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contrast ratio...confused??

Discussion in 'Projectors, Screens & Video Processors' started by (GTV)Chris, Jan 13, 2004.

  1. (GTV)Chris

    (GTV)Chris
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    Ok, I have been doing a bit of research about contrast ratio.
    Reason being, I want the AE500 when it comes out and at a ratio of 1300:1 against my AE100 at 500:1 it sounds amazing.
    But then there are people who tell me that it doesn't mean it will deliver a better black? Is this really the case?
    I understand that it is the difference between the brightest white and the blackest black. The higher the ratio the better the definition in the dark areas with better colour seperation.
    Is that basically it or did I miss something.
    Will an LCD unit create a better black at 1300:1 than a DLP unit at the same ratio?
    I would like someone with an enormous brain to help explain the subtleties of it all.

    Cheers
    Chris
     
  2. Tempest

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    Whilst I have no idea technically I don't think these figures really make the difference you would suspect in reality. although the figures are massivly difference, most seem to say it's a bit better.

    Re the dlp, well as far as I understand, in a LCD you have a bulb who's light is being blocked by liquid crystals and they can only be made so dark (opaque) so there will be a small amount that squeezes through the liquid crystals.

    A DLP on the other hand has those mirrors which I guess on a black area are turned away from the screen, hence the light is not being shone in the screen direction in the 1st place

    I could be wrong though !!!!!!!!!!
     
  3. buns

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    in theory you could increase the contrast ratio in an unlimited fashion without the black level ever changing. If your white was 1000 times birghter yet black the same, then the contrast ratio is going to be enormously higher yet black the same! Of course, chances are you would get a suntan and perceive better black, but that is anoher matter.

    I thought with lcd the LC should have parallel polarisers on either face, thus when the LC is in the off position, then the plane of polarisation is rotated by the LC thus the polarisers are realtively speaking crossed to the light so there should be little or no transmission? (of course i could have the process reversed there). I thought that the main source of leakage was through the gaps between pixels?

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  4. Daftboy

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    From what I hear it depends on the manufucaturer of the panel.
    Epson poor, Sony good. Damn shame Sony have now stopped making LCD panels for projectors (Except their own with that special technology).

    Doesn't bother me now I've getting my Z-200. DLP baby.
     
  5. (GTV)Chris

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    Do you suffer from rainbows at all.
    I do, which is why I have to stick to LCD for the time being :(
     
  6. tbrar

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    Seen some interesting posts on AVS, with regard to measuring relative black levels (under the >$3500 Digital Projectors forum). I am at work so do not have time to link to them. Apparently (as far as I remember, anyway ?) the way to work it out is the peak brightness figure stated, devided by the contrast ratio stated, for example:-

    a PJ with a brightness of 700 and a CR of 3800:1 will be:-

    700/3800 = 0.18 Black Level

    Where as a PJ with a brightness of 1000 and a CR of 2700:1 will be:-

    1000/2700 = 0.37 Black Level


    Using this method, the lower the black level the better (in terms of trying to attain a true black as possible). As said, this is from memory, so please exuse if completely daft, I am also unsure as to whether this applied to DLP's only or both LCD & DLP's, but may be a good starting point ??
     
  7. Daftboy

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    Hi Chris,

    I suffered in the early days of DLP, but the Z-200 was very good. Watched a Infocus and the Z-9000 they had and could see the colour wheel breakup V badly.
     
  8. popeckia

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    On/Off contrast ratio (the number typically quoted) is really about as close to a measure of black level as you can get.

    If the projector is very bright, then the typical black level will be raised, but this can always ben countered by filters. The point is that getting a good black level is entirely simple in any projector, by placing a heavy neutral density filter over the lens. This will of course however reduce the whites by the same amount and leave you with a very dull picture.

    If you are after good black levels, then seek high contrast ratios (taking into account of course the general outrageous lying from manufacturers). If the black level is still too high for you on a high contrast machine, then use a filter/screen to adjust it.

