IR photography; focussing problems.

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Had a play about with my new Hoya R72 this weekend, on an A80. Got some decent shots, but having a few issues and wondered if anyone can shed some light on them?

Out of about 35 shots, 2 were sharp, well focussed and came out very nicely. However using the same technique the rest were all blurry to varying degrees. My workflow was as follows:

Set up tripod, attach filter, set custom white balance using grass.
Remove filter, compose shot and auto-focus then lock the focussing point, by pressing "MF"
Re-attach filter, set up exposure time at f/2.8
Take shot using 2 sec timer to eliminate any camera shake.

Test shots without the filter are sharp as expected, so the focus seemed to be right before attaching the filter. Any ideas on how to improve my technique? My current thoughts are to try auto and manaul focus through the filter, but not sure weither will be very successful, but I wonder if the filter is causing refraction, effectively changing the optical path and messing up the focus?
 
The error in your technique is quite simple : you need to focus closer. SLR focusing systems won't account for the focusing error you encounter when using IR filters, simply because they cannot focus with a filter attached. Compacts can...

So how do you focus? I actually tend to focus completely manually... guessing focus distance, but that's because I use a rear polyester filter on the lens I do most IR with, and it's a right PITA to remove it to compose.

Basically you just need to focus a lil' bit closer. "Decent" lenses have an Infra Red focusing marker on the focus ring, it'll say something like "R" or "IR" - if you're lenses have one of these, then just focus without the lens, flip to MF, and then adjust the focus distance so that the "measured" distance is in line with the infra red marking.

If you're lenses don't have this marking, then just approximate. Go outside and take a photo of a tree with the IR filter, first of all using the autofocus measurement and then gradually moving the focus distance closer. Then go and see what's sharpest on a PC - that way you can "get to know" your lens, and make the right adjustment without the need for barrel markings.

Hope that's of some help ;)
 
The problem you're getting is possibly due to the fact that infrared light having a longer/shorter (can't remember which) wavelength than the visible spectrum is bent more/less (again can't remember which) by the glass in your lens. Upshot of all this is that when the visible light image looks sharp and well focused you're actually focusing too long/short for the infrared light. I guess there's two ways round this:

1) Use a smaller appature (i.e. bigger f number). This will get you a larger depth of field and should get round the problem
2) Manually adjust the focus to account for the extra/less strong focusing required. Some slr lenses actually have a little red line on the panel that indicates focusing distance to show you what the effective focusing distance for ir is.

To be fair though if you're taking landscape shots of objects quite a distance away (>10m) the effect of playing with focusing distance will be quite small so you might want to rule out problems such as camera shake first.

edit: doh beaten to it!
 
Hmmmm i've just re-read your post and realised you're using a powershot A80...

In that case, then, I think the issue is that you're focusing with the IR filter off. Most compacts can focus through IR filters, especially the less dense R72 - give it a go.
 
Thanks for all your input: next time i'll try focussing through the filter and see what happnes. I also though that setting the aperture at f/8 to increase DOF might help, as might setting the camer a to it's hyperfocal length at f/5.6 was also worth a shot.

I'll try all three and see what the results look like.
 
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