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john.l.barford
19-Jul-2017, 07:23
There has been a lot of chat and controversy about the use of fresnel lenses in combination with the ground glass on large format cameras in attempt to brighten the image formed on the screen thus assisting focusing and composition.
I decided to do a series of experiments ion a 4x5 camera to find out what all the fuss was about.
My findings are as follows:
THE FRESNEL LENS
Most fresnel lenses are plastic and can be bought on ebay. These are designed as reading aids and have a focal length between 5 and 10cm. They can easily be cut to size using scissors or a craft knife.
PLACING THE FRESNEL LENS
There has been much speculation on this topic. After much experimentation trying all four possible posiitions,
1) Camera lens side. Grooves facing back of camera lens smooth side touching screen.
2) Camera lens side. Grooves touching screen smooth side facing camera lens.
3) and 4) Same as above but fresnel lens put on viewer side of ground glass screen.
Because of the machining process in the manufacture of fresnel lenses the lens will not lie completely flat to the ground glass screen and may be found to be bulging slightly in the middle or sides.
A further series of experiments with the fresnel lens being deliberately distorted and moved up to half a mm from the screen were done to find the effects of this on image an focusing.
THE RESULTS
The image using the fresnel lens was noticably brighter in ALL positions.
The accurate focus of the camera was NOT affected by any of the fresnel lens positions.
The bulging of the fresnel lens, up to half a mm, had no effect on focusing or image.
DOWNSIDES
Some fresnel lens grooves were noticable, under loupe magnification especially on lenses over 180mm focal length.



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EdSawyer
19-Jul-2017, 09:53
There are much better quality fresnel lenses to be had, than those found on eBay such as what you have used. Some have much finer grooves, like the ones Used on Graflex cameras. There's also integrated fresnels with screens, like the ones made by Maxwell and used by canham. Edmund optics also sells fresnels in various sizes and different focal lengths as well as different groove pitches, which might be worth exploring.

Randy
19-Jul-2017, 10:19
The accurate focus of the camera was NOT affected by any of the fresnel lens positions.This confuses me. The ground-glass (frosted side) has to be located exactly at the film plane in order to achieve exact focus, especially when shooting at wide apertures and close distances. If placing a plastic fresnel lens of 1-2mm thickness under the ground-glass moves the ground-glass 1-2mm from the film plane (because now the fresnel is at the film plane and the ground-glass is moved farther away from the film plane by the thickness if the fresnel), will that not affect the accuracy of the focus? I don't see how it can't. What am I missing?

Michael Graves
19-Jul-2017, 10:22
This confuses me. The ground-glass (frosted side) has to be located exactly at the film plane in order to achieve exact focus, especially when shooting at wide apertures and close distances. If placing a plastic fresnel lens of 1-2mm thickness under the ground-glass moves the ground-glass 1-2mm from the film plane (because now the fresnel is at the film plane and the ground-glass is moved farther away from the film plane by the thickness if the fresnel), will that not affect the accuracy of the focus? I don't see how it can't. What am I missing?

I was pondering the same thing. The first time I replaced a ground glass/fresnel combo, I got the two bass-ackwards and had to get several out of focus 8x10 negatives before I figured out my stupidity. Switched them around and all was well.

Well, except for composition, exposure, subject matter and other things that went wrong. But those had nothing to do with the ground glass.

Bob Salomon
19-Jul-2017, 10:43
There has been a lot of chat and controversy about the use of fresnel lenses in combination with the ground glass on large format cameras in attempt to brighten the image formed on the screen thus assisting focusing and composition.
I decided to do a series of experiments ion a 4x5 camera to find out what all the fuss was about.
My findings are as follows:
THE FRESNEL LENS
Most fresnel lenses are plastic and can be bought on ebay. These are designed as reading aids and have a focal length between 5 and 10cm. They can easily be cut to size using scissors or a craft knife.
PLACING THE FRESNEL LENS
There has been much speculation on this topic. After much experimentation trying all four possible posiitions,
1) Camera lens side. Grooves facing back of camera lens smooth side touching screen.
2) Camera lens side. Grooves touching screen smooth side facing camera lens.
3) and 4) Same as above but fresnel lens put on viewer side of ground glass screen.
Because of the machining process in the manufacture of fresnel lenses the lens will not lie completely flat to the ground glass screen and may be found to be bulging slightly in the middle or sides.
A further series of experiments with the fresnel lens being deliberately distorted and moved up to half a mm from the screen were done to find the effects of this on image an focusing.
THE RESULTS
The image using the fresnel lens was noticably brighter in ALL positions.
The accurate focus of the camera was NOT affected by any of the fresnel lens positions.
The bulging of the fresnel lens, up to half a mm, had no effect on focusing or image.
DOWNSIDES
Some fresnel lens grooves were noticable, under loupe magnification especially on lenses over 180mm focal length.



