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11-23-2008, 01:25 AM   #1
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Tamron 28-75 sharpness @ f2.8

I just got a new Tamron 28-75 f2.8.
First impressions are that the image quality at 75mm f2.8 at infinity or distances larger than 15ft. is bad. Stepped down to f4.0 there is great improvement, although infinity doesn't improve till f8.
At close distances, the IQ is very good, even wide open - no complaints.

Is it reasonable to expect such bad IQ for a lens in the $350 price range at its longest and wide open? I've heard zoom lenses are especially bad at infinity range, but cannot find any article on the web to confirm that.

Here are pictures (100% central crop) of a parrot at f2.8 and f4, and an elephant at f2.8 and f4. The parrot was about 15ft away and the elephant was at infinity.

Is this normal lens softness or a lens defect?

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11-23-2008, 01:38 AM   #2
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Those are the kind of results you should expect for those kind of shots if you use F2.8 or F4, you just aren't giving yourself enough DOF. f8 is probably what you should have been using for the first shot and maybe f11 or f16 for the elephant shot. Zooms are always going to be weak wide open and fully zoomed.
If you really want to test the lens, use a tripod (especially if you want to use wider apertures) and also try shooting at 75mm and then say 70mm or 65mm, you should see the level of softness and then the gradual sharpening as you move away from the extremity.
Of course, you could also possibly have a faulty lens but again, testing on a tripod should help confirm if that is the case.


BTW, this thread should have been in the lens subforum, I flagged it so a Mod can move it.

Last edited by Damn Brit; 11-23-2008 at 01:43 AM.
11-23-2008, 02:18 AM   #3
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Are you sure the lens if focusing properly? The lens may be focusing past infinity. Try taking several pictures, manually changing the focus slightly each time. Or if you have a K20D, you can use live view and zoom in.
11-23-2008, 05:33 AM   #4
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Not sure what is going on there, but generally speaking, that is not the kind of performance you should expect from the Tamron 28-75mm, at f2.8 or otherwise. It seems something is not quite right, either with the settings or the lens. If you have other fast lenses, you can compare the Tamron to those lenses at the same settings, and see what it shows. Or take some other photos with the Tamron and see what it shows.

11-23-2008, 05:33 AM   #5
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I have a Tamron 28-75. I dont think at F2.8 the picture quality is this bad, especially in daylight. What I experienced is that at night or under tunsten lighting conditions or stage lighting conditions, the lens will tend to go OOF at F2.8 but then again, i guess this is kinda expected in situations like this.

Maybe u should try to adjust your BF/FF using your k20D (if you have one).
11-23-2008, 08:11 AM   #6
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I have always been curious as to how one gets a good 100% crop. The way I have been cropping is using Elements and zoom in until it shows zoom level and try to crop accordingly. There has got to be a better way.

stanjo,
if you can explain how you achieve 100% crop I will do so and post some examples. I think I have a few photos saved for some results. Although, I don't think any of them are with tripod and mirror lock up.

BTW, I think results should be better than what you have posted. I hope I can be of some help. I haven't use my lens extensively and am amateur at best. This lens does however get some pretty good reviews. 28mm seems to be its weakest point but still a good preformer I would say. I haven't used my lens for a little while and this is a good excuse to take it out today. Hoping for good weather.

This is a testing on a Nikon but should be simular.
Tamron AF 28-75mm f/2.8 SP XR Di LD Aspherical (IF) (Nikon) - Lab Test Report / Review
11-23-2008, 09:45 AM   #7
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That parrot shoot looks pretty bad at 2.8. I would do some informal testing to see if the focus might be off.
11-23-2008, 10:25 AM   #8
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These are about a 50% crop of a 75mm f/2.8 shot from the Tammy:




Very shallow DOF, but as you can see, the parts of the flower in focus are very crisp.

I think I use a +4 AF adjustment on my Tammy with the K20D, although really,
most shots are great without any adjustment.

You should do some focus chart testing. Are you shooting a K20D?


.

11-23-2008, 04:45 PM   #9
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QuoteOriginally posted by OrenMc Quote
I have always been curious as to how one gets a good 100% crop. The way I have been cropping is using Elements and zoom in until it shows zoom level and try to crop accordingly. There has got to be a better way.
If your browser supports java, you can try using the tool here. It will create a 100% crop. I haven't used elements, so I can't help there.
11-23-2008, 05:15 PM   #10
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QuoteOriginally posted by OrenMc Quote
I have always been curious as to how one gets a good 100% crop.
Crop until it's small enough to fit on most screens - say, under 600 pixels in the largest dimension. As long as you don't also resize it, the result is a 100% crop. It's as simple as that. You shouldn't have to touch the zoom controls or worry in the slightest bit about how big you are viewing it on screen when you do the cropping. as long as there aren't too many pixels to fit on other people's screens, nd you don't resize the picture, it's a 100% crop.
11-24-2008, 12:24 AM   #11
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That is NOT the kind of results I achieved with my Tamron 28-75 when I was using Canon. It was sharp from wide open. There is something really wrong with that baby!!! Take it back!

Ben

Image enclosed shot at f3.2 at 800 iso as well as a cropped image. I classify the Tammy in the "Brutally Sharp" category.
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11-24-2008, 02:31 AM   #12
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Thanks to everybody who replied.

The camera used is K100D.

As I said, the image quality problem is only when the subject is far, more than 15 feet. Up close, the lens is very sharp.

Too shallow DOF cannot be a cause, because at 20ft distance, the DOF is more than 1ft behind the subject for f2.8.

To the person who asked about 100% crops: I did it in Lightroom. Just crop the desired part and export to jpeg with "Constrain Max Size" unchecked.

I will come up with a comprehensive test plan and provide more shots.

Regards,
Stanjo
11-24-2008, 05:37 AM   #13
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QuoteOriginally posted by stanjo Quote
Thanks to everybody who replied.

The camera used is K100D.

As I said, the image quality problem is only when the subject is far, more than 15 feet. Up close, the lens is very sharp.

Too shallow DOF cannot be a cause, because at 20ft distance, the DOF is more than 1ft behind the subject for f2.8.

To the person who asked about 100% crops: I did it in Lightroom. Just crop the desired part and export to jpeg with "Constrain Max Size" unchecked.

I will come up with a comprehensive test plan and provide more shots.

Regards,
Stanjo
This is shot at 75mm
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11-24-2008, 09:36 AM   #14
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stanjo: send it back for replacement. Looks like my first copy. Make sure you test for BF/FF w/ any copies you get as well. I'm 0 for 3.
If you search in this forum, you can see the loads of fun I had...
11-24-2008, 11:20 AM   #15
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I'm on my fourth copy of this lens, yours is not an isolated problem.
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