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Old Dec 14, 2010, 5:20 PM   #1
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Default Help needed please - Is this a defective lens?

It seems I have the worst luck when it comes to prime lenses. My first Nikkor 35mm 1.8G would front focus horribly. So I held off on getting a replacement and decided to try my luck with a Sigma 30mm 1.4




Even though portraits are fine, it tends to front focus and focus more sharply on the right side while the left side is blurry. Strangely enough its a lot better than my older Nikkor 35mm which is why I'm asking.

What do you think I should do - return it for an exchange or a refund or RMA to sigma or live with it? What would you do if you were me?
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Old Dec 14, 2010, 10:56 PM   #2
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The first thing I noticed is that the chart is not square to the camera, and that is critical to doing these kinds of tests.
For side to side focus, you probably need a different test image, like newsprint, and it also must be square to the camera in order to really evaluate it.

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Old Dec 15, 2010, 8:36 AM   #3
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Lilacfire-

Brian is very correct. Only a direct overhead shot where the page was in the same focal plane would be a proper test. That way focus sharpness, as well as edge to edge sharpness could be checked.

Sarah Joyce
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Old Dec 15, 2010, 2:28 PM   #4
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Hmm redid the test and same result. I placed a bunch of batteries on a coffee table and I tried to focus on the white battery but its clearly off by quite a bit:



Same thing happened with a portrait today, instead of focusing on eye which was my aim, it focused on the nose. The only work around seems to be not using it wide open but if I'm paying for it, I really would like it to work correctly.
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Old Dec 15, 2010, 7:03 PM   #5
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It does appear to have a front focus issue. Before you send it back, though, and since you had a similar problem with another lens, you could maybe check a couple other things to be sure. First, are you using auto or manual focus for the tests? Sometimes, when the diopter adjustment on the VF is not correct for your vision, it can look like the best focus is forward or back of the actual focus point. To adjust it, you should aim the camera at a blank wall, and adjust so the VF information is a sharp as you can get it. (if the diopter is incorrect, it will never be exactly in focus, but the best point can be wrong)
Next, try the same test with kit lens or other lens that you have which you haven't noticed a problem with. The smaller aperture may mask a problem with the camera, and if you haven't checked it, you could end up spending more time and trouble buying and returning lenses.

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Old Dec 15, 2010, 11:20 PM   #6
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Tried your suggestions brian and with the 18-105 no focus problems except if I try to focus on a blank point on a wall, then focus doesn't lock but that's the same with the sigma. However I can pinpoint focus on the chart and real performance with the 18-105. So I'll send it back.

I have a question if anyone can help.

There are 4 lenses that might address my needs for low light street photography.

1. 35mm 1.8 nikon (which I tried out but it was defective, however I had it long enough to know that it was a bit too telephoto for me)
2. 30mm 1.4 sigma
3. 28mm 2.8 nikon AF-D lens
4. 28mm 1.8 sigma

I think my personal preference is for the Nikon 28mm 2.8 lens but I can get that focal length with my 18-105mm at f/4.0. Is it worth the upgrade there for just 1 stop? I'm a bit iffy about Sigma since I just had the front focus problem with them.

An alternative might be going with a 28-75mm from tamron which is a constant aperture 2.8 as it would fit my requirements and all the way zoomed out would behave identical to a 28mm 2.8 prime.

What would you experienced people recommend?

Thanks,

Sandy.

P.S. I thought about the 35mm 1.8 again since its the best for the price, its tempting but it was just the wrong focal length for me. I know primes are all about walking around and composing but I want it wide enough that I can take a picture while sitting across a table of the people in front of me.
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Old Dec 16, 2010, 8:14 PM   #7
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I didn't mean you should attempt to focus on a blank wall, just point the camera, and look at the info readout in the viewfinder, to adjust the diopter on the viewfinder for best focus for your eye. (it won't affect autofocus)

As to lenses, I can't be much help, but for your stated purpose, I think you should be looking at something shorter than 28mm for the DX format. The Nikon Lens forum would likely be the best place to ask, as the members will have experience with the type of shooting, and lenses you are asking about.

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Old Dec 17, 2010, 8:20 PM   #8
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just to add a bit... I purchased the sigma 30mm 1.4 about a month ago for my pentax. I sent it back a few days later due to a LOT of front focus. It is a known issue that many people complain about with this particular lens. I decided not to ask for a replacement because of the huge majority of people with this problem.
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Old Dec 17, 2010, 8:30 PM   #9
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can i make suggestion that you retry your tests but not under tungsten lighting - it can often result in front focusing due to the lights frequency - might discover that under natural lighting that the lens works fine
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Old Dec 19, 2010, 1:45 PM   #10
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Sorry didn't see the reply. I did not shoot under tungsten. It was under auto. And I sent it back because it was hopelessly bad. I'm now cautious of tamron too and am thinking of just saving up for Nikkor lenses.
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