spinning glass?

[Scot McPhie]

This is probably a dumb question but witht he mini35 adaptor why do you need a spinning glass plane ? I take it is so the image actually forms onto a surface so it can be filmed - but why can't the light be fed straight out of a 35mm lense into the video lense (perhaps via a condensor) and then collected on the ccd with the 35mm DOF characteristics?

Scot

[Chris Hurd]

Scott

The spinning glass creates the moving film grain effect. It can be switched off if desired. Hope this helps,

[Rob Lohman]

1) you need the ground glass to *retain* the 35mm DOF characteristics. These will be lost if you simply beam the light onto the CCD or into the camera's lens

2) it is spinning to actually reduce a lot of grain and structure which would otherwise be visible and probably ruining the picture too much. Usually you don't remove all and it will move a bit as Chris posted above

[Don Donatello]

ground glass has very very fine patterns ( it's called GROUND glass) some can see these patterns/etchings when image is focused on the glass .. if you spin glass then the patterns in the glass are constantly changing positions so your eye doesn't find/see it.

not sure why they can't do a aerial image .. they used it in optical printers back in the 80's when they had more then one image to be optical printed at same time ( effects) ... !

[Scot McPhie]

Yeah I guess the reason why I was asking is that it may be easier to just adapt a 35mm lense onto a video camera and then add the grain effect later through something like Grain Surgery. I don't understand why you need the disc to preserve the DOF characteristics - because won't that be inherant in the light coming into the video camera's lense once it's been created in the 35mm lense?

Scot

[Mizell Wilson]

Don,

There is a device that uses an aerial image, the Angienuex Cine Lens Adapter, but with an aerial image you lose the DoF and AoV characteristics.

Moving to Scot's ponderings...

DoF and AoV, while influenced by many variables, is most directly controlled, for our discussion purposes, by the size of the frame.

As discussed in other lens articles on the subject, a focal length is a focal length. It's all in how the taking format interprets it.

If you take a 50mm lens on a 35mm camera you get one composition, and in fact get the "normal" composition for Academy 35mm film. If you take that same lens and mount it physically onto, let's say, the XL1s, the 50mm is still a 50mm, but the 1/3" CCD sees an AoV (and the resultant DoF) that is quite telephoto. This all has to do with the image circle created by the lens. I won't go into details becuase there are many posts on this subject.

So we use the image screen first so that the lens creates it's full size 35mm filmic image, with all the characteristics intact, and then the prism block to reduce, flip, and transfer the image back to the camera. Another problem with aerial images is that they deliver the picture to the video cameras flipped upside down, which most cameras cannot correct for.

We spin the ground glass so that the static pattern of the screen is not visible. This helps to add back to the image what Spielberg would call the "life of the grain."

mizell

[Alain Aguilar]

When I didn't set the adapter to spin the glass the video would capture the grain and even fine dust particles in the glass. I would suggest to keep it on all the time, but it's only a suggestion.

Al

[Matt Phillips, CSC]

Yes, keep the glass spinning and avoid stopping down below a 4. Increasing your DOF can reveal the GG texture.
Also, if you a going to be shooting exteriors in extreme cold, -40c, like I did last winter, make sure to contact ZGC. The lubricant used on the motor/bearings is NG in those temps. An alternative lube is available for this.
What happens is, the GG slows down to a very low RPM thus revealing the texture. I had to jury rig and insulative blimp for the mini35 connected to a hairdryer to keep it warm enough to get through my five day, exterior heavy shoot.

Keep Warm.