Someone please clarify this for me (Fresnel Lenses)

[clark_king1]

I have heard that if the fresnel lens is behind the gg(lens side) that the images will be out of focus. Then also I have heard (from this board) that it does not matter on some cameras. I have asked this queation before but no one gave a real solid answer. I am new to LF and have been making test exposures on pol. 559. I have noticed some softness while using nikkor 210 and at a subject distance of about 9 inches. While the image is very sharp on the gg. I am wondering if the softness is not just a product of the polaroid film. Would the same result from neg or trans film?

 

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Sorry about the newbie type questions, thank you very much!

[clark_king1]

I have thought of another question. The gg and the fresnel on the

camera back have a small amount of play, meaning that with the camera

back in a vertical position (on the camera) if you touch the gg it

moves back and forth toward the lens. The focus does not change when

it is moved. But is this okay?

 

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Thanks, CK

[robert_a._zeichner1]

If a camera was designed to use a Fresnel between the gg and the lens,

removing the Fresnel will cause focus problems unless the gg is

repositioned. Conversely, if a camera not designed to use a Fresnel is

fitted with one between gg & lens, once again, the gg must be

repositioned to compensate for the rearward shift of focus. A Fresnel

screen is a lens, essentially. It shifts the image rearward by an

amount roughly 1/3 of the thickness of the screen. Some cameras are

designed in such a manner that will allow the user the choice of

Fresnel or no Fresnel. This is accomplished by 1. "Hanging" the

Fresnel off the gg and not between it and the mounting pads of the

camera back and 2. by putting shims equal in thickness to 1/3 the

thickness of the Fresnel between the gg and the mounting pads. With

Fresnel and shims in place, the image that comes into focus on the gg

will be in focus on the film. If you were to remove the Fresnel, you

would need to also remove the shims. This would move the gg forward

toward the lens by a dimension equal to 1/3 the thickness of the now

removed Fresnel screen. Again, image on the gg would be coincident

with what would strike the film. If you want to test a paricular

camera, you might read my article in the Nov./Dec. 1997 issue of

ViewCamera. I describe a home made test target and a procedure that

will confirm proper alignment.

[clark_king1]

After further investigation I have found that on my camera

back(horseman 4x5 approx. 14 years old), there are shims on the top

and bottom that are placed on the photographer side of the back.

Because these shims have been eroded away because age and the

material. It seems that I can simply unscrew the screws and replace

the shims but can I do this without precision equipment to measure

this?

The placement of Fresnel lens is lens side to the GG and the

smooth side of the Fresnel lens is toward the lens board. I think can

you actually feel the rings on the fresnel.

I have shot some poaroids and they seem to be soft at f/5.6. I

have wondered that this might be because the polaroid film is not as

sharp as neg or trans film. But could it be because of the gg/fresnel

placement. As I've mentioned before I use a 10x loupe and the image

is very shrp on the gg but not on Polaroid test shot. Also because

the shims have been "eroded" could this "play" or movement be causing

a shit in focus?

 

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Thank you very much for bearing this message I know it's long but I'm

trying to learn so that I can truely enjoy the fruits of this

wonderful labor.

 

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Clark King

[andrea_milano]

In the case of your Horseman, if I were you, I would get in touch with

Jin Yamaguchi at Horseman in Japan, he will be able to advise the best

procedure. As far as I am aware , Horseman cameras mount fresnel at the

back of the GG, therefore facing the photographer, just slightly

unscrew the retainers and slip the fresnel underneath, then screw tight

and voila'! At least this is what I did on my Horseman EM, other models

might have a different construction.

 

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Film and GG allinement have been in the past widely discussed onn this

site and you should find in the archives all the necessary information.

 

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Concerning the poor grammar and typing skills of the contibutor of any

site, which another contributor to this thread quoted, I am afraid

that an International and well-meaning array of contributors produces

contributions to the best of their abilities, in other words the

content is more important than its form.

[andrea_milano]

OOOOPS! it was another thread which spoke about poor grammar and typing

skills, the point is valid anyway!

[armin_seeholzer]

Hi Clark

 

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Just about your last post. If you take a picture with f 5,6 you always

will get a soft picture on almost every camera in LF.

You have to stop down the lens at least up to f11-16 and make your

tests on a high resolution film like Tmax 100, Techpan ,Ilford Deltas

100 or the Gigabitfilm.

Good luck!

[bob_salomon]

Where you place the fresnel does not matter. As long as the

ground glass is adjusted to lie in the film plane.

 

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When the camera is set up for the GG to be placed closer to the

lens you cannot switch the position and put the GG behind the

Fresnel unless the shims the GG sit on are re-adjusted for the

shift in GG position.

 

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It is usually better to place the fresnel closer to the eye so it can

be removed when the lines in the fresnel are bothersome or

when it makes focusing difficult with some length lenses.

[doremus_scudder1]

Clark,

There is a quick test you can do to determine if the film is really

in the same plane as the groundglass. Set up a close-up shot of a

ruler lying flat or at a nearly horizontal angle. Choose a lens

(longer is better), aperture (i.e. wide open) and subject distance

(closer is better)that will give you the minimum depth of field.

Focus carefully on, say, the six-inch mark, noting how much the other

marks in front of and behind the one you focused on are out of focus.

Expose a couple of sheets of film and develop them. A quick

examination of the developed negs will tell you if your focusing

plane is the same as the film plane. If the mark you focused on is

exactly as you focused it, you are in great shape.

 

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If closer marks are in focus, the film plane is behind the focusing

plane (this happens only rarely, and not because of the fresnel, but

because of some other mechanical positioning error such as too many

shims or faulty back). In this case, you would have to remove shims

or otherwise find a way to move the ground glass/fresnel sandwich

closer to the lens.

 

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If marks farther away than you focused are the sharpest, then the

film plane is closer to the lens than your ground glass (this is the

most common case, especially when adding a fresnel screen for the

first time). In this case, you would have to add shims or otherwise

find a way to move the ground glass/fresnel sandwich farther from the

lens. Usually the error is around 1/3 the thickness of the fresnel.

 

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I usually do these tests while I am printing and have a tray of print

developer handy. I expose two sheets (one holder) of B&W film, take

them to the darkroom, unload the holder and toss them in the print

developer for 3 minutes or so (safelights off, of course). Time isn't

critical, you only need an image to check for focus. If adjustments

are necessary, these can be made and subsequent tests done quickly

(much faster than normal film developing).

 

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After making the adjustments, you must test again. Sometimes it takes

two or three tries to get things just right. I had to add shims to a

factory-designed gg/fresnel made especially for my camera. Apparently

some camera manufacturers don't take this focusing problem into

account when designing!

 

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Polaroids should work for this, but if the sharpness of the Polaroid

material is in question, I would recommend using conventional B&W film

 

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Hope this helps, ;^D)

[trevor_crone1]

Clark I have an Ebony wide angle fresnel permanently mounted between the GG and protective glass on my Ebony SW45. No problems negs are as sharp as a pin.

Regards,

[clark_king1]

Thanks to all who have responded, I appreciate the information!

 

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CK