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Congratulations! You wasted all that time and energy, not to mention expense, doing an absolutely worthless test since you were not testing large format Fresnel lenses.
Try again but this time buy the proper type of Fresnel for your camera.

john.l.barford
19-Jul-2017, 10:46
I absolutely understand that the frosted side of the ground glass screen must be exactly at the film plane. If you place the fresnel lens flat onto this frosted side of the ground glass screen without moving the screen from its position.

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Dan Fromm
19-Jul-2017, 10:52
I absolutely understand that the frosted side of the ground glass screen must be exactly at the film plane. If you place the fresnel lens flat onto this frosted side of the ground glass screen without moving the screen from its position.

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Doing this shifts the plane of best focus by ~ 1/3 the fresnel's thickness. Which raises a question. How did you determine that putting a fresnel in front of the GG didn't shift the plane of best focus?

MAubrey
19-Jul-2017, 10:56
Congratulations! You wasted all that time and energy, not to mention expense, doing an absolutely worthless test since you were not testing large format Fresnel lenses.
Try again but this time buy the proper type of Fresnel for your camera.
Is there some advantage to sarcastic mockery in forum conversations that I'm simply unaware of?

john.l.barford
19-Jul-2017, 11:12
I hope this helps to clear up some confusion

The total cost of this project was about $5 and two hours work. As an added bonus it annoyed the wifehttps://uploads.tapatalk-cdn.com/20170719/6007d8a42d4deaacd5545b4f4ea79bbb.jpg

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Bob Salomon
19-Jul-2017, 11:32
Is there some advantage to sarcastic mockery in forum conversations that I'm simply unaware of?

Not really sarcastic. He simply did not do a valid test and presented his findings as if they are valid. He could have easily checked before wasting the .time, cost and effort before the test to verify what Fresnel was necessary.

Randy
19-Jul-2017, 13:27
I absolutely understand that the frosted side of the ground glass screen must be exactly at the film plane. If you place the fresnel lens flat onto this frosted side of the ground glass screen without moving the screen from its position.

Sent from my SM-T210 using TapatalkOK, I understand now what you did...or I think I understand.
I think where my confusion came in was - if you take, like a standard 4X5 back that has no fresnel, is not made to use a fresnel, but you then remove the gg, put the fresnel in it's place, then place the GG on top of it, you have moved the GG past the film plane, and the result could be slightly out of focus images.

~ but ~

If you can mount a fresnel in front of the GG with out moving the GG, you will maintain the GG at the exact film plane...correct...?

Neal Chaves
19-Jul-2017, 13:53
I found that with a 4X5 Crown Graphic, when I removed the Fresnel I had to insert shims under the ground glass to maintain accurate focus. The shims were thinner than the Fresnel, about .0025". Recently I replaced the broken ground glass and Fresnel in a Toyo G with a plain ground glass. I expected that I would need shims, but I was surprised to find that focus was perfect. There must be differences in the Fresnels themselves.

Greg
19-Jul-2017, 16:50
OK, I understand now what you did...or I think I understand.
I think where my confusion came in was - if you take, like a standard 4X5 back that has no fresnel, is not made to use a fresnel, but you then remove the gg, put the fresnel in it's place, then place the GG on top of it, you have moved the GG past the film plane, and the result could be slightly out of focus images.

~ but ~

If you can mount a fresnel in front of the GG with out moving the GG, you will maintain the GG at the exact film plane...correct...?

In the 1980s I was doing a lot of Photomicrography and Photomacrography (shooting 4x5 chromes) with my Nikon Multiphot. On the Multiphot, the fresnel lens is mounted in front of the GG. One time I took the Multiphot apart to clean it. Shot some 4x5 chromes and they came back out of focus. I called Nikon's Instrument Division to trouble shoot my problem. First question they asked me was if I had put the fresnel lens in front of or behind the GG. Told them I put it in front of the GG (I had made that mistake once before). Then they asked me which direction was the fresnel facing. I told them towards the GG. Was told the fresnel should be facing towards the lens. If the fresnel was reversed (how I had mounted it), the result would be that my images would noticeably be out of focus. This wouldn't be noticeable if I were using a regular view camera (depth of field more than compensates), but since I was shooting between X1 and X100, with its essentially minuscule depth of field, my film images would be noticeable out of focus.

