PDA

View Full Version : Checking film position?


mpirie
21st August 2009, 05:01 AM
Hi guys,

Does anyone have a reliable and accurate way of making sure the ground glass screen and film lie in exactly the same position relative to the lens?

I've heard of various situations where (for example) a standard GG has been replaced by a "better" one but because of shimming etc, the two may not have had the same position as the film when inserted.

I know Barry Thornton suggests a wall at 45 (ish) degrees to the lens/film plane which is marked with vertical lines with one being a central point of reference. But this approach would be very consuming of film for LF cameras.

I guess there are no laser tape measures that would be accurate enough to use?

Anyone resolved (no pun intended) this problem before?

Thanks,Mike

Dave miller
21st August 2009, 06:02 AM
Using the method suggested by Barry Thornton will confirm the accuracy, or otherwise of the screen placement, and only require one exposure. Other than that I can only recommend careful measurement from the front standard with the lens removed.

Bill
21st August 2009, 07:32 AM
You do not mention which camera you have and whether it has a removable ground glass holder fitted. If it does have then it makes it easier to check.

From Way Beyond Monochrome by Ralph Lambrecht and Chris Woodhouse page 109 (in the edition I have) the depth from the mating face at the rear of the camera, where the film holder sits, to the ground glass should 0.197 inches or 5 mm. If you can remove the ground glass holder you can check the depth by holding a straight edge across the mating face of the holder and measuring to the ground glass. If you cannot remove the ground glass holder then it is more complicated and Dave's suggestion of measuring from the front standard may be the way ahead. The difficulty may be in measuring exactly to the mating face joint.

Remember it is the ground side where the image focusses not the fresnel side, if fitted.

It was using this measurement that lead me to the discovery that the ground glass in my Wista way installed the wrong way round. I have turned it over and the difference in negative sharpness is remarkable.

Hope this helps.

Bill

Trevor Crone
21st August 2009, 08:16 AM
Mike, as a preliminary test for accuracy of focusing perhaps you can try using b&w photographic paper in the film holder. This will of course give you a negative image. And of a high contrast subject such as a sunlit wall will help you with the accuracy of your setup. Then perhaps use film to confirm this.

Alan Clark
21st August 2009, 10:02 AM
Bill,
The critical distance from the mating plane to the film plane is not 5mm as you say, but actually 4.5mm, for 5 x 4 and 10 x 8 .
There is a very simple way to check this. Open the slide on a film holder, and place a straight edge across the mating faces of the film holder. Now take a 4.5mm drill bit lay it in the film holder and roll it under the straight edge. It should just go under. A 5mm drill bit definately won't.

Mike,
To check if your ground glass is in the correct plane perform the same test with the ground glass screen, though it will only work if you have direct access to the ground side of the ground glass, which won't be the case if there is also a fresnal lens in there. You will have to remove this to do the test

Alan

Bill
21st August 2009, 10:41 AM
Bill,
The critical distance from the mating plane to the film plane is not 5mm as you say, but actually 4.5mm, for 5 x 4 and 10 x 8 .


I would be interested to know where these figures are from. Lambrecht and Woodhouse definitely quote 0.197 inches for 4 x 5 which is 5.0038 mm. The ANSI standard also quotes 0.197 +/- 0.007 inches. They also quote 0.260 +/- 0.016 inches for 8 x 10, again to the ANSI standard. This is from the mating face to the middle support in the film holder. Are you measuring to the emulsion side of a sheet of film? HP5 Plus is quoted on the Ilford fact sheet as being 0.180 mm thick which does not account for a 0.5 mm difference.

Your figure of 4.5 mm equates to 0.177 inches but we need to be sure we are talking about the same two points of measurement.

I have just checked my empty Fidelity Elite holders and they measure 5 mm, a 4.5 mm drill would rattle around.

I don't intend to fall out with anybody over this I just know the 5 mm figure works for me.

