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Discussion Starter · #1 ·
Hey everyone..


Lately there have been a couple of reviews where we've seen the use of digital cameras in comparing the image of two or more projectors. Unfortunately unless carefully used a digital camera is practically useless for any kind of objective comparison. I'll try and touch upon some of the problems.


The most crucial problem is really light sensitivity. A digital camera is not sensitive enough to low lighting in order to give you a proper representation of the image you see. If you look at some of the shots of dark scenes you'll notice camera artifacts, and incorrect color/gamma balance. This is because while the light sensitivity of the CCD is the same - digital cameras have self adjustment for gain. That means that if you put two projectors side by side and one has a higher light output the camera will try and adjust the gain for each picture.


Most still picture CCDs aren't sensitive enough to low light in the first place. I'm thinking that instead of using a still picture camera a better idea might be to use a digital video camera with low light sensitivity. For those it's also possible to turn off auto gain etc.


Cameras might also have internal color adjustment - some might even skew the color of the image slightly so that what looks fine in reality could become more yellow or green.


There's also the camera positioning issue. As everyone knows there are screens that have a sweet spot for viewing. Making sure that the camera is in the exact same place each time a comparison picture is taken is a very good idea. This should preferrably be done with a tripod, and be placed where the head of the viewer normally would be. Use the same settings for zoom etc.


When pausing the image on your HTPC/DVD player make sure the player shows the entire frame - not just a line doubled field. You'll get obvious aliasing artifacts and an incorrect representation of what it really looks like if you don't.


Finally - why not try using a regular camera with a high contrast/light sensitivity? Use a scanner - preferrably one with negative capability - to digitize it. That should give you much more consistent results with low light scenes. For scenes with a lot of light then digital cameras obviously look good as we can see from some of the pictures taken. I'm still not sure if the resolution is high enough to accurately represent the differences of two or more projectors unless it's glaringly obvious however.


If a picture from two different projectors is taken with care it's actually possible to take the digitized image and analyse it. You can get very useful information as to contrast, brightness and whether it has a color problem or not. This can also be used as a helpful tool to figure out what adjustments to make on the projector in order to get a better image.


ehmm.. this post somehow turned into an interesting digression on my part:

What I would like to see on "super projectors" in the future is some kind of auto color/brightness/contrast adjustment where the projector actually has a CCD and a processor that analyses the projected image in order to get the best adjustment. Imagine putting in Avia on your DVD-player and just hitting the 'analyse' button on the projector. That should not only calibrate for the incoming signal, but also based on ambient lighting and the type of screen used.


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/frode
 

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Interesting and informative comments Frode. For the scaler review I just posted I used a tripod and took the photos of both my D-ILA and CRT monitor to ensure the results were consistent. This seemed to work fine to capture certain aspects of the scalers performance, although capturing some of the on-screen anomolies were difficult. For instance, the banding pictures really don't reflect the extent or severity of the banding, and the chroma delay pattern actually looked worse in person.


Used for the purpose of illustrating anomolies and within the context of "here is what I looked for..." the digital camera seemed to be quite helpful. But based on your comments above, I now wonder if posting the actual pictures from each of the respective scalers is at all worth while? Or are your feelings above limited to the use of digital cameras to capture projector performance? Your thoughts would be appreciated.


Thanks,


Tom

 

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I think what Frode is saying has merit, but I think the digital pics can also be a useful tool. I believe the pictures I have taken are quite representative of what I actually saw. I can tell the difference between the quality of different DVDs by looking at the pictures I took. A bad transfer is easily discerned in the pictures. I think the main problem is that when left in automatic mode, the camera compensates to make the exposures equal by leaving the shutter open longer for the dimmer picture.
 

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Discussion Starter · #4 ·
Tom,


Note my use of the word 'carefully' in the original post http://www.avsforum.com/ubb/smile.gif . While I do feel there are limitations in digital camera technology, that's not to say that they cannot be used at all. My original post was meant more as an examination of our testing methodology. If you base your own reviews solely (or mostly) on the pictures from a digital camera without doing a proper subjective round of your impressions then the review is IMO essentially useless.


Digital camera technology is not yet at a point where it can be used in this type of setting without a guiding hand. For scaler reviews for instance the camera should be used to point out obvious artifacts that you notice. A picture of a combing scene for instance can be very illustrative in the amount of combing seen. Followed up by a picture of the same scene with a different scaler. This is something that is easily detectable and can definetly be shown on a digital camera.


For checking out color, brightness and contrast however I feel that is very difficult if not impossible on consumer level products. It's possible that some of the real high end (read: extremely expensive) cameras are good enough in this regard.