Dan Fromm
19-Jul-2017, 17:01
If you can mount a fresnel in front of the GG with out moving the GG, you will maintain the GG at the exact film plane...correct...?

Stands to reason but it ain't so.

Randy
19-Jul-2017, 17:26
Stands to reason but it ain't so.Trying my best to translate that statement...I give up.

Jim Jones
19-Jul-2017, 18:02
Dan is right. The length of the optical path is shortened when transparent material is inserted between the lens and ground glass (or film). You commonly see a similar effect in the apparently broken straw in a water glass.

john.l.barford
20-Jul-2017, 02:03
There are obviously many factors in fresnel lens placement that I have failed to understand. Could someone explain to me that if a fresnel lens has to have a focal length related to the focal length of the camera lens

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john.l.barford
20-Jul-2017, 02:10
There are obviously many factors in fresnel lens placement that I have failed to understand. Could someone explain to me that if a fresnel lens has to have a focal length related to the focal length of the camera lens why camera manufacturers fresnel lenses seem suitable for all lens focal lengths?

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xkaes
20-Jul-2017, 04:29
Here's what Darkroom Techniques said about it in 1990:

167422

www.subclub.org/groundglass.JPG (http://www.subclub.org/groundglass.JPG)

john.l.barford
20-Jul-2017, 07:43
Excellent article especially the "light spreading screen" definition.

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Emmanuel BIGLER
20-Jul-2017, 11:18
Could someone explain to me that if a fresnel lens has to have a focal length related to the focal length of the camera lens

Hello from France.

Actually you have two different ways to understand how Fresnel lenses work in combination with a ground glass.
The first idea is to consider that the Fresnel lens "bends" peripheral rays to force them to enter into the photographer's pupil.
See this diagram
https://www.flickr.com/photos/43175600@N00/36006774246

Now which is the proper setting in terms of focal length for the Fresnel lens?

Imagine that there is no ground glass (GG) and that you simply want most camera rays to enter your eye's pupil.
Like in a telescope, or in binoculars, where your pupil is centered at the center of an eyepiece.
Then you would chose a Fresnel lens with the appropriate focal length able to transfer the image of the exit pupil of the camera lens into your eye's pupil. In technical terms, the Fresnel lens acts as a field lens; catches all rays emitted by the camera lens exit pupil, and forces them all to pass through an image, which is the smallest size of the beam, this image of the camera lens exit pupil should be located at the operators' pupil, in principle.

But for a view camera, this is not the right choice, because you want to be able to see the whole image at all points of the ground glass.

Hence the usual setting for the Fresnel lens is to take a focal lens equal to the distance between the exit pupil of the camera lens and the image plane.
In this situation, rays sent by the exit pupil of the lens exit approximately parallel to the optical axis, hence illuminating the GG as evenly as possible.
The other, and more scientific explanation is to look where the image of the exit pupil of the camera lens, seen through the Fresnel lens, is located. If the exit pupil is set at the focal point of the Fresnel lens, the image will be sent to infinity, but this is hard to understand ;) Imagine that the image of the exit pupil is a large white circle located far away, and of a very large size!

Mission: impossible, since we change lenses all the time and re-focus lenses for all distances!
You would need an adjustable zoom Fresnel lens!!!

Well, actually the setting which is chosen is a compromise, the Fresnel's focal length is close to ... the camera lens focal length. This works quite well.

Why does it work?
Actually most LF lenses except telephotos are quasi-symmetric and the distance between the exit pupil and the focal plane of the camera lens is equal to the focal length, and most of the time we focus to object distances much larger than 2 focal lengths, the sharp image is located close somewhere behind the focal plane. Hence the distance between the exit pupil and the image plane is, say, between 100% and 130% of the camera lens focal length (for a quasi-symmetrical design).

Fresnel lenses with a focal length equal to their diameter, or to their diagonal, are standard.
In other terms, most Fresnel lenses are F/1 single element lenses, something almost unconceivable with a real thick glass lens!
And will be perfectly suited for use with a standard focal lens e.g. 150 mm (6") in 4x5" image, diagonal = 150 mm (6"), focusing at large distances, focal length of the Fresnel lens = 150 mm.
In principle with wide angle lenses you would need a shorter focal length for your Fresnel. f/0.7 Fresnel lenses do exist, but are more difficult to find and are somewhat at the limit of present technology.

