Focusing with SFX film and SFX/R72 filter in a SLR camera

[pentaxuser]

I have done a search of the forums and opinions differ somewhat. Some say that it is as good as impossible to see through a R72 filter, although at least one member says that looking through a R72 for about 30 secs reveals the scene enough to focus. However this was the intense sun of Australia and not the much weaker sun of the U.K.

At least one other has mentioned that his autofocus camera will work with a R72.

So does the AF system effectively "see" sufficiently well to focus even if the eye fails to do this and is it possible for the eyes to acclimatise to the R72 filter sufficiently well to allow for manual focusing after waiting about half a minute.

I am aware that both autofocus and manual focus are perfectly possible with a red 25A but if the "wood effect is desired then the 25A really won't "cut it"

Thanks

pentaxuser

 

[Neal]

Dear pentaxuser,

I use a B+W 092 filter for most IR photography and I can easily focus manually with it under the type of conditions one would want to use IR. I have not attempted to use my autofocus cameras with this filter as I find a tripod necessary for good results. I find SFX gives a pleasing result hand held with a B+W 091 and I would be surprised if the autofocus did not work in bright sun.

I hope the above is of some value.

Neal Wydra

 

[pentaxuser]

Thanks Neal. I'd have to re-read the info on the various filters but I will take it that a B+ 091 is the equivalent of a Hoya R72 which I know to be the same as an Ilford SFX filter.

What attracts me to the SFX film is that while it is extended red only and not full IR it does give a good "faux" IR effect with a SFX filter and seems to be the only IR film left that is just about handholdable.

pentaxuser

 

[HowardDvorin]

I use Cokin filters and tripod. Focusing is done without the filter and then I slidethe filter into the holder.

Typically,I use a 24 mm ,28 mm or a 35mm lens. Set at F/8 or f/11 gives great depth of field.

Then bracket.

That's it.


HowardDvorin

 

[polyglot]

I have a chinese-generic R72. My Minolta 5 could focus through it, and I could see through it with full sun (or at least f/11-light = EV14) and an f/2.8 lens. However, 99% of the time I would just set f/16 and hyperfocus and because there's generally a tripod involved, focus before putting the filter on.

 

[Tony-S]

I have done a search of the forums and opinions differ somewhat. Some say that it is as good as impossible to see through a R72 filter

I thought 89b (Cokin 007) was the filter of choice for SFX 200? I only use it on my Bronica GS-1 so I can't comment on autofocus lenses.


Mission Mountains, Montana 2

 

[pentaxuser]

TonyS I think that you are saying that through the 89b you can manually focus. I ask this as several have said that while focusing with the filter on is/may be possible they focus first and then apply the filter.

Clearly this method may be more foolproof than with the filter on and is simpler if the filter is a drop-in to the filter holder but I had hoped to use a screw-in type which isn't so easy and may alter the focus setting.

Nice shot by the way. Is the foreground green grass that has the silvery-white "wood effect" and not say ripe wheat that can look quite white even with an ordinary red filter?

Thanks

pentaxuser

 

[Maris]

I have got the focus badly off on my Mamiya RB67 using the 360mm lens wide open with an IR720 filter on the front. The very dark red image in the viewfinder is hard to see but if there is a specular highlight on the subject I can reliably focus on that....but it's wrong! The light that the eye sees, dark red, is mainly not what the film sees: infrared. The two species of electromagnetic radiation focus in different planes; at least they do on my Mamiya lenses.

Now I focus without the filter, stop way down, add the filter, and hope. So far so good.

 

[Bill Burk]

Hi pentaxuser,

Did you know about the red mark on the lens barrel? At least all my old Super-Takumar and SMCT lenses had them. It's an infrared focus mark. You should focus optically (without a filter), make a note in you mind where the feet/meters focused and then rotate the focus away from infinity to line up where you focused onto that red line.

 

[Tony-S]

TonyS I think that you are saying that through the 89b you can manually focus. I ask this as several have said that while focusing with the filter on is/may be possible they focus first and then apply the filter.

Yes, this is what I do.

Clearly this method may be more foolproof than with the filter on and is simpler if the filter is a drop-in to the filter holder but I had hoped to use a screw-in type which isn't so easy and may alter the focus setting.

