fresnel lens placement

[raucousimages]

I am rebuilding a toyo 5x7 back. Does the fresnel lens go in front (lens side) or in back (film side) of the ground glass?

 

[Dave Parker]

Normally if the camera was equiped with the fresnel as orginal equipment if would be mounted first, then the ground glass.

Dave

 

[MichaelBriggs]

Cameras have been made with either placement. You must use the original placement or you will cause a shift in focus. Perhaps mount the ground glass and Fresnel both ways, and make a test photograph with each setup. Hopefully one test setup will show the object you focused on in pefect focus, and the other the object clearly out of focus, and you will be sure what the correct configuration is for your camera.

I think the best design choice is to have the Fresnel on the photographer's side, with the ground glass closer to the lens. With the configuration, you can remove the Frensel if you don't like it, without shifting the focus. Others prefer the Fresnel on the lens side. The argument in favor of this configuration is that the glass protects the plastic Fresnel lens.

 

[Charles Webb]

Michael,
Just for the record, I would like to know what camera manufacturer ever mounted the fresnel lens behind the ground glass? This defeats the whole therory of the fresnel lens. Please bring me up to date and enlighten me!
Without solid proof that such a practice has been used by a major manufacturer I simply don't buy it. I know of several folks that removed the GG and FL to clean them, then proceeded to install them backwards which indeed will cause a focus shift. Please Show me the error of my ways!
I say B. S. to your argument as to why some folks do this, A fresnel lens was not designed to work behind the GG. Show me where Linhoff, Deardorff, Hasselblad, Graflex, Korona, Kodak, Ansco, Mamia, Omega, Toyo, Wista or
Sinar ever recommended that the FL be placed behind the GG!



Charlie.......

 

[j_landecker]

My Technikardan came with the fresnel behind the gg (photographer's side), and Linhof puts clips there for just that purpose. I don't see how it defeats the theory of the fresnel lens. Light hits the gg at an angle (steeper with wide angle lenses, causing the dreaded "hot-spot"), and the fresnel helps redirect this light towards your eyes, making the illumination of the screen appear more even. It works on either side of the gg.

 

[gwatson]

I think the main problem is that some people don't realise that the fresnel is a lens that distorts light. If placed lens-side of the gg, the light passes through the fresnel, is distorted, and the image then forms on the gg. Because the light is distorted as it passes through the fresnel, you are in fact focussing on a distorted image rather than the true image. Once the dd goes in and the fresnel is removed from the equation, the image hitting the film is not the same image you focussed on. Of course, you will also get focus shift if you put a fresnel on the film plane (rather than the gg). Personally, I've found that fresnels are great for composition, but are best removed when focussing.

Geoff

 

[Dave Parker]

In working with ground glass as much as I do, I have seen cameras with the fresnel mounted on either side of the ground glass, the old graflex cameras from the factory mounted on the lens side of the ground glass, I have seen other brands with them on the outside of the ground glass, and I have seen cameras where the fresnel was the ground glass with a clear protective glass over it, with most Toyo's in which this question was asked about, the fresnel is on the inside of the ground glass, if equipped from the factory with a fresnel.

Dave

 

[MichaelBriggs]

Charles, others have given examples of cameras that ship from the manufactuer with Fresnel lenses on the photographers side. This configuration will function. Consider a light ray arriving from the lens to the corner of the ground glass. The ground glass scatters to some extent, depending on the coarseness of the glass, but the predominant direction of the cone of scattered rays is still in the original direction, from the lens to the corner of the ground glass. If the photographer's eye is behind the center of the screen, not many of these rays will reach their eye. The Frensel lens behind the screen functions as a lens and refracts the bundle of rays towards the center line so that more of them reach the photographer's eye and the ground glass image appears brighter (as j_landecker explained). So a Fresnel lens will brighten the image when placed on the photographer's side and this placement doesn't defeat the purpose of using a Fresnel lens.

