Fresnel Question

[alanmcd]

I just want to get clarification about Fresnel screens

I have a plastic one for my Shen Hao.
I’ve watched Alan Brock’s video but I’m not outlaying for a Maxwell yet.
So if someone can let me know if I understand the following:
1. The screen needs to always be ribs to the lens – flat side to the eye.
2. If you put the screen under the GG, you may need to allow for focus since you are still focussing on the GG not the fresnel and the fresnel is now moving the focal plane 2 mm away.
3. If you put the fresnel on the eye side of the GG, the improved brightness may not be as good as inside the GG.
4. If I operate well within my DOF, the focus issue is not really in play
5. Obviously if I use tilt/shift movements, the DOF (hyperfocal distance) is harder to gauge and therefore moving the Fresnel to the outside would be better under these circumstances.

I’ll do some tests but I just wanted to know what to expect.
Thanks
Alan

[Huub]

I am afraid that your points 4. and 5. are not valid and the vocal issue will be in play, even when using larger appertures. Some of the other forum members will probably have some mathical formulas to illustrate the case.

The good news is that the Shen Hao has probably a feature that allows you to put the fresnel lens in front of the GG, without moving it's position. At least min has. When you take the GG of the camera you will see two slots that will allow for the fresnel lens and letting you put back the GG to it's original position.

[Huub]

I am afraid that your points 4. and 5. are not valid and the vocal issue will be in play, even when using larger appertures. Some of the other forum members will probably have some mathical formulas to illustrate the case.

The good news is that the Shen Hao has probably a feature that allows you to put the fresnel lens in front of the GG, without moving it's position. At least min has. When you take the GG of the camera you will see two slots that will allow for the fresnel lens and letting you put back the GG to it's original position.

[Bob Salomon]

1 all photo fresnels are always plastic.
2 if the fresnel is under the gig you have to have the gig focus positioned adjusted and, should you want to remove it for some reason, you can't without readjusting the gig.
3 the effect of the fresnel s the same, in fact, it is probably brighter as the fresnel correction does not then have to pass through the gig. And the fresnel may be removed at any time without changing gig position.
4 depth of focus changes with focal length of your lens, the shorter your lens the LESS depth of focus. So, since the fresnel has some thickness you cannot rely n depth of focus to correct for proper gig positioning.
5 the fresnel on top of the gig is far superior. One of the main reason for positioning original fresnels under the gig was that they were easily scratched. That s not as true today. And today means the last 3 or 4 decades.

[R.K]

On the Shen Hao cameras there are a special space provided for the Fresnel lens under the ground glass. If yours Fresnel made specifically for the Shen Hao, just remove ground glass, put Fresnel in with grooved side facing the ground glass, not the lens, put ground glass on the top of a Fresnel, place the screws back and you ready to go. If the Fresnel made specifically for the Shen Hao cameras and have a right thickness, the focusing of you camera will not be affected in any way.

[R.K]

If you want to know more on Fresnel's and how they work, follow this link http://www.fresneltech.com/pdf/FresnelLenses.pdf . Or open http://www.fresneltech.com/visible.html page and follow to the linked pdf file. I hope you will understand that grooves must be faced photographer and not the lens, and if you don't want grooves to be damaged because the Fresnel made from plastic and grooves usually very small and can be damaged easily, just keep your Fresnel under the ground glass.

[Jim Jones]

On the Shen Hao cameras there are a special space provided for the Fresnel lens under the ground glass. If yours Fresnel made specifically for the Shen Hao, just remove ground glass, put Fresnel in with grooved side facing the ground glass, not the lens, put ground glass on the top of a Fresnel, place the screws back and you ready to go. If the Fresnel made specifically for the Shen Hao cameras and have a right thickness, the focusing of you camera will not be affected in any way.

Beware, if the Fresnel is placed between the ground glass and the lens, it changes the image focus on the ground glass by roughly 2/3 the thickness of the Fresnel. This might not matter with very thin Fresnels and lenses well stopped down. The link provided above is informative about Fresnel lenses in applications other than the special case of camera ground glasses.

[Tin Can]

What I find amusing is my favorite camera maker Horseman, couldn't decide where to put fresnel. I have 2, which have the fresnel in each location. One front, one back of GG. They both look original and NOS.

Since I shoot them stopped way down I can't SEE the difference, but I
have poor vision. Ymmv

In a couple decades I'll have time to examine the problem.