I see where the could be a problem. I use a Cokin filter holder, so it's not an issue for me.

Nice shot by the way. Is the foreground green grass that has the silvery-white "wood effect" and not say ripe wheat that can look quite white even with an ordinary red filter?

I can't recall, actually. It was in August so I suspect it was turning white at that time.

 

[pentaxuser]

Maris, it was actually your much earlier post on another IR film and filters thread to which I was referring when I said that at least one APUGer had said that it was possible to focus through a R72 in the intense Aussie sun if you wait about 30 secs .

Did the focus problem only arise with that particular lens and was the film a true IR one? As Bill Burk has said there is a difference in the focus point with true IR film but as others have said and hopefully correctly, this problem goes away with SFX film as it is extended red and not true IR.

For that reason, as well as its speed and ease of loading in subdued light, it is Ilford SFX in 120 that I am tempted to try.

pentaxuser

 

[Maris]

Maris, it was actually your much earlier post on another IR film and filters thread to which I was referring when I said that at least one APUGer had said that it was possible to focus through a R72 in the intense Aussie sun if you wait about 30 secs .

Did the focus problem only arise with that particular lens and was the film a true IR one? As Bill Burk has said there is a difference in the focus point with true IR film but as others have said and hopefully correctly, this problem goes away with SFX film as it is extended red and not true IR.

For that reason, as well as its speed and ease of loading in subdued light, it is Ilford SFX in 120 that I am tempted to try.

pentaxuser

After some hundreds of exposures on Ilford SFX film behind an IR720 filter I've built up a personal mythology about it and a bunch of opinions. Here goes:

SFX looks very much like Ilford HP5+ except it has an IR sensitising dye added.

The infrared sensitivity is about 5 stops less than the panchromatic sensitivity so exposures without a filter look "ordinary". With an IR720 filter on the lens I set EI=6 on my lightmeter and hope there is a decent correlation between visible light and infrared.

The Wood effect, luminous foliage for example, starts at about 730 nanometres wavelength and Ilford SFX certainly works up to 750~760nm so with the right filtration and the right subject matter it resembles a true infrared film. But it doesn't match the wonderful IR sensitivity of Kodak High Speed Infrared, nothing does.

The IR 720 filter I use is a dyed-in-mass high pass filter (not a dichroic) so it leaks a tiny bit of red light at about 680nm. That whiff of red light is what my dark adapted can use for focussing, sometimes. SFX is sucking up infrared at 720nm and beyond and not much of the the red light I'm focussing with. There is a small focus shift between 680nm and 720nm+ and a non-apochromatic lens offering shallow depth of field (eg Mamiya 360mm wide open) will find it.

Shooting Tmax 100 behind an IR720 filter delivers an image (try 5 minutes @ f4.5 on a sunny day) but the negative looks like an ordinary red filter job, not infrared. I guess this proves I can't really see infrared and neither can Tmax 100.

The only precaution I've used for loading SFX rollfilm in broad daylight is to do it in my own shadow; no fogged frames ever.

 

[keithwms]

Focus ~10% closer and stop down a bit more for good measure; that will be sufficient for most of the current film/filter combinations.

If you rig an apo process lens on there, chances are good that it'll not need any refocus. I proved this some time ago on an LF field camera using a wide-open lens and fairly liberal tilt and nailed the focus with no fuss.

 

[Tony-S]

Focus ~10% closer and stop down a bit more for good measure; that will be sufficient for most of the current film/filter combinations.

I'm pretty sure with SFX this will throw it out of focus. It is not IR film and thus does not need focus changed from the visible.

 

[keithwms]

I'm pretty sure with SFX this will throw it out of focus. It is not IR film and thus does not need focus changed from the visible.

Yep, experimentation is definitely necessary. It really depends on the filter used, and unfortunately there doesn't seem to be much agreement between the different filters. Anyway, with my #87, the prescription is about right. It does vary with glass and filter though, of course.

N.b. the 87 is quite a bit deeper than the 72 but actually the naming convention is confused between different manufacturers and seldom gives what one might think. 87 does not mean 870 nm cutoff, nor does 72 mean 720nm... alas we just have to try and see what we get.

Anyway, what I keep saying and too few people are believing is that the old apo process lenses usually don't need any refocus when shooting the current IR films... or UV, for that matter.