So most of the time, f/1 Fresnel lenses in combination with the GG, are chosen, as follows
6x9 cm (2X3") - Fresnel = 100mm (4") focal length
4x5" - Fresnel = 150mm (6") focal length
5x7" - Fresnel = 210mm (8"1/4) focal length
8x10" - Fresnel = 300 to 360 mm (12" to 15") focal length

100mm f/1 Fresnel lenses are easy to find. Same for 300-360 mm, under the form of an A4 or letter-size reading loupe. All overhead projectors have a Fresnel lens in the range of 300 to 450 mm focal length.
150 mm Fresnel lenses are more specialized to LF cameras and so far I did not find any cheap one, outside the specialized market of LF accessories.

Actually, the constraints are the availability or sub-f/1 Fresnel lenses for properly illuminating a ground glass when using wide-angle lenses of quasi-symmetrical design.
Fresnel lenses are more useful with WA angles, but even if the focal length is too long, there will be an improvement.
And you do not want to change your Fresnel lens any time you change your camera lens!

I am not aware of specialized ultra-short Fresnel lenses for large format cameras, but I have in mind at least one example for medium-format cameras.
For use with the Hasselblad SWC and its 38mm biogon lens, there was a GG accessory that you would substitute to the film back, to focus and compose like with a view camera. The standard focal length for Fresnel lens in 6x6 cameras is about 80 mm [difficult to say, for example in the post-1958 Rolleiflex, the GG is a single piece of molded plastic combining the function of a Fresnel lens and a GG, impossible to check the focal length easily!], hence for the 38 mm Biogon, a quasi symmetrical lens design, you would need a much shorter focal length that 80. Ideally = 38 mm, distance between the exit pupil and the focal point, but this would imply a f/0.5 lens ...

Regarding the proper assembly of a GG + Fresnel combination, the one I have on my Arca Swiss is as follows, from the lens exit lens to the photographer's eye

Fresnel polished side
Fresnel corrugated side
Ground glass ground (frosted) side
Ground glass polished side

This combination requires a proper placement of the GG, according to this diagram, the image being pushed backward by about 1/3 of the thickness of the Fresnel lens. This offset is built-in the springback design.
https://www.flickr.com/photos/43175600@N00/30929837496

This combination has the following advantages:
The polished side of the glass is the only one accessible to dust and operator's finger prints. Glass is harder than plastic and will resist better to scratches when manipulating a loupe.
The corrugated side of the Fresnel lens as well as the frosted side of the GG are protected, laying one against each other. The frosted side of the GG is more sensitive to dust and scratches than the polished glass side.
The flat side of the Fresnel lens is less prone to catch dust than the corrugated side. The Fresnel lens in plastic is more fragile than the glass GG and is located inside the camera, and cannot easily be damaged ... except if you collapse the bellows to a point where the lens touches the Fresnel lens.

All combinations where you simply lay the Fresnel on top of the GG are fine, no focus shift at all, but doing so the Fresnel Lens is more easily scratched.
However, I have found 310 mm focal length, A4-size plastic Fresnel lenses for €4.90 (e.g. here) (http://astromedia.eu/Material-fuer-Selbermacher/OPTI-Media-Fresnel-Linsen/OPTI-Media-Fresnel-Linse-F-53::169.html), taking into account the price of a single sheet of 8x10" film ;-)


Hope this long explanation helps!

xkaes
20-Jul-2017, 11:37
Mission: impossible, since we change lenses all the time and re-focus lenses for all distances!
You would need an adjustable zoom Fresnel lens!!!

All combinations where you simply lay the Fresnel on top of the GG are fine, no focus shift at all, but doing so the Fresnel Lens is more easily scratched.

An even crazier idea would be to have to carry a different Fresnel for each lens. Maybe some perfectionists have the time and money to do this -- as well as the pack animals.

As to the latter point, #1, we can just be more careful, or #2 purchase a "groundglass cover" -- which in this case is a "Fresnel cover".

Your long explanation has cleared a lot of dust -- I hope!!

Emmanuel BIGLER
20-Jul-2017, 11:47
An even crazier idea would be to have to carry a different Fresnel for each lens.

