Yashica Mat and Infared - focus adjustment?

Quote:

---

Originally Posted by EASmithV

I'm shooting handheld ir portraits and they are focus critical, I'm getting clearly unsharp results at close distances. I'll try the dof trick. I wish ir film wasn't more expensive.

---

It just occurred to me to wonder what sort of shutter speeds you wind up with? I might suspect some of the focus problem working hand held could be camera motion given what I recall of my exposures; ISO 6 or whatever is not perzactly fast. Of course, I was using a tripod. Maybe a couple of test shots with a solidly anchored camera to isolate variables would be useful. Shoot a yardstick angled at 45º to the lens axis; focus at the middle and see where the results fall.

It does feel sort of perverse that the film that seems to need the most experimentation costs substantially more than the ordinary stuff! :confused:

Quote:

---

Originally Posted by DWThomas

It just occurred to me to wonder what sort of shutter speeds you wind up with? I might suspect some of the focus problem working hand held could be camera motion given what I recall of my exposures; ISO 6 or whatever is not perzactly fast. Of course, I was using a tripod. Maybe a couple of test shots with a solidly anchored camera to isolate variables would be useful. Shoot a yardstick angled at 45º to the lens axis; focus at the middle and see where the results fall.

It does feel sort of perverse that the film that seems to need the most experimentation costs substantially more than the ordinary stuff! :confused:

---

Bright sun or sun filtering through leaves I can shoot handheld f/3.5 at 1/60 or 1/30 if I feel I need more. I have shot sharp results handheld at 1/15th though. I've been getting away with around ISO 8, but lately I haven't even been metering, just shooting at 1/60 and wide open for the most part.

There are listings and formulas around to correct the focus-difference depending on wavelenght andf model of lens.

I simple rule-of thumb-formula for short distance exposures is this:

IR focus-setting = visible light focus setting x distance lens (shutter)/film plane divided by focal lenght

On page 222 in View Camera Technique, Leslie Stroebel, 7th Edition, Focal Press, the author recommends focusing, and then increasing the lens-to-film distance by 1/400th to correct the focus for infrared film.

http://books.google.com/books?id=71z...roebel&f=false

This might be difficult to implement on some cameras. It can be done more easily for finite subject distances by working with the subject distance instead of the image distance.


Let

s = lens-to-subject distance

f = focal length of lens

Then the corrected infrared subject distance is

s’ = 401sf/(s + 400f)


Example: s = 3 meters = 3000mm and f =75mm

s’ = 401*3000mm*75mm/(3000mm + 400*75mm) = 2734mm


For several distances we get the following combinations:

s = 1.5 meters, f = 75mm, s’ = 1432mm

s = 1.75 meters, f = 75mm, s’ = 1658mm

s = 2 meters, f = 75mm, s’ = 1880mm

s = 2.5 meters, f = 75mm, s’ = 2313mm

s = 3 meters, f = 75mm, s’ = 2734mm

s = 3.5 meters, f = 75mm, s’ = 3142mm


With no infrared film to test this, I cannot confirm its usefulness. The above formula is equivalent to Mr. Stroebel’s infrared focus-correction recipe.

Quote:

---

Leslie Stroebel recommends focusing, and then increasing the lens-to-film distance by 1/400th to correct the focus for infrared film.

---

Leitz once recommended a correction in the range of 1/200 to 1/300.


As there are quite a bit recommendations out there, the best way would be to fix several markings (strip of millimeter lined paper or so) for a IR-focussing mark at the barrel and to do test exposures at a critical target.

Any correction you might make would be dependant on the wavelength of the infrared or near infrared light your film is sensitive to.

This is the reason that the adjustment is much smaller for the current films (with a peak sensitivity near 720nm) then with the old HIE (with a peak sensitivity nearer 850nm).

The formula for the adjustment is out there.

Yes, I concur with the comments from AgX and Matt. A bit of testing surely would make the most sense. And shooting at or nearly wide open means there is not much depth of field to cover error either.

Quote:

---

Originally Posted by MattKing

This is the reason that the adjustment is much smaller for the current films (with a peak sensitivity near 720nm) then with the old HIE (with a peak sensitivity nearer 850nm).

---

It's not necessarily the peak spectral sensitivity we are after. One of those films in question goes even up to about 800nm, though with much, much less sensitivity. But we can use that by very strong filtration.

Guys...
The math is great (and extremely good to know, and try) if i'm shooting still life or landscapes or LF but it won't fly on a model shoot with a TLR.
I think looking into that Rollei IR "focus corrected" filter may be the best option yet.

Quote:

---

Originally Posted by EASmithV

Guys...
The math is great if i'm shooting still life or landscapes or LF but it won't fly on a model shoot with a TLR.

---

Once you have established the IR-focus mark (by whatever formula or by test exposures), you only have to shim the groundglas (or the viewing lens) to that new focal plane. From then on you can use your TLR as before:


Establish that new focusing mark.
Focus a target of any kind visually without filter.
De-focus the taking lens so that its focusing mark alines with that IR-mark.
Shim the groundglass or de-adjust the viewing lens until your groundglass image is sharp again for that test target.

Now you got a IR-only TLR (of course do not forget to mount that IR taking-filter).

In case you did the method of de-adjusting the taking lens, you might be able to adjust that lens between two marks. One for visible, one for IR-light, then you even got a dual purpose TLR.