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12-09-2010, 07:44 AM   #1
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Long lens - what effect is this?



Hi, above is a test shot of my cheapo lens Sigma A400/5.6 , taken @6.7 from 100 yards, no filter.
Not bad for the price, I think.

However at 100% crop it seems to be spraying yellow paint all over the sign.
What is that due to: flare? diffraction? lens resolution? or something? Thanks.



12-09-2010, 07:53 AM   #2
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Do you want to say that in reality the sign is white?
12-09-2010, 07:59 AM   #3
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QuoteOriginally posted by blende8 Quote
Do you want to say that in reality the sign is white?
No no, the sign is yellow letters on blue as shown, I'm just curious about the yellow dots/artifacts on the blue blackground where they shouldn't be. Thanks
12-09-2010, 08:10 AM   #4
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I note a slight border/halo around the letters; this may be due to slightly wrong focus and/or camera shake. Else there is some chroma noise within the blue.

12-09-2010, 09:34 AM   #5
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Looks like chroma noise to me as well. Part of the reason is the soft sidelighting. The side of the building and part of the tree is yellow as well.
12-09-2010, 09:34 AM   #6
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I don't think the lens is defective. Was the shot hand held? If so it is pretty hard to shoot that focal length hand held without getting at least a bit of blur and even if that is the case, that 100% crop is not that bad. Maybe a bit more blur than there should be be again I do not think it's a defective lens. Try it on a tripod and then decide if it's the lens or the photographer.
12-09-2010, 10:47 AM   #7
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The halo effect around the cropped detail looks very similar to what I get from a Sigma 170-500 at the 500mm end which seems to be just the way the lens is, soft.

As hard as I try, I cannot seem to get better IQ at a detail level. Old Takumar showed it up. Link to thread https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/117740-image-q...170-500-a.html

On your image there seems to be noise added to the halo as well as the rest. What was your ISO?

Perhaps just a soft effect of less than perfect glass??
12-09-2010, 10:52 AM   #8
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My guess is a combination of noise, focus and movement.

12-09-2010, 01:03 PM   #9
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And possibly the lens is not top class.
12-09-2010, 01:52 PM   #10
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Every lens has a sweet spot, just got to find it...

The yellowish tint might be due to the time of day the photo was taken, it looks from the shadows, it's either right after dawn or before dusk, which will be a little more yellow. And when I look closely, is that grass or astro turf in the lower left side of photo. The light reflections off this surface maybe causing some color abnormalities.

I think the photo looks pretty sharp and would clean up with PP easily. Is this a converted RAW or a JPG? If this is a handheld shot, bravo! At 400mm that's pretty steady...

I'd take the lens

Last edited by theunartist; 12-09-2010 at 02:10 PM.
12-09-2010, 08:53 PM   #11
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Thanks everyone for your thoughts.

I forgot to mention it was ISO 200 in bright light unlikely iso noise, no tripod but pressed hard against a tree & ~1/500sec unlikely due to shakes, f/6.7 from 100 yards the dof must be several metres I don't think I could have missed focus.

I will read more about halo and chroma noise as suggested. Is there anything we can do to minimize these effects?
12-10-2010, 11:36 AM   #12
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I think the resolution of long telephoto lenses is typically much lower than lenses in the normal range. That is certainly true of long telephoto lenses that I can afford. I have a Tokina 400/5.6 that's a similar vintage to your lens, and that's pretty much what I get.

You might make sure that your rear element is clean. On most long telephotos, the rear element is a long way from the lens mount. Dust that gets down there tends to stay. I think a speck of dust on the element becomes greatly magnified by the time the image gets to the sensor.
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