Nice article, Jonathon! Pardon my ignorance, but is having a 36X24mm GG image a kind of a 'Holy Grail' concept that cannot be altered? What if it's slightly smaller? Just wondering...

Anyway, to zoom in to the correct size (let's say 36X24), you can print a 36X24 square in a circle on a transparency sheet, and stick it right after the GG. Remove after zooming.

About Holo diffusers: well, I've tried them last night. Don't ask how I got them, that's a whole other story... I had 3 samples to play with, 5deg, 20deg, and 60deg diffusion angle:

1. 5deg is the brightest, but exibits a strong halo, and the texture is too rough for the DV lens not to notice. With the SLR lens fully open at 2.8, the out-of-focus objects looked like they have 'crawling ants' all around their edges. This could be also a 'feature' of my crap Sony PC100.

2. 20deg is OK, but just OK. Somewhat smallish halo is visible, but if you don't mind zooming in a bit, could be OK. Still, I think it would require a field lens. The results are NOTHING compared to what Alain had produced with his finely ground GG. With my camera (1/4.7 CCD size, what a joke!), you get a lot of digital noise.

3. 60deg is way too dark to be of any use. So dark, in fact, I couldn't even get a focus (manual, of course!).

After this fiasco, I started thinking: if Halo diffusers are so great, how come no manufacturer uses them for typical photo applications as a GG replacement? I'm sure they could spare $100 or so for a quick test :D

Bottom line: I think Alain's solution (lots of elbow grease) is the one. Unless, of course, we get hold of the supplier of this magic 'crystalline wax'. Since I'm lazy, I had high hopes for Holo diffusers, but...

Wow - good info on the holos... we'll all just assume that they "fell off the back of a truck." You make a good point about the photo applications not using them - and sticking with the tried-and-true ground glass. Once again, Alain finds the best solution first shot out of the gate, and then we all run around with our heads cut off trying other things only to learn he had it from the beginning :)

I certainly don't think that the 36x24 gg image is a holy grail. You can certainly focus your entire image on any size ground glass you can get and focus in to wherever you'd like. I just thought it was an interesting point of reference as 35mm cameras certainly are aimed at this size, so their focusing systems (mainly the viewfinder focusing system) would be aimed at minimizing optical distortion only to this size... not the entire circle. It would seem like we'd be pushing our luck to expect no optical distortion outside of this frame.

Speaking of focusing - this is another area of concern for me. Since our cams will be hard-focused to the ground glass, all of the actual image focusing will be done manually with the 35mm lens. I, for one, put little faith in the ability to consistently get good focus on my crappy (and at the moment, inverted) viewfinder. It is common practice in medium format photography to have a ground-glass 'focusing screen' which the photographer actually scopes with a magnified loupe to inspect for focus. There has also been specialized focusing screens that either line up two lines or their color evens out when in focus - a number of tricks which make focusing much more quantitative - less subjective. Given our manual focus situation, it'd be interesting to see what ideas might pop up for some kind of magnified or non-visual focusing systems.

I certainly noticed that much of the example footage I saw on the original thread had some pretty serious focus issues. If anyone ever intends to blow any of these shots up for projection, they'd better figure out how to have some hard focus :)

I agree with the focus issue and the first thing that poped in my head was the idea of taking the focusing aid circle, that you see often in the middle SLR viewfinders, and off setting it to the very top or bottom of the screen where it would hide behind the area where black letterbox lines normally are. You cant have the circle in the middle because it will ruin all your shots and since alot of us already shoot 4:3 and add our letterboxing in post anyways we would have the best of both worlds. Just a quick idea. I'll give it more thought.

