Mini35 Oscillating Ground Glass Idea

[Brett Erskine]

Thought you guys might like this multi format DOF comparison.

To give you a true sense of what each format will give you in terms of DOF I've kept the FOV the same and the distance between the camera and actor the same - thus different focal lengths between formats. I also used real world lenses commonly available for each format which explains the different minimum f/stops used. For the following comparison an actor is placed 10 ft from the cameras postion. The camera is never moved nor is the FOV changed. I choose a common composition - in this case a medium/CU shot (mid chest to top of head) of the actor. Here are the focal lengths and DOF of available lens in each format:

Medium Format (645) - 200mm(FL)/f2.8 = 2.07inches(DOF)
35mm (Still Camera) - 135mm(FL)/f2.0 = 1.92inches(DOF)
35mm (Motion Cam) - 85mm(FL)/f1.3 = 3.62inches(DOF)
16mm (Super 16) - 50mm(FL)/f1.3 = 6.31inches(DOF)
2/3" (CCD Video Cam) - 35mm(FL)/f2.0 = 2ft, 2inches(DOF)
1/3" (CCD Video Cam) - 18mm(FL)/f1.7 = 3ft, 4inches(DOF)

Again the reason why their are different FL for each format is because they all give the same FOV from the same subject distance so in other words they are the same equivalent lens for its format.

Medium format image size/gg is 2.7X larger than 35mm (Still)

[Charles Papert]

Brett:

Just want to clarify something--your 35mm and 16mm numbers seems to be off, but that's probably something to do with the different formats available within those gauges. Were you using a calculator based on field of view? If so, this is a horizontal measurement, but the vertical height of the frame is more relevant when considering a shot of an person (i.e. mid chest to top of head). As an example, standard 16mm and Super 16mm frames have the same height, but the Super 16 has a greater horizontal field of view.

Nominally, the difference between 35mm and 16mm should be a factor of .5x.

[Brett Erskine]

Yeah thats exactly right. I was basing my FOV on a 35mm 1:66 frame since it was the closest to all the other formats. I knew that everyone has their preferance between full screen, 16X9, 1.85 and 2.39 so I decided to try and keep the horizontal FOV the constant in each example but you right the important thing here is the vertical FOV because we are talking about shooting actors. If however you keep everything pretty much a 1:66 ratio for each format then these numbers should give you a pretty good idea of the DOF for each. If you would like to post the exact numbers by all means please do.

[David Parker]

Good morning everybody.

I am the owner and producer of Satin Snow(TM) Ground Glass.

If you would like to contact me, our email address is:
gglass@satinsnowglass.com

I have received a few emails about these types of projects and currently would like to find out, which sized ground glass screens would work best for the applications you folks are using them for, I have been told that 55mm and 52mm are two of the sizes that are being used.

Would you require these in regular screw in filter rings, such as used with 35mm cameras?

Anyway, if you would like, I will try and see what I can put together that may be of assistance to the types of projects you are doing, currently our main focus has been Large Format Film Cameras, but have sold a few screens for use in the DV industry and I am sure I can come up with a solution for those of you that are interested.

Thanks again.

Dave Parker
Ground Glass Specialties
Satin Snow(TM) Ground Glass
www.satinsnowglass.com

[Bob Hart]

David.

There is another method which uses a spinning groundglass to furthur enhance apparent resolution by moving the "grain" of the groundglass through the image faster than the camcorder can resolve it, virtually eliminating it. However, a coarser texture to the groundglass will still affect resolution.

Awhile back, I bought in raw cut optical disks from Ohara in Japan. These were supplied as a special order of 10 with a 15mm diameter center hole and outer diameter of 120mm, a whisker over 5 inches which replicates the dimensions of a plastic compact disk. The thickness was 1.3mm and dressed down to about 0.9mm before the cut marks polished out. The concentricity is an issue for balance but a groundglass texture of uniform quality across the glass means the rpm can be kept down to 1500 or thereabouts which limits vibration.

If you can replicate the compact disk profile with your product at an affordable price, there may be a few making AGUS35s (nicknamed spinners) who might be interested. AO5 dressed spinning groundglass seems to yield about 600 TV lines of resolution at best. Finer than AO5 makes the groundglass too transparent. Thanks for the interest in visiting.

There are some on-site images you can find here at
www.dvinfo.net/media/hart and www.dvinfo.net/media/mellor There may be others here but I don't know of them.

