IR focus shift.

[fidget]

Some lenses have a mark for the IR focus point. It seems that all of those that I want to use don't.
Is there a general rule for the IR point on a lens, sort of X% nearer?
I'm thinking of doing some more MF IRish images using Ilford SFX and want to get the focus a little better. I generally have to open the lens a bit to gain the 5 or 6 stops of speed, so lose some of the DOF.

Dave....

 

[projectbluebird]

Which lenses were you thinking of using?

Here's what I found in my old leica manual

"Images formed by infra-red rays focus slightly further back of the focal plane formed by visible light. To compensate for this the lens should be racked out about 1/200 of it's focal length, with the exception of the 35mm and 28mm lenses whose depth of focus takes care of the matter."

 

[fidget]

Hi, I will probably use a 6x6 folder with 75mm or 80mm lens or a 6x9 with 105mm lens. This way I can dedicate one of my folders to SFX.
It seems that when I get the odd chance at a suitable scene, I've not got the right film loaded.

 

[fidget]

Which lenses were you thinking of using?

Here's what I found in my old leica manual

"Images formed by infra-red rays focus slightly further back of the focal plane formed by visible light. To compensate for this the lens should be racked out about 1/200 of it's focal length, with the exception of the 35mm and 28mm lenses whose depth of focus takes care of the matter."

Just re-reading this I am not sure what this means.
Is it, if 50mm then extend the lens by 1/200 of 50mm = 1/4mm?

I wonder if this is the "rule of thumb" that i am looking for🙂

 

[HenningW]

Some lenses have a mark for the IR focus point. It seems that all of those that I want to use don't.
Is there a general rule for the IR point on a lens, sort of X% nearer?
I'm thinking of doing some more MF IRish images using Ilford SFX and want to get the focus a little better. I generally have to open the lens a bit to gain the 5 or 6 stops of speed, so lose some of the DOF.

Dave....

IR focus shift depends on various lens factors as well as the part of the spectrum being imaged. SFX is not that IR sensitive, so not as much shift as usual is to be expected.

As a starting point, I use the dof markings for f/4 or f/5.6 as a start. With a new lens and film, you always have to do a focus series. Take a picture at infinity and set the infinity mark to the dof marking for f/2.8, f/4, f/4-5.6 and f/5.6 at the apertures you intend using. Develop and check.

If you are using a lens like the 75 Summicron, you don't have to shift focus at all for SFX as it's well enough corrected into the near IR.

If you use HIE with an 87c filter, your starting focus points would be a little bit further up the scale and you should then go to f/8. You should also stop down a bit more since lenses aren't usually corrected that far into IR.

Henning

 

[HenningW]

If you use HIE with an 87c filter, your starting focus points would be a little bit further up the scale and you should then go to f/8. You should also stop down a bit more since lenses aren't usually corrected that far into IR.

The above is not particularly clear. What I meant is that you should set the distance measured at the near f/8 (or whatever) dof mark, instead of the central line or arrow indicating the point of focus. Ie, if shooting at infinity, set the infinity mark at the near f/8 dof mark.

Henning

 

[gnashings]

Here is my experience: if you shoot with a true IR filter (opaque to the eye), you should correct and the above advice is as good as I have seen anywhere. However, if you are shooting SFX or any IR film with a deep red filter, the majority of the light that exposes your film is of the visible variety. In that case, I find that you should focus as per visible light. I did a side by side test (not with an RF, but it demonstrates the principle) with a 100mm portrait lens at the same apperture, with a r25 filter doing a head and shoulders portrait. One was done with, one without IR focus correction. The one without was properly in focus. The one in which I used the IR mark was out. Again, this will not be the case with a full IR filter.

 

[fidget]

Thanks for the suggestions, I was aware that the shift for SFX + filter will be not so much as true IR. I wasn't aware that lens design meant that it could be quite different for lenses of similar focal length.
As an interesting point, I recently checked a bulk film (Agfa APX400s) for it's IR properties when I read about APX200s/400s (very confusing). To test for this I ran a series of exposures through the SFX filter with generous extra exposure. All the frames were blank. This makes me think that the filter passes light at least outside the range of some film. (But it worked well on an old digi that I had)


Dave.....