Weird IR reflections

I recently purchased a Hoya RM-72 IR filter to experiment with. I like the effects I can get after post processing, however, I have noticed a reflection in the very center of the shots with it. I'll post an example below. The reflection disappears for the most part once I do my post processing, but I'm still not quite sure what's causing it. The camera gear is a 6D with Sigma 35 f/1.4 Art, and the shot settings are ISO 800, f/8, 25s. I haven't been able to see if it appears on shorter exposures as I haven't been able to go much lower than a 20 second exposure at noon on a sunny, cloudless day.

The circle reflection is visible in the center of the first attachment. It is in the same spot and has the same intensity regardless of the position of the sun, cloud cover (direct vs indirect light), and it almost always clears up after post processing. Does anyone with experience with this filter have any input on what it is and how to mitigate it?

Sounds like it is a hotspot with the lens when used with IR light.

adam8080 wrote in post #16440054
Sounds like it is a hotspot with the lens when used with IR light.

Interesting, I wouldn't have guessed that. So to test this, I'd need to use it on my other lenses. I'll be sure to report back. Thanks!

adam8080 wrote in post #16440054
Sounds like it is a hotspot with the lens when used with IR light.

this.

Some lenses are worse than others about generating hotspots. Somewhere there was a list of lenses ranked by whether they generated a hot spot when used on an IR camera or not. If I can find it I'll post it up.

Also a note, I'm not using an IR converted camera.

Thanks for the help guys!

Here ya go.

http://dpanswers.com/c​ontent/irphoto_lenses.​php

http://www.kolarivisio​n.com/lenshotspot.html

imsellingmyfoot wrote in post #16440067
Also a note, I'm not using an IR converted camera.

Thanks for the help guys!

your still photographing IR light.

Scatterbrained wrote in post #16440076
your still photographing IR light.

Correct, but they way I read your response made me think that we weren't on the same page. Thanks for the links, I've got some reading to do now.

try stopping way down F7-F16. IR hot spots are caused by several things. The coatings on the lenses do not transmit ir properly so ir light builds up between the elements. Your sensor still has it's highpass filter so it is reflecting ir light off of the front element. Stopping down helps to reduce internal reflections and it also makes it easier to get the ir light into focus. Because most lenses are designed to focus RGB, it is a little more difficult to get ir into focus because the focus range is narrow and the viewfinder will be very dim. What bandwidth filter are you using? 950nm on my full spectrum T2i is VERY dark. I cannot see through this filter with my eye in broad daylight. 500-750nm is not to bad. I can shoot these types of filters handheld with the T2i, and performance is not too bad with a tokina 11-16mm. 800-950nm is usually were things get a little tricky and hot spots are bad even when you stop down the troublesome lens, forcing you to use one of the so called "recommended lenses".

calypsob wrote in post #16464804
try stopping way down F7-F16. IR hot spots are caused by several things. The coatings on the lenses do not transmit ir properly so ir light builds up between the elements. Your sensor still has it's highpass filter so it is reflecting ir light off of the front element. Stopping down helps to reduce internal reflections and it also makes it easier to get the ir light into focus. Because most lenses are designed to focus RGB, it is a little more difficult to get ir into focus because the focus range is narrow and the viewfinder will be very dim. What bandwidth filter are you using? 950nm on my full spectrum T2i is VERY dark. I cannot see through this filter with my eye in broad daylight. 500-750nm is not to bad. I can shoot these types of filters handheld with the T2i, and performance is not too bad with a tokina 11-16mm. 800-950nm is usually were things get a little tricky and hot spots are bad even when you stop down the troublesome lens, forcing you to use one of the so called "recommended lenses".

I'm using a 720 nm filter. When I hold it up to my eye, I can barely see a few highly IR reflective things after my eye adjusts for a few seconds.

Then it should be fixable by stopping down to f11 for sure

I agree with calypsob, f11 should do

Life Pixel had some info on the hot spot problem and lenses that were prone to it. If I remember correctly stopping down makes it more pronounced. Just sent them my 50D today for conversion.