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Songyun
4-Dec-2009, 11:00
Since the other thread is so hot. I am just curious what are the design used in different brand cameras.
For what I know,
1. Shen Hao (old model 45, I haven't played with any new models) offer only ground glass with a slot for fresnel (no adjustment)
2. Tachihara fresnel is between glass and lens (I don't know if any adjustment was made)
3. Canham use the screen has fresnel on one side and ground glass on the other side and a cover glass.

Gem Singer
4-Dec-2009, 11:23
Tachihara and Shen Hao designed their backs with a recessed cutout so that a Fresnel can be mounted between the lens and the ground glass. These are factory adjusted for proper depth.

Tachi includes a Fresnel, frosted on one side, grooved on the other side, with their cameras. Shen Hao does not.

The 4x5 and 5x7 Canham Traditional backs allow an accessory Fresnel and cover glass to be used instead of the original ground glass. The Fresnel is mounted between the frosted surface and the photographer's eye. No adjustment needed.

The later 8X10 Canham backs have a recessed cut out that allows an accessory Fresnel to be mounted between the lens and the ground glass. No adjustment needed.

I have owned all four of these cameras with Fresnels. Still own the Canhams. No problems with focus.

Mike1234
4-Dec-2009, 11:26
I've decided to buy an acid etched focus panel (no grain and no melting parafin) and have at least three fine quality fresnels of differering FL's used on the back side (viewing side) with quick/easy temporary clips to hold them in place and swapped as needed to match the FL of taking lens. It seems like the best of all scenarios.

Songyun
4-Dec-2009, 11:59
Tachihara and Shen Hao designed their backs with a recessed cutout so that a Fresnel can be mounted between the lens and the ground glass. These are factory adjusted for proper depth.



Gem, I just have a question about ShenHao. Since it doesn't offer fresnel as standard purchase. Does the recessed cutout is shallower than the thickness of fresnel to push the ground glass back, or it equals the fresnel thickness?

Gem Singer
4-Dec-2009, 13:05
The Fresnels that I use are made out of plastic and have a frosted surface on one side and Fresnel grooves on the other side.

The Tachihara, Maxwell and the Canham Fresnel's are made that way.

As long as the frosted surface of the Fresnel is placed in the same position as the original ground side of the ground glass, the total thickness doesn't matter.

A cover glass is used to prevent the focusing loupe from scratching the softer plastic screen. Available in plain or gridded.

Using a ground glass as a cover glass will cut down transmission of the light.

Recessed cut outs are usually milled to high tolerances to accept a factory furnished Fresnel, that is placed between the ground glass and the lens, in order for it to focus correctly.

Tim Povlick
4-Dec-2009, 14:03
The 4x5 and 5x7 Canham Traditional backs allow an accessory Fresnel and cover glass to be used instead of the original ground glass. The Fresnel is mounted between the frosted surface and the photographer's eye. No adjustment needed.

The later 8X10 Canham backs have a recessed cut out that allows an accessory Fresnel to be mounted between the lens and the ground glass. No adjustment needed.


Hi Gem,

I have a Canham 8x10 Traditional (recent model) that I placed the Fresnel in the recessed cut out as you mention. I assume the late models one still uses the ground glass, is this correct? Everything seems to work fine, focus / brightness.

It cracks me up when folks look under my dark cloth at this screen and comment that's it's like a computer LCD only better.

_ .. --
Tim

Gem Singer
4-Dec-2009, 15:28
Correct Tim.

The Canham Fresnel for the 8X10 has a clear back instead of a frosted back.

Therefore, the light doesn't pass through two frosted surfaces.

RichardRitter
5-Dec-2009, 06:37
The most common way is to put the Fresnel lens (clear) on the ground glass closes to the viewer eye. This way the image is focused on the ground surface. The problem with putting the Fresnel between the lens and the focusing area is with very short and very long lenses focusing starts to shift on to the ridges of the Fresnel lens. Causing a focusing problem. Problems also crop up when using swings and tilts with the Fresnel lens on the inside of the camera. Cameras that have the Fresnel on the inside of the camera often have very little in the way of movements and are limited to the type of lens used on the camera.