    High and Low lumen projectors can to a degree be compensated for with filters and screen options, but other than a slight colour correction filter benefit, you can't increase contrast ratio, and speaking personally... lack of contrast is still the thing that annoys me more than anything else in my projector.

    Also, when comparing contrast ratios, I have read that you perceived benefit of going from 500:1 to 1000:1 will be the same as going from 1000:1 to 2000:1. i.e. you are looking at the ratio of ratios, rather than absolute increase.

    With that in mind, don't draw too many conclusions regarding the difference in contrast between a 2000:1 projector, and a 2500:1 projector as the difference will probably be hard to perceive.
     
  9. buns

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    well if on off contrast ratio is as good a measure of black as you can get, surely it should work with crt as well!? We know it is ill defined in that case because black is in fact black, so the light output is zero hence the contrast is infinite irrelevant of how bright white is.

    I agree with what you say that you could make a horrentdously bright machine and filter it to the point that black is black, but I see no reason to prefer this to actually designing a way to have full off as being true black. I also suspect that a true black solution is going to be a hell of alot cheaper than a heavily filtered bright source.

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  10. Gordon @ Convergent AV

    Gordon @ Convergent AV
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    It should also be pointed out that even although contrast is important it is not a measure of how good dark or light detail will be as it is perfectly possible to have high contrast and a complete inability to differentiate graduations in detail as light output increases or decreases.

    Gordon
    back from CES and trying to wake up and have breakfast.
     
  11. (GTV)Chris

    (GTV)Chris
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    (In an Oliver Hardy accent) "Tell me that again?"

    So, basically...High contrast ratio doesn't mean better blacks, but a percieved bertter black.
    But also doesn't mean a better definition between the full on and off range...
    I want my mummy :(
     
  12. Chris Frost

    Chris Frost
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    As Gordon said, contrast ratio is simply a measurement of the difference in brightness between to points. It has less to do with the absolute level of blackness than most people imagine.

    You could measure a high brightness projector where maximum white light output is blindingly bright, but the blacks look a sort of mid grey. The contrast ratio might be say 1000:1 but for home cinema use this wouldn't look good because the blacks are poor.

    Next you could then measure a low ANSI lumens projector which produces a much deeper black and come up with the same 1000:1 ratio. Whites wouldn't look so bright, but for home cinema this would be a more pleasing picture because the level of black is darker.

    There then comes the fudging of the figures. You can use measurement methods that enhance the final result (full on/off, contrast control to max etc) or choose to set-up the projector for real world viewing conditions and come out with much lower figures. I'll leave you to guess what most manufacturers do ;)

    As always, the best advice is to view a product rather than relying on specs.

    Regards
     
  13. buns

    buns
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    my lord, ive been shouting this at anybody and everybody for the last month!

    The Z2 is prime example of fudging. I cant get a contrast ratio of any better than about 900:1 unless I measure the white level on full power and contrast set way high and with black level measured on low lamp and irish closed and brightness set low....

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  14. theritz

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    Gordon...

    .................... would love a report on any intersting stuff you've seen...... (you lucky sod...:D )



    Sean.
     
  15. popeckia

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    "You could measure a high brightness projector where maximum white light output is blindingly bright, but the blacks look a sort of mid grey. The contrast ratio might be say 1000:1 but for home cinema use this wouldn't look good because the blacks are poor."

    The thing about this statement that I was trying to get across is that it rather confuses matters regarding black levels.

    Say you have two projectors, both 1000:1 contrast (we'll assume both manufacturers lied equally). One is rated at 500 lumens, and one is rated at 2000.

    In this instance, the second projector (out of the box) would have a higher black level, but the addition of a neutral density filter of the correct value, would (all else being equal) bring the two projectors into line with each both in terms of black level and brightness of image.

    The point being that black level is simply a direct calculation based on the real contrast ratio and the real lumen output. And given the lumen output can be adjusted either via filters or via the screen, the only real factor in terms of a floor level for black, is the on/off contrast ratio.

