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uk.tech.digital-tv (Digital TV - General) (uk.tech.digital-tv) Discussion of all matters technical in origin related to the reception of digital television transmissions, be they via satellite, terrestrial or cable. Advertising is forbidden, with no exceptions.

projecting an image



 
 
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  #41  
Old April 4th 15, 09:17 PM posted to uk.d-i-y,uk.tech.digital-tv
Norman Wells[_6_]
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Posts: 937
Default projecting an image

"Bill Wright" wrote in message
...
Jim Lesurf wrote:
In article , Bill Wright
wrote:
alan_m wrote:
On 03/04/2015 20:30, Bill Wright wrote:

But surely I need a much longer focal length? A normal 35mm camera
lens will cover an area about 1" by 1.5" at a distance of about 50mm.
And if you reverse the lens?


The image is very small and is 50mm from the lens.


You've probably now read what Norman explained. If in doubt, think of this
in terms of opening the back of a camera and placing the picture you
already have where the film would normally go. Illuminate this picture and
it will make at image at the distance you can set using the camera's focus
control. i.e. ant any distance from its minimum setting up to infinity.

Then consider using the lens without the rest of the camera to make it
easier to illuminate the picture you have, etc.


Yes you could do it that way round. It would just amount to using the camera
as a 35mm projector. The original is actually quite big though. Almost life
size. I suppose I could make a very small copy of the original, but then I
might as well dig the projector out of the loft, which I was trying to avoid.

That isn't what Norman explained by the way.


If you place the original, well-lit, within the focal length of the lens, and
preferably as close to it as possible, then you will be able to get a complete
image of it at whatever size you want on the medium on which you want to
reproduce it. The further away from the lens you place the medium, the larger
the image will be, though of course correspondingly fainter.

If you don't understand the theory, just try it.

  #42  
Old April 5th 15, 03:58 AM posted to uk.d-i-y,uk.tech.digital-tv
Bill Wright[_2_]
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Posts: 9,381
Default projecting an image

Norman Wells wrote:

If you don't understand the theory, just try it.


Sounds as if I understand optics a lot better than you.

Bill
  #43  
Old April 5th 15, 05:44 AM posted to uk.d-i-y,uk.tech.digital-tv
Bill Wright[_2_]
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Posts: 9,381
Default projecting an image

I've decided to do it with a grid. So far it's going well. I have a 64 x
64 grid on the original and the same on the board, but scaled up 8x. It
isn't much harder to copy onto the board than it would be to draw round
the picture elements (having cut them out).

I've done the head but I decided to leave the teeth. I might put some
yellow on them. I'm going to do the eyes last, in red. As for the bones,
well I like the idea of two femurs rather than the rather bland generic
bone shapes that are traditional. The offset heads of femurs are rather
cute I think.

Bill
  #44  
Old April 5th 15, 09:26 AM posted to uk.d-i-y,uk.tech.digital-tv
Norman Wells[_6_]
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Posts: 937
Default projecting an image

"Bill Wright" wrote in message
...
Norman Wells wrote:

If you don't understand the theory, just try it.


Sounds as if I understand optics a lot better than you.


Well, I admit I was wrong in some respects, but not as wrong as you think. With
a single convex lens, the object actually has to be positioned at a distance
from the lens of greater than its focal length in order to form a real image
that can be captured on the medium you want to copy onto. To obtain an enlarged
image, the object has to be between the focal length and double the focal length
away from the lens.

If the focal length of the lens is 50mm and you position the object at, say,
60mm from it, you will get an in-focus image formed at 300mm on the other side
of the lens, and the magnification will be 5 times.

 




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