Originally Posted by DavidHir
Gotcha, that clears it up.
So, are the terms "native" contrast and "on/off" contrast meant to be the same?
"Native Contrast" generally refers to what the chip/light/engine/system can pull off on it's own, without a Dynamic Iris.
Typically we talk about "native on/off" contrast, which is sequential contrast, meaning a measurement first with a full white screen image ("on" measurement) and then after that, measure the projector projecting a full black image ("off" measurement). Do that gives you the full on/off dynamic range of the native system. So for instance a JVC projector can be measured with a 50,000:1 on/off contrast range (even higher depending on who is measuring). So that's the distance between the darkest black it can put out and it's brightest white.
Ideally that would mean that you could have that kind of simultaneous contrast as well - in other words, say you have a shot at night with a pitch black background, and a very bright searchlight peircing the black night. If the projector could simultaneously produce it's darkest black and it's brightest white, that is do 50,000:1 contrast in that single image, it would be a very dynamic image. Unfortunately, there are various ways in which the bright part of the image is not perfectly contained and the light bleeds and diffuses in to the dark areas. It happens at the chip level, at the lens level etc (others can get more technical about that). The upshot is that actually maintaining that kind of simultaneous contrast, due to light bleed in the system, doesn't really happen.
ANSI contrast measurements attempt to determine how much simultaneous contrast (the less light bleed in the system the better) by displaying dark squares and bright squares together, measuring how dark and bright they are simultaneously.
However, as some very smart folks on the forum have pointed out, the type of pattern used to measure the simultaneous ANSI contrast is not typical of most movie images, and since the light and dark areas of many movie images are more spread out, the light bleed becomes less severe in so you get higher contrast. This is why ANSI contrast doesn't really tell you how contrasty an image will be. It turns out that a higher native on/off contrast ratio comes into play here, which is why the JVCs can in certain dark scenes with light objects look more contrasty and dynamic than another projector that measures "higher ANSI contrast" in the ANSI patterns.
Which is what I was getting at before: In making a dark room you are preserving these type of contrast dynamics that a high contrast projector like the JVC can deliver.
Now, as to whether "native on/off" is the SAME as "on/off" contrast...not really. The distinction to be made is generally due to the existence of projectors that use Dynamic Irises to increase their on/off contrast ratio. A Sony projector has much lower native on/off contrast than a JVC, but in adding a DI to close down and cut off more light for certain dark scenes (and full black screens) the Sony is able to produce a deeper black level at times than if it didn't have the DI. So in measuring the on/off contrast with the DI engaged, on a full black screen the DI will close down lowering the light output and hence lowering the measured black level. Put on a full white screen and
the DI opens up no longer limiting the light output, so you measure the brightest the projector can put out.
Hence using the DI you get a higher on/off contrast measurement than the native contrast of the system.
That's why people will refer to "native" on/off contrast to distinguish it from contrast possible adding a Dynamic Iris which dynamically lowers light output for dark images. Some projectors look to have really high on/off contrast with the DI working. Turn it off and their native on/off contrast is much lower.
The issue with DI's as I believe you know is that in order to lower the light level for dark scenes, they are also cutting off all the light output, so making everything dimmer including the objects that should be bright in the shot.
Whereas a high native contrast projector (e.g. JVC) can produce a higher contrast of dark and light in the same image without a DI. (And to compensate for this, projectors employing a DI will often manipulate the gamma to boost the brightness of lighter parts in a dark scene, to boost the contrast).