[R.K]

Beware, if the Fresnel is placed between the ground glass and the lens, it changes the image focus on the ground glass by roughly 2/3 the thickness of the Fresnel. This might not matter with very thin Fresnels and lenses well stopped down. The link provided above is informative about Fresnel lenses in applications other than the special case of camera ground glasses.

So in accordance with post above, the Fresnel on the cameras is a special Fresnel, not just a regular Fresnel lens. Well, some people on this great forum like to make a posts even when they not sure are they right or not.
So, some information for the poster above. On the cameras the Fresnel lens used for the same purpose as on the light houses and any other lighting devises – to combine the rays of light falling on it, make those rays parallel to each other and perpendicular to the flat face of the Fresnel lens. As known the ground glass not only a focusing device, it is in the same time a good reflector and diffuser. So when the Fresnel placed under the ground glass it can combine all light falling on it and send a powerful parallel ray of light though the ground glass to make the focusing work much easier. Of course as any other surfaces the Fresnel also reflecting some light back, but because it is clear and made from the special optical material it reflect much less light then the ground glass. So when the Fresnel located under the ground glass it really help. Some people like to locate Fresnel on the top of the ground glass. This is possible to. But what the Fresnel really doing in that arrangement, almost nothing. In that arrangement the ground glass already reflect some sufficient amount of light back in the camera and this light will never any more reach the Fresnel and the photographer.

[Doremus Scudder]

So in accordance with post above, the Fresnel on the cameras is a special Fresnel, not just a regular Fresnel lens. Well, some people on this great forum like to make a posts even when they not sure are they right or not.
So, some information for the poster above. On the cameras the Fresnel lens used for the same purpose as on the light houses and any other lighting devises – to combine the rays of light falling on it, make those rays parallel to each other and perpendicular to the flat face of the Fresnel lens. As known the ground glass not only a focusing device, it is in the same time a good reflector and diffuser. So when the Fresnel placed under the ground glass it can combine all light falling on it and send a powerful parallel ray of light though the ground glass to make the focusing work much easier. Of course as any other surfaces the Fresnel also reflecting some light back, but because it is clear and made from the special optical material it reflect much less light then the ground glass. So when the Fresnel located under the ground glass it really help. Some people like to locate Fresnel on the top of the ground glass. This is possible to. But what the Fresnel really doing in that arrangement, almost nothing. In that arrangement the ground glass already reflect some sufficient amount of light back in the camera and this light will never any more reach the Fresnel and the photographer.

Most of the above quote is false and/or misleading. I'll try to address things in order:

First, although all Fresnel lenses work on the same principle, they are not all the same. Some are thicker than others and, more importantly, Fresnel lenses have a focal length, just like other lenses. For cameras, a Fresnel lens should ideally be matched to the focal length of the lens being used. This is rarely practical, but a Fresnel with an intermediate focal length works well with all but the shortest and longest lenses usually used on a specific format. Which focal length that is depends on the format, of course.

Yes, Fresnel lenses on cameras work the same as Fresnel lenses on a lighthouse in principle, however, it's function in a camera is slightly different, i.e., to take the angled rays coming from the lens and project them back toward the viewer's eye at an angle that lets the viewer see a larger and brighter image.

The diffusing and reflecting properties of ground glass don't change just because a Fresnel is placed between it and a lens (or between it and the eye of the viewer for that matter). Therefore, it doesn't matter really which side of the ground glass the Fresnel is placed on PROVIDED THAT allowance is made for the focus shift introduced by the Fresnel lens when it is placed between the ground glass and the lens. If this is allowed for, then the brightness and other viewing characteristics of the Fresnel/ground-glass combination are the same.

If you place a Fresnel between ground glass and lens, the position of the Fresnel must be slightly different than that of a plain ground glass without the Fresnel. Not adjusting for this will cause significant focus shift (approx. 2/3 of the thickness of the Fresnel screen) and result in unsharp photographs. Stopping down won't focus the lens in the right place, only reduce the size of the out-of-focus circles of confusion. As mentioned above, shorter lenses (although they have greater depth of field) have a shallower depth of focus and the softness will be most apparent when using these. However, long lenses aren't immune and, unless you're contact printing or only enlarging slightly, you'll be a lot better off having a Fresnel that is positioned properly.

For the OP: If your Shen Hao has a recessed seat for the addition of a Fresnel lens between ground glass and lens, you still have to be sure it is the proper thickness, i.e., the thickness that the manufacturer has designed the recessed seat to compensate for. If you aren't sure of this, then you'll be better off mounting the Fresnel between the ground glass and your eye. Often, in the latter configuration, a protective clear glass piece can be added to protect the Fresnel from scratches.

Best,

Doremus