35mm adapter newbie questions???

yzmotoxer807

yzmotoxer807

Well-known member
I understand how the 35mm adapter works but I dont understand two things: Why does the adapter need the ground glass? Can't it just focus on the incoming light?

The other question is how does the ground glass not totally block the image? Wouldn't this be the equivalent of looking through a frosted window?

Obviously I am not questioning the fact that this design works because its proven but I am just trying to understand why.

Thanks.
 
yzmotoxer807 said:
I understand how the 35mm adapter works but I dont understand two things: Why does the adapter need the ground glass?
Click to expand...

Re think your first sentence :).

The SLR lens projects it's image onto the GG. The GG in this sense acts as the film plane the SLR lens would normally focus on. Your camera then focuses and records this image.

To the eye the GG does look frosted by itself but the materials used allow a picture to be projected onto it. Imagine the screen for a rear-projection projector, or look through the viewfinder of a 35mm still film camera. That is some form of GG that looks translucent (not transparent) but the image looks fine. Look at Timur's thread at the top of this forum for more in-depth explanations.
 
Hi,

The groundglass acts as a projection screen for the 35mm lens to project its image onto. It usually spins or vibrates so the grain and rough surface of the glass is not visable on the image.

The glass is like tracing paper, the light is just caught by the frosted surface. The light does pass through but the image is caught on the surface which creates the projected image.
 
The reason you want the Ground Glass, instead of attaching the slr lens directly to the video camera, is to be able to achieve the shallow focus (depth of field) and other characteristics of film lenses.
If you were to attach the lens directly, you would get a similar image to the Canon EOS adapter, which maintains "the video deep focus", and we don't want that...well not here (this forum) anyway :)
 
Yes but why does using the ground glass give it a shallower depth of field? Is it because the distance from the 35mm lens to the GG is included in the focal length? I know that a longer focal length gives shallower depth of field.

And by the way William_Robinette, my first sentence makes perfect sense to me. I understand HOW it works but not WHY it works. I see what you thought though :happy:.
 
Ground glass does not give it SDOF.
The ground glass has nothing to do with the SDOF.

there are two systems, your camera and the adapter.

the camera does not have very SDOF

the adapter, depending on the lens you use and the aperture it's set at can ...

Now how do you get your camera to film what the adapter can do ... you project it on a little movie screen inside the adapter ... i.e., the ground glass.

So your camera is filming a little movie screen inside the adapter ... now what matters in terms of the DOF happens in front of the adpater.

If the GG gave you shallow depth of field then you would be stuck with extreme SDOF as your only choice, which would be even worse than not having it at all.

But instead, since all the magic happpens in front of the screen .i.e., ground glass, you can CONTROL the DOF. A long lens wide open will give you the maximum effect if SDOF. A wide angle lense closed some will give you much less shallower DOF.
 
I think I can see where you are getting at as people have said the same to me multiple times, re: video can have shallower depth of field when zoomed in BUT trust, it does not even come into comparison with actual 35mm lens depth of field.
 
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yzmotoxer807 said:
I understand how the 35mm adapter works but I dont understand two things: Why does the adapter need the ground glass? Can't it just focus on the incoming light?
Click to expand...
A camera needs an object to focus on, and the adapter needs a surface to project on.

The camera focuses on the grains in the ground glass. The grains then virtually disappear when you turn the adaptor on and it vibrates. Now you have calibrated your camera to focus on a given focal plane.

The adapter happens to project onto this focal plane which must be semi opaque.

yzmotoxer807 said:
The other question is how does the ground glass not totally block the image? Wouldn't this be the equivalent of looking through a frosted window?
Click to expand...

I think it's because you're not really looking through the glass, you are looking at an image projected onto it.
 
Thanks for your help.

Jack, I understand what you are saying now, but why cant you just attach a 35mm lens to the front of the dvx? According to Charles (above) its because this would not give a shallower dof. I dont see how this can be because by directly attaching the 35mm you are lengthening the dof.
 
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yzmotoxer807 said:
Thanks for your help.

Charles, I understand what you are saying now, but why cant you just attach a 35mm lens to the front of the dvx? According to Charles (above) its because this would not give a shallower dof. I dont see how this can be because by directly attaching the 35mm you are lengthening the dof.
Click to expand...

It has to do with with the size the devise receiving the image.

The larger it is the shallower the depth of field (area in focus)

Video cameras come with CCDs 1/3", 1/2" or 2/3".

The 2/3 CCDs have shallower area in focus than the other 2 because the CCD seeing the image is bigger than the other 2.

When you add a 35 mm adapter you are in fact doubling the size of the device receiving the image. The camera is just recording a flat image that it sees from the GG in the adapter.

If you didn't have the adapter with a GG the image would still be received by a small CCD.

I hope that helped.
 
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Thanks so much, I finally get it! The way I was thinking about it was taking for granted a large imaging surface like 35mm film. Gotcha.

Thanks again.
 
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