Yes, but in an era where everybody tells you "I simply take pictures with my mobile phone", accumulating ultra-specialized gear like, a whole set of high-quality Fresnel lenses of all sizes and all focal lengths and all possible f/numbers (including rare f/0.5 models), and preferably stored in a elegant real-leather porfolio, is the distinctive sign of the discerning LF photographer who refuses to follow the mainstrem of ordinary photographers ;)

Peter Collins
20-Jul-2017, 12:17
Coucou 'manuel! Merci beaucoup!
Peter

xkaes
20-Jul-2017, 15:29
Yes, but in an era where everybody tells you "I simply take pictures with my mobile phone", accumulating ultra-specialized gear like, a whole set of high-quality Fresnel lenses of all sizes and all focal lengths and all possible f/numbers (including rare f/0.5 models), and preferably stored in a elegant real-leather porfolio, is the distinctive sign of the discerning LF photographer who refuses to follow the mainstrem of ordinary photographers ;)

Hey if it was good enough for W. H. Jackson, it's good enough for me! My guess is that he was a real chick magnet in his day!

167440

Emmanuel BIGLER
21-Jul-2017, 01:20
Coming back to rare f/0.5 Fresnel lenses, I found one from Edmund Optics that would be well-suited for wide-angle lenses in 4"x5"

https://www.edmundoptics.com/optics/optical-lenses/fresnel-lenses/6.7quot-x-6.7quot-3quot-focal-length-fresnel-lens/

Effective diameter = 6" = 150 mm, focal length = 3" = 76 mm. This is a f/0.5 lens !
$89 ... not cheap.
If you look at Edmund's offer, most of Fresnel lenses they have on catalog (https://www.edmundoptics.com/optics/optical-lenses/fresnel-lenses/) are in the range of f/1 - f/2, only few of them are f/0.7 or even f/0.5.

This one is a f/0.7, 5" [127 mm] square, diagonal 7", focal length 5" [127 mm] could be a good candidate for a general-purpose Fresnel lens in 4"x5". Still $50, though.
https://www.edmundoptics.com/optics/optical-lenses/fresnel-lenses/5.0quot-x-5.0quot-5quot-focal-length-fresnel-lens/

John Layton
21-Jul-2017, 10:33
I actually find that a fresnel with a focal length of appx. five inches is, for me, the best compromise for the 5x7 format - with my most-used lenses for this format being 120 and 210, followed by 90 and 305. I'd probably go with something longer if I also used something like a 450 for this format.

I did have to modify my film back in order to place the fresnel between the ground glass and lens...with the cut fresnel field facing backwards (in contact with the ground glass). Forgot exactly how much I needed to recess the assembly into the gg frame...but Bill Maxwell can give you this info.

EdSawyer
22-Jul-2017, 06:47
Emmanuel, great information! Thanks for the detailed write up and sharing of all that data. Very helpful.

Doremus Scudder
22-Jul-2017, 11:30
So let's summarize and clarify for the sake of those possibly misled by the OP's original post:

1. Placing the Fresnel screen between ground glass and camera lens WILL change the focus and result in out-of-focus negatives, so beware! The Fresnel shortens the optical path by ~1/3 of its thickness.

There are two ways to wrongly place the Fresnel between ground glass and lens (assuming here that the camera has a simple ground glass correctly positioned to start with): A. The Fresnel screen is exactly the same size as the ground glass and sits in the recess that normally holds the ground glass; the ground glass is then placed on top of it. This will result in the ground glass being optically spaced too far back by about 2/3 the thickness of the Fresnel screen. If this is a desired configuration, the camera back will usually have to be modified to position the entire sandwich closer to the lens than the original position. B. The ground glass stays where it is originally positioned and a Fresnel screen is (somehow) affixed to it between it and the lens. This results in the optical path being shortened by ~1/3 the thickness of the Fresnel. This latter, of course, could be rectified by shimming the entire sandwich out by the appropriate distance (measure carefully and shim or find someone with an optical bench in both cases).

2. Placing the Fresnel screen between ground glass and eye will NOT change the focus. This is the safest and easiest way to add a Fresnel screen for those that have cameras that originally came with just a ground glass.

Replacing an OEM Fresnel/ground glass sandwich that has the Fresnel positioned between ground glass and lens with a regular ground glass will shift the focus from correct. Better to try and find a factory or after-market dedicated replacement.

3. Cheap Fresnel screens designed for reading help are (thanks Emmanuel) usually in the 300-360mm focal-length range and, therefore, most helpful with 8x10 cameras. Using this focal length Fresnel on a 4x5 (or even 5x7) will work for long lenses but present viewing difficulties with short focal-length lenses. The rule of thumb is that a Fresnel with the focal length of the "normal" lens for any format is close to the ideal compromise, i.e., 150mm for 4x5, etc.