Fresnel lenses can be made to different focusing lengths. The length used for 4 x 5 camera covers lenses from 90 mm to 270 mm lenses with little to no problems. Some times problems accrue with a 90 mm or a 270 mm lens when tilt and swing are used. Having the Fresnel lens on the outside of the camera makes it easier to remove. Sinar camera backs are made so the Fresnel lens can be removed very quickly.

Best thing to do is keep the ground glass clean and use a very dark dark cloth big enough to wrap around the camera back and seal out all the light.

I took the Fresnel off may camera 15 years ago and have not missed it at all. I was always having problems with it when using a 305 mm lens with swings and tilts.

RichardRitter
5-Dec-2009, 10:17
There is on more thing that gets miss informer from time to time that putting a Fresnel lens on the camera can produce up to a stop more light to see the image with. This is false. The lens is only projecting “x” amount of light onto the focusing area. This does not change when you replace the ground glass with a Fresnel. The only thing that changes is that the Fresnel lens makes the image on the ground glass look evenly lighted.

The reported increase of viewing light out put by others can be for the following reason.
1 Did the testing wrong.
2 Ground glass was dirty, poorly made, made on the wrong type of glass or some other problem and replace with a Fresnel that was a ground glass and Fresnel lens in one.

Fresnel are made by cutting the lens into optical grade plastic. This type of plastic is what they make glasses that ever one wears. The optical transmission is about 2% more then what is used to make the standard ground glass. But most people offset this 2% gain by putting the wrong type of cover glass on the Fresnel. The only way to save the gain is to buy an optical grade cover glass.

Dirty ground glass can cut the see-able light by up to 15% or more.

Mike1234
5-Dec-2009, 12:03
Richard... is your ULTIMATE solution to use a non-frosted fresnel on the focusing side of a super-fine ground or acid etched screen? Furthermore, if one has a vast range of lenses, one should have at least three fresnels from which to choose... one for 38-75mm lenses, another for 90-200mm lenses and yet another for 300-600mm lenses? I'm just trying to get this straight in my feeble mind.

RichardRitter
5-Dec-2009, 13:08
The best is to have a clear Fresnel lens that is removable. That you could put on the outside of the ground glass between you and the glass. Do final focusing with out the Fresnel.

Mike1234
5-Dec-2009, 13:40
Thank you, Richard... what fresnel(s) do you recommend?

bobwysiwyg
5-Dec-2009, 14:17
It might seem off-topic, but related. Assuming one mounts the Fresnel behind the GG, that is between the GG and your eye, and you are using a loupe, is the loupe's fixed focus going to shift to the Fresnel rather than the GG? In other words if the Fresnel is 1mm thick, is your loupe view now off by 1mm?

Peter K
5-Dec-2009, 15:11
IMO a fixfocus loupe is good enough for pictures but for focusing a LF-camera one needs a loupe with an adjustable eyepiece. Not only if one uses a fresnel lens but also it helps to focus on the grains of the gg and the image. It helps with myopic or hypermetropic eyes too.

bobwysiwyg
5-Dec-2009, 15:16
Peter, thanks. Can you give me an example of an adjustable focus loupe make?

Mike1234
5-Dec-2009, 15:18
One needs a loupe which can be focused to adjust for such things and to one's eyes. This is simply an adjustable diopter available on many, if not most, loupes.

Peter K
5-Dec-2009, 16:18
Peter, thanks. Can you give me an example of an adjustable focus loupe make?
I'm using old Linhof loupes 8x made by Schneider. This loupes have a diameter of only 34mm, so one can also focus at the edges of the gg.

The new Schneider (http://www.schneideroptics.com/pdfs/photo/LoupeData.pdf) and also Rodenstock (http://www.rodenstock-photo.com/mediabase/original/e_Rodenstock_Magnifiers_74-75__8229.pdf) loupes have IMO too big diameters so I would choose a Peak (http://www.peakoptics.com/index.php?main_page=product_info&cPath=66&products_id=10) loupe without reticle.

Peter

bobwysiwyg
5-Dec-2009, 16:59
One needs a loupe which can be focused to adjust for such things and to one's eyes. This is simply an adjustable diopter available on many, if not most, loupes.

Not my cheapo.:)

Peter, again, thanks for the info.

