Photoclubalpha

I have a Sigma 18mm 3.5 that seems to create a purplish spot at the center of the image especially shooting against the sky or something. Is this normal for this lens? Will a polarizer help?
Thanks for any info.

Its a problem with the rearmost lens element not having a coating designed for use with digital cameras. Light is bouncing between the sensor's anti-aliasing filter and the rear element. I don't think you can do much about it. This a problem common to many lenses which were made originally for film cameras where the problem does not exist. A different (anti-reflective) coating is used on lenses designed for digital cameras.

Thanks for the info Roger! I guess I can only use it without issue on a film body then

There may some change with different apertures or when strong direct light is not entering the lens. I had a lens which sometimes but not always exhibited the problem - but it's while ago and I can't remember the details. Of course you may be able eliminate the spot in Photoshop

I had one of these lenses when I got the KM 7D. It was fine - sharp and generally good colour and contrast - on any 'normal' scene but produced a distinct sharply focused flare spot whenever a light source appeared in the pic or just out of the frame. It had to go, because too many pictures were spoiled by the flare patch.

David

Thanks for the response David. I might also try it for just night time slow exp shots.

That's why I sold mine - I tried some night time shote and every single light in the image was mirrored with a reflection...

David