Bill

mpirie
21st August 2009, 11:05 AM
Hi guys,

Thanks for all the responses so far, plenty for me to check out.

The reason for asking is that I recently sold my soul for an Ebony, and am still getting to grips with the front axis tilt instead of the base tilt used my other cameras.

I was sure i focussed on a particular point, yet after processing the images, found that the image focus point seemed to have moved. It may be that I didn't lock everything down correctly before placing the film, or that it moved during the film placement.

That's what started me thinking about differences in film placement and GG focussing.

I'm surprised that there's a difference in the figures of 5.0038 and 4.5mm and although this is not a big difference in house-building, it is in terms of critical focus.

I'll look into the mearurements mentioned when I get home. It'll be an interesting exercise.

Bill - I doubt anyone will fall out over this, it just show there are many ways to reach the same end!

Mike

Trevor Crone
21st August 2009, 11:13 AM
Mike, what are the results like without using any movements?

mpirie
21st August 2009, 11:48 AM
Mike, what are the results like without using any movements?

Good question Trevor,

It's second nature for me to use some front tilt in almost all my shots.

I will give it a try and report back.

Mike

Alan Clark
21st August 2009, 12:35 PM
Bill, I have re-checked. With film in place 4.5mm is definitely the distance with my Toyo darkslides. I also tried a Fidelity, since this is what you measured. This is bigger. This almost takes a 5mm drill bit -not guite, maybe 4.8 or 4.9.

My first post contained an error. I should have said "6.5mm for" 10 x 8

Alan

Bill
21st August 2009, 02:00 PM
No problems Alan. Just shows that there are variations in all these things.

Bill

Alan Clark
21st August 2009, 03:54 PM
Quite big variations too, Bill.
I am pleased that when I made my 5 x 4 camera I took the film distance measurement from a Toyo filmholder, as I now have 7 of these, and only one Fidelity which I am now reluctant to use.

Alan

alfie
22nd August 2009, 01:22 PM
The most accurate way to measure the releative positions of the film vs. the focussing plane is to use an autocollimator, not too difficult to make (google will find a couple of DIY ones). Then you will find each darkslide is different! If your fresnel is on the inside of the GG it will shift the focus by about 1/3 of it's thickness, this is taken care of wth a collimator.

Another way is to sacrifice a darkslide, drill a hole in the middle and use a piece of fixed film as the focussing surface observing through the back of the darkslide. You may need to remove the GG to do this. And use a short focal length lens as the depth of focus is less than a long one.

alfie
22nd August 2009, 01:23 PM
Just turning on email notification...

Michael
22nd August 2009, 04:22 PM
And use a short focal length lens as the depth of focus is less than a long one.

Other way round, no?

Oops, no! I should have commented that focal length is not a factor where depth of focus is concerned.

Sandeha Lynch
22nd August 2009, 07:53 PM
This is a reference I've used ...

http://home.earthlink.net/~eahoo/page8/filmhold.html

Bill
23rd August 2009, 06:20 AM
This is a reference I've used ...

http://home.earthlink.net/~eahoo/page8/filmhold.html

Thanks Sandeha, a very useful reference.

Bill

alfie
24th August 2009, 06:14 AM
Long lens, short depth of field (the world end)
Short lens, short depth of focus (the film end)

This means any errors (of GG to film) will be made larger and easier to see.

Michael
24th August 2009, 10:27 AM
No, alfie. From Stroebel: "Depth of focus is not affected by focal length. At the same f-number, all focal length lenses subtend the same angular cone of light. Note that the term focal length does not appear in the formula".

The formula he refers to: Depth of focus=2x(f-number)x(acceptable circle of confusion)

alfie
25th August 2009, 12:04 PM
I stand corrected, always good to learn. (Bloody obvious when I thought about it)

Larger aperture short focal length lenses are easier to get than long ones. So I correct myself and say 'Use the largest aperture lens you have'.

The world (depth of field) side still holds, I wonder how to explain that words of one sylable...