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05-29-2011, 12:47 PM   #1
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Sigma 50-150mm f2.8 AF doesn't work right in backlit situations?

I was taking some portraits last night of my brother-in-law with my Sigma 50-150mm f2.8 and the shoot was going well but when I had the sun behind him or a little to his right the lens wouldn't lock focus correctly. It would lock but it wasn't locked on him, he was blurry. This happened like 10 times, I had to manual focus sometimes but that is just as bad because I can't tell if he's sharp (he was around 20 feet away and I was at 70mm zoom). Anyone experience this with their Sigma 50-150mm f2.8?

05-29-2011, 01:06 PM   #2
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I have the same lens, and have been shooting with it for about six months. I haven't noticed a back-lighting focus problem, but with the weather just now getting nice, I've mostly just shot with it indoors.

Are you sure the problem was the lens, and not the autofocus system in the camera? Did you try any other lenses when you were having the problem, and did they give any better results?
05-29-2011, 01:36 PM   #3
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No Issues Here

I've used mine extensively and haven't seen this kind of behavior. I used it for an entire shoot against the sun with some brides in the water images and it was rock solid as always.

It might be your copy.
05-29-2011, 04:49 PM   #4
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I've only had mine a short time and so far it has performed flawless in every situation I've put it through. If you post some samples with the exif we might be able to get a better idea of what's going on.


Last edited by JHD; 05-29-2011 at 05:13 PM.
05-29-2011, 04:51 PM   #5
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All my lenses struggle in backlit situations against the sun, depends how extreme the situation is, if the sun is actually in frame then you're likely going to be MF'ing.
05-29-2011, 07:52 PM   #6
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Thanks everyone. The lens works great but this one was the first time I noticed this focus issue. My other lenses work fine.

Twitch - Your Sigma 50-150mm f2.8 struggles with this too?

I hope I don't have a bad copy.
05-29-2011, 08:08 PM   #7
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QuoteOriginally posted by crossover37 Quote
Twitch - Your Sigma 50-150mm f2.8 struggles with this too?

.
Sorry I should have bene clearer. I have a DA50-135 not a Sigma, but all my lenses struggle to find focus when the sun is in the frame. It's progressive though, no problem when it's a bit backlit to completely hopeless backlit and the sun directly in the frame. The AF in the camera can't find contrast when it's blasted by light.

No idea if that's what happening for you, but perhaps you could post a photo ...
05-29-2011, 08:38 PM   #8
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Most of my lenses also find it hard to find contrast enough to focus on - such is the nature of the AF system. And the longer the lens, the harder it will be to nail focus in those conditions.

05-29-2011, 11:23 PM   #9
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Ok I uploaded some of the images. The first set of pictures show how the subject is blurry, the camera said it focused correctly though. The second set of images is more in focus but not backlit as much...I haven't really experienced this issue before this.
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PENTAX K-5  Photo   
05-30-2011, 12:14 AM   #10
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QuoteOriginally posted by Ash Quote
Most of my lenses also find it hard to find contrast enough to focus on - such is the nature of the AF system. And the longer the lens, the harder it will be to nail focus in those conditions.
+1 to this statement
05-30-2011, 12:26 AM   #11
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QuoteOriginally posted by axl Quote
+1 to this statement
For this reason I tried focusing on his sweater too where the red lettering met the black part, it still won't work that way?
05-30-2011, 12:38 AM   #12
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It doesn't really matter where you are trying to focus, if you have strong lighgt shining down your lens, the chances are the AF system will go silly...there is simply too much light plus bouncing light reflecting from element to element etc. Resul is that contrast is lowered so much that AF simply struggles to lock

Plus, consider the size of the cross sensors. You might have the little red indicator where you want to AF but actual cross sensor is much bigger so it might focus somewhere else. In the first shot though, the jumper/shorts is better spot to AF anyway
05-30-2011, 12:41 AM   #13
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Here's something worth trying... zoom in tight, focus, and then pull back. Obviously this won't work with primes.
05-30-2011, 03:17 AM   #14
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QuoteOriginally posted by crossover37 Quote
I was taking some portraits last night of my brother-in-law with my Sigma 50-150mm f2.8 and the shoot was going well but when I had the sun behind him or a little to his right the lens wouldn't lock focus correctly. It would lock but it wasn't locked on him, he was blurry. This happened like 10 times, I had to manual focus sometimes but that is just as bad because I can't tell if he's sharp (he was around 20 feet away and I was at 70mm zoom). Anyone experience this with their Sigma 50-150mm f2.8?
Hi there! I too have the same lens and have no problems shooting under similar conditions as how you were shooting. Here are some shots indoor but against strong lights which were taken at a FL around 70mm, aperture of f2.8 and ISO800 with my K200D:










May I suggest that you do the following in sequence:

1) remove the lens from the body and then reinstall it;

2) focus on a close subject at different focal lengths to check if focusing is aligned;

3) focus on a distant subject subject at different focal lengths, then if focusing is correct;

4) focus on a subject which is backlit.

If the problem occurs again, well you may need to send in the lens for a check up.

Hope this helps.
05-30-2011, 03:27 AM   #15
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Um...mightn't the camera model be a relevant variable in this discussion?
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