normat
Premium
join:2000-08-02
Boynton Beach, FL
clubs:
| Tamron 28-75 f2.8 Wide angle focus
In looking at a few reviews I've seen a few mentions of focus issues at wide angle and wide aperture. I'm not sure the aperture matters as much but would accentuate any problem.
I did a few tests and I did find this to be the case. I did some distant test shots focused while at 28mm off in the distance. I then refocused on something close by to change the distance, recomposed to the original subject zoomed to 75, focused and zoomed back to 28 for another shot. I found that in most shots (about 75%) that zooming out to focus produced better results. In one case it was worse (also the closest distant object I tested). The rest were about the same.
I'm not sure if it is a problem with the lens or just something inherent in focusing on distant subjects and shooting at a wide aperture. After all, more distant detail is available at 75mm.
I didn't have any problems within 10-20 feet as many people claimed.
In any case I'm not worried as I tend to not shoot distant subjects at 28mm and wide apertures. If I do I'll just zoom in first. |
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B52GUNR
KM 7D love and D3 Nirvana
Premium,MVM
join:2001-03-06
Alameda, CA
clubs:  | It could be the focus sensor focusing on the wrong part of the scene. Zooming in allows you to pick the point of focus more precisely, then you can lock the focus and zoom out. |
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normat
Premium
join:2000-08-02
Boynton Beach, FL
clubs:
edit:
September 19th, @03:45PM
| Many of the review sites discuss the issue extensively. I wouldn't have noticed otherwise so I gave it a test. It could easily be mistaked for a soft f2.8 shot. Though, this lens is capable of producing surprisingly sharp images that wide.
Zooming out isn't bad, especially when * is used for focus lock. What's interesting it the motor always adjusts a little given the same focal point at 28 and 75mm. Perhaps it is supposed to do that but it usually looks better when locked from 75mm.
I'm not sure if it is specific to the Canon version or not. The complaints I've seen are from 300D, 350D and 20D users.
When I get around to it I'll post some examples. |
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B52GUNR
KM 7D love and D3 Nirvana
Premium,MVM
join:2001-03-06
Alameda, CA
clubs: | Please do, and I'll try to duplicate with the KM 28-75 (which is the same lens, it just has Minolta's wizbangery added to it). |
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normat
Premium
join:2000-08-02
Boynton Beach, FL
clubs: | Do you have the Tamron KM mount or one labeled as KM made by Tamron? I'm not sure if it exists as a pure Tamron but I did see it listed on Tamron's web site. |
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B52GUNR
KM 7D love and D3 Nirvana
Premium,MVM
join:2001-03-06
Alameda, CA
clubs:
edit:
September 19th, @04:20PM
| It's KM badged, made by Tamron. The Tamron one is the Tamron 28-75mm f/2.8 XR DI |
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sykocus
Watashi Wa Shashin O Toru Ga Suki Desu
Premium
join:2002-11-21
Guam, USA
| reply to normat
said by normat  :Zooming out isn't bad, especially when * is used for focus lock. What's interesting it the motor always adjusts a little given the same focal point at 28 and 75mm. Perhaps it is supposed to do that but it usually looks better when locked from 75mm. Unless there's a custom function in the 20d or something, the * isn't a focus lock. it's exposure lock and partial metering mode at least on the 300d. also i'm pretty sure changing the zoom after focusing will change the focus, depending on the distance and amount the zoom changed it may get it close, but I don't think you could say focus on a object at 100mm switch to MF and zoom at out 30mm and take a picture and have it be at optimum sharpness.
in regard to the overall problem, aperture shouldn't have anything to do with it, since the lens stays wide open until the brief moment before the shutter opens.
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My Gallery
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normat
Premium
join:2000-08-02
Boynton Beach, FL
clubs:
| said by sykocus :Unless there's a custom function in the 20d or something, the * isn't a focus lock. it's exposure lock and partial metering mode at least on the 300d. Yes, there are 4 ways to configure AF/AE lock on the 350D and 20D.
said by sykocus :also i'm pretty sure changing the zoom after focusing will change the focus, depending on the distance and amount the zoom changed it may get it close, but I don't think you could say focus on a object at 100mm switch to MF and zoom at out 30mm and take a picture and have it be at optimum sharpness. I've read some lenses are able to do this. I'm not sure why some can and others can't. However, that's the odd thing in the case of this lens. Objects around infinity focus better when using the zooming technique. The difference is strikingly better.
