green splotch in long exposures? (sigma 18-50)

[frozenintime]

hey there

i recently bought a sigma 18-50 for my nikon d50 and promptly left the country for a week.
everything seems fine with it, but when i went out with my tripod for some longer exposures, a truly odd green splotch (i don't know what else to call it) started showing up.  i'm attaching an image with one and a crop at 100% of the splotch alone.

the size and location of the splotch is not consistent. there were also a few images where i don't see one.  i was shooting RAW, at ISO200, exposures in the 8-10 second range generally, but i can't find any relation to shutter speed or aperture settings as to the size/appearance/etc of the splotch.

any ideas what's going on here?

thanks
b

[DarkPenguin]

Looks like lens flare to me.

[Richowens]

Did you also stick a UV filter on the front of the lens?
I am with the Dark One on this, the specular light in the upper right of the photo is causing flare, and judging from the color, is bouncing off the front element and flaring on the back of the filter.

This is just an educated guess, experienced many moons ago when I first started shooting.  

Rich

[frozenintime]

no filters or anything, and this is the first i've heard of 'lens flare' (i'm quite uneducated and underexperienced).

any idea why i've never seen this with the d50 kit lens (nikons pretty cheap 18-55)?

[DarkPenguin]

Have you put a light directly in frame before?

[frozenintime]

mr penguin - yeah, i've taken many photos with the sun or artifical lights in frame.

the sigma did come with a petal lens hood, which i didn't bring with me. a cursory web search for lens flare leads me to believe that this would mostly take care of the problem in the future..

[DarkPenguin]

Probably so.  The thing about the sun being in frame is that the general loss in contrast (veil flare?) can make it harder to see the flare.

But a hood will help immensely.

[howiesmith]

mr penguin - yeah, i've taken many photos with the sun or artifical lights in frame.

the sigma did come with a petal lens hood, which i didn't bring with me. a cursory web search for lens flare leads me to believe that this would mostly take care of the problem in the future..
[a href=\"index.php?act=findpost&pid=93364\"][{POST_SNAPBACK}][/a]

A hood will help with light sources outside the frame.  

It will not help with light sources in the frame.  Otherwise, the shade would be in the frame too.  All shades does is cast a shadow on the front of the lens.

[DarkPenguin]

No idea what I was thinking there.  Does it help for lights out of frame if you have a light source like that in frame?

[frozenintime]

so i guess my next question is - what contributes to a lens being more or less prone to lens flare?


looking back on images shot over the last year with the D50 kit lens (the rather cheap and often disparaged 18-55), there are many, many shots with direct light (usually artificial) in the frame.  i've never seen flare in these images.  now, with this new lens, there's a great deal of it.

is it well understood why that is?

[howiesmith]

so i guess my next question is - what contributes to a lens being more or less prone to lens flare?
looking back on images shot over the last year with the D50 kit lens (the rather cheap and often disparaged 18-55), there are many, many shots with direct light (usually artificial) in the frame.  i've never seen flare in these images.  now, with this new lens, there's a great deal of it.

is it well understood why that is?
[a href=\"index.php?act=findpost&pid=93610\"][{POST_SNAPBACK}][/a]

Big question.  

Lens coatings are a major factor (better coatings reduce flair).  

Light hitting the front of the lens, like the sky (use a hood) and lights in the frame (good luck).

More lens elements makes flair more likely.

Filters (added glass surfaces not accounted for in lens design).

Internal lens and camera designs (light bouncing around inside).

The list likely goes on.

[frozenintime]

howie - i see, thanks for the reply.
i guess i'm surprised that this relatively well respected sigma exhibits much more flare than the generally reviled D50 kit lens.  live and learn i suppose.

[howiesmith]

howie - i see, thanks for the reply.
i guess i'm surprised that this relatively well respected sigma exhibits much more flare than the generally reviled D50 kit lens.  live and learn i suppose.
[a href=\"index.php?act=findpost&pid=93714\"][{POST_SNAPBACK}][/a]

You're welcome.  Tests are usually run under rather ideal conditions that would minimize flair.

I have no other ideas why your lens would flair (if that is what this is) and why other less regarded lenses do not.  But a Sigma lens is produced for many cameras.  The kit lens may be produced for more specific cameras and internal reflections.

A Leitz lens produced during WW II was thought to be the sharpest lesns ever made.  However. it was useless during daylight because of flair.  It was used for night recon photography where flair was more easily controlled.