View Full Version : Focus shift when mounting fresnel?
I have use a Toyo GII for years and it has a fresnel attached to the ground glass with two black clamps so they form a "sandwich". The G-series along with most Toyo view cameras come with fresnel from the factory.
Some months ago I got my hands on a very low priced Toyo 45CF field camera that does not have a fresnel on the ground glass. Since I have used the CF most of the time lately (long outdoors trips) I wondered the other day if I just could move the whole gorundglass/fresnel "sandwich" from The GII over yo the CF. It was done in 5 minutes and I was very happy with the bright ground glass on my light field camera.
Until 10 minutes ago. I read a chapter in Leslie Stroebels "View camera technique" about fresnels (page 257 if anyone has any interest). He says that on some backs you will experience a focus shift equal the width of the fresnel, and the focusing surface will no longer be in the film plane.
This made me seriosly worried. :eek: Am I in big trouble here? I have made 12-15 exposures after the switch, but I have not developed them yet.
David A. Goldfarb
As long as the groundglass is between the fresnel and the lens, then you should be okay.
If you put the fresnel between the lens and the groundglass on a back not calibrated to work that way, you'll have problems.
Bob Salomon - HP Marketing
There would only be a focus shift if the frenel is beneath the ground glass. You are fine if it is above the GG (nearest to your eye).
I know that worried feeling, Eirik!
I bought a beautiful, lightly used Zone VI 8x10 from Midwest Photo several years ago. First thing I did was remove the fresnel that was betwen the GG and the lens (I don't care for them.)
Then just lately I got to thinking...did I get a focus shift because I removed it -- and only getting good negs because of using f64 or f90? I measured the distances and they are correct (the GG and the film seem to be on the smae plane). So whoever had the camera before me must have put the fresnel in the wrong place.
Crap. The fresnel is between the GG and the lens. It was so on the Toyo GII so I just did the same on the CF. Does this mean that the GG mounting brackets on the GII is somewhat "calibrated" to give correct focus with a fresnel on beneath the GG? And not on Toyo CF since it does not come with a fresnel?
Should I loosen the fresnel and remount it on the other side of the GG? (closest to me). I guess the "rugged" surface should point towards the GG?
> Does this mean that the GG mounting brackets on the GII is somewhat "calibrated" to give correct focus with a fresnel on beneath the GG? And not on Toyo CF since it does not come with a fresnel?
Yes on both counts. You could slice up the fresnel and use pieces of it as spacers to get the GG back in place.
I had the same quandary when I made a custom back for my Gowland camera; do I need to account for the fresnel, and if so, how much? I found this link to be useful:
Basically, you measure the distance to the plane of the film holder, and compare it to the distance to the ground side of the GG (the focus plane). If there is a fresnel between the lens and the GG, its index of refraction needs to be accounted for as shown in the link. This is also a good double check to perform to convince yourself that the GG is correctly located.
yup it should always be ground glass then fresnel or light modifiers. I learnt this through building mini35 adapters for DV cameras. I made a lot of GG and cut a good chunk of bellows up until I learnt my lesson.
I must admit though a reasonably fast lens with a fresnel is much better especially on a 90mm for getting a properly focussed image quickly. Plus having a nice viewing evenness of brightness.
I have really jumped acrosses the river to get water. The backs on the Toyo GII and 45CF seem to be identical (the one you remove when you use a roll film holder). I switched GG in the first place, but will I be on safe ground if I return the GG´s back to where they originally were and just switched backs between the two cameras?
Brian C. Miller
Here's how to check if your GG is registered correctly:
Grab some corrugated cardboard. Make a step wedge with the cardboard, maybe 2 inches thick or so, with the wedges being at least a 1-inch square. Glue newspaper or similar to each wedge. Position your camera so that you can read the print on the GG with a loupe. Focus on the middle square. Expose the film. After developing it, see if the middle square is the one in focus. If not, time to screw around with it.
I replaced the non-original GG in my Graflex with Betterlight GG. The fresnel is facing me, then the GG with the ground surface towards me. I had to shim it a little bit, but now its wonderful.
To get a quick idea if you have the right spacing is to take the back off the camera and put a stiff ruler across the back (on the side normally towards the lens) -- then measure the distance from the ruler down to the GG. Then slip in a film holder, remove the dark slide and measure the same way down to the bed of the film holder (put a piece of film in the holder if you can measure that accurately). Both measurements should be the same. If the measurement is off by the thickness of the fresnel, then you know which way to go.
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