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photographing a computer screen?



 
 
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  #34  
Old October 18th 04, 12:31 AM
Big Bill
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On Sun, 17 Oct 2004 20:00:05 +0000 (UTC), (Dave
Martindale) wrote:

writes:

It's easy. You'll need a tripod, positioned with the camera as parallel
as possible with the screen, & a shutter speed that's an *exact*
multiple of the displays refresh rate to prevent dark bars appearing in
the photo. Eg: to photograph an American TV screen (60Hz field rate,
30Hz refresh rate), you'd use a shutter speed of 1/30th, 1/15th, etc.


Mechanical shutters are just not that accurate on their own. Some
high-end video cameras have electronic shutters that can be adjusted to
almost exactly match a computer monitor, so there's only a very fine
dark or light line left moving up or down the screen, but still camera's
can't get exposures that accurate.

The simple way to get around this is to expose for 1/2 to 1 second.
That way, any spot on the screen will get about 15 or 30 refreshes of
the same image, and although there will be some portion of the screen
that gets 14 or 16 instead of 15 (29 or 31 instead of 30) the difference
becomes small enough not to see.

The more sophisticated method, used in video-input film recorders, is to
switch the video instead. They open the camera shutter with the signal
blanked, wait for vertical retrace, turn on the video and count some
integer number of video frames, then blank the video and close the
camera shutter. This obviously requires a dark enclosure for the camera
and CRT.

Dave


I've taken many screen shots with my C3030; no problems at 1/60 or
1/30 sec.
I also did this with my older Konica FT-1 Motor.
I've never heard that mechanical shutters aren't good enough for this
before. Where did you get this info?

Bill Funk
Change "g" to "a"
  #37  
Old October 18th 04, 02:21 AM
Al Dykes
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In article ,
wrote:
Kibo informs me that Hugh Nagle stated
that:

Now, I know I could do a screen capture, but I was wondering if anyone had
any ideas on how best to photograph a computer screen.


It's easy. You'll need a tripod, positioned with the camera as parallel
as possible with the screen, & a shutter speed that's an *exact*
multiple of the displays refresh rate to prevent dark bars appearing in
the photo. Eg: to photograph an American TV screen (60Hz field rate,
30Hz refresh rate), you'd use a shutter speed of 1/30th, 1/15th, etc.



Not to diminish the fine photography advice here, but there's gotta be
a way on a MAC to grab the screen and save it to a file in some
graphics format.

I don't do MAC. In a windoze system I can save the entire screen, or
just the active panel to a BMP file, and then do anything I want with
it.


--
Al Dykes
-----------
adykes at p a n i x . c o m
  #38  
Old October 18th 04, 02:21 AM
Al Dykes
external usenet poster
 
Posts: n/a
Default

In article ,
wrote:
Kibo informs me that Hugh Nagle stated
that:

Now, I know I could do a screen capture, but I was wondering if anyone had
any ideas on how best to photograph a computer screen.


It's easy. You'll need a tripod, positioned with the camera as parallel
as possible with the screen, & a shutter speed that's an *exact*
multiple of the displays refresh rate to prevent dark bars appearing in
the photo. Eg: to photograph an American TV screen (60Hz field rate,
30Hz refresh rate), you'd use a shutter speed of 1/30th, 1/15th, etc.



Not to diminish the fine photography advice here, but there's gotta be
a way on a MAC to grab the screen and save it to a file in some
graphics format.

I don't do MAC. In a windoze system I can save the entire screen, or
just the active panel to a BMP file, and then do anything I want with
it.


--
Al Dykes
-----------
adykes at p a n i x . c o m
  #39  
Old October 18th 04, 04:50 PM
Dave Martindale
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Big Bill writes:

I've taken many screen shots with my C3030; no problems at 1/60 or
1/30 sec.
I also did this with my older Konica FT-1 Motor.
I've never heard that mechanical shutters aren't good enough for this
before. Where did you get this info?


Personal experience.

Now, if you're shooting with a P&S or rangefinder camera with an in-lens
shutter, the shutter actually opens gradually and closes gradually
over the space of several milliseconds, and it is in a position where it
controls light over the whole frame area at once. At speeds higher than
1/60, less than the whole screen will be illuminated (as you'd expect)
and the illuminated area will have a soft edge where brightness
increases or decreases over several scan lines. When you shoot at 1/30,
this means that any overlap or lack of overlap will have a soft edge,
making it less visible.

But a focal-plane shutter passes or blocks light very rapidly. If you
shoot a CRT at faster than 1/60, you will see an illuminated band with
very crisp edges (it will be diagonal with a horizontal-running shutter,
horizontal with a vertical-running shutter). And if you try to expose
for exactly 1/30 second, you'll almost certainly see either a dark band
because the exposure was really less than 1/60, or a light band where
some part of the screen got exposed twice because the exposure was
slightly more than 1/60. I've seen this myself in film SLR shots.

Photography magazines used to test shutter accuracy. A shutter that was
within 1/10 stop was considered very good - but a 1/10 stop exposure
error is still a 7% time error. Mechanical shutters just aren't that
precise.

Dave
  #40  
Old October 18th 04, 04:50 PM
Dave Martindale
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Posts: n/a
Default

Big Bill writes:

I've taken many screen shots with my C3030; no problems at 1/60 or
1/30 sec.
I also did this with my older Konica FT-1 Motor.
I've never heard that mechanical shutters aren't good enough for this
before. Where did you get this info?


Personal experience.

Now, if you're shooting with a P&S or rangefinder camera with an in-lens
shutter, the shutter actually opens gradually and closes gradually
over the space of several milliseconds, and it is in a position where it
controls light over the whole frame area at once. At speeds higher than
1/60, less than the whole screen will be illuminated (as you'd expect)
and the illuminated area will have a soft edge where brightness
increases or decreases over several scan lines. When you shoot at 1/30,
this means that any overlap or lack of overlap will have a soft edge,
making it less visible.

But a focal-plane shutter passes or blocks light very rapidly. If you
shoot a CRT at faster than 1/60, you will see an illuminated band with
very crisp edges (it will be diagonal with a horizontal-running shutter,
horizontal with a vertical-running shutter). And if you try to expose
for exactly 1/30 second, you'll almost certainly see either a dark band
because the exposure was really less than 1/60, or a light band where
some part of the screen got exposed twice because the exposure was
slightly more than 1/60. I've seen this myself in film SLR shots.

Photography magazines used to test shutter accuracy. A shutter that was
within 1/10 stop was considered very good - but a 1/10 stop exposure
error is still a 7% time error. Mechanical shutters just aren't that
precise.

Dave
 




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