4. There are a lot of readily-available Fresnel screens for 4x5 and larger formats from many manufacturers, both OEM and after-market, that are specifically designed for particular cameras and formats as well as others in various focal lengths that can be utilized by those that have the skill and knowledge to install them correctly. These are vastly superior to the cheap Fresnel screens for reading pages.

Best,

Doremus

Emmanuel BIGLER
22-Jul-2017, 14:14
Thanks to Peter Collins and Ed Sawyer for their appreciation!
Well, I'm happy if we can record here various ideas, experiments, and tests regarding Fresnel lenses and share them with the community!

Thanks to John Layton for sharing his experience with the 5x7" format. Generally speaking, a Fresnel lens with a focal length of 5" covering a diagonal of 8-1/4" [210 mm] for the 5x7" format is roughly a f/0.6 lens, suitable for wide-angles taking lenses, an extreme Fresnel lens in terms of f/number, not so easy to find!

And thanks to Doremus for his concise explanation!
Explaining the principles of a Fresnel lens in "Twitter-mode" with not more than 140 sign is the next challenge for you, Doremus ;)

Rich14
22-Jul-2017, 19:19
I've really been intrigued by this thread. I've never used a Fresnel lens. Always wanted one, but never got around to it.

I just got a Sinar 4x5.

Reading all the opinions here, I figured, now's the time. What the hell, just try a cheap lens and see what it does. What could it hurt? I like empiric solutions to problems.

So I got this one from Amazon. $6.50 (big ticket item)

https://www.amazon.com/gp/product/B01MXF0RI2/ref=oh_aui_detailpage_o00_s00?ie=UTF8&psc=1

It came today. 5" x 4.5". Much thicker than I expected - 0.024" - that's as thick as 6 sheets of copy paper. It's advertised as a 3x magnifier so I figure its focal length is about 3.3", 80mm

I cut it down to 5" x 4" to fit behind (on my side of) my ground glass. Surprisingly hard to cut. I had to use a light tin snips.

Secured it under the same clips that hold the glass. Ruled side toward the glass

It works great!

It's much easier to compose now with my 75mm lens as I can easily see the image to its periphery and it causes no problems up through a 300mm which is the longest lens I have right now.

Focusing is straight forward. The thickness of the Fresnel required a tiny adjustment of my 10x loupe to focus on the rules on the glass which are on the ground side. The concentric lines of the Fresnel are exceedingly fine and seem to be in focus in the loupe even though they are slightly above its plane of focus, against the rear side of the glass.

Sometimes the simplest solutions are the best.

Rich

Emmanuel BIGLER
23-Jul-2017, 05:43
From Rich14:
... I figured, now's the time..... Sometimes the simplest solutions are the best.

Hi, Rich, and many thanks for your feedback! I'm glad that you found a source for Fresnel lenses suitable for the 4x5" format !

It's advertised as a 3x magnifier so I figure its focal length is about 3.3", 80mm

Yes, exactly!
In principle, the focal length of a loupe in millimeters is

(250 mm) / (commercial magnification)

250 mm / 3 is about 80 mm

But since the definition of "commercial magnification" might be ... fuzzy, you can easily double-check by simply focusing on the palm of your hand (or on a piece of white paper) the image of a small-size light source, the image should be located somewhere 80mm behind the lens.

Beware: if you focus the Sun, you'll be able to ignite a fire! (this caveat is often printed on those large reading loupes)

Enjoy your wide-angle Fresnel lens for the 4x5" format!

Sweep
23-Jul-2017, 08:38
Well the thread title does say your worries answered, so I'm presuming I can add 'on-topic' questions:
I have just stripped out the clear Perspex cover and Yanke Fresnel from my Wilderness 8x10 to clean out the dust and I have noticed that the Yanke was a very tight fit in the recess. This caused the Yanke to convex towards the lens leaving a gap in the middle between it and the clear Perspex. On examination the two sheets are slightly different dimensions. The Perspex was a nice clearance fit.
Before I try trim to size and return the Yanke to true flat, am I doing the right thing or is the Yanke designed to be convex such as you would find in any lens?
I guess not but thought I would ask.

thanks ...Sweep

Emmanuel BIGLER
23-Jul-2017, 15:26
This caused the Yanke to convex towards the lens leaving a gap in the middle between it and the clear Perspex.

Hello!

Before you re-assemble your Fresnel with the Perspex, you should, just for curiosity, try to estimate your Fresnel's focal length by focusing a distant light source.