Randall,


I agree with you that digital cameras can be somewhat used to showcase a bad transfer - but they're not good enough to show the severity of the problem. Most cameras will soften it up slightly making it more difficult to see what the fuss is about.


You're confusing digital camera technology and ordinary cameras btw. Digital cameras don't have shutters, but instead a CCD chip that's constantly on and capturing incoming light. The result is then passed through a analysing chip that adjusts the gain. For normal use this gain adjustment is actually necessary because without it everything will look crap. Imagine having to adjust the gamma setting of your camera every time you want to take a picture - not going to happen.



Mark,


Good point about the compression. Compression can hide artifacts - and sometimes even create artifacts that aren't there in the first place. A serviceable analogy could be taking the analog input from two high end hi-fi systems and compressing it with MP3 - and then comparing them. Not only is the compression reducing the information the available, but you're limited in the sampling capabilities of the sampler/soundcard.


You also mentioned one important point I forgot to elaborate properly on - resolution. Resolution should be as high as possible. If say you use 1024x768 on both camera and projector, you can't aligne the pixels of the camera perfectly with the projected pixels. You'll need a resolution that's _at least_ twice as high as the projected image, preferrably higher. Sampling theory and all that. Even then the limitations above still apply.


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/frode
 

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One of the problems with scanned images is that they can be very large and take-up most of the space allocated in the free internet web hosts.


I agree with Frode that the picture taking method has to be consistent for the comparisons to be valid. Thereby everything being equal differences can be discerned.


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cai
 

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Frode,

I don't think the digital cameras are that bad. I agree that you shouldn't base a purchase on seeing photos made with the digital camera. I think the pictures I took are quite good representations of what I saw. I'm not really up on digital camera inner workings, but does not the CCD stay on for different lengths of time for different exposures? I know I can change my camera to manual setting and control the length of exposure. Are you saying the length of exposure is always the same? If I look into the lens there appears to be a shutterlike mechanism that closes down, I can't see it well, but maybe I am seeing the diaphragm.


Anyway I think we kind of agree. The digital camera can be a useful tool but must be taken for what it is. There is no way to copy what we are seeing on our screens without a margin for error.
 

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Whenever we have the report of a "shootout" - it's the

evaluation of the reviewers that's important - not the

pictures.


The reviewers doing the reporting are the ones that

actually saw the demo.


The pictures are just a "bonus" and should not be used

to do a shootout "online". After all, in addition to

the quality of the camera, the quality of the monitor

on which the online digital picture is being viewed, also

affects what is perceived.


The real information contained in one of these shootout

reports are the comments of the reviewers.


Greg
 

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Discussion Starter · #8 ·
Randall,


The digital camera does not have an exposure part at all. It's instantly on. Without it you wouldn't see anything in the LCD viewfinder at the back of the camera. When you adjust the exposure on a digital camera you're in reality giving adjustment commands to the analysis/gain chip. You give it a set of input parameters to modify it's normal operation somewhat in order to simulate exposure. These setting aren't absolute however, but more as a guideline/limitation for how much the chip can auto-adjust.


A normal camera however is very reliant on exposure because the chemicals in the film react to light based on the amount and the time exposed to it. Since this is fixed a normal camera should not only give you a more controlled environment, but also be better at representing differences in light output, contrast differences for dark scenes, and color differences.


Anyway - I fell into the trap myself of analysing pictures when I commented on the shots Les made of the LP350 and LT150. This is a clear example where the digital camera just cannot properly represent the difference. While I could see clear differences - it's impossible to tell whether those are due to the camera or what Les really was seeing. Several others were also picking up on this and really commenting on the pictures.


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/frode
 

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Frode,


you are SORELY mistaken about how digital cameras work! They DO have ALL the characteristics of traditional cameras. Shuttertime, aperture, exposure, everything works just the same way it does with a film camera. Just that the sensitivity of the film (ISO 100,200,400 etc) is 'emulated' by gain adjustments of the sensor chips (CCD or CMOS).


While the shutter is open, light is accumulated on the sensor chip. If short exposure times are used in darker environments, a lot of 'gain' is needed to yield the correct exposure of the image. This 'gain' (through ISO adjustments or an AUTO mode) will result in a high level of image noise. If OTOH a longer exposure time is used (which will make a tripod necessary), noise will be at a more acceptable level.


