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Posted 01/26/2004

 

Uncle Bill's Guide to El Cheapo Computer-Generated Slides

by Bill Higgins, Fermilab

In September 1999 I was asked about making 35mm slides from a computer monitor. Below is my reply.

I have been shooting slides off the screen of my Mac for some time. They're not great, but they're good enough.

A professional would use a "graphics camera" to make slides, or perhaps take his image files to a "service bureau," but I'm operating on a shoestring, and there are a lot of nice images available on the Web. I'll tell you what has worked for me over the past few years.

I'll call this (*ahem*):

Uncle Bill's Guide to El Cheapo Computer-Generated Slides

1. THE BIG IDEA: OPEN THE SHUTTER FOR MANY REFRESH CYCLES

The essential thing to keep in mind is this: The pictures on your screen are built up by a moving electron beam, about 75 times a second or so depending on your scan rate. (Television makes a new picture 30 times a second.) If your shutter is open for less than the time it takes to make a picture, such as 1/100 second, the camera will see only *part* of a picture. If your shutter is open for just a little more than this time, that's not good either-- part of the picture will be scanned once during the exposure, part will be scanned twice, and your slide will show a bright horizontal band somewhere in the picture.

So you want to have the shutter open a long time, time enough for many scans to build up. I typically use 0.5 second or 1 second. The image will show an average brightness over dozens of scans.

To do this, I set the camera's aperture to be fairly small (in other words, set the f-stop number to be large, or at least in the middle of the camera's range). Then I set the shutter speed by paying attention to what the light meter tells me.

2. COOKBOOK FOR CHEAP COMPUTER-GENERATED SLIDES

Now, some more details.

I shoot at night, with all the lights off in the room, because I want to capture only the light from the computer.

I set up my tripod and single-lens reflex (SLR) camera to view the screen of my computer. A bigger, higher-resolution display is better, but I get adequate results with a 13-inch display showing 640 by 480 pixels.

Of course you want the camera at a distance from the screen so that the screen's image nearly fills the viewfinder. And you want the height of the tripod set so the center of the display is in the center of the viewfinder. But what about alignment?

You don't want your camera's line of sight to be tilted, which would produced a trapezoidal or "keystoned" image, but rather your camera should be looking squarely at the center of the screen.

A photographer taught me a trick: Turn off the computer's display. Look through the SLR viewfinder at the REFLECTION of your camera in the screen. Tilt or pan the camera, and move the tripod, until the reflection of the camera lens is seen exactly in the center of the viewfinder. Then you're aligned correctly.

As for film, I usually use Kodak Ektachrome of ASA 100 or 200. I don't worry about accurate color reproduction, but a professional photographer would, and probably add many more steps to this list of instructions!

I also use a "cable release" to trigger the shutter without touching the camera. The shutter speeds I'm using are so slow that any camera motion will result in blurring.

Switch on the computer's display and bring up the images you want to shoot.

Set the maximum color "depth" you can (the maximum number of colors your computer can display) if you're looking at JPEGs. If you're displaying only GIFs, 8 bits (that is, 256 colors) are adequate.

I like to use the freeware "JPEGview" for the Macintosh. It can display one picture, all by itself on the screen, with black borders, and scale it so it spans the screen. There are probably fancier programs that will do this, both in the Mac and PC world. Anybody got suggestions? JPEGview will handle JPEGs and GIFs--the most commonly encountered image formats on the Web-- and a couple of other formats.

I have trouble hiding the cursor arrow, but usually sneak it into a corner.

Turn off the lights in the room.

I use a zoom lens to size the image just the way I want it. Focusing is very important, but once you get the first shot focused you won't have to adjust it at all, in theory.

The first time you try this, keep a record of your shutter speeds and F-stop settings for each slide. "Bracket" your shots: take one at the speed recommended by your light meter, and others one stop slower or faster. Very bright shots (like a diagram on a white background) may cause the light meter to underestimate the exposure time needed to see the details. So getting a little experience with the performance of your own screen/camera/light-meter system is a good idea. When you get the slides back from the camera store, note which settings gave you the best-looking slides. Also, for your first roll, try to shoot a mixture of mostly-dark and mostly-bright images, to help understand these effects.

After all your work setting this up, you'll find that the process goes pretty fast: call up the next image, set up the exposure, check the focus, click the shutter.

3. COPYRIGHT

Be sure you have permission to use the images. This topic is probably worth more discussion, but, briefly, NASA policy is that images made for NASA projects are in the public domain, and you don't need to ask for permission.

However, for other pictures you need permission from the copyright holder, and there may be strings attached. For example, I put together a talk about the hypothetical ocean on Europa, and Woods Hole Oceanographic Institution was kind enough to send me a slide set of robot submersibles and hydrothermal vents. But WHOI told me that I can only use these slides if I'm speaking for free. If somebody wants to give me a fee for my Europa talk, I must either remove these slides, or go back to Woods Hole and ask (and possibly pay a fee) for permission to use them.

4. DISADVANTAGES

The method I've described should yield you a nice bunch of slides. Maybe someone more expert can give me tips on how to improve what I'm doing.

There are some disadvantages in doing things this way, which I accept because it's cheap.

First, the spatial resolution of pixels available on the computer is much less than what the slides could potentially show.

Second, the color range of the computer's display is limited and may be less than you'd get on a good photographic transparency.

Third, I'm paying no attention to color accuracy, and the colors on my slides may be bluer, or redder, quite different from the original image that the creator put into the computer.

Fourth, while I've taken care to eliminate "keystone" geometric distortion, I still get some "pincushioning" by shooting a curved screen onto flat film, and maybe other geometric distortions I haven't thought about. A good graphics camera would correct for much of this.

If you don't mind these problems, and you have access to a computer and an SLR camera, you can make pretty good slides on a very tight budget.

-- Bill Higgins | higgins@fnal.gov | Fermi National Accelerator Laboratory |

"Get the dinosaurs in, Martha, they're predicting comets." -- Dr. Barry D. Gehm