Excellent idea... I wonder if we could stretch your idea even slightly more and place the focusing circle (I've got one from the SLR I just trashed for parts...) just outside the full 4:3 frame. One could zoom out with the camcorder and set the focus, then zoom back into the full frame. If you were good, you could use this technique multiple times to set a few focus marks somewhere on the 35mm lens for a follow focus shot... Of course, if the circle were in the shot the whole time, you could just keep your eye on it. Who needs 4:3 anyway ;)

microcrystalline focus screens & Hol diff's
 

http://www.stabitech.nl/Bosscreen.htm#top

The manufacturer was not easy to find, it took a bit of Sherlock Holmes-ing, but got there in the end!

They're expensive, (120 euro for a 6x6) - but hey, they have all the right properties,

- excellent light tranmission
- no hot spots
- good dynamic range
- no grain

You can specify no markings, however I would recommend asking for a 36x24mm frame guide to be printed in the middle.
They take around a week to make, and the company is based in Holland.

Ive persoanlly tried commercial fine GG, (6 micron), Holo diff, Opal Diff. The best out of these is by far the Hol diff, but as someone else noted, they don't transmit nearly as much light as GG.
Not tried Bosscreen yet, but contemplating it...

great article...
 

jonathan, great article on Field of View (FOV) effect it's VERY relevant to us XL1/XL1s guys. i also believe there may still be something in Halo Diffusers, and i know they come in a large variety of degrees (5, 10, 20, 30, 60, 80), but that requires testing. if anyone is interested, you can try going to www.poc.com, they will send out FREE samples of their goods to ONLY to "engineers" and "researchers," in, just by coincidence, 2"x2" diameters(!).

I suggest any "engineers" and "researchers" out there might inquire with them... ;)

onward,
D

Alain's got new footage and pics he'd like to share:

Footage

Pic 1

Pic 2

- jim

Ah.... isn't a little less DoF wonderful :)

Nice Alain. What's the music?

strangely, in the scene of the window with the red brick wall, there is some stuff in the top right corner that moves very fast like some spots on a 8mm movie.
amazing, no vignetting...
what kind of gg are you using ? is it moving ?

Looks great Alain, how did you get around the vignette?

Thank's again Jim.

Jonathon, I did the sound track.


Francois,( there is some stuff in the top right corner that moves very fast )
This is some drop of melting snow falling from the roof.Here the spring is comming. :-)
The gg is static , from alu oxyde 9Y( 9 micro Bryte capsule) it's a bit smaller than 1000 grit.

John ,(how did you get around the vignette) One condenser.

Alain

Bosscreens
 

Simon- Thanks for the link to the Bosscreens. For the longest time I wasnt able to find them in anything smalled than 4 X 5 with gridlines so I didnt plan on using them. But with this new info the plain 60mm X 60mm sounds very interesting to me and it looks like custom sizes can be made (at a price). As far as putting the 36mm by 24mm markings I can fully understand why you would want to do that but personally I would just leave it blank which opens me up to using medium format lenses as well. Another advantage is the ability to zoom back alittle with the video camera's lens which will give me the choice of having or not having some very beautiful vignettes. Now that they seem more practical for our designs Im going to go check one out first hand at a local camera store. The claim that these screens dont need a fresnel and at the same time are very bright seems to defy the laws of light and optics so I'll be checking on that and just how grainless these screens are. I've always thought that they relatively expensive but if this gets the job done right then thats what I'll do. Too bad we'll have to baby these screens on hot days because after all they are going to be sealed inside a flat black tube.

-B

Alain with that last video did you check to see that you were shooting a 36mm X 24mm frame?

Also when you say you used a single condenser is it a achromat? What camera did you get it out of?

Brett, for what its worth, I was reading a medium-format camera forum the other day (can't remember where it was), and they were debating the value of Bosscreens as focusing screens. Someone brought up the same issues about heat and possible damage (having heard that it can happen), but about 4 people immediately chimed in just after that, saying that they'd used them for a long time, never with any problems...

Bosscreens
 

Thanks John. Yeah I read the same forum (photo.net). It does seem a bit unlikely under normal use but you have to remember that on Medium and Large format camera the ground glass is mounted at the end of the camera and not trapped inside of a back tube thats likely to get alot hotter.