I came across this quote in the BenQ W7000 thread, and was going to reply there but thought I'd start a new thread to discuss the "surround" for home theatres.
Originally Posted by tallnick
Having said that, pretty much everyone one on this forum and every reviewer I've ever read seems to put black level and contrast at the top of the list of importance. I, simply, don't.
I don't either. It's a valuable characteristic of any projector, but other considerations are more important. It's an attribute that may appear to enhance the experience, but it's a bit of a fraud, especially the ridiculous numbers quoted for contrast ratio. The Holy Grail of dense black levels is an analogue of the Loudness control found on some amplifiers. "Let's boost the bass so it sounds better". The loudness control does have some technical validity (it can compensate for the "bass loss problem" when music is played at low volumes, see http://hyperphysics.phy-astr.gsu.edu/hbase/acoustic/baslos.html
), but when used at high volumes the concept is a bit sus if you are really trying to experience the music as recorded. Darkening the black levels is a similar phenomenon, and what price is being paid to achieve it? Irises buzzing in and out, scenes lightening and darkening unnaturally.
Improving black levels has a long history. Kodachrome was designed to improve black levels (when projected) by seriously increasing the density of the shadows, sort of like the effect of a variable iris in a digital projector, but the effect was achieved chemically (see pages 10-12 of my PDF on scanning Kodachrome: http://www.mediafire.com/download.php?qc67n2gkdz3viyp
). Scientists in the Kodak labs were compensating for the effect whereby the shadow areas of an image projected in a dark room appear less dark than they should be, not because they were actually less dark, but because of the characteristics of human vision. To make shadow areas of slides appear acceptably dark, those areas had to have their blackness increased by as much as a hundred fold as the slide was being exposed.
Some other considerations pertaining to black levels:
1. Our eyes are more sensitive to changes in brightness in dark areas than in light areas. Make the overall scene darker and suddenly what was an acceptable black in the image is now distinguisable from the deeper blacks or areas outside the image.
2. I don't know the exact figures, but contrast ratios of image-reproducing devices are really quite limited. Paper is about 100; Kodachrome under ideal projection conditions can improve on that, as can a film in a movie theatre, both of which might achieve 500; and a good LCD screen in a darkened room with dark surrounds might get close to a 1000. And yet users complain about a projector, such as the BenQ W7000, as having poor black levels when it has a native contrast ratio of 1000.
3. The human visual system operates relatively, not absolutely. Meaning: if we have nothing in view to compare an image with, we accommodate to what we see in terms of how bright it appears, the colour balance, and how much contrast we perceive. So, it is quite difficult to see the "yellowishness" of the light when you walk into a room lit by incandescents, but it becomes very obvious when you photograph such a room on a daylight setting and then view the image on a screen. Similarly, if you concentrate on nothing but the image from a digital projector, black levels, I think, are of not much importance as long as you have a certain level of contrast. You'll perceive the shadow areas as acceptably black – until you look at the black wall over to the side and begin comparing the blackness.
What interests me in all of this is: by aiming for surrounds that are as dark as possible when projecting, are we actually reducing the perceived contrast? I'm not talking about the absolute value of how black the shadow area is, but how black it is perceived. I don't know how comparable digital viewing conditions are to the way slides were projected and tested, but it is well-documented that the darker the surrounds, the lighter the appearance of the shadows will be. For a simple example, see the figure on the bottom right on p12 of my Kodachrome PDF.
I'd like to read some research about perceived black levels versus the lightness of the surroundings for digital projection. Does anyone know of such research? A good starting point is the paper by Fairchild (http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.39.6955
). I imagine a room where there is a digital projector projecting stills onto a screen which is surrounded by nothing but:
1. White walls
2. Gray walls
3. Black walls which absorb as much light as possible.
Then ask a number of ordinary viewers to rate the contrast of each scenario, particularly how black the blacks appear.
As an aside: the human visual system is a strange beast. Here's a surprising phenomenon (http://www.cis.rit.edu/fairchild/PDFs/PAP13.pdf
), which shows how what we think we see can change if it is viewed in isolation. Fairchild says:One very interesting case is for the perceptions of colors such as brown and gray. These colors only exist as related colors. It is impossible to find an isolated brown or gray stimulus, as evidenced by the lack of a brown or gray light source. These lights would appear either orange or white, when viewed in isolation.
I assume there is something similar for blacks: that the darkest area of an image, under suitable conditions, will be perceived as black when the whole image is viewed in isolation. It is only when compared to a darker black that we can see that what we thought was black, could be blacker.
This black levels thing is a complicated matter, much more so than just measuring how much light is coming from the screen when the projector is projecting black. That comes into play of course, but what is more important is how we perceive that black. It wouldn't surprise me if it turned out that perceived contrast improves when the surroundings are not an all-absorbing black. Maybe the optimal perceived contrast is achieved when the surrounds have the same blackness as the blackest possible part of the image. That way, you've got nothing blacker with which to compare the shadows, and the overall viewing experience would improve.