Tim Povlick
5-Dec-2009, 18:58
Correct Tim.

The Canham Fresnel for the 8X10 has a clear back instead of a frosted back.

Therefore, the light doesn't pass through two frosted surfaces.


Thanks for the information Gem.

Best Regards,

Tim

Mike1234
6-Dec-2009, 05:38
I'm going to try using some +6 (thick) reading glasses instead of a loupe. It seems easier to me.

bobwysiwyg
6-Dec-2009, 06:06
I'm going to try using some +6 (thick) reading glasses instead of a loupe. It seems easier to me.

I'd be interested in knowing how it works for you after you try them.

Mike1234
6-Dec-2009, 06:26
I'd be interested in knowing how it works for you after you try them.

I'll post my opinion either here or in a new thread in the next few days. I'll be ordering a set today on-line since I haven't been able to find any this powerful OTC.

bobwysiwyg
6-Dec-2009, 06:58
Thanks, Mike.

RichardRitter
6-Dec-2009, 07:18
I use a pair of +3 reading glasses paid all of a dollar for then. So if I loss then no great loss. The +3 through the +6 are great you can look at the ground glass with both eyes. The stronger the magnification the closer to the view screen you will have to be.

Mike1234
6-Dec-2009, 10:12
I use a pair of +3 reading glasses paid all of a dollar for then. So if I loss then no great loss. The +3 through the +6 are great you can look at the ground glass with both eyes. The stronger the magnification the closer to the view screen you will have to be.

Great news, Richard. My myopic eyes already require +1.5 for casual reading. Do you think a +8, +10, or +12 are better options, in your experience? Or maybe I'll just stack the +6 on top of some +2's so I have dual vision... flip up the +6 and use the +2 for corser viewing of camera controls? BTW, I just ordered a pair of +6.

r_a_feldman
7-Dec-2009, 13:06
Richard... is your ULTIMATE solution to use a non-frosted fresnel on the focusing side of a super-fine ground or acid etched screen? Furthermore, if one has a vast range of lenses, one should have at least three fresnels from which to choose... one for 38-75mm lenses, another for 90-200mm lenses and yet another for 300-600mm lenses? I'm just trying to get this straight in my feeble mind.

It seems to me that if you have the fresnel on the outside of the GG (between your eye and the GG), then it should not matter what focal length the fresnel is, since you are using it to view the flat plane of the GG. Only when the fresnel is between the lens and the GG, where the light would be striking the fresnel at different angles for different focal length lenses, would fresnels of different focal lengths be useful.

Bob

GPS
7-Dec-2009, 15:22
Since the other thread is so hot. I am just curious what are the design used in different brand cameras.
For what I know,
1. Shen Hao (old model 45, I haven't played with any new models) offer only ground glass with a slot for fresnel (no adjustment)
2. Tachihara fresnel is between glass and lens (I don't know if any adjustment was made)
3. Canham use the screen has fresnel on one side and ground glass on the other side and a cover glass.

You must have miraculously short memory then. I'm very sure you also know -
4. Chamonix 45N-1 camera with Fresnel lens between glass and lens, with no adjustment and a focus error as a result...

Peter K
7-Dec-2009, 15:23
Bob, if a focussed ray coming from the taking-lens strikes the gg it will be scattered by the grains in a small angle. If the gg is viewed in the direction of the optical axis of the taking lens the image formed on the gg looks fairly bright in the center but at the outer aerea of the gg it is as dimmer as bigger the tanking-lens' angle of view is.

If a field-lens of the same focal-lenght as the taking lens is mounted between gg and the eye the rays are bendet perpendicular to the surface of the gg in the direction of the optical axis again. And so the eye looking on the center of the gg can see all the light coming from the exit-pupil of the taking lens without moving around.

This will only work with the maximum brightness hitting the eye when
a) the field-lens has the same angle-of-view as the taking lens, this means it has the same focal-lengt and is as big as the gg and
b) the optical axis of taking-lens plus field-lens are the same and perpendicular to the gg.

Because a field-lens made from a plan-convex glass-lens in the size of the gg is heavy, breakable and expensive a fresnel-lens is much more convenient. Also the imaging quality possible with a fresnel-lens is sufficient for the eye because our "build-in image processor" ignores distortion, color fringes etc. made by such a lens. And a field-lens placed behind the gg meets not only it's optical function, it doesn't disturbes the focal-plane on the gg resp. on the film too.