    Of course, given the examples above the bright projector would be preferred, because you can use a low gain screen to bring the total light output back down to a similar level as the first projector, but the low gain screen would have less hotspotting and would dramatically help to reduce the effects of ambient light in the room. This is obviously preferable to using a filter, which would adjust light output, but without the benefits of reducing the effects of ambient light.

    So we come to another quote

    "So, basically...High contrast ratio doesn't mean better blacks, but a percieved bertter black."

    I don't agree, I think that if you are seeking better black levels, what you really mean, is that you are seeking higher contrast ratio. Taken in isolation, nobody is really seeking lower blacks, because that can be achieved with a 10 pound filter, but only at the expense of brightness, which is also important.

    Of course, as was also pointed out, a lower absolute black level tells you nothing about the ability to reproduce shadow detail
     
  16. buns

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    My physics background tells me that I define something which I can actually make a direct change to as opposed to an indirect change. I can state the light intensity at full black, I can also change it by adding more light. In doing so, black level has maintained its definition. Were i defining black in terms of contrast ratio, the contrast ratio would be indirectly changed by the addition of light to full black. However, stating an increase in contrast ratio in this instance tells me nothing physical..... i simply know that either white has decreased or black has increased. So my contrast ratio has now lost its definition in this case.

    The addition of filter is simply a shift in your space and needs no redefinition, that would be akin to saying that the laws of physics arent invariant.

    I can see what you are saying, if you wish to look at it in a big picture, yes, I want increase in contrast ratio BECAUSE of decrease in black level. Without the because, it means nothing at all.

    However what I am looking at is different, im looking at a cause and effect. I dont want to add someting that makes further mess of my signal (filter), I want to remove the need for the filter altogether. Hence I (me, Adam.....) am definitiely striving for the better black, the contrast ratio is down the road and will simply improve as a result of what i desire.

    I think we'll have to agree to disagree because we are attacking this from totally different perspectives!

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  17. popeckia

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    ok, one more comment from me, then I'll shut up ;)

    If you have projector A, which has a contrast ratio of 3000:1 and a lumen output of 1000

    and projector B which has a contrast ratio of 2000:1 and a lumen output of 500

    In this case, projector B would have the lower black level, which is what you say you are seeking. Hence, in this measure, it is preferable to projector A for you.

    However, if you were to project A onto a 0.5 gain screen, and project B onto a 1.0 gain screen, then A would have the lower black level in a perfectly light controlled room, whilst retaining the same peak white level due to its increased contrast ratio.

    Also, in anything other than this theoretical pefectly light controlled room, you will get less light reflection washing out your picture due to the lower gain screen, which means the black level will actually be even lower relative to B. The worse your light control, the better real world black you will get with A, not perceived black... real black.

    So projector A has the lowest black level when you lined them up side by side on the same screen, but is it still really the one you want for "low blacks"?
     
  18. buns

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    but im only talking about black level! I only said black level. I didnt actually mention filters, i didnt mention gains of screen, i didnt mention the white level or washing out. The way I have said things is I want lower black level. I havent mentioned the other directly controlled variables because i dont want them to change (contrast ratio is not something i am directly controlling, it is a result of something else changing so if it changes, thats fine, im not considering it so i dont care!). What I want is a projector with a given peak white output but with decreased black. So in this case, the only thing I have directly altered is the black level. The contrast has addmittedly changed, but it is an effect of the black level changing. Contrast ratio has no physical meaning. It is a ratio, a comparison of 2 other quantities, it has no units, it cannot be measured in isolation.

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  19. popeckia

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    "What I want is a projector with a given peak white output but with decreased black. "

    How is that not a definition of "higher contrast"?

    Let me try another approach on this... black level is not an absolute on any projector. It is a function of the projector itself, the size of image you are projecting, the screen you are projecting on to and the ambient light conditions in the room you are viewing it.

    These things are so heavily intertwined, how can you assess one outside of all the others?

