Homemade 35mm -- Edited Copy for Reading

[Bob Hart]

Groundglass disks. Been doing some homework and have found there are two types of precision glass disk made in numbers for industrial applications, a third if you count CD mastering disks but these are too big at about the size of an old 45rpm vinyl record.

Some advanced computer hard drives now use a glass disk which has an outer diameter of about 3 and a bit inches and an internal hole of about 3/4" Its thin stuff at 0.6mm. It is supplied as blanks ( hard disk substrates )to the industry. They offer toom small an image target between the inner and outer diameters for our purposes.

There is another product which is probably most promising to us if we could get our hands on it. It is a glass disk made for testing computer hard drive read heads. It is available in varying outer diamaters with varying internal diameter centre holes. It is a bit thick at 2mm. (You could probably get something made up as 2mm glass is fairly common.) This stuff however is made to spin.

If there were such items as rejected disks which don't meet the surface standards but would suit us to make our groundglasses, or worn out disks, buyable cheap???

Otherwise it might be necessary to form some sort of group to buy in a batch at economic prices.

This product is called a glass disk for flying height tester "GD-FHT". made by ohara of Japan. There is a US website www.oharacorp.com The Japan website is www.ohara-inc.co.jp

Anybody here involved in R & D or product quality control at IBM???

[Nicholi Brossia]

A couple of people have experimented with zooming straight into the rear of a 35mm camera lens. I've watched Spencer's video and think it looks like video, and accomplishes the same as attaching a telephoto/wide angle lens to the camcorder. Others feel that it is acceptable. Either way, if you like it, you like it. But, for experiment's sake, maybe it would be a good idea to get a controlled group of shots from both Spencer's setup and Agus's setup. Spencer's keyboard/note shot and the guy at the door could be easily reproduced with a ground glass adaptor for comparison. Unfortunately I don't have my adaptor completed to do a controlled experiment here, but if Agus or any of the other guys that have completed their adaptor want to try this, then it might help clear something up.

here's something to try...

1) Find out your camera's 35mm focal length equivalent. You can usually find this information on the internet. GL2's are 39.5mm, PC-101s are 42mm, PD150s are 43.2mm equivalent at their widest settings.

2) Use a 35mm lens with a close focal length. The lens that most people have been using on their adaptors have been 50mm focal length (35mm refers to the size of the film frame, not the properties of the lens). 50mm lenses should be close enough to the 42mm of the camcorders to work for this experiment.

3) Record identical shots with each of the two setups as well as the stock camera lens. Keep the camera lens front at the same distance from the subject with both versions. Depending on the length of the adaptor, you may have to move the camera back or forward a little to keep the 35mm lens front at the same distance from the subject. The shots should be framed the same. For test purposes, if you have access to both setups (that would be most controlled) get a shot with the 35mm ground glass adaptor, 35mm non-ground glass adaptor, and the camcorder's normal lens (you may try zooming in just a tad to match the other shots... but be precise).

4) Keep track of which shot is which and compare, taking note of the distance between objects that are "in focus" relative to "out of focus"

5) Post still frames (with no post production) of the different setups with descriptions of what the shot was. That way others can see your findings.

Unfortunately, not everyone has access to everything, and not everyone has identical camcorders. However, this can still be done in a convincing manner... if you have an adaptor, you can contribute. Just be sure to mention which adaptor, 35mm lens, distance from subject, and camcorder model.

[Bob Hart]

Have experimented with a microscope slide using loose aluminium oxide grit 600 grade lapped on a sheet of glass. Glass is a much more controllable medium to work with than the CD disks which seem to get injured at the drop of a hat. The result across the glass is also totally consistent.

For those thinking of the fixed glass option and can't get the Zenit screens, the microscope slides are almost 16:9 frame. Used with a large or medium format camera lens and videoing the larger image across the whole slide, you could look forward to a good blemish-free result. I'm not sure how good the light from the lens will be as the image is spread over a larger area thus dimmer.

The frosting texture with 600 grade aluminium oxide might still be a bit course but the light transmission is heaps better than the plastic CD lapped with the 600 or pressed with 600 grade silicone carbide paper. With glass, the frosted surface is not injured by encounters with finger sweat or cleaning cloth.

[Bob Hart]

Have tried the new groundglass as a stationary screen. Method = Pringles chip can, slots for glass slide to fit through it. PD150 videocam, with close up lenses x4 + x2 + x1 stacked, looking in from one end, 55mm Nikon f3.8 lens in other. Lens is crudely secured in end by rolled up sock cuff, video camera secured in end by rolled up sock cuff (True, the socks did have no heels left - honest and were washed).

In bright daylight which would require f16-f22 with ASA 320, the Nikon lens wide open, there does not appear to be any difference in the quality of the image at centre from that of image into PD150 direct. Contrast and colours remain the same though blue seems perhaps a little truer. This effect is reminiscent of late seventies 16mm Agfa neg stock.

There seems to be a slight diffusion of highlights. White objects like a flying moth seem a bit fuzzy.

There is a bit of a hotspot over 80% of the image from centre with the corners seeming about two f-stops darker.

Granularity becomes apparent in these darker corners. If the Nikon lens is stopped down, the granularity becomes apparent across the screen. The dark corners might be because I can not zoom close enough to the screen before focus crashes in the last 10% of the zoom range and I am covering an area larger than the 35mm camera frame.