Placing a Fresnel lens in front of the ground glass causes a focus shift, not so much because it is a lens, but because of its thickness. If you placed a uniform thickness sheet of plastic or glass in between the ground glass and lens, you would also get a focus shift. This is because the index of refraction of the plastic or glass is higher than air. Assuming an index of refraction of 1.5, the shift is 1/3 the thickness of the sheet. This applies whenever a sheet is inserted into a converging beam and is something to be aware of when using a thick filter behind a lens, or even in front of a lens in macro photography.

When I first read Satinsnow's first answer, I thought it more sweeping than it is meant to be. Of course "the camera" is supposed to be the Toyo of the question, not any camera with a factory installed Fresnel, which is the meaning of the sentence if read in isolation.

If there is the slightest uncertainty of the correct placement of a focusing screen, then a photographic test to confirm focus would be wise. There have been other questions where photographers have discovered focus problems after taking important photographs.

 

[Dave Parker]

I always recommend to take a test shot or two after dismounting the focusing screen, that way you can make sure no problems exist before the next important shot.

Dave

 

[Early Riser]

Michael,
Just for the record, I would like to know what camera manufacturer ever mounted the fresnel lens behind the ground glass? This defeats the whole therory of the fresnel lens. ......... A fresnel lens was not designed to work behind the GG. Show me where Linhoff, Deardorff, Hasselblad, Graflex, Korona, Kodak, Ansco, Mamia, Omega, Toyo, Wista or
Sinar ever recommended that the FL be placed behind the GG!



Charlie.......

Charlie, the fresnel that Sinar makes pops on behind the GG, that is on the viewing side of the GG, not the lens side. It's made to pop on and off easily and does a decent job.

 

[Charles Webb]

Thank you Gentlemen for bringing me into the 21st Century. No studio camera I have ever seen or used has the Fresnel behind the GG. I have used Sinar, Deardorff, Kodak, Burke & James and Graphlex's etc. over the past fifty five years and never saw one so equipped. I assume that these inovations came about after I left the commercial photography field. There is a bit of shock involving this new idea (new to me) but I do appreciate everyones response. I bow to your superior knowledge! Charlie.......

 

[Dave Parker]

Just my guess. The camera focuses on the ground glass. I don't think you can move it from that plane or you will mess with focus. I think you would have to modify the ground glass holder by machining out the depth of the Fresnel lens to put it in front of the glass. Like I said, this is my guess, but I think I am right here.

You are right to a point, if your going to put it in front of the ground glass on a camera that was not orginally equiped with a fresnel, there is one guy out of Seattle that is sell expensive fresnels that he claims are thin enough to not shift the focus, but I found them to actually shift the focus a bit.

Dave

 

[Dave Parker]

The average fresnel is about a 1/16 of an inch thick and can cause quite a problem in focus plane if not properly installed...and when placing in front on a camera not designed for a fresnel, you will notice a focus shift of approx 2/3 the thickness of the fresnel.

Dave

 

[MichaelBriggs]

Just my guess. The camera focuses on the ground glass. I don't think you can move it from that plane or you will mess with focus. I think you would have to modify the ground glass holder by machining out the depth of the Fresnel lens to put it in front of the glass. Like I said, this is my guess, but I think I am right here.

The issue for the original question from raucousimages is how his 5x7 camera shipped from Toyo (assuming that no one has modified it in the intervening years). If Toyo shipped it with the Fresnel in front of the ground glass, then the camera should be reassembled in this configuration. (And this seems to be the general practice of Toyo for cameras with Fresnels, as stated by Satinsnow / Dave in a previous answer.) It is very likely that Toyo got the focus correct when they made the camera.

If someone had a camera without a Fresnel, or with a Fresnel on the photographer's side of the ground glass, and wanted to install a Fresnel on the lens side of the ground glass, then for perfect focusing accuracy, then yes, it would be necessary to adjust or machine the seat of the focusing screen.

 

[Struan Gray]

My Sinar Norma 5x7 has a factory Fresnel installed on the lens side of the ground glass. The Fresnel has lugs at the edge that insert between the machined surface of the back and the ground glass, moving them apart by the correct amount to compensate for the focus shift. Adding and removing the Fresnel is not as simple as with the later, framed Fresnels on the outside of the camera, but there is no need to adjust the seating of the metal portions of the back.