But if the two optical axes, the taking-lens' and field-lens', aren't inline anymore - the gg-plane is tilted or shifted as often with LF-cameras in use - the brigthness of the combination taking-lens plus gg plus field-lens decreases. If the gg or taking-lens is only shifted the field-lens can be shifted in the opposite direction to get the best brightness again. But if the taking-lens or the gg are tilted, there is no easy way to keep the brightness on the gg.

http://picz.to/image/Jf8

Peter

mortensen
8-Dec-2009, 16:08
You must have miraculously short memory then. I'm very sure you also know -
4. Chamonix 45N-1 camera with Fresnel lens between glass and lens, with no adjustment and a focus error as a result...

Please, sir GPS, I am sure by now all of us and each and everyone of us plus loads of innocent european ski tourists know (in explicit details) your positions on that company!

... and regarding the topic, thanks Peter K and Richard Ritter for interesting information.

GPS
8-Dec-2009, 16:19
Please, sir GPS, I am sure by now all of us and each and everyone of us plus loads of innocent european ski tourists know (in explicit details) your positions on that company!

...

Please what then? Stop carrying about innocent European tourists and get your senses together...

Mike1234
10-Dec-2009, 16:58
I just received my +6 reading glasses and gave them a try on my Toyo 810G GG. They work GREAT!! I'll never use another loupe of any sort including the tilting type. I can move my head around freely to few all corners with ease and both my hands are free. Plus I can view with both eyes for greater resolution. My eyes are +1.5 in error myopically so this compares to +4.5 to others without this issue. That said, I could use +8 or +10 for ultra-fine focus. I'm going to buy a +4 set for corse viewing and stack the +6 to slide on top for fine focusing. To HECK with loupes!!

bobwysiwyg
10-Dec-2009, 19:16
Mike,

Where did you find them? Not much selection where I am.

wclavey
11-Dec-2009, 08:06
I am going to bungle the words here, so please forgive me that... I know this in advance.

Back on the question and response about putting a fresnel between the lens and the GG, it was my understanding that a fresnel lens transmitted the light at a rate that was equivalent to only 67% of the equivalent amount of air... in other words, if you inserted a 2mm thick fresnel into the light path, that was the equivalent of moving the GG forward by 0.66mm (2.0 - (.67 * 2.0))mm.

Perhaps that is not true. But if it is, then inserting a screen that is fresnel on 1 side and GG equivalent on the other so that the GG surface is in the same place as a plain GG would have been is the equivalent of shifting the GG surface forward (toward the lens) by 33% of the thickness of the viewing screen, isn't it?

Again, my apologies for the brutally layman description of what I'm trying to say - - but I'm sure you who know what you are talking about will understand.

Peter K
11-Dec-2009, 08:43
Back on the question and response about putting a fresnel between the lens and the GG, it was my understanding that a fresnel lens transmitted the light at a rate that was equivalent to only 67% of the equivalent amount of air... in other words, if you inserted a 2mm thick fresnel into the light path, that was the equivalent of moving the GG forward by 0.66mm (2.0 - (.67 * 2.0))mm.
This will only work with a fresnel-lens with infinte focal-length, a clear plate.

But a fresnel-lens is at first a lens with the focal-lengt of e.g. 150mm that works together with the taking-lens as I've described here (http://www.largeformatphotography.info/forum/showthread.php?t=56287&page=4) #36. So if you won't to calculate the compensating factor for not only every lens you're using but also for all different bellows-extentions, put the fresnel-lens behind the gg. It's easier. ;)

Mike1234
11-Dec-2009, 09:51
Mike,

Where did you find them? Not much selection where I am.

I bought these on-line. I couldn't find anything stronger than +2.75 locally OTC here either.

http://www.speert.com/closeup.cfm?ID=1392&cfpage=reading-glasses-products.cfm&startrow=1&SubCategory=Reading%20Glasses%20%2D%20High%20Power&readingpower=&sort=1373&keywords=

bobwysiwyg
11-Dec-2009, 10:17
Thanks, Mike.