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DPChallenge Forums >> Hardware and Software >> Anyone else have this problem? (soft focus)
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Showing posts 1 - 11 of 11, (reverse)
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 09/10/2004 11:13:47 PM · #1
Okay, so I finally received my brand new Tamron 28-75mm f/2.8 XR Di lens (supposed to be very good!) and I take it out for a spin. I'm feeling like images at f/2.8 are a bit soft, so... I decide to try out this lens test with my new lens.

Now, I didn't set up the exact distances and all, I just wanted an image with varying resolution lines that I could use for comparison between my lenses.

So... I set up my new lens and took several pictures, at 2.8, 3.5, 4.0 and so on. Every time, the 2.8 images were soft and the 4.0 images were sharp.

So... I put my Canon 50mm 1.8 II lens on the camera and took some pictures, again at various aperatures. And again the 1.8 end is soft, even 2.8 is still fairly soft (but better) and 4.0 looks pretty sharp.

I tried my kit lens, I tried my Canon 75-300mm USM IS lens.

Every one of them had a "soft" focus at he widest open aperature.

So then I thought... What if I turn my camera so that it is at a 45 degree angle to the paper and reshoot. GUESS WHAT?!?!?!?

It appears that my camera is focusing IN FRONT of the center focus position!

Every one of my lenses produced a sharp image on the left side of center, with soft focus in the center and blurry to the right. (the left side of the paper was closer than the right due to the 45 degree angle)

So, my question is... Is anyone else experiencing this? Is it a Digital Rebel thing? Any other cameras? Should this be considered a defect or is it "within spec"?

Thanks for any insight.
 09/11/2004 12:02:24 AM · #2
That's weird! I'm going to have to try and duplicate your experiment.
 09/11/2004 12:06:23 AM · #3
I don't know about all that, but most lenses are not very sharp wide open. I think they are sharpest in the middle of their aperture range.

June

BTW, is that Tamron lens 2.8 at all focal lenghts?

Message edited by author 2004-09-11 00:07:30.
 09/11/2004 12:37:36 AM · #4
Originally posted by chiqui74:
BTW, is that Tamron lens 2.8 at all focal lenghts?


Yup! I really want to love this lens, but I want it to be sharp even at f/2.8. I may experiment a bit with manual focus (all of my tests were with automatic focus on the center focus).
 09/11/2004 12:39:44 AM · #5
Originally posted by dwterry:
Originally posted by chiqui74:
BTW, is that Tamron lens 2.8 at all focal lenghts?


Yup! I really want to love this lens, but I want it to be sharp even at f/2.8. I may experiment a bit with manual focus (all of my tests were with automatic focus on the center focus).


Well, I don't think it has anything to do with the actual focusing process, I think "softness" just is. I've read alot of different lens reviews and that's usually the main complain people have with their lenses, being soft at wide apertures.

June
 09/11/2004 12:49:40 AM · #6
Not to argue but what is the point of having a fast lens if it's just going to be soft wide open? 'Splain it to me please :-)
 09/11/2004 02:20:22 AM · #7
It's a trade off I guess, but that's how it usually is. Normally it is not so soft that images are unusable, but they are as sharp. Don't ask me why, I'm on the same boat everyone else is! I have the Sigma 24-135 2.8-4.5 Aspherical IF and it is a tad soft at 135mm. I'm not complaining though, they images are still great,it's just that they are their best around the midrange of the focal lengths. Most of the time the same concept applies to aperture.

June

BTW, this doesn't only happen with fast lenses, but most lenses.

Message edited by author 2004-09-11 02:21:04.
 09/11/2004 10:31:05 AM · #8
David,
Here's something to try. you've shot at one of four possible orientations of "45 degrees". You shot with the pattern being nearer on the left side; try this with the near side on the right. You might also try it with the near side top and bottom to see what happens there.
Two things are possible... either the camera is front-focusing, or the focus point is not centered where you think it is. The above test will tell you which is the case.
Front- or back-focusing is certainly not unknown on 10D or 300D cameras, and Nikons are not immune either. It's been discussed to death for the 10D over at DPReview. If you can document that there is a focusing issue, save the pics and document your method, contact Canon and send it for service.

Edit:
This si more prominent with fast glass, since the DoF is so small. the focusing system on the 10D & 300D is only accurate to "within the DoF" whereas the focusing on the 20D, 1D MkII & high-end Canon film bodies is accurate to 1/3 of the DoF, with f/2.8 or faster glass.

Message edited by author 2004-09-11 10:32:49.
 09/11/2004 11:31:19 AM · #9
I had auto focus problems with a Nikon N90s on a Kodak DCS460 digital back.
The source of the problem was IR light causing the focus sensors to misread the scene.
Adding a "Hot Mirror" greatly improved the auto focus and had a secondary benefit of improved saturation of green leaves,
as well as noticeably lower image noise. I would hope that newer cameras would not suffer this malady!
Here are examples of the difference using a hot mirror:

hot mirror: No hot mirror:

Message edited by author 2004-09-11 11:32:50.
 09/11/2004 11:55:25 AM · #10
Originally posted by kirbic:
...This si more prominent with fast glass, since the DoF is so small. the focusing system on the 10D & 300D is only accurate to "within the DoF" whereas the focusing on the 20D, 1D MkII & high-end Canon film bodies is accurate to 1/3 of the DoF, with f/2.8 or faster glass.


With the centre of focus being approx. 1/3 into the DoF (front to back), leaving 2/3 behind it, like this -|-- (-=unsharp; |=sharp), whereas most appear to expect the centre of focus being smack in the middle of a shallow DoF?


09/11/2004 05:19:59 PM · #11
Excellent advice Kirbic, thank you. I'll give those ideas a try and see what I get.
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