera instructions or reporting focus distance or focal length)

This user also tried more than one body and Tamron lens with identical results at the dealer he purchased it from (they front focused, and the Minolta lens did not). The dealer ended up giving him the Minolta Lens at the same price after confirming the problem.

I suspect that the problem is the way a camera is talking to one lens versus another. If the camera is reasonably sure of how out of phase it is, it may move the lens a certain amount, without double checking the final move (resulting in BF or FF if the instructions are not being interpreted properly, resulting in frontfocus or backfocus).

P.S.

Part of the problem is likely how a manufacturer compromises on AF speed versus reliability, and I suspect they're probably using a combination of things like aperture, focal length and focus distance to decide how to handle it (more DOF won't require as much accuracy).

So, if a lens is not reporting the variables used accurately for AF, the camera may misintrepret how much to move the focus for the final move to increase speed, assuming a greater DOF in some conditions).

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Old Mar 6, 2006, 5:54 AM   #24
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JimC

If you do a search in this forum, you'll find as many Canon lenses reported with front/back focus as 3rd parties

The best way to describe (or experiment to see) the AF is to put on a macro lens when everything is in "slow-motion" - the camera (not the lens) will hunt back and forth until focus is achieved (i.e. it does not move a preset distance or angle, but look for the phase far/near differences). When the camera can't achieve focus it'll timeout and at this point you'll have front or back focus!

I'm in no way saying a lens can not be defective, but the autofocus is a closed loop system remember? You can easily 'break' a lens by inserting a teleconverter. Others put diopter(s) in front of the lens to modify its myopia, and the camera can still maintain focus correctly after all theses 'mods' to the lens. :-)

Also if you check the camera's manual, if the AF is in any of the AI servo mode for action shots, the camera is now in shutter priority and will release the shutter before focus is achieved (hence front/back focus) while subsequent shot will be in focus priority, provided the AF can be achieved lock within a specified time - This AF timeout period varies with USM and non-USM lenses (if I recalled it's about 200-250ms to meet the 5 fps rate of the camera)...


-> A lens can always be at fault, but usually it's the last culprit in the chain but not the first (the camera's quick return mirror too...)! :idea:

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Old Mar 6, 2006, 9:12 AM   #25
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I just couldn't let something go uncommented on because I dislike technically incorrect (while practically some times correct) statements.
tlmiller10 wrote:
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When used in combonation with a higher ISO setting (400 or above) it produces a nice DOF (it increases the more you go up with the ISO).
It is not the changing of the ISO which increases the DOF. ISO has no direct effect on depth of field. If you are in aperture priority, increasing the ISO will increase the aperture linearly (both are a doubling scale. So going from 200ISO to 400ISO doubles the light, which allows the aperture to double from f2.8 to f4.) And increasing the aperture will increase the depth of field.

But if you are shutter priority then increasing the ISO does not effect the depth of field one bit. This is because increasing the ISO increases the shuter speed (not the aperture.) And shutter speed has no effect on depth of field.

You can learn more about depth of field here:
http://www.dpreview.com/learn/?/Glos...f_Field_01.htm

I'm sure that tlmiller10 understands all of this well, but I didn't want others to be mislead by his statement.

Eric
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Old Mar 6, 2006, 11:05 AM   #26
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eric s wrote:
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I just couldn't let something go uncommented on because I dislike technically incorrect (while practically some times correct) statements.
tlmiller10 wrote:
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When used in combonation with a higher ISO setting (400 or above) it produces a nice DOF (it increases the more you go up with the ISO).
It is not the changing of the ISO which increases the DOF. ISO has no direct effect on depth of field. If you are in aperture priority, increasing the ISO will increase the aperture linearly (both are a doubling scale. So going from 200ISO to 400ISO doubles the light, which allows the aperture to double from f2.8 to f4.) And increasing the aperture will increase the depth of field.

But if you are shutter priority then increasing the ISO does not effect the depth of field one bit. This is because increasing the ISO increases the shuter speed (not the aperture.) And shutter speed has no effect on depth of field.

You can learn more about depth of field here:
http://www.dpreview.com/learn/?/Glos...f_Field_01.htm

I'm sure that tlmiller10 understands all of this well, but I didn't want others to be mislead by his statement.

Eric
I do stand corrected... =o) I do seem to noticed a greater DOF when I go from 800 ISO to 1600 ISO when shooting basketball. Both at f2.8.Interesting read...

-tlmiller10
Tim Miller Photography
http://tmillerphoto.com
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Old Mar 6, 2006, 11:25 AM   #27
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tlmiller10 wrote:
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I do stand corrected... =o) I do seem to noticed a greater DOF when I go from 800 ISO to 1600 ISO when shooting basketball. Both at f2.8.Interesting read...
My best guess... less blur because you've got faster shutter speeds versus greater depth of field.

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