Focusing LF in the Infrared Sprectrum

[tgtaylor]

Hi all,

Two quick questions:

What's the best way to focus LF using an IR filter? The 35mm and MF lens have an IR focusing offset, but those are lacking on LF since you focus with the bellows.

I have a Cokin Z007 IR filter on the way and intend to experiment first with a 35mm camera with Ilford SFX. However MF ad LF are my main camera's and both the Rollie and Efke products look promising and the latter is available in 4x5, 5x7 and 8x10.

Finally, your input on IR filters suitable for current IR film would also be appreciated. I purchased the Cokin filter because it is an 89B 4x4 filter that would work with all my lens, was relatively inexpensive ($69.95 with free shipping), and was available. The Hoya R72 was recommended most often in the literature but it and the B&W 92 are round filters which, with suitable step-up rings, would work for most of my lens except for the WA MF and LF lens that I have.

Thanks in advance,

Thomas

[David A. Goldfarb]

According to Ansel Adams' _The Camera_, for IR you should extend the bellows by 1/70 the focal length. If you can focus on both standards on your camera, I suppose you can mark a zero point and then an IR focus correction for each lens on the bed or the rail of the camera for the standard you don't usually focus with.

[vinny]

It depends on the lens. If its a modern lens, contact the manufacturer for specs and they'll tell you how much to compensate. In other words DON'T WORRY ABOUT IT!
This has come up before, try a search too.

[Daniel_Buck]

extend the bellows by 1/70 the focal length.

for those of us not very mathematically inclined, I think that might roughly translate to: "nudge the front standard forward, and stop down"

[David A. Goldfarb]

You could also make a little ruler with 1/70 the focal length marked for each lens you carry.

[Bryan Lemasters]

Focus shift is not nearly as great on Ilford SFX as it would be on, say, Kodak HIE. With the dark red filter you are still exposing some visible light. Focus as you normally would and stop down to f16 or 22. You should be OK. My 2 cents.

Bryan

[Marko]

Focus shift is not nearly as great on Ilford SFX as it would be on, say, Kodak HIE. With the dark red filter you are still exposing some visible light. Focus as you normally would and stop down to f16 or 22. You should be OK. My 2 cents.

Bryan

The longer the wavelength, the greater the shift. Ilford SFX is mostly getting the deepest red and barely scratching the IR, and the HIE is good past 900 nm, depending on the filter.

Using an Apochromatic lens should also help a bit, especially when stopped down.

[Ash]

By the looks of things, unless you're going shallow focus, I wouldn't worry.

If you've stopped down the lens to have the majority of the scene in focus, the offset for IR will be compensated for anyway.

[Brian Ellis]

I used a lot of 35mm and 4x5 HIE 4x5 before it was discontinued. I haven't used any of the other infrared films. With Kodak's infrared I didn't find that the focus shift was worth worrying about so I stopped bothering with it even when using 35mm lenses that had the red adjustment mark on them. Adams' advice was useless to me, I'm incapable of making adjustments in increments of 1/70th of the focal length of the lens. But if you feel compelled to make some adjustment, do what Daniel suggests. I did the "nudging" routine for a while before concluding that I didn't need to bother with it.

[David A. Goldfarb]

I've got my 4x5" set up and pointed out the window, so I just tried this little experiment--

Focus a 150mm lens at infinity, but leave room to back it up (i.e., set the lens a bit back from the infinity stop, if you've got infinity stops). 1/70 the focal length is a little over 2mm. Now back up the lens 2mm and what does the horizon look like? Now stop down, and how far do you have to go to get acceptable sharpness. I thought it was okay again at f:32.

Maybe some of that "infrared glow" is just being out of focus. It wasn't very hard to measure 2mm of bellows travel using an ordinary ruler.