Fresnel - Focal length?

[oliver_vogler]

<p>Hey guys!</p>

<p>I'm playing with the thought of building my own LF camera and am currently gathering information about the topic. At the moment I'm trying to figure out the cheapest ways to make a ground glass for the back of the camera. The glass itself should be no problem but I have some questions about the fresnel layer. I don't want to spend 200$ on this but I found lots of cheap plastic fresnels that would do the job just as well, I think. I would need it to see a brighter picture and then remove it for focussing.</p>

<p>However, they all have different focal lengths, and that's what confuses me. I didn't find a single question about this in the forums nor anywhere else. Is there anything to consider when I choose one or doesn't it really matter what focal length I get? Is there any connection with the lens I'm going to use? For example, I found some with a focal length of +330 and others with +118 and I'm sure they have a different influence on the picture but I can't figure out how it will affect the brightness or enlargement.</p>

<p>I think I'm going to attach the fresnel on the photographer's side of the GG. Like that it doesn't mess with my focus and is easier to remove when I don't want to use it. Plus, since those plastic fresnels cost next to nothing, they can protect the GG which would be more of an effort to replace.</p>

<p>What exactly are the effects of a fresnel sheet's focal length on the image brightness or quality? Can you suggest any other cheap alternatives to the professional fresnel lenses? Thanks a lot!</p>

[Bill C]

[Oliver] However, they all have different focal lengths, and that's what confuses me. I didn't find a single question about this in the forums

nor anywhere else. Is there anything to consider when I choose one or doesn't it really matter what focal length I get? Is there any

connection with the lens I'm going to use?

 

Hi, yes it's related to the camera lens. Essentially you can pretend that there is no ground glass, and it is your intention to use the Fresnel

lens to project an image of the lens (pupil) into your eye. But... this would mean you could only see the image from one exact eye position.

So it's better to let the Fresnel project straight out the back, so you can freely move your eye around. This means that the Fresnel focal

length should be roughly the same as the camera lens. A little longer should be good when photographing things that are moderately close.

 

If that got too complicated, let me repeat - a moderate amount longer than the camera lens. It's not too critical because the ground glass

scatters the light so much.

[oliver_vogler]

Thanks Bill, that makes perfect sense!

I'm sure it would be much easier if I

had all the parts already in front of me

but looking at pictures of online stores

doesn't really help understanding the

effects. Thanks for your help!

[BrianS1664879711]

Before you get too focussed on that aspect of the

camera I would urge you to try just the ground

glass and a loupe. It works just fine. I've never

found fresnel to be of much value unless trying to

GG focus without a dark cloth... And I've never

found that to work well either.

[john_shriver]

<p>Also, unless the fresnel lens has fine enough lines, it will be a major detriment when focusing. It just gives you more brightness to look at the whole image from more distance.</p>

 

[oliver_vogler]

<p>I know there's a big discussion going on about this. What I'd like to do is make it flexible. If I need to see a bright picture I can attach the fresnel, if I need to focus it I'll remove it. I know about the disadvantages but would like to try it for the experience of it and because I think the effect is fascinating. I might end up not attaching a fresnel sheet at all in the end but we'll see!<br>

Anyway, thanks for your help, I really appreciate it! I have a clearer understanding of it now and am looking forward to trying it!</p>

[Bill C]

Good luck on your project!

[rodeo_joe1]

<p>A Fresnel works best when it's placed in front of the GG, i.e. on the lens side of the GG screen. For this to work you need to know the thickness of the screen and the refractive index of the material it's made from. For most plastic screens the index will be around 1.5, and you need to move the GG screen+Fresnel forward by (Fresnel_thickness/Refractive_Index) to get the focus right. So if your Fresnel is 1mm thick you need to push the GG+Fresnel assembly forward towards the lens by 1mm/1.5 = 0.66mm.</p>

<p>If you mill out the back by this amount it isn't the end of the world if at some time you want to remove the Fresnel. It's easy to shim the GG back out with spacers of card or paper.</p>

<p>Nearly all commercially made GG+Fresnel viewing screen assemblies are made with the Fresnel on the lens-side. Putting it on the viewing side does very little to increase image brightness, since light has already been widely scattered before it hits the Fresnel condenser lens.</p>

<p>BTW, sandblasting or wetblasting is the easiest way to get a good even "grind" on a piece of plain glass. Just make sure the transparent side is well protected with masking tape and cardboard.</p>

[bob_salomon]

Linhof, Sinar, and others, place the Fresnel on the eye side, not the lens side of the gg. This way it is easily removed and

replaced, as desired and its thickness is not an issue.