[Richard Mellor]

hi : welcome a board david. after trying many types of ground glass the best so far is the optmo sigma 1500 it is 50mm and will fit perfectly in a static tube system . this is the one to beat .
it,.s price is $24 the $ 14 thorlabs is on order and will let you know how that looks maybe your technolgy can better these options. for static devices . but I just got a chance to test out the device that we all hope to equal .the $26,000 ps teknik hd pro model and saw first hand the quality of the ground glass in the $26,000device.
and the optmosigma equaled it .as a matter of fact I think they use a lower grit size than the 1500. for a brighter image
it was amazing the amount of grain that you could see when you powered down the motor . the ocillating disk was about 55mm . I think with future devices the biggest concern is precision of the focal plane. the camera rental place told me that the first generation of the ps tecknik with spinning glass
had a lot of complaints .1 was the vortex of hell. this is caused
by the image not spinning at the same rate the outside of a disc will spin much slower than the inside this was corrected with the $26,000 ocillating adapter . motor vibration was the 2nd complaint . brushless motor and precison parts, and we can make this. maybe there is away to make a piece of glass so good we will not need to ocillate it. that would be a dream

[Jim Lafferty]

maybe there is away to make a piece of glass so good we will not need to ocillate it. that would be a dream

That's the idea behind the microwax design -- my shipment of wax is pending, but it seems to me to be the only realistic setup that promises a grainless static adapter.

For someone like myself, moving parts, a motor and power supply (not to mention a larger profile) make an oscillating adapter out of the question. For these reasons, if microwax doesn't work, I'll just abide the grain.

- jim

[Karel Bata]

I've just spent the last hour reading this thread.

It's fascinating.

I'm sure you know that this business of a video camera looking at an image formed on a ground glass has been around for years in the form of video-assist cameras taking a peek at the ground glass in a film camera. (In fact, by some very weird coincidence I posted in this very forum yesterday about how clients often mistook the resulting image on the playback monitor as being a telecine of the film rushes!)

Here's an off the wall idea. Suppose you keep the gg stationary, take the lens off, and record an image of the gg illuminated from a light source in front of the camera assembly. You then subtract this image fom the image you get when you have the lens on and focussed on a target. You would have to ensure that the gg stayed absolutely in position.

Also, you might find it interesting to check out the Spintec Lightweight Rain Deflector

Good luck!

[Valeriu Campan]

I have found a self adhesive sheet of frosted material from AVERY the same people that make the labels. Probably any sign shop have some scraps. Worth to have a look.

[Karel Bata]

The problem with such materials Valerie is that they have a thickness. Once the light hits one side it will form a sharp image, but will continue to diffuse as it travels through, and become slightly blurred by the time it is viewed from the other side.

A ground glass has a diffusing surface on only side and avoids this.

I personally don't understand how a hot spot is avoided - hotspots are a real bugbear when rear projecting film onto a traditional cinema screen.

[Dogus Aslan]

hi david...

i have mailed you twice but have recevied a reply that my ail was not sent..(the date can be seen on a few posts earlier)..

i am in the need of 2 pieces of 6cm diameter snow glass for testing purposes..can u tell me the price with included shiping to turkey?

and david...your grains ..what are there sizes scientificly? can u go finer?

thanx..

[Bob Hart]

Vortex from hell ??

I wonder how much of the bad rap Gen 1 of the P+S Technik was due to mismanagement of the system. I found after I had about four of five hours of messing with the early Agus version I could get better results with my primitive setup un-modified. So I imagine the P+S Technik would have required at least that time for the operator's fluency and comfortable familiarity to build up.

Rentals cost money and practice time might be kept to a minimum in some tight budget situations

[Anders Floe]

To Les - I'm really anxious to hear about your osc. device. When is the webpage up and running??? Do you think it would be possible to finish the osc. devices in december???

[Dogus Aslan]

hi..i have colected the parts..and i have put them together..the system is working fine..i have used a dummy ground glass..since there is still some welding to do..so i cant screw my slr lens on..and for cant do any testing..i have found out that i have used the bearings to tight..(there are looser bearing with the same diemensions..only the turning parts turn easier)..so i have to change them..the friction is much more than i had imagened..but a mechanical engineer friend told me that changing the bearings would solve the problem...i still havent gone in the motor part so i can not test vibration yet..put it works when turning by hand..of course at verrrrryyyy looooowwwww speeds:)

when all done it will be painted with matte black paint..decreasing the reflections inside the device...

here are a few snapshots of where the project is...

www.tumgorsel.com/dosealas/dodo35/dodo_1.jpg
www.tumgorsel.com/dosealas/dodo35/dodo_2.jpg
www.tumgorsel.com/dosealas/dodo35/dodo_3.jpg
www.tumgorsel.com/dosealas/dodo35/dodo_4.jpg
www.tumgorsel.com/dosealas/dodo35/dodo_5.jpg
www.tumgorsel.com/dosealas/dodo35/dodo_6.jpg
www.tumgorsel.com/dosealas/dodo35/dodo_7.jpg

(can some tell me how to put links:)

[Jesse Roberts]

is anyone going to be offering these kits?