PDA

View Full Version : DIY fresnel



BennehBoy
14-Oct-2009, 10:07
After searching on the bay for a fresnel to fit my 8x10 p2 I found a seller who had a fresnel sheet for sale that could be 'cut to fit'. The seller was charging somewhere in the region of US $25. I noticed that the seller had left the brand of the item within his listing photo and realised that the item was simply an a4 full sheet magnifier. Very enterprising.

2.35GBP later and with a little bit of cutting I now have a transformed focusing experience when shooting my 165SA on the P2.

This may be common knowledge already but I thought I'd share anyway.

Bosaiya
14-Oct-2009, 10:27
I went to my local office supply store and bought a couple of the 8.5x11" fresnel magnifiers for next to nothing. I cut them to size with a utility knife and so far they work pretty well.

Struan Gray
14-Oct-2009, 12:43
I never seem to be able to find mine when I need it......

JimL
14-Oct-2009, 23:36
Also try this place (http://3dlens.com/). Their fine pithc acrylic fresnels are good quality - much better than the soft flexible page magnifiers. Tip: for wide angle lenses, try stacking two fresnels - you may not believe what you see!

ic-racer
15-Oct-2009, 17:47
A question for you 8x10 fresnel users. I have fresnels on all my cameras from 6x6 and up through 4x5. I was on line recntly looking to try one of these cheap options but realized I view the 8x10 GG from close-up (8" with my eyesight) and scan the GG that way. Will a fresnel work like this or, do you only get the benefit when viewing from a central viewpoint, 20 inches or so back from the GG?

The fresnels on all my other cameras seem to only work well from a central view location.

john wilton
15-Oct-2009, 21:41
Is it true that Beattie and Boss etc screens increase brightness by 2+ stops as claimed? Or would it be more accurate to say that they improve brightness marginally at best over a good ground glass in the center(and all other points on the screen when the point viewed is in a direct line between eye and lens), and greatly improve brightness in the corners only when eye is perpendicular to the screen?

Sascha Welter
15-Oct-2009, 23:36
Fresnels increase the perceived brightness of the image: The light rays from the lens are spread out in an angle. You see a lighter spot on the ground glass, that's where they get directly to your eye. Where the gg is darker, these rays pass your eye, but they light up the ground glass too. With a fresnel these "spread out rays" are diverted back towards a central point. Place your eye there and theoretically everything should be bright at once.

How much this will work only in a central point depends mostly on the focal length of the lens - a wide angle spreads out more, so the fresnel might not "centralize" enough. As mentioned, two fresnels behind each other could help in that situation. Also no fresnel is perfect - which is a good thing, as otherwise you'd see things only from that central point.

Fresnel lenses for large format cameras should in my oppinion be a bit of a compromise, as people expect to be able to still move around viewing behind the camera. Fresnels on medium format expect the eye to be much more reliable in one position or central line of view.

IanG
16-Oct-2009, 00:27
Is it true that Beattie and Boss etc screens increase brightness by 2+ stops as claimed? Or would it be more accurate to say that they improve brightness marginally at best over a good ground glass in the center(and all other points on the screen when the point viewed is in a direct line between eye and lens), and greatly improve brightness in the corners only when eye is perpendicular to the screen?

Compared to even the best plain screens, yes, adding a fresnel does the same but by slightly less, over-all screen brightness is very significantly improved.

You're right though in that all a fresnel is doing is bringing the brightness of the edges and rest of the screen closer to hot spot of a plain screen.

With my Crown Graphic a fresnel makes a huge difference to it's usability, particularly hand held, even focussing a 90mm f8.8 Angulon, of 75mm f8 Super Angulon is now easy.

Ian

ChrisN
16-Oct-2009, 05:12
Which side of the ground glass is the fresnel placed?

BennehBoy
16-Oct-2009, 07:03
eyeball side

IanG
16-Oct-2009, 07:43
Who sells the 2.35 fresnel in the UK ?

Ian

BennehBoy
16-Oct-2009, 08:30
This is the one I picked up (http://cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=180415250715&ru=http%3A%2F%2Fshop.ebay.co.uk%3A80%2F%3F_from%3DR40%26_trksid%3Dm38%26_nkw%3D180415250715%26_fvi%3D1&_rdc=1), but there are plenty of other sellers

IanG
16-Oct-2009, 08:46
Thanks, I looked for them on my last visit back to the UK with no luck, the Staples version has been mentioned on this site a few times, but they weren't a stock item in the UK Staples stores. At that price they are worth a try :D

I've ordered 3, try on the spare 10x8 when I've made a new screen for it and a couple of 9x12's :)

Ian

jbenedict
16-Oct-2009, 09:19
I'm a little bit confused about where the fresnel goes- none of my cameras have had them.

These go on the front side of the ground glass (where the image is formed) with the flat side of the fresnel against the ground glass?

Jeff "I have questions" Benedict

BennehBoy
16-Oct-2009, 09:34
well they _can_ go on the front side, but it's easier to just place them against the rear side, my p2 has little clips to accomodate one.

bobwysiwyg
16-Oct-2009, 09:49
OK, now this novice is confused. If you put it in front (lens side) of the GG, aren't you moving the GG back from the film plane, wouldn't there be a focus difference? I'm assuming, quite falsely perhaps, that even with the fresnel screen, you are still focusing the image on the GG. :confused: :confused:

Robert Hughes
16-Oct-2009, 11:18
Hello bobwysiwyg (hope I spelx'd that right). Adding a fresnel lens adds a bit of complication to your focus point calculations if you place it between the lens and your GG; where is the lens supposed to focus, on the fresnel or the GG? Does addition of the fresnel throw off the actual focus point from the film plane? Which was do the fresnel elements go?

If you place the fresnel between the glass and you, these issues disappear. But the additional space taken by the fresnel may interfere with your ability to close any protective doors or glass coverings.

IanG
16-Oct-2009, 11:46
The Fresnels we are talking about are very thin, so no problems with closing the back of a Crown/Speed Graphic, although you may well need to trim a bit it's not perfect :D

Some backs are designed for the fresnel to be the film side, this does alter the focus plane if you put a different one or remove it.

Ian

bobwysiwyg
16-Oct-2009, 12:58
Phew, OK, the novice feels better. :) I use one and it is mounted between me and the GG.