Yashica Mat and Infared - focus adjustment?

[EASmithV]

Hello all,

I am considering making my Yashica Mat an infrared only camera, or at the very least shooting a good deal of infrared with it. I've shot about 3 rolls so far, and they look great, especially portraits... However, I have a problem. Shooting wide open at 1/60th is often my perfect exposure, however, that gives me little or no depth of field and at close distances the focus error becomes evident. I know I could use a tripod and stop down, but I'd ideally like to keep the ability to handhold. Is there a way to adjust for shooting infrared? I don't see a dot on the focus knob. Also, would it be feasible to have someone shim the GG for infrared? Or would it be easier to just back focus all my shots a bit? How much, if so? I want to be able to go all analog with my shots but it seems they need heavy sharpening using alternative techniques to get good images...

thanks

 

[summicron1]

infrared marks don't seem to be as common as they once were -- i found one on my 50-year-old summicron 50, but not on the rolleicord or yshicaflex.

Where it is found it is usually a titch to the right of the zero point so that when you move to it, yu actually focus a bit closer to allow for the different focus point of the infrared light. Perhaps lenses are better correctd and the mark is no longer needed? Or only needed for longer lenses where the depth of field is more critical?

I'd do a test, shoot half a roll focusing dead on, and half a roll adjusting each shot 1 mm to the right (closer) on the focusing dial and see what comes out.

 

[LiamG]

You could try finding a Rollei Infrarot filter which is supposedly optically compensated for the focus shift on a TLR; they should exist in Bay I. Changing the relative focus of the viewing lens might work in theory, but it would be quite a delicate operation, and would certainly require a collimator.

 

[EASmithV]

Wait.... Rollei made a focus shifted corrected filter thingy?! Does it work? What is the IR cutoff?

 

[Dan Daniel]

The Minolta Autocord had a red arrow on its focus lever for infrared focus. I made a composite of shots form someone who asked about this. It might give you a good starting point as to what the compensation is?

http://www.flickr.com/photos/18067251@N04/4339520944/in/set-72157624555271098

 

[DWThomas]

I have used my 124G with an 89B (695 nm cutoff) and didn't really worry about it; unlike the earlier IR films, the light spectrum for today's IR films barely extends outside the visible spectrum and as such I don't think compensation is that important. Now I admit that is an opinion, not carefully researched fact.

That said, I think on many lenses the IR adjustment amounts to using approximately the next depth of field mark toward closer focus as the indicator mark.

 

[EASmithV]

I have used my 124G with an 89B (695 nm cutoff) and didn't really worry about it; unlike the earlier IR films, the light spectrum for today's IR films barely extends outside the visible spectrum and as such I don't think compensation is that important. Now I admit that is an opinion, not carefully researched fact.

That said, I think on many lenses the IR adjustment amounts to using approximately the next depth of field mark toward closer focus as the indicator mark.

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.

 

[Alex Muir]

I managed to find the Rollei IR filter in bayonet 1 size for my Rolleicord. It was new and came from Mr Cad in Croydon England. It was expensive( about 50GBP). It is quite opaque, like an IR720 and gives good results with the Rollei film. I have heard that it is "focus-corrected" but I have never seen this in any Rollei publication. I have used it for landscape, and therefore have not seen the effect with the lens at large apertures. The shots I have are in focus without making any adjustment. In many ways the TLR is the ideal IR camera as you can work with the filter in place and see to compose/focus.

 

[LiamG]

I managed to find the Rollei IR filter in bayonet 1 size for my Rolleicord. It was new and came from Mr Cad in Croydon England. It was expensive( about 50GBP). It is quite opaque, like an IR720 and gives good results with the Rollei film. I have heard that it is "focus-corrected" but I have never seen this in any Rollei publication. I have used it for landscape, and therefore have not seen the effect with the lens at large apertures. The shots I have are in focus without making any adjustment. In many ways the TLR is the ideal IR camera as you can work with the filter in place and see to compose/focus.

The 1960 edition of Rollei: The Practical Accessories mentions "ground-in focus compensation." What that means in practice, well, who knows.

 

[Alex Muir]

I have found some information on the Rollei IR filter which confirms the cut-off value as 700m/u. Unfortunately, this source gives no more information to clarify the effect of the focus correction.

 

[DWThomas]

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:

 

[EASmithV]

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.

 

[AgX]

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

 

[Ian C]

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=71...red focus view camera leslie stroebel&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.

 

[AgX]

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.

 

[MattKing]

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.

 

[DWThomas]

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.

 

[AgX]

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.

 

[EASmithV]

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.

 

[AgX]

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.

 

[DWThomas]

In my case I run all sorts of film through my TLRs, I would rather just make an alternate focus mark or scale than tinker with the basic calibration. My Yashica Mat 124g has knob focus with a depth of field scale where one could add a sticker or a stroke of white paint or just visually gauge by one of the existing depth of field indicators.

The ancient Flexaret I have has a swinging lever below the lens on the front for focus. That might be a bit more challenging, but I imagine I could come up with an overlay for the focus scale (if I was actually using the camera!)

Having shot a few rolls with the 124g this past summer, I will say TLRs are nice for IR work since you don't have the opaque filter in the viewfinder path.