IR question

[Mark Glendenning]

Long-time forum lurker, first-time poster. So ... hi!

I don't have much experience with IR, but when I was in Valencia a few weeks ago I found myself with some time to kill and an IR filter in my bag, so decided to have a play. The shots aren't amazing but I didn't expect them to be (I didn't have a tripod, for starters) but I'm curious about the dot in the middle of the images - would this have been somehow caused by the long exposure? I've taken the odd IR shot before (see image4) but have not come across this. Would anyone be able to shed light on why it happened?

[miketoll]

Looks like a reflection off the inside surface of the filter of the taking lens at the moment of the actual shot with the aperture blades stopped down giving the six sided shape, if you follow my somewhat convoluted sentence.

[yelvertoft]

As Mike says, from the shape, it looks like it's a reflection of the blades that form the aperture.

You may find this thread useful:
http://www.worldphotographyforum.com...hread.php?t=63

[Mark Glendenning]

Thanks for the quick responses. What would I need to do to prevent the reflection from appearing in the future? Would a lens hood help?

And Yelvertoft, thank you for the link - I'd actually seen that thread already, but it's got some useful info in it.

[yelvertoft]

Mark,

Keeping the lens' front element and the filter scrupulously clean will help. A hood may help, removing other filters you may also have fitted such as a UV/skylight will help. Keeping the sun to the sides or behind you will help.

[postcardcv]

For some reason (whcih I don't know) some lenses produce hot spots like this when used for IR photography. If you google IR hot spots for yoru brand of camera you should be able to find a list of which lenses suffer with this problem. I use Canon and am interested in trying IR, here's a link to a Canon list - http://www.lensplay.com/lenses/lens_infra_red_IR.html

[miketoll]

Interesting link, thanks Peter.

[Don Hoey]

Mmmm ........ these are quite different from my hot spot experience.

I have a few lenses that do not like IR, but the hot spot is larger than in these images and does not show signs of the aperture blades despite being taken at f11. So more like the central area having considerably more exposure. This I believe is light bouncing back and forth from the AA filter on the sensor and the lens rear element.
The quality of the lens for visible light photography appears not to be a factor here so there may be something in the lens coating.

My Nikon lenses that hot spot on IR are 12-24 zoom, 24mm f2, and 28mm f2.8.

20mm f3.5 is fine on the 6mp D100 but hotspots on the 12mp D2X, yet the 35mm f2 is brill on both cameras.

Unfortunately I seem to have binned my tests but can re-do if wanted.

Don