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projecting an image



 
 
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  #31  
Old April 4th 15, 08:12 AM posted to uk.d-i-y,uk.tech.digital-tv
Norman Wells[_6_]
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Posts: 984
Default projecting an image

"Bill Wright" wrote in message
...
Norman Wells wrote:
"Bill Wright" wrote in message
...
I need to project or otherwise place an image on a wooden board so I can
paint it on the board by using it as a guide. I don't have an overhead
projector. I have some transparent media for a laser printer and I have a 60
year old 6cm projector in the loft but I don't want to dig it out and the
bulb will blow and the whole thing will be annoying.

Isn't there some way the old painter guys used to do this, just with a
mirror or something?

Come on gentlemen and ladies, ideas?


You need a lens from an old 35mm single lens reflex camera. Mount it, eg in
a hole through another board, so you can securely position it where necessary
between the well-illuminated original and the target board so as to form an
image on it of the right size.

You may need to darken the room, especially if the image you want is large.


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.


Not so. If the focal length is 50mm, what that means is that light from any
infinite point striking it will be focussed 50mm the other side of the lens.
You think the image is just 1" x 1.5" simply because that is the size of the
film you place at that distance. If you placed a bigger bit of film there, the
whole of the film would capture the image, but it would cover more of what is
visible to the lens.

To get a magnified image, as with a magnifying glass, the object you want to
magnify has to be within the focal length, and the closer it is the larger the
image produced. You must know that from experience.

All you need to know are the schoolboy optics formulae:

1/u +1/v =1/f

where u is the object distance from the lens, v is the image distance from the
lens, and f is the focal length

and

magnification = -v/u.

See for example:

http://www.physicsclassroom.com/clas...tics-of-Lenses

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  #32  
Old April 4th 15, 08:43 AM posted to uk.d-i-y,uk.tech.digital-tv
Jim Lesurf[_2_]
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Posts: 4,326
Default projecting an image

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.

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

  #33  
Old April 4th 15, 10:05 AM posted to uk.d-i-y,uk.tech.digital-tv
nightjar
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Posts: 12
Default projecting an image

On 03/04/2015 19:54, michael adams wrote:
"Roger Mills" wrote in message
...
On 03/04/2015 18:25, Bill Wright wrote:
I need to project or otherwise place an image on a wooden board so I can
paint it on the board by using it as a guide. I don't have an overhead
projector. I have some transparent media for a laser printer and I have
a 60 year old 6cm projector in the loft but I don't want to dig it out
and the bulb will blow and the whole thing will be annoying.

Isn't there some way the old painter guys used to do this, just with a
mirror or something?

Come on gentlemen and ladies, ideas?

Bill


Do you know anyone who would lend you the sort of projector that plugs into a computer?
If so, you could take a digital photo of the image and then project it from a laptop.


On second thoughts...

If possible scan the image

Load it into a DTP or Paint program

Blow it up to the require final size. Or even bigger

Trace around any lines you need as a guide, with a thick black line.

Flood fill the rest with white so as not to waste ink.

If the program has a "poster" printing option, use that.

Otherwise break it into panes manually and print it onto bank
paper. The problem with the transparent medium may be that that is
too thick to press the impression.

Print all the panels out and join them together. Turn it over and trace
over the lines with an B6 or chinagraph pencil. Offer that up to
the board and go over the lines again presing hard same as with old
fashioned tracing.


The problem with that is that chinagraph is wax based and you end up
with wax residue on the surface you want to paint. The art technique is
pouncing - prick holes along all the lines you want to transfer, fix the
image over the surface to be painted, then force chalk or graphite dust
(depending upon the background colour) through the holes by placing the
dust in a muslin bag that is banged (or pounced) over the prick marks.


--
Colin Bignell
  #34  
Old April 4th 15, 11:34 AM posted to uk.d-i-y,uk.tech.digital-tv
Graham.[_5_]
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Posts: 488
Default projecting an image

On Sat, 04 Apr 2015 04:37:53 +0100, Bill Wright
wrote:

Capitol wrote:
Bill Wright wrote:
Andrew Mawson wrote:

Google 'camera obscura'

The problem is finding a lens of suitable focal length. This isn't the
method I'm half remembering anyway.

Bill


It doesn't use a lens, it's a pinhole camera.


There's no way it would be bright enough, unless I did it by sunlight
and rigged up a darkroom. Too much hassle. A large lens would mean I
could do it indoors.

The camera obscura as used by artists used lenses once they were available.

Bill


The large camera obscura in Edinburgh uses lenses.



--

Graham.

%Profound_observation%
  #35  
Old April 4th 15, 12:02 PM posted to uk.d-i-y,uk.tech.digital-tv
Syd Rumpo
external usenet poster
 
Posts: 2
Default projecting an image

On 03/04/2015 18:25, Bill Wright wrote:
I need to project or otherwise place an image on a wooden board so I can
paint it on the board by using it as a guide. I don't have an overhead
projector. I have some transparent media for a laser printer and I have
a 60 year old 6cm projector in the loft but I don't want to dig it out
and the bulb will blow and the whole thing will be annoying.

Isn't there some way the old painter guys used to do this, just with a
mirror or something?

