Actually, the title of this thread isn't 100% accurate. It should be more like "Why
Most Screenies don't matter"; but I'm getting ahead of myself.
First, a few definitions:
Screenie:
A digital photo taken of a projection screen displaying a paused movie or television scene.
Reference image photo:
Similar to a screenie, but the still or paused image is not meant to entertain, but is designed to show an attribute of the screen or projector. Most such images are meant to aid in correctly calibrating television sets or projectors for contrast, brightness etc..
Reference photos are necessary to determine screen performance. For an example see this post.
http://www.hometheatershack.com/foru...html#post97077
Unless you are total newcomer to DIY screens, and internet forums dealing with such, you have seen a number of examples of screenies. The things are all over the place as people try to show off their home theaters, home-made screens or newest projector. There is nothing wrong with this, it can be fun to see such images;
but, what do they really tell us? Most of the time, not much.
I want to be clear that I'm not talking about photos that show how to build a screen, or images of entire home theaters. These usually are quite informative, and some are even down-right gorgeous. Many HT's I've seen on forums should be in
Home and Garden magazine! They are works of art!
One of the primary reasons most screenies don't give us much usable data is the fact that taking them is not an easy thing to learn; it ain't Rocket Science, but the only place to learn how is in forums like this one.
Most people just use the auto setting on their camera. That's a bad way to take screenies or reference photos, and will lead to great frustration if the person is trying to show anything like the image they are seeing on the screen with their naked eyes.
The first step is to use whatever mode your camera has that will allow the user to
manually set the white balance. Go into that mode and then project a solid white image onto your screen. There is one on the Video Essentials DVD, and probably other video calibration DVD's as well. After you have this image on your screen, go through whatever procedure your camera uses to set this screen as the white reference. This is the only way to be sure your camera will produce photos close to what you see.
Now we come to setting the exposure time for the photos. Many times the camera will automatically set the proper exposure for the shot, but many times it won't either. The only thing to do is to take multiple photos of the screen at different exposure times and later select the one that looks closest to the true screen image.
Even with white balance and exposure taken care of we still have a problem, one I haven't found a good solution to yet. I'm talking about color saturation. I have taken screenies of a movie that seem pretty good as far as proper colors and exposures are concerned, but dad-burn it, the things are over-saturated! I'll show what I mean by this in a moment. Sometimes a camera will let you compensate for this, but most won't. In my case, my camera will only let me choose to have a
more saturated picture!
Now comes the two biggies; contrast and dynamic range. Very few camera's, at least in the "point and shoot" category (read that as the cameras most of us have
), will let you manually set contrast levels, and NO camera I know of can manually adjust dynamic range; you either have an adequate amount or you don't (most don't).
Contrast and dynamic range are related, but it will take someone smarter than me to explain it properly. My first set of photos will show you what I mean though. they're coming up shortly.
In the end, our digital cameras are designed to make the best "normal" photo of whatever the camera is seeing. This is at odds with what we are trying to do with screenies. We are trying to show what our screens really look like to our eyes, but the camera is trying to make each photo look like it was taken in a perfect textbook setting, such as outside on a clear day in the middle of the afternoon with the sun behind the camera. In short, the camera is trying to make each photo "perfect" and not real. Sorry, at the moment I can't explain it any better than that.
All the following photos are from the movie
The Fifth Element full-screen version.
The first two will show my camera's lack of dynamic range, meaning I can't get proper shadow detail and highlight detail at the same time. The first photo is exposed the way the camera thought it should be. In general, it's a good photo for shadows and mid-tones, but the highlights (glare on foreheads and the doctor's shirt) are overexposed. The second photo had a shorter exposure time and the highlights look better, but the shadows have gone too dark. If my camera had a wider dynamic range this wouldn't happen.
Camera auto exposure 
Manual exposure 
This next photo also has dynamic range problems (I could see more detail in the mans eyes), it also has a more evident color saturation problem. The colors are too vivid and are not what I was seeing on the screen.
Auto exposure 
Colors again too vivid.
Auto exposure 
While this next photo looks like it was properly exposed, in reality, the image on the screen was brighter yet.
Auto exposure 
One more example of lack of DR (dynamic range) and over-saturation by the camera, not the projector.
Auto exposure 
Now another example of why most screenies don't mean much. The screen used in these photos was made up of 5 panels, each about 12 inches wide and 4 feet tall. Each panel was painted with a different paint. There isn't a lot of difference between them, but there are differences. Especially in the second panel from the left, it hot-spots. A bit of the bare wall of my shop can be seen as the far-right edge of the screen image.
Composite screen under white PJ light 
The more discerning of you might have noticed the composite screen in the photos above. Here are some photos in which it is very hard to see the separate panels. If 5 different screens can all look the same, what does that tell you about using screenies to show how one screen is better than another? That is what reference photos are for.
Several shots where the composite screen is quite apparent.
One last photo that just plain looks good
.
The final kicker is that NONE of the above photos looks like what I was seeing on the projection screen. The real image didn't have the contrast or saturation the photos have. The camera was doing it's job and making a silk purse out of a sow's ear, or at least doing it's best. Most cameras do the same thing.
There are times when screenies can show a
relative difference between two or more screens and give meaningful results, but these are few and far between on the forums. The scenes must be chosen with care and photographed properly. Such photos will almost never look "good", they will be lacking in contrast or color; but they should, many times, if they are truly showing what the screens looks like. My hat goes off to the folks that take the time, and have the expertise, to take such photos.