Hey guys, I have been experiencing a particularly annoying problem lately. I have been using my sigma 180mm f/3.5 APO [IF] EX with a Pz-AF Uniplus 25mm extension tube* and at focus settings approaching 1:1 I get a rather nasty hotspot:
Sigma 180mm f/3.5 [IF] APO EX - 1/180th ISO 100 f/16 1:1.5 with 25 mm extension - wireless AF540 with sto-fen diffuser - lens hood used, No filters.
It only seems to show up on occasion - it is easier to see when I am using flash, even using ring flash can cause the hot spot to rear its ugly head. The Hotspot doesn't appear when I use my Pentax FA*200mm f/4 ED Macro lens so I'm thinking it is an issue specific to this particular lens/extension tube combination. As more than a few of us are aware Sigma's lenses aren't exactly well known for being resistant to flare. I have used this extension tube with the FA77 and a loaned Carl Zeiss 100mm f/2 Macro for Pentax K mount without issue.
* The reason why I use this particular extension tube is because it transmits lens focal length correctly, as well as aperture information and it also allows for full P-TTL flash functionality.