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-   -   circle of bright area on ground glass, with homemade kit (https://www.dvinfo.net/forum/alternative-imaging-methods/98204-circle-bright-area-ground-glass-homemade-kit.html)

Jay Cowley July 4th, 2007 04:10 PM

circle of bright area on ground glass, with homemade kit
 
i recently got a cheap 35mm lens off ebay, and have been playing around with it in an enclosed box and a piece of ground glass. the issue i'm having right now is that when looking through the end of the box, at the projected image on the ground glass, I can see through to the hole in the lens, so the projected image is there, but in the middle it is much brighter, in the form of a blurred circle, and then the rest is darker

how do you get around being able to see the actual hole in the lens where light is coming in, so that the projected image is uniformly bright, and not just bright in the middle?

Mike Dulay July 4th, 2007 06:15 PM

Jay, what kind of 35mm lens are you using and how wide do you have it open? What you are seeing is probably normal. At about 1.8 the lit area for a 50mm prime lens isn't very large ... maybe 2.5cm(?). You're supposed to zoom in with the camcorder. The higher the stop the more vignetting you get.

Wayne Kinney July 5th, 2007 03:33 AM

You really need to add some type of condenser lens into your light path. Most use a PCX (planoconvex) lens close to the ground glass, in between the ground glass and camcorder.

Jay Cowley July 5th, 2007 03:52 PM

Quote:

Originally Posted by Wayne Kinney (Post 707281)
You really need to add some type of condenser lens into your light path. Most use a PCX (planoconvex) lens close to the ground glass, in between the ground glass and camcorder.

Quote:

Originally Posted by Mike Dulay (Post 707185)
Jay, what kind of 35mm lens are you using and how wide do you have it open? What you are seeing is probably normal. At about 1.8 the lit area for a 50mm prime lens isn't very large ... maybe 2.5cm(?). You're supposed to zoom in with the camcorder. The higher the stop the more vignetting you get.

i'm using this lens

http://cgi.ebay.ca/Classic-Canon-FD-...QQcmdZViewItem

i've got my lens hooked up now to a makeshift box, and I was able to get the picture bigger by placing a square magifying glass between the ground glass and my video camera. The issue i'm finding now is that everything seems to be in focus on this lens.

I'll turn the ring on the lens, and everything will go out of focus, and I can turn it into focus, but it seems at the best position, everything is clear, far and close. The whole idea of this lens was so that the background was blurry and near objects are clear. What am I doing wrong that is giving me such shallow depth of field?

Svyatoslav Pylypchuk July 5th, 2007 05:00 PM

like this
 
Quote:

Originally Posted by Wayne Kinney (Post 707281)
You really need to add some type of condenser lens into your light path. Most use a PCX (planoconvex) lens close to the ground glass, in between the ground glass and camcorder.

How to brake Hotspot? I have lens Mamiya 645 2.8 and Bosscreen GG in my 35mm adopter for HV20.
What kind of condenser I should use (D = 60-80mm)?
http://www.thorlabs.com/NewGroupPage...Visual_ID=1998
http://www.anchoroptics.com/catalog/product.cfm?id=34
http://www.optosigma.com/miva/mercha...herical+Lenses

One OR Two:
Lens l) GG (l camcorder
Lens l)(l GG camcorder
Lens GG l)(l camcorder
Lens GG (l camcorder
Lens GG l) camcorder
Lens (l GG camcorder
Lens l) GG camcorder ?

Jay Cowley July 6th, 2007 08:08 AM

the issue with the brighter light in the middle has seemed to be better with a different piece of ground glass I am now using.

The issue i'm now getting though is that everything seems to be either clear or blurry with this lens. If I adjust the focus on the lens so that the image is clear, everything from 10feet -Infinity is clear. The only way I can get depth of field is to place things 3 feet from the lens, but I can do this with my Regular Camcorder anyways.

how do I get a more shallow depth of field, I thought with these 35mm lenses, they all have that shallow depth of field that you see in movies

Ben Winter July 6th, 2007 11:27 AM

Quote:

Originally Posted by Jay Cowley (Post 707899)
the issue with the brighter light in the middle has seemed to be better with a different piece of ground glass I am now using.

