Infrared focus

[ulrich.von.lich]

Hello,

I wonder if there's a way to mesure the infrared focus precisely instead of using large DOF to cover it.

And is an APO lens exempt from the IR focus shift issue?

Thanks & Regards

 

[sol33]

You could use an M8 and bracket for focus. Or if you have the time you could make a table to record the correction factor for each focus distance.

I think all lenses experience focus shift.

 

[JohnTF]

I've tried IR from time to time for a long time, sometimes carefully using the IR mark to correct the focus. I have used orange, red, deep red, and the "opaque" (to visible light) filters + near IR, Kodak IR, and about four brands of IR films.

It often seemed the correction was not at all helpful.

I often was shooting something at distance, so I do not see much of a point there regardless.

I shot with a 30mm Fisheye and an orange filter (only thing I could find close to an IR filter of any sort) and was surprised at the effects using the old Konica 120 IR-- got the dark sky and white trees and the fisheye has great DOF.

The Orange filter fit behind the lens and was a Russian filter that came with the lens, which I used adapted to Mamiya 1000s in 645 format.

With the M8, I ordered an IR cut filter, but had trouble focusing with almost everything, so after years, am still experimenting. Some have said the filter is not a good one--

Sort of like pinhole, it involves degree of experimentation, and possibly a bit of time with Google and searching on RFF?

Perhaps I can try an IR filter with my pinhole on the M8?

With a Hasselblad, + expensive, probably at least a 720 nm expensive filter, the focus compensation seemed to be worse than no compensation. I think I was using mostly the 50mm FLE T*.

Hope something in here sheds some light in the "right" direction, I know there are folks who have been much more thorough than I in their experimentations.

I got away from the word "precise" in regards to IR.

Regards, John

 

[Blanco Dos]

Just use the IR mark on the lens barrel. But as John says, there is not much difference in real world terms except for very close, wide open 'bokeh' shots.

 

[pintpot]

Hi
Just move the point of focus ftom normal to between f4 / f5.6
on the DOF scale. This seems to work for me, but any older lens will have an IR focus mark at about the same plce on the DOF scale.

Tony

Dunham Park

 

[oftheherd]

The few times I used Kodak HIE, I found it not uncommon to have a little fuzziness. However, small apertures will help. I remember getting some pretty good examples of writing when experimenting with it forensicly.

 

[charjohncarter]

Just use the IR mark on the lens barrel. But as John says, there is not much difference in real world terms except for very close, wide open 'bokeh' shots.

I did this one wide open (f3.5) using the ground glass on one of my TLRs: real world:

 

[Chris101]

I did this one wide open (f3.5) using the ground glass on one of my TLRs: real world:

That's a beautiful picture CJC! What, if you don't mind my asking, film did you use?

 

[ulrich.von.lich]

Thanks for the advices.

I was expecting to know some military device that gives accurate focus and costs not too much...

I just checked: none of my lenses has the IR mark. Could someone please show me a picture or describe how far it is off the normal focus (for a 50mm lens)? Is IR focus always closer than normal focus.

I was told the necessity to use correction depended on the wavelength of the filter. The IR mark on most lenses is calibrated to 800nm, so applying it to a R25 filter would lead to misfocus. The filter I'm going to use is a 780nm one, so the IR mark should help.

 

[sevo]

I just checked: none of my lenses has the IR mark. Could someone please show me a picture or describe how far it is off the normal focus (for a 50mm lens)? Is IR focus always closer than normal focus.

I was told the necessity to use correction depended on the wavelength of the filter. The IR mark on most lenses is calibrated to 800nm, so applying it to a R25 filter would lead to misfocus. The filter I'm going to use is a 780nm one, so the IR mark should help.

The shift is generally towards closer to normal (hypothetical lenses with a overcompensating red correction might behave different, but I have never encountered such a beast - a Apo lens that errs into the opposite domain would be putting lots of extra effort into a detrimental effect), so that the mark is towards the far side. The exact spot depends on the correction of the lens, so there is no common value for it, but usually the IR mark is somewhere between the f/4 and f/5.6 DOF mark. True apo lenses have no shift, but some "apo" lenses are only calculated for the visible band.