Regarding the curvature of the Fresnel lens resulting from being bent in its holder, I do not think that it can have any significant effect on the focusing properties of the lens.
The argument is very simple: if you consider a standard f/1 Fresnel lens (e.g. focal length 300 to 360 mm for the 10x8" format), on the edges rays, between input and output, are bent by about 25.
The fact that the Fresnel lens is slightly bent by a fraction of a degree in the edges with respect to the centre, cannot change things significantly. Moreover if you look at the structure of the Fresnel lens, made of tiny prismatic elements, when you rotate a prism by a fraction of a degree, in fact you do not change the total (input/output) bending angle very much in any case.
However you can imagine Fresnel grooves fabricated on a curved surface, but I assume that your Fresnel lens is simply flat and parallel, with equal thickness everywhere, when released from its mount.

I have no clue to explain what you have observed, but my feeling is that it is simply a parasitic effect due to some dimensional instability of the Fresnel's plastic when ageing.
I would do nothing and simply re-install the Fresnel, slightly convex!

stawastawa
23-Jul-2017, 17:54
can we get Doremus's post number 29 (http://www.largeformatphotography.info/forum/showthread.php?140260-Fresnel-lenses-and-ground-glass-focusing-screens-your-worries-answered&p=1399808&viewfull=1#post1399808) and the article linked above added to the LFP.info homepage?

thanks folks!

Rich14
23-Jul-2017, 18:06
This caused the Yanke to convex towards the lens leaving a gap in the middle between it and the clear Perspex.


I have no clue to explain what you have observed, but my feeling is that it is simply a parasitic effect due to some dimensional instability of the Fresnel's plastic when ageing.
I would do nothing and simply re-install the Fresnel, slightly convex!

My Fresnel screen is slightly convex. This causes no difficulty. To focus with my loupe, the screen gets pressed flat against the back of the ground glass. As it is flattened, there is barely no change in the appearance of the image.

The degree of tolerance of the Fresnel/main lens/ground glass "system" is quite broad and not critical.

Sweep
24-Jul-2017, 04:49
Thanks for your replies Emmanuel & Rich though I am not sure I fully understand what you mean. Please note, Rich, that I do not have a separate ground glass only the combination Fresnel and clear cover so, I guess, I am actually focusing on the fresnel
The Fresnel isn't actually sandwiched between anything and because it oversized in the long dimension, compared to the camera back, when the clear perpex cover is placed on the Fresnel bows towards the lens. Surely this no longer replicates the true film plane and must effect focus?
I have removed the Fresnel from the camera and it is starting to 'relax' back to become flat but should I trim it to the correct size so it stays flat?

Sweep
24-Jul-2017, 05:12
here are a few pics of the Fresnel and clear Perspex. I have actually flipped the Fresnel to demonstrate the bulge as you can see there is a high spot in the middle and the edges have lifted. In reality the edges would touch and the centre bulge would be away from the Perspex.
167515
167516
167517

Rich14
24-Jul-2017, 07:18
Sweep,

At first I did not understand the "anatomy" of your Fresnel lens/ground glass/cover glass sandwich.

Just to clarify my set-up, my ground glass sits with its frosted side, defining the film plane, toward the lens (of course). My thinner plastic Fresnel lens sits behind the ground glass with the concentric prism facets facing forward, pressed against the smooth back of the ground glass.

The Fresnel lens bulges very slightly away from the back of the ground glass. It doesn't lie perfectly flat as the Fresnel lens is slightly convex toward its back or smooth surface. I don't think it's intended to be that way, but the manufacturing process leaves the plastic with uneven tension. That slight bulging does not seem to affect its functioning. I don't think I could permanently remove that bulging, although keeping it pressed flat for some time might. When I focus with my loupe, the Fresnel gets pressed tightly against the back of the ground glass, flattening out the bulge. The image I see through the Fresnel does not change much from bulging condition to flat.

In your case your Fresnel/ground glass (one single unit) is bulging toward the lens, right? The bulge is the result of its having been forced out of flat because it is too long for the channel in which it has been restrained, yes? Therefore, it is not lying perfectly in the film plane and your focusing cannot be accurate.

I hope you can get it to relax back to its intended flat plane.

Rich

Sweep
24-Jul-2017, 09:16
Sweep,
In your case your Fresnel/ground glass (one single unit) is bulging toward the lens, right? The bulge is the result of its having been forced out of flat because it is too long for the channel in which it has been restrained, yes? Therefore, it is not lying perfectly in the film plane and your focusing cannot be accurate.