The issue i'm now getting though is that everything seems to be either clear or blurry with this lens. If I adjust the focus on the lens so that the image is clear, everything from 10feet -Infinity is clear. The only way I can get depth of field is to place things 3 feet from the lens, but I can do this with my Regular Camcorder anyways.

how do I get a more shallow depth of field, I thought with these 35mm lenses, they all have that shallow depth of field that you see in movies

Jay, a more extensive understanding of lenses may help you. The lens you are using has a 35mm focal length, meaning it is wide angle. Wide angle lenses have considerably deeper depth of field, meaning more is in focus at the same time. You need to change lenses to something more telephoto, like 50mm or 85mm, if you want to have focus separation between an object and infinity. You can get a general understanding of how much a lens keeps in focus just by looking at the side of the lens on the focus ring. If the numbers on the focus ring are grouped together closely, depth of field will be fairly deep. If they are spread out, you'll know you have to rack through a longer range of focus to reach a certain distance.

Don't confuse focal length of a lens with the image plane size. "35mm lenses" are called such because that is the size of the image they project onto the film, and they come in many focal lengths such as 28mm, 50mm, 85mm, 100mm, etc. not just 35mm.

As you learn about lenses, you'll see how the aperture setting affects depth of field also, and how you can control lighting to allow you to open up the lens for shallower depth of field, as well as how shallow depth of field is not always a good thing as it makes focus-pulling difficult and the overall image look soft--and therefore, how a careful balance between shallow and deep depth of field is what makes a good image.

Jay Cowley July 6th, 2007 08:05 PM

thanks for your explanation, i did notice that the lens I was using did have a very wide image.

These two lenses I found

http://cgi.ebay.ca/2-Canon-35mm-Came...QQcmdZViewItem

they seem to be the same make and model as the lens I have, but are 50mm and 135mm. Would these give me more shallow depth of field?

I guess 35mm would be the equivilent of my video camera zoomed all the way out (or even farther then it will go). What would 50mm be the equivilent of? And would 135mm be like have my video camera zoomed all the way in the entire time?

Svyatoslav Pylypchuk July 7th, 2007 03:08 PM

"DOF versus focal length
The phrase "a wideangle lens has more DOF than a tele lens" is frequently heard. Is it true? Sometimes it is, often it is not. It all depends on the comparison. We may assume that the lenses are compared for the same format and at the same F-number, but what about the object distance? If the object distance is kept the same the wideangle image will indeed show a greater DOF, but also a larger field and, consequently, a smaller subject. On the contrary, if the subject is framed the same way we have an entirely different situation. Now the depths of fields may be comparable or even identical. However, in both cases the photographs are not the same and the comparison is one between two different pictures. The former case concerns a comparison with preservation of the perspective (viewpoint) but with different magnifications, the latter preserves the magnification (the size of the subject on the film) but not the perspective. Ironically, the only case where the images are the same is also the only case where the depth of field is of no importance: the reproduction of a flat object like a stamp or a painting. For such a subject has no depth.

Whenever the depth of field is compared between lenses, formats, or whatever, a complete framework must be sketched within which the comparison takes place. If not, the comparison is meaningless."

http://www.vanwalree.com/optics/dof.html

Jay Cowley July 10th, 2007 08:48 PM

i purchased an f1.8 50mm canon MF lens, and have been experimenting with it. I am finding it no different then the 35mm lens.

If I focus it to affinity, set it to 1.8 aperature, and the line up my ground glass so the image is in focus, everything is in focus from about 2m and farther out.

It's prettty much everything is clear, or everything is blurry, with the exception of things within 1m, which i can focus on, and blur the background, but I can do this with a regular video camera too. What am I doing wrong, am I just buying the wrong lenses?

Jay Cowley July 11th, 2007 07:43 AM

i'm also still having this issue with the circle of bright light right in the middle of my projected image, that is overexposed.

basically what is happening, the lens projects the image on the ground glass, this looks good. But I can see through the ground glass right into the little hole in the lens and out through that hole, so the very middle part of my image has a bright overexposed circle. How do you avoid this? Wouldn't this happen with everyone's kit?