Come on gentlemen and ladies, ideas?

Bill


Toys R Us for a working toy projector. I think I've seen a toy
epidiascope somewhere too.

Or cardboard box around a monitor with a suitable convex lens.

Cheers
--
Syd
  #36  
Old April 4th 15, 01:42 PM posted to uk.d-i-y,uk.tech.digital-tv
michael adams[_4_]
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Posts: 12
Default projecting an image


"[email protected]" wrote in message
b.com...
On 03/04/2015 20:13, michael adams wrote:

The resulting image would be upside down and reversed left-right.

That's where all the mirrors, prisms and all the rest of the
tricky stuff comes in.




Like rotating it 180 degrees?


More than that, I suspect. All I know is that French Daguerreotype
images which were direct were upside down and reversed left
to right. The upside down bit was solved by rotating the image
180% degrees but that didn't solve the left right bit.
That was solved by the Englishman Fox-Talbot who introduced
negatives, which could be flipped over and printed from behind.
You still needed to rotate the negative or the resulting print
through 180% though, or they'd still be upside down. Hence the
"more".


michael adams

....


  #37  
Old April 4th 15, 04:39 PM posted to uk.d-i-y,uk.tech.digital-tv
[email protected]
external usenet poster
 
Posts: 47
Default projecting an image

On 04/04/2015 14:42, michael adams wrote:
"[email protected]" wrote in message
b.com...
On 03/04/2015 20:13, michael adams wrote:

The resulting image would be upside down and reversed left-right.

That's where all the mirrors, prisms and all the rest of the
tricky stuff comes in.




Like rotating it 180 degrees?


More than that, I suspect. All I know is that French Daguerreotype
images which were direct were upside down and reversed left
to right. The upside down bit was solved by rotating the image
180% degrees but that didn't solve the left right bit.


Of course it does, just imagine lying on your side then its upside down
and is fixed by rotating it 180 degrees.

That was solved by the Englishman Fox-Talbot who introduced
negatives, which could be flipped over and printed from behind.
You still needed to rotate the negative or the resulting print
through 180% though, or they'd still be upside down. Hence the
"more".


michael adams

...



  #38  
Old April 4th 15, 06:13 PM posted to uk.d-i-y,uk.tech.digital-tv
michael adams[_4_]
external usenet poster
 
Posts: 12
Default projecting an image


"[email protected]" wrote in message
web.com...
On 04/04/2015 14:42, michael adams wrote:
"[email protected]" wrote in message
b.com...
On 03/04/2015 20:13, michael adams wrote:

The resulting image would be upside down and reversed left-right.

That's where all the mirrors, prisms and all the rest of the
tricky stuff comes in.



Like rotating it 180 degrees?


More than that, I suspect. All I know is that French Daguerreotype
images which were direct were upside down and reversed left
to right. The upside down bit was solved by rotating the image
180% degrees but that didn't solve the left right bit.


Of course it does, just imagine lying on your side then its upside down and is fixed by
rotating it 180 degrees.


Put an apple in your right hand pocket. Turn a cartwheel 180%
degrees so you're balanced on your hands upside down.

Before it falls out in which pocket is the apple ?


michael adams

....


  #39  
Old April 4th 15, 06:45 PM posted to uk.d-i-y,uk.tech.digital-tv
Bill Wright[_2_]
external usenet poster
 
Posts: 9,381
Default projecting an image

Norman Wells 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.


Not so. If the focal length is 50mm, what that means is that light from
any infinite point striking it will be focussed 50mm the other side of
the lens. You think the image is just 1" x 1.5" simply because that is
the size of the film you place at that distance.


No I think that because lens design gets increasingly difficult if the
size of the area covered is bigger. Distortion gets worse. So a modern
lens is designed with accurate coverage restricted to not much more than
the intended target size.

If you placed a bigger
bit of film there, the whole of the film would capture the image,


Only within limits. The optics would give out completely beyond a
certain point.

but it
would cover more of what is visible to the lens.

To get a magnified image, as with a magnifying glass, the object you
want to magnify has to be within the focal length, and the closer it is
the larger the image produced. You must know that from experience.

All you need to know are the schoolboy optics formulae:

1/u +1/v =1/f

where u is the object distance from the lens, v is the image distance
from the lens, and f is the focal length

and

magnification = -v/u.

See for example:

http://www.physicsclassroom.com/clas...tics-of-Lenses


So it boils down to what I said, which is that if you want a focussed
image from an object that is effectively near infinity using a lens with
a focal length of 50mm the surface on which the image is to form has to
be 50mm away from the focal plane of the lens. Since I need an image 600
x 600mm, placing the surface on which I want the image to form 50mm from
the focal plane of the lens will result in the lens having grossly
inadequate coverage. In practice for a normal compound lens designed for
a 35mm camera you'd get a circular image about 100mm across in the
middle of the 600mm x 600mm surface.

Bill
  #40  
Old April 4th 15, 06:50 PM posted to uk.d-i-y,uk.tech.digital-tv
Bill Wright[_2_]
external usenet poster
 
Posts: 9,381
Default projecting an image

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.

Jim


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.

Bill
 




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