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Any idea on what is causing this??

Here is one of my first test images with the lens, much higher contrast than today, early morning light striking the leaf.

SDIM0029.jpg


No evidence of the problem that i can see.

Cheers

Steve
 
I have been looking at what is similar in the images that display the problem.

I think i have narrowed it down to small apertures, in some of the boat house pics, i forgot to reset the aperture after i opened up the lens to focus, most of the problem shots would have been shot at f16 or f22. Does this shed any light on the problem?

Steve
 
Steve,

That would coniside with Steaphany and my statements above concerning the IR shots...when shooting IR you get hotspots with certain lenses...not all lenses do this...and even two copies of the same lens may or may not do this....

I don't know the technical reason why you get this...maybe something to do with how the lens bends the light to fit on the film or sensor produces this issue...(really have no need to technically know why...I just know it happens)...

When playing with my IR modified camera's when I get this I usually dial up the f-stop from 1.8 to like 4 then 6 or 8 and it usually gets rid of this issue...(again back to how the lens handles the light comming into it)...so I have played with my MF lenses and I keep a note book with each lenses pros to cons and what is needed to take sharp images with a particular camera.....(I am a little anal)....so when I grab a lens I quickly check my notes to re-familiarize myself with it...so I don't get any issues....

By the way...great shot above....

Tony C.
 
Section 4.7: Recognizing and Correcting Equipment Problems, Page 105, of "The Handbook of Astronomical Image Processing" by Richard Berry and James Burnell has a nice graphic explaining the origins of Hot Spots, Field Flooding, and Vignetting:

View attachment 1781

If the rear structure of the lens body is providing the reflective surface causing the hot spot, the sharp angle away from the dust protector/hot mirror normal may be why this manifests as being spectrally dependant.
 

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Steaphany is awesome...!!! :z02_respekt:


Thank you I actually wanted an answer to that question...tried looking one time but no-one could explain-it or show why it happens....

Tony C.:z04_cowboy:
 
BTW, my Sigma 28mm DG EX Aspherical Macro lens intermittently produces an IR hot spot dependant on exposure.

For some IR exposures, the image has a hot spot, at other settings there is none. I still have to run some specific tests to understand what's going on.

Understanding exposure dependant IR hot spots may explain why the same model lens used by different photographers yields IR images with and without hot spots.
 
Just an update for you all, Luis has spent some time looking at my images and believes it is stray light causing the issue. He recommended trying to keep the lenses at f5.6 - f8 to maintain maximum sharpness and faster shutter speed and minimise the chances of this problem happening again.

One of the things i want to start playing with, is night exposures, so Luis' recommendation of faster shutter speeds wasn't what i wanted to hear!

So with the 28mm mounted, i wandered down to a little church near me to shoot some high contrast night shots. this shot was taken at f8 and 10 seconds at iso 200, no hint of the problem :z04_breakdance.gif:

Here is the shot:

View attachment 1791


Cheers

Steve
 

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Steve nice shot...what does it look like in B&W...??? sorry sorta on that kick lately..:z04_yes:

Tony C.
 
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