Ben Winter July 11th, 2007 11:27 AM

Quote:

Originally Posted by Svyatoslav Pylypchuk (Post 708501)
"DOF versus focal length
The phrase "a wideangle lens has more DOF than a tele lens" is frequently heard. Is it true? Sometimes it is, often it is not. It all depends on the comparison. We may assume that the lenses are compared for the same format and at the same F-number, but what about the object distance? If the object distance is kept the same the wideangle image will indeed show a greater DOF, but also a larger field and, consequently, a smaller subject. On the contrary, if the subject is framed the same way we have an entirely different situation. Now the depths of fields may be comparable or even identical. However, in both cases the photographs are not the same and the comparison is one between two different pictures. The former case concerns a comparison with preservation of the perspective (viewpoint) but with different magnifications, the latter preserves the magnification (the size of the subject on the film) but not the perspective. Ironically, the only case where the images are the same is also the only case where the depth of field is of no importance: the reproduction of a flat object like a stamp or a painting. For such a subject has no depth.

Whenever the depth of field is compared between lenses, formats, or whatever, a complete framework must be sketched within which the comparison takes place. If not, the comparison is meaningless."

http://www.vanwalree.com/optics/dof.html

I think it is fair to assume that in general, the lens is switched out while the camera remains stationary. Also, the higher the focal length, the greater distances there are in which the near and far limit of depth of field are of comparable closeness as to provide focal separation.

Quote:

Originally Posted by Jay Cowley (Post 710304)
i'm also still having this issue with the circle of bright light right in the middle of my projected image, that is overexposed.

basically what is happening, the lens projects the image on the ground glass, this looks good. But I can see through the ground glass right into the little hole in the lens and out through that hole, so the very middle part of my image has a bright overexposed circle. How do you avoid this? Wouldn't this happen with everyone's kit?

Ok, I am seeing what is wrong with your adapter. If you can see through your ground glass like that than your ground glass does not diffuse enough. This would also cancel out the DOF effect which is the trouble you're having. If the glass doesn't diffuse enough, then too much of the 'aerial' image is let through that is not diffused and the image appears unaffected by depth of field.

Try setting your adapter up with an object exactly 2m away. According to a depth of field calculator, everything between 1.92m and 2.09 meters should generally be in focus, and everything else blurry. If a great amount more than that is in focus, you are using a focusing screen that does not diffuse enough and does not work for adapter purposes.

If you set your lens to focus at infinity, everything about 23m and closer should be out of focus.

What are you using for a focusing screen?

Bob Hart July 11th, 2007 11:32 AM

Jay.

If you can see though your groundglass, then its surface is not coarse enough and aerial image is coming through.

To work properly, no "aerial image" (that which you can now see through your groundglass) must come through the groundglass to the camera.

Only the projected image on the groundglass should be visible to the camera, otherwise all you achieve image-wise is something like a combination of vaseline in your eyes and a promist filter.

Jay Cowley July 11th, 2007 07:33 PM

oh i found a clear seethrough plastic dummy cd, and then used sand paper to rough it up so that the image wasn't so trasparent.

is there something better I should be using?

Bob Hart July 11th, 2007 10:24 PM

Your clear CD-R is fine. A sandpapered texture is not. The texture comprises a bunch of scratches not pits.

The optimal finish seems to have shakened down to that achieved by aluminium oxide powder, grade five micron (AO5 in previous posts here) in a water slurry. The disk is moved across a flat surface, usually glass, on the slurry which has to be kept sloppy and not allowed to thicken.

To get the disk to dress across the entire surface, you need to trim the inner ridge and outer ridge off so the disk sits flat. Spacers from DVD recordables might not have that inner ridge.

The best clear spacers are those which are diverted from actual CD-R and DVD+R or DVD-R production. These are identifiable by holding up to the light. On one surface is the guide track visible as a faint blue sheen. Dress this surface.

The newer disks have a toughened plastic so you will find getting a good consistent dressing will take a lot of work.


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