How to measure ground glass and film plane differences.

[jimgalli]

Got into this today and wrote a short article with photos about how to accomplish these measurements very quickly and plenty accurately enough. This is very easy to do and can be very telling, especially with ULF and other cameras that have left their "factory status" far behind.

Anyways the article is HERE. I didn't know how to make it work on the APUG article page so I did it this way for now.

Ask questions, make corrections, let me know what you think.

 

[bobfowler]

Got into this today and wrote a short article with photos about how to accomplish these measurements very quickly and plenty accurately enough. This is very easy to do and can be very telling, especially with ULF and other cameras that have left their "factory status" far behind.

Anyways the article is HERE. I didn't know how to make it work on the APUG article page so I did it this way for now.

Ask questions, make corrections, let me know what you think.

Very well done Jim! That's the exact way I check my holders and backs.

I found I had to shim the GG on my sliding split back for my 5X7 about 1/64th, but at least I had to add material, it would suck having to shave it off.

 

[sanking]

Got into this today and wrote a short article with photos about how to accomplish these measurements very quickly and plenty accurately enough. This is very easy to do and can be very telling, especially with ULF and other cameras that have left their "factory status" far behind.

Anyways the article is HERE. I didn't know how to make it work on the APUG article page so I did it this way for now.

Ask questions, make corrections, let me know what you think.

Jim, this is not a comment about your method of measuring the displacement of your grond glass but a comment about the holders. I am very concerned that if you try to put a groove in the S&S holders to lock into the ridge on the camera back you will cut through the wood into the light trap area. Be sure to check the exact position of the area where you would have to cut the groove and compare it with the depth of the baffle cover and position of the cut out to make sure this is possible before making any cuts.

Sandy

 

[jimgalli]

Jim, this is not a comment about your method of measuring the displacement of your grond glass but a comment about the holders. I am very concerned that if you try to put a groove in the S&S holders to lock into the ridge on the camera back you will cut through the wood into the light trap area. Be sure to check the exact position of the area where you would have to cut the groove and compare it with the depth of the baffle cover and position of the cut out to make sure this is possible before making any cuts.

Sandy

Thanks Sandy. I'll double and triple check. It's the last thing to do on the project except figure out how to make up 1/16th inch. I guess I'll just roll forward a 16th each time. Actually at f64+ when you're trying to make up a 1/2" focus differential between near and far that 1/16th isn't the end of all.

 

[mikeb_z5]

Thanks for the info Jim!

I've been wanting to check this for some time now. I haven't really had terrible sharpness problems but feel my negs could be sharper.

I have a beattie fresnel installed between the glass and lens where would i measure the depth? (Toyo 45cx, back has a recessed area where fresnel sits so GG has not changed its original position.)

Thank you,

Mike

 

[removed account4]

thanks jim!
i always wondered how to do that sort of thing
and ... now i know

-john

 

[KenM]

It's interesting how some holders can be spot on, and others can be off by quite a bit.

I recently did a check on my Master Technika, and found that the previous owner had mucked around with the GG position - he had installed the combination fresnel/GG backwards, and must have noticed the softness of the negatives....I had replaced the crappy Super screen with a plain GG, and had to make an adjustment. But how?

Unfortunately, the technique shown in Jim's article (and also shown in Ralph Lamprecht's book) wouldn't work, since there is no easy way to position a ruler such that you can measure the distance to the film/GG plane - not without removing the back from the camera, something I wasn't willing to do. What I did instead was create a lensboard-sized wooden block our of 3/4" plywood that would fit into the front standard. I drilled a series of holes into the block that match the size of the thin metal shaft on a digital caliper. I then moved the front standard fairly close to the rear standard, and using a film holder with a sheet of film, made four measurements - one at each corner. Since the holes are just slightly larger than the shaft on the caliper, the caliper remains perpendicular to the front standard - this is important for accurate measurements. I then removed the film holder, and made the same measurements against the GG, and adjusted accordingly.

One of the nice things about this technique is that measuring the distance from the front standard also ensures that the front and rear standards are parallel.

I doubt if this technique would work with a ULF camera, since there would probably not be any way to get the standards close enough together to use a digital (or vernier) caliper. If you could remove the bellows, you'd probably have a shot, but with? Doubtful.

It should work for any 4x5 that can handle a 90mm lens.

 

[James Bleifus]

Fasinating article, Jim. I'll have to check mine for the heck of it.

Cheers,

James

 

[jimgalli]

Mike, With a Beattie or other device in place, the image still lands on the frosted surface of the glass. The screen just brightens it on the way. jg

 

[Ole]

Hang on there Jim - anything transparent has a difference between the physical thickness and the optical thickness. In fact that's what refractive index is...

So measuring to the surface of the GG with a fresnel in front of it is just as wrong as measuring to the front of the fresnel! That's why it's a specialist job to mount a fresnel on the inside of the GG.

 

[mikeb_z5]

Where would I find such a specialist to check this for me?

Mike

 

[Nige]

Slightly off topic but close enough to add here...

I've never managed to grasp this fresnel either side of the glass topic. My 2nd hand Zone VI came with a fresnel installed between the GG and the lens. So far I've not been super impressed with the sharpness of my prints. They seem to be in focus but just not sharp, not compared to my Mamiya 645 enlargements or Nikon MF gear. I have done a few things to try to locate the source including printing someone elses negs (which proved I could produce a sharp print, taking the enlarger out of the equation somewhat!).