Absolutely right. It may have deformed over time but I suspect that it has never been the correct size as the camera is only 3 years old.
It will easily trim with a pair of scissors and then I will flatten it under a few heavy books and see how it goes.

Doremus Scudder
24-Jul-2017, 15:09
Sweep,

Your Fresnel sandwich has to include a frosted surface for the image to be focused on. In essence, it is a ground glass and Fresnel lens in one. Wista screens are made this way as well. So if it is deformed, the frosted image-forming surface is as well and your focus will be slightly off. Do try to get it back to flat.

Best,

Doremus

Emmanuel BIGLER
25-Jul-2017, 08:45
Coming back to the discussion!

From Sweep, Yorkshire:
Absolutely right. It may have deformed over time

Well, sorry, I did not understand that your Fresnel lens was also the ground glass! And that your PerspexTM (** note 1) sheet was just a protective overlay.

Exactly like in the post-1958 Rolleiflex TLR's focusing device, which is a molded plastic Fresnel lens with a very fine pitch and a frosted "look".

Hence, as usual ;), Doremus is right, it's better if your "ground glass" is perfectly flat!

However, the situation is not really catastrophic, taking into account a reasonable estimation of depth-of-focus in the 10"x8" (** note 2) format.
For far-distant objects, the focusing tolerance around the sharp image plane is plus or minus N c, where "N" is the f-number and "c" is the diameter of the circle of confusion chosen as a sharpness criterion.
If we take c = 0,17 mm (0,17 mm = 300 mm /1720, 1/1720 = 2 minutes of arc) and N=22, we get a depth of focus of plus or minus 22x0,17 = plus or minus 3,7 mm. Hence you can safely tolerate about 2-3 mm of bulging for your Fresnel+GG combo.



(** note 1) The first time I read the word "PerspexTM " was in a book entitled Spitfire at war by Alfred Price. (http://www.worldcat.org/title/spitfire-at-war/oclc/3898747)
I had never heard about "PerspexTM " before, because Poly-Methyl-Metacrylate (PMMA) plastic changes name when crossing the borders!
In France we sometimes call it AltuglasTM ;) but more often "Plexi" for PlexiglasTM.
For me, "PerspexTM " is like the RADAR, legendary British technology of WW-II!
In Spitfire at war, the author explains how the ground staff had to carefully clean and polish "PerspexTM " canopys, it was a matter of life and death for RAF pilots, stray light being the enemy, according to the legendary rule established during WW-I: "The Hun is always in the Sun".

(** note 2) 10"x8" is supposed to be the proper British denomination for a film format that we call 20x25 cm in France ;)

Dan Fromm
25-Jul-2017, 09:37
Lucite™

Sweep
25-Jul-2017, 12:39
Thanks Doremus for your guidance and thanks to Emmanuel for your entertaining reply :)
I have only shot eight sheets of 10x8 so far and all appear fine on the focus, although potentially having focus appear to change radially from the centre, apparent at fully open diaphragm, doesn't help my learning curve much!
Whilst you say tom"ay"to and I say tom"ar"to, and you say 8x10 and I say 10x8, it appears that ICI chemists in England first discovered PMMA in the 1930's and bestowed upon it the name Perspex so, being English, I will cling on to this small comfort and recall the days when we actually developed and made things that were useful.

Daniel.E
28-Jul-2017, 22:36
Some cameras are designed so that you can place the fresnel in between the lens and the ground glass with a proper slot to sit the fresnel. I remember reading that if your ground glass is fine then it benefits from having the fresnel in front of it, between the lens and the ground glass. I remember doing that with a Hopf ground glass and there was a noticeable difference. My camera has a provision for the fresnel to be put in between the lens and ground glass.

Jim Jones
29-Jul-2017, 05:16
Some cameras are designed so that you can place the fresnel in between the lens and the ground glass with a proper slot to sit the fresnel. I remember reading that if your ground glass is fine then it benefits from having the fresnel in front of it, between the lens and the ground glass. I remember doing that with a Hopf ground glass and there was a noticeable difference. My camera has a provision for the fresnel to be put in between the lens and ground glass.

Remember that placing the Fresnel between the ground glass and lens will change the optical path as Doremus so thoroughly described in post #29.

consummate_fritterer
29-Jul-2017, 12:30
Years ago, I bought a Chamonix 45N-1... the one with the focus problem. I bought a Maxwell screen and had it installed by the technician Maxwell always recommended. It's a custom fit to the Maxwell screen.