BUT, once the exposure time gets too long, another effect is kicking in. Images seem to exhibit noise that looks like 'stuck pixels' on LCD projectors. Thats because the sensor elements (there are seperate elements for red, green and blue per pixel) reach a point of saturation. The resulting stuck pixels are quite obvious (more so than the normal noise), because they appear in fully saturated colors (red, green, yellow, etc...). The phenomenon is sensitive to the temperature of the CCD sensor. The cooler the sensor, the less stuck pixels. While letting a camera cool off before taking critical low light is a good idea, putting it in the fridge is not! http://www.avsforum.com/ubb/smile.gif


Digital cameras differ VASTLY in their low light performance. Newer models are getting better and better in this regard. The general signal to noise floor also differs greatly from model to model. Its a function of the sensor used AND the electronics involved. The exposure time that is needed to get the pixels 'stuck' is probably around 1 second for a lot of todays cameras. For some it can be as early as 1/4th second, for others as late as 15 seconds!


A good start for taking usable digipics:

- ISO to the lowest possible value (80 or 100 on most cameras) if adjustable; there might be another name for it like auto-gain ON/OFF, if so, switch to OFF

- best optical performance (detail) of most lenses is around F4

- exposure time needed is usually something between 0,1 and 1 second, depending on how bright your setup is

- use the AVIA white needle pattern and choose a exposure time so that the near white bars are still visible in the resulting image, otherwise bright scenes are completely clipped

- test if this exposure time also reveals the black needles; if it doesn't, you will loose some black level detail in the pics (common problem); you can either be content with the result or you can adjust black level (brightness) and white level (contrast) on your setup until both black and white bars are just visible with AVIA; this will result in the best possible pictures

- if picture seem oversatured, you can check if your camera has a 'saturation' parameter that can be adjusted in a lower position


If anyone is interested, i could write a more thorough tutorial on how to get good digicam pictures. Seems like most here don't know much about photography and simply use the point-and-shoot approach http://www.avsforum.com/ubb/smile.gif


You can take a look at my Fifth Element pictures in the link below and in my HT setup thread there are a lot more pictures.


Regards

Bjoern



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My HT in action | Fifth Element Shots | My Ultimate Guide to Edge Enhancement | My DVD/LD SPL site
 

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Discussion Starter · #10 ·
Hi Bjorn Roy..


Seems I was wrong. Thanks for correcting my assumptions on how digital cameras work. What I've posted about black levels etc. now makes even more sense.


Btw those tips you mentioned how does that affect the comparing of images from two different projectors? You're essentially tuning the camera for the first projector - do you tune it again for the second? If so would that not skew the results? How good a camera do you need in order get accurate results for comparison of color, brightness and contrast? Do you feel they can be used in such a setting?


This is exactly why I started this discussion - in order to look at our review technique using digital cameras and hopefully improve on it.


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/frode
 

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Assumptions? It sounded more to me like you were stating a fact. It would be wise of you in the future to understand your subject matter before beginning the lecture. Otherwise you should state that it is only an assumption.
 

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I just don't understand how you guys can say a digital camera is useless in a shootout or for comparison. I agree it should not be used to make your final decision on buying but there is useful information to be seen. Maybe my camera just works better than most, I don't know. If you look at my posted frames #1 and #8 of Demi Moore it can easily be seen in her face. The first is with the blackout screen, and the second is with the Highpower. The Highpower is clearly brighter. It is not the exact same frame, but it is the same lighting. I could take the pictures a dozen more times and get the same result. When you guys do your shootout, I want to see pictures. You guys that don't believe in them, don't look.
 

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Any photographic process be it digital or film has way too many variables including the bias and skill of the photographer to be taken seriously. A photo is only a representation or approximation of reality and not reality itself. Certainly consumer or pro-consumer cameras cannot be trusted from an equipment point of view and even if professional equipment is used it would have to be used by a professional photographer with rigid published protocols that are always followed exactly on every shoot to grant any credibility to the results.


[This message has been edited by leckian (edited 06-17-2001).]
 

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Hey Bjoern!


damn you are an encyclopedia!! (btw, how about those Lost World dvd/ld graphs ? http://www.avsforum.com/ubb/smile.gif )


I'll take shots tommorrow on my HT (postponed to Monday) with a Sony XGA digicam. any special quick recommendation for the screenshots ? will use a tripod. don't ask the model of the cam, it's of a friend.


cheers!




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for cinema sound in your HT, use cinema speakers and cinema amps! unbeatable.
 

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I find that a digital camera rarely helps in a shootout. The possible exception is when screen door is being evaluated and then only if the photographer does a very careful job.


I find that most digital cameras don't have enough dynamic range to give an accurate impression of black level.


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Ken Elliott
 
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