I have read enough posts about the fresnel positioning that I'm confused! The other day I took it off to clean it (washed the fresnel as mentioned in a thread here somewhere and dried with cool hair dryer.. came up loverly, but managed to not seat the GG holder lugs properly when putting it back and manged to crack the glass, but that's another story...) and decided to measure the depth of a film holder, a Grafmatic and the postion where the GG sits. The DD and the Grafmatic looked pretty close to 5mm (eye balled with decent steel ruler only so not accurate) but was unable to measure the position of the GG due to not understanding the fresnal/GG relationship. It seems the fresnel sits approx 5mm off the camera rear standard due to a 5mm (approx) rebate in the camera back, then the GG over that. The fresnel and GG are close to 5mm thick by themselves but I think that's a furphy. From the pictures in the link, I can see I should have taken the back off the camera and measured from the front with the fresnel?GG installed, but my question really is... which of the 4 surfaces in the fresnel/GG sandwhich am I looking at thru a loupe or with unaided eye? From my calcs, it has to be the on nearest the camera?

Anyone who can clear this up for me, please do!

 

[jimgalli]

I'm at a loss as far as the fresnel's go. Sorry. Thought I knew, but apparently I don't. Hope someone can chime in here and give us a little theory regarding how the fresnels change where the light lands.

 

[Ole]

A fresnel is a lens, cut into many concentric rings and then all extraneous material removed. At any point it has the same slope as an "ordinary" lens of the same focal length - the cheaper ones have no curvature.

So they focus light just like any other lens. What they do in a camera back is very similar to what a condensor does in an enlarger: It doesn't really focus so much as change the direction the light is travelling in. Instead of continuing out through the GG in a straight line from the lens, it gets bent a little towards perpendicular to the back. That makes more of the light end up where your eye is, making the whole thing seem brighter.

Now the whole "focusing" trick isw based on the difference in refractive index between air and glass/plastic or whatever, which incidentally is also the relative speed of light. We all know that the speed of light is a constant, but most people don't know that that is only half the statement: The speed of light in a vacuum is constant. Differences in light speed in different media give refraction.

Put a thick slab of glass on a newspaper, and use a camera to focus on the print through the glass. Then remove the glass, and you'll see that you have to refocus. Same thing with the fresnel lens: Put a thin piece of plastic between the ground glass and the lens, focus the lens so that something at infinity is sharp on the ground glass. Then remove the plastic: If you reseat the GG to where the front of the plastic was you'll have to move the GG back about 2/3 the thickness of the fresnel, if you just pull the plastic off the inside of the GG you'll have to move it about 1/3 forwards.

These numbers should not be taken seriously, they would be close to correct if fresnel lenses were made of crown glass. Since they're not, the numbers are out. I don't have better numbers off the top of my head, but can look up some when I get home next week.

 

[GeorgesGiralt]

Hi !
My main motivation in using a Fresnel, was to be able to *see* something when using my F/6.8 90 mm Schneider Angulon of 1960 vintage.
Is it usefull to drill a hole in the center of the Fresnel in order to focus accuratelly, and keep the outer "rings" to gather the light in order to check composition ?
In other words, is the center of a Fresnel lens mandatory ?
Oh, and while we are at it, what about putting the Fresnel on the outer face of the ground glass ?
TIA !

 

[Ole]

Georges, both of those things can and have been done. I have heard that there are even some who have a clear spot or a hole in the center of the GG, and focus on the aerial image...

On the other hand, I have used a 90/8 Super-Angulon on a 5x7" camera and have managed both focusing and composition without a Fresnel (you're right by the way, it's a name and should be capitalised). It's difficult, but possible.

 

[Donald Miller]

Slightly off topic but close enough to add here...

I've never managed to grasp this fresnel either side of the glass topic. My 2nd hand Zone VI came with a fresnel installed between the GG and the lens. So far I've not been super impressed with the sharpness of my prints. They seem to be in focus but just not sharp, not compared to my Mamiya 645 enlargements or Nikon MF gear. I have done a few things to try to locate the source including printing someone elses negs (which proved I could produce a sharp print, taking the enlarger out of the equation somewhat!).

I have read enough posts about the fresnel positioning that I'm confused! The other day I took it off to clean it (washed the fresnel as mentioned in a thread here somewhere and dried with cool hair dryer.. came up loverly, but managed to not seat the GG holder lugs properly when putting it back and manged to crack the glass, but that's another story...) and decided to measure the depth of a film holder, a Grafmatic and the postion where the GG sits. The DD and the Grafmatic looked pretty close to 5mm (eye balled with decent steel ruler only so not accurate) but was unable to measure the position of the GG due to not understanding the fresnal/GG relationship. It seems the fresnel sits approx 5mm off the camera rear standard due to a 5mm (approx) rebate in the camera back, then the GG over that. The fresnel and GG are close to 5mm thick by themselves but I think that's a furphy. From the pictures in the link, I can see I should have taken the back off the camera and measured from the front with the fresnel?GG installed, but my question really is... which of the 4 surfaces in the fresnel/GG sandwhich am I looking at thru a loupe or with unaided eye? From my calcs, it has to be the on nearest the camera?

Anyone who can clear this up for me, please do!

I had always heard that when one is adding a fresnel lens to a camera that did not originally have one installed that the fresnel should be placed behind (away from the lens) the ground glass. The reason is that if the fresnel is installed ahead of the ground glass (nearer the lens) you will affect the focus dimension/Tdimension of the film holder relationship.

Prior to sticking my neck out and making that statement as I recalled it, I checked the Boss site. They state on their site that there are two types of cameras. The first (type one) is the type that did not come with a fresnel as manufactured...In that case my first statement applies. The second (type two) is the type that did have a fresnel installed as manufactured. In that case the fresnel should be installed in it's original orientation.

 

[Ole]

... precisely because of the difference between physical and optical thickness of any transparent material - like plastic and glass.