Infrared Focusing Issue!

[JeffGreene]

You guys inspired me! Well, I went out and got an R72 filter and enjoyed taking some infrared shots, but I am unable to focus clearly. Things are crystal clear in the viewfinder and I am getting that wonderful white, cottony look, but it's blurry! I remember reading somewhere that there were some focus issues to be aware of in infrared photography, but I thought I would consult you guys who have been shooting infrared successfully. Any thoughts, suggestions or tips? Thanks in advance.

 

[brucek]

You've got two possible causes for the blur. First would be long exposure times. Second would be focusing issues. If you're lucky enough to be using a lens that has an IR focusing mark on it then all you need to do is to focus normally and then shift the focus to that mark before taking the picture. I'm finding that most lenses don't have that mark, though, so it's going to be a matter of guessing at the correct focus and/or using a small enough aperture to give you gobs of DoF so that everything you want is in focus.

I hope that helps.

Bruce

 

[Terao]

Its out of focus because IR is a different frequency to the visible spectrum. Think of it effectively making your camera short-sighted (or is it long, I forget my optical physics!) IR focus depends on the lens used and the IR sensitivity of the film (or in this case the sensor). If the lens doesn't have an IR focus mark there's no hard and fast rule except experimentation. At least this is free and instant with digital.

I assume you're shooting landscapes, in which case I'd recommend you stop down to increase the depth of field and use hyperfocal distance on a suitably wide lens.

 

[pfogle]

For infinity focus on my Elmarit 21mm, I have to focus the infinity mark on the barrel to between the 4 and 5.6 marks on the depth of field scale.

This should provide a starting point, then you have to experiment to fine tune it.

 

[ampguy]

You're going to have to focus a little closer. You can get some kind of guess head start by finding a picture of a RF lens of similar focal length and design that has the IR mark, and going from there.

Also there are books and websites that talk about making a template for IR focusing that you can tape to your lens. If you have a 40mm lens, or even 35mm, I can tell you what the approx offset is at several distances as my Rokkor 40/2 has the IR red index mark.

 

[JeffGreene]

Thanks again. You guys nailed the problem! It's definitely both. I was stopped wide open and I was using longer exposures without a tripod. Duh!

Anyway, I'll try to use all your great advice, and will post the results if I'm successful. Thanks as always!