There is a greater depth of field effect but nothing I cannot replicate with a close-up lens on the DP150 direct. However with a close-up lens, one cannot pull focus to infinity, so there is the bonus to using the Agus35 principle.

The PD150 will tolerate being operated upside down and the viewfinder can be positioned to display correctly if the combination is used at waist height like an old-fashioned box Brownie or medium format camera with top window viewfinders. With the eyepiece jammed into the stomach, the whole thing can be held quite steady. (An ample gut helps).

[Bob Hart]

In low light levels, with the 600 grit fixed groundglass, I have shot a couple of comparative tests also one test in good light. I'll send these images to kennelmaster Chris to see if he can post them.

The groundglass test pattern seems softer yet on closer inspection, resolution seems to be the same across both test patterns in poor light. The groundglass image is about a half-stop darker, a slight lowering of contrast and there is a faint fawn coloured tint.

Camera gain was left on automatic for the low-light tests. It's probably not a valid test except it might be more representative of practical low-light conditions where you want every bit of gain you can get. Defects on the groundglass visible in strong light were not apparent in low light.

Some slightly darker streaks were apparent in some of the bright light images. These came from oils in the can lining from the crisps it once contained.

Dressing the groundglass surface with a hint of some sort of oil or wax might be an option for better contrast. The oily bits seem to contain light spill from adjacent areas.

[Nicholi Brossia]

I read on the large format camera forum that one thing to do is smear vasoline on the ground side of the glass, then wipe clean with a soft, smooth cloth. The vasoline will fill in the holes and provide a "wet" look to the glass, and improve brightness. I haven't personally tested it, but it might be worth a try.

[Jim Lafferty]

GREAT NEWS!

I've discovered where to get pre-frosted CDR's like Agus uses -- Maxell's 48x/700mb 50 spindle has it!

I found this on sale at Tower Records today ($15.99), and since I just ripped the label in order to open the pack, here's an idea of what it looks like:

http://ideaspora.net/agus35/frosted.jpg

- jim

[Jim Lafferty]

Daniel,

Are these the clips you speak of at bobflash.com.br:

Clip 1

(bobcam-natalbarrozo-01.wmv -- 4.07mb)

Clip 2

(bobcam-natalbarrozo-02.wmv -- 3.47mb)

?

For future reference -- when posting clips, please include the file name and/or size in your post -- that way we know we're getting the right material.

- jim

[Bob Hart]

If having trouble getting the CD disk to run true, three solutions - (1) Use the whole of the CD spindle system including the pressure plate which clamps from the opposite side. This is supposed to let the CD find its correct position through gyro effect with the inner hub acting more like a clutch and centre rather than an alignment. The motor remains hard mounted. (

2) If this can't be done because of limited space and you have to mount the CD rigidly to the spindle, try making a diaphragm mount for the motor if the motor is of face-mount Mabuchi style construction. Make a little thin disk or square out of something really light and pliable. Shim steel would be best. Ice cream or milk container is okay but it will deform out of position in time. Face-mount the motor to this. You'll need to be able to precisely drill 4 holes through it, one for the motor spindle shaft, three for the screws.

( A quick and dirty cheat for making a template is to place an identical motor face down on the platen of a scanner or photocopier. The spindle shaft will have to have been removed though some photocopiers or scanners will accurately scan an object up to half an inch away from the platen. - So you'll probably have to have a second motor to dismantle for this. Slide-tray CD players often
have similar motors for tray and transport drives so these could be used. Print the scan and check the distance between the holes against the motor. Glue the print to the metal or plastic you want to drill, mark or centrepunch through the paper then drill through it. Printing to a sticky label makes the job even easier.) Mount the plate or glue it to the case. When the CD runs up to speed and straightens out, the motor will be seen to be vibrating slightly.

(3) Make a foam rubber sock out of hotwater pipe
insulation. Fix your spindle motor inside this and mount the foam inside a piece of plastic pipe glued to the case. It may be harder to keep the motor is position though as the material will slump in time.

[Bob Hart]

For Aussie visitors, with Chris kennelmaster's help, there may soon be a .jpg of a partly assembled early prototype of the Agus35 non-inverter rotary groundglass principle, using Australian standard PVC plumbing bits for case and lens tube structure, plus a Decor potplant saucer as a soft motor mount. (Yes the sock will be there).

Once the motor and disk image plane positions have been tied down by testing, the lens tube will be shortened to accept a proper lens mount for the objective lens and some sort of attachment, probably 58mm filter thread for the PD150 at the other.

There will be observed a cut-out in the lens tube insude the enclosure. This provides assembly clearance for the motor and disk mount which clips into place. The enclosure case comprises two riser caps and a short piece of pipe inside one purely as an internal guide sleeve for case assembly.

The disk sits ahead of the orange saucer driven by the motor mounted behind - not fitted in this illustration. The battery holder will fit in the rear enclosure with the motor and the switch will be mounted to the rear cover which will be the removable cover for maintenance.

Fixing of the covers will be by long pillar nuts and screws as used to mount circuit boards.

[Chris Hurd]

Bob Hart's images now online at www.dvinfo.net/media/hart.

Note from Chris Hurd: This is an edited, locked version of the original "Homemade 35mm" thread located here. It has been edited by removing off-topic posts, meta-data (such as "great job!" etc.), repeated questions, and so on. This should make your reading of the entire relevant technical discussion a bit easier. If you'd like to add to this discussion, or post a question which has not yet been asked, please do so at the original thread located here. Hope this helps -- CH