    You yourself said, you want lower blacks WITHOUT LOSING PEAK WHITE LEVEL which is not just seeking lower black levels, it is seeking two things at the same time, and these two things are exactly what is represented by a contrast ratio. The point about screens is that it can affect total effective light output, up or down (when up, at the expense of off-axis viewing etc), but nothing can increase effective constrast ratio of a projector. It is basically your limiting factor, when finding black levels to suit you.

    It seems like the greatest mistake people make in purchases is to consider the projector in isolation without taking into account it's environment.

    Saying one projector has better black level than another means absolutely nothing in isolation, because in many cases it will simply be a low lumen output projector, which does not satisfy your requirements to have lower black levels and keep peak white levels.
     
  20. buns

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    What i gave there was a definition of contrast ratio..... but the contrast ratio has been defined by the white and black, not the white and black defined by the contrast ratio.

    The black and white should be independent frankly. If they are dependent, there is a flaw in the system. By definition, black is the absence of light, so to be dependent on a quantity of light is nonsensical. So if you want me to say it, current technology is poor in that it results in 2 independent quantities becoming dependent. From a theoretical standpoint (which is what we are doing), a change in black level is all i want because it should be assumed that white will remain the same

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  21. (GTV)Chris

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    RIGHT....That's it!.....At the weekend I am buying a Bell and Howell 16mm film projector.;)

    c
     
  22. popeckia

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    black and white sadly are entirely dependant on each other in LCD and DLP projectors, hence the on/off contrast ratio being important when assessing digital projectors.

    In CRT as you mentioned earlier, there is theoretically the possibility of emitting absolutely no light at a particular point on the screen, hence rendering as black as you can get.

    In LCD and DLP, if you increase the intensity of the light source, you leak more light out onto the screen when trying to project black. In this regard, black and white are not independent whatsoever, black is simply a calculation of the peak output of the projector (as determined by the light source and optics) divided by the contrast ratio.

    This I suppose is the flaw in the system that you refer to.

    Going back (or trying to) to the original point of whether or not a higher quoted contrast ratio will mean "better blacks" to the average consumer, then really the answer has to be yes, and the reason is because, like you, they want the picture to be of a certain brightness, and then when there are dark scenes, they want them as dark as possible. The ability for a projector to due this is perfectly represented by on/off contrast ratio and hence it is a valid measurement.

    Of course.... this is just the absolute lowest light level the projector can produce, and no measure of the quality of the projector in low light situations and shadow detail reproduction.
     
  23. buns

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    we'll have to agree to disagree! I am actually starting to believe we are both right, but neither right. Neither of our perspectives actually allows the other arguement to be valid! At least we can agree that the current technology is seriously crap! :D

    on a side note, i actually know how to block 100% of the light :p in fact, i have a friend who is doing it so :p

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  24. Gary Lightfoot

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    I've not read the entire thread, so thought I'd post my thoughts on the subject. :)

    IIRC, in some cases, you can increase contrast by fitting a brighter lamp, though this won't improve black level (providing it stays the same, or the black doesn't get significantly brighter).

    How much this effects the black level, I couldn't say, but you can get better CR increases with smaller improvements in black level, as you need to increase the brightness a fair bit to get a good increase in CR with the black level remaing the same.

    For example: the X1 has a contrast of 2000:1 (manu figures) which will drop down to 1400:1 when the white segment is switched off. So in this case, you can get a slightly better black level, but with reduced contrast. - Black level and CR aren't necessarily related..

    Does that help or should I just delete it all? :)

    Gary.
     
  25. (GTV)Chris

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    LOL. It does help, as did all the other replies.
    I still find it a BLACK area?
    I guess, when it all boils down to it, it really is subjective. Whether percieved better black or actual.
    It really in the eye of the beholder. What looks great to one person may look completely different to another.
    I guess I'll just have to demo a few before buying :)

    Cheers

    Chris
     
  26. buns

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    Add in that many actually dont much worry about the CR or balck level...... other factors are much more important to them.

    By the sounds of it, if you want the best of all worlds, hold out and watch for 'The eclipse' from immersive ;)

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    ps. contrast ratio isnt just something to do with video.....
     

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