Focus compensation was more relevant in the past, when there were films like Kodak HIE that went deep into the IR range at high sensitivity. Nowadays you only need to bother if you shoot digital, as no current IR film has significant sensitivity past 800nm - Ilford SFX and Rollei IR are already cutting off by 800nm, and Efke, while reaching deeper into the 800's, has so low a sensitivity to start with that operating it with strong filters will place you in reciprocity hell.

For current films, you'll be best off with nothing stronger than a 89B (730nm) filter, which will on most lenses do without focus correction at any f stop sensibly used with IR - you'll have to stop down in any case to get reasonable sharpness out of lenses operated outside their proper corrected range.

 

[ChrisN]

...Could someone please show me a picture or describe how far it is off the normal focus (for a 50mm lens)? ...

Here's what it looks like on the Canon 50mm f/1.2 rangefinder lens.

 

[former member]

APO and Mirror lenses in general can be used with IR without the need to refocus.

APO lenses are corrected for three wavelengths, one of them is usually in the RED. Achromats are corrected for two wavelengths.

Some "regular" lenses have a good enough color correction to be used without refocussing- the type 1 Rigid Summicron is an example. The IR index is within the F2 DOF marks.

 

[pintpot]

The I.R. point of focus mark was, in most lenses, midway between the f4 / f5.6 marks on the DOF scale. I use this and with the DOF you'll get at f4 onwards the focus should be fine. if you are using an I.R. cut filter from 850nm you will need a tripod as the exp. times will be long, so you could use a smaller f stop and get bigger DOF. I get by with a 695nm filter and can use that hand held at about 45th at f4 ISO 320 on my M8. If you are digital then a white balance of 2000 to 2200K will help the get the best I.R. shots.

Happy shooting!
Tony

 

[charjohncarter]

That's a beautiful picture CJC! What, if you don't mind my asking, film did you use?

Thanks, Chris, I used Efke IR820, Hoya R72 filter, F 3.5 @ 1/10, so tripod, develop HC-110h 14 minutes, 30 seconds initial agitation, and then 3 inversions each minute, this was full sun.

 

[JohnTF]

APO and Mirror lenses in general can be used with IR without the need to refocus.

APO lenses are corrected for three wavelengths, one of them is usually in the RED. Achromats are corrected for two wavelengths.

Some "regular" lenses have a good enough color correction to be used without refocussing- the type 1 Rigid Summicron is an example. The IR index is within the F2 DOF marks.

I recall I heard that, and it seemed to be that the T* Hasselblad lenses were in that category, at least the ones I used.

I think the only IR film I had in those days in 120 was Konica.

Will see if I can find the one IR shot I made with the Hasse and post it.

John

 

[ampguy]

With the Canon 28/2.8 ltm, the IR mark (and corresponding focal point with the R72 filter) is between normal focus and the 2.8 barrel marking.

 

[Chris101]

Thanks, Chris, I used Efke IR820, Hoya R72 filter, F 3.5 @ 1/10, so tripod, develop HC-110h 14 minutes, 30 seconds initial agitation, and then 3 inversions each minute, this was full sun.

Thank you for the information. I've shot a few rolls of Efke Aura 820. I dig the look, but hate the (lack of) speed. I've been thinking about mercury vapor sensitization for about a year now.

 

[Phil_F_NM]

Each lens is different with respect to the amount of correction as has been stated previously.
This last semester for my physics class, I used the M8 + 10.5cm Nikkor to demonstrate the difference that wavelength has upon focus.
Using that lens, wide open at 1.25m, the point of focus shifted back 81.2 mm when shooting through a 720nm filter.
The correction factor is about 6.5% of the distance that should be added for critical focus. That's just for the 10.5cm Nikkor. Other lenses run a range of correction from better to worse and only testing can tell you how your particular sample will behave.

and playing with an M8 and only IR to test lenses is just fun too...

Phil Forrest

 

[sevo]

With the Canon 28/2.8 ltm, the IR mark (and corresponding focal point with the R72 filter) is between normal focus and the 2.8 barrel marking.

Yep, but it is a 28mm. The focus offset is proportional to the focal length. On long lenses, it may be around or past the f/8 DOF mark, for the same reason.

 

[delft]

I've found no need to correct for the CV 90mm APO lanthar and the 21/4 color skopar when using Rollei IR and a 89B filter

Dirk