The only part aperture plays is at the narrower apertures it brings the off focus subject, from the above case, into the focal field. |
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sykocus
Watashi Wa Shashin O Toru Ga Suki Desu
Premium
join:2002-11-21
Guam, USA
| good to know about the * button.
if your lens was stopping down as you were composing the picture, you would see viewfinder darken/brighten as change the aperture. being a SLR, there is only one lens, and the image you see and the image the AF system sees is the same. If you hold down the dof preview and change the aperture, you can see (and hear) what it's like when this is happening.
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My Gallery
Great photography is about depth of feeling, not depth of field. --Peter Adams |
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Mauricio
Premium,MVM
join:2001-12-04
Vancouver, BC
| reply to normat
said by normat :said by sykocus :also i'm pretty sure changing the zoom after focusing will change the focus, depending on the distance and amount the zoom changed it may get it close, but I don't think you could say focus on a object at 100mm switch to MF and zoom at out 30mm and take a picture and have it be at optimum sharpness. I've read some lenses are able to do this. I'm not sure why some can and others can't. However, that's the odd thing in the case of this lens. Objects around infinity focus better when using the zooming technique. The difference is strikingly better. Only parafocal lenses will maintain focus when zooming in or out. Parafocal lenses are very reare and expensive. All other lenses, the 28-75 among them, will shift the focus plane when zooming in or out. The behaviour you describe seems to be a compensation for backfocus. My Konica-Minolta 28-75 focuses with the same precision at either end of the range.
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normat
Premium
join:2000-08-02
Boynton Beach, FL
clubs:
| I did read some about parafocal focus. The lens is not parafocal but when focused that way the results seem so much better. Though you would not want to focus wide and then zoom.
Unfortunately, I deleted my initial test shots. I was planning on doing another set to post here but I've been waiting for the weather to clear up from Rita.
I'm wondering if it's a compatibility issue with the lens and camera. After all, it is the camera that controls AF. Though I have read it's an issue with many zoom lenses at wide angle. Given that it's a wide aperture for a zoom in this range it probably aggravates the situation.
Anyway, I'm not complaining since I love the lens. I'm just trying to understand how to work with it. |
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javaMan
Premium,MVM
join:2002-07-15
San Luis Obispo, CA
| reply to normat
Interesting thread as I am having the same type of issues with a Sigma 17-35 on my 300D. Since I'm a novice I have assumed I just don't know what I'm doing, which could still be the case I guess. But I have had an extremely difficult time getting sharp focus on anything beyond 30 feet or so. No matter what I've tried the shots are soft.
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Woe unto them that call evil good, and good evil; that put darkness for light, and light for darkness. . . Isa. 5:20 |
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normat
Premium
join:2000-08-02
Boynton Beach, FL
clubs:
edit:
September 24th, @07:24PM
| reply to normat
Canon EOS DIGITAL REBEL XT
28mm 1/2500th F2.8 ISO100
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These are 100% crops. |
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normat
Premium
join:2000-08-02
Boynton Beach, FL
clubs:
| reply to javaMan
I'd try the same experiment and see what your results are. Even though this lens is not parafocal it's clear to see that using a parafocal technique for wide aperture, wide angle distant shots produces dramatically better results. The middle image is nice and sharp for a f2.8 zoom. It even looks a little better than the f8 shot on the right. |
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B52GUNR
KM 7D love and D3 Nirvana
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join:2001-03-06
Alameda, CA
clubs:
 ·DSL EXTREME
| reply to normat
I put my new that I received on Friday through some paces today. It seemed to focus fine at all focal lengths. I did notice that the 7D picked different focus points than I wanted it to, but apart from that it worked great.
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C'est la vie
Unabashed Minolta Fansheep
A man who cannot stand alone cannot stand for long. |
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