ND2 Calibration question

[sniffer66]

Digging through a box of kit last night I came across an old Hoya HMC ND2 I used on my old Sharp Z200.

I decided I'd give it a shot on my current HD72i to improve the blacks. It looks OK to my eye currently. Question is, my kit has been ISF'd but Im assuming I will only need to recalibrate bightness and contrast again. Ive had a play with DVE but according to my recalibration Im hardly changing either figure - Im calibrating brightness the usual way on a DLP using the mirror activity. Contrast Ive notched up a small amount

Is this correct that my settings shouldnt alter that much with the filter on ?

 

[Peter Parker]

You shouldn't have to recalibrate the brightness and contrast, and there is only a very small imbalance in the RGBs with an ND2 so you shouldn't have to get it ISF'd again either.

Gary

 

[sniffer66]

Gary

Thats excellent, just what i needed to know, thanks

I've been watching a few films tonight (Strictly Ballroom with the wife - class film ) and the ND2 definitely makes the PQ more "filmic". With the DLP it also seems less noisy

 

[Peter Parker]

I find the very same thing, so adding a filter to get the reflectance to cinema levels rather than having them at higher tv/plasma levels does make a big improvement in image quality IMHO, so I'm glad you're seeing the same results.

I've found that having less than 12 foot lamberts is a good guide, and around 7 to 9 foot lamberts of reflectance is very similar to local cinemas, so that's the number I try to aim for with DLP. Image noise from SD sources tends to get highlighted the brighter you go, so aiming for lower levels seems to make things appear more cinematic and less noisy, and gives a far more watchable image.

Gary

 

[Alan Mac]

The spatial frequency response of the eye is affected by the picture luminance level. The response (MTF) is better at high frequencies with higher luminance levels. That the picture looks less noisy at low luminance levels is a consequence of the eye’s impaired high frequency response. This impairment may be acceptable, even desirable, for lower resolution sources but for high definition, low noise sources it is arguable that higher peak luminance levels (say 100 cd / m^2) might be more appropriate.


Alan

 

[Peter Parker]

That's very true. Seeing HD on bright projectors such as the C3X (or HT5000) on something like an 8ft wide screen (or smaller) is certainly jaw dropping, since the the newer codecs like VC1 have very little artefacting in most cases, so the brightness isn't going to enhance noise in the same way it does with SD. High ANSI DLPs with HD content have an impressive depth of field in high APL scenes so brighter could indeed be preferable in those cases for some people.

Having the image at lower reflectance levels can still have a more cinematic look compared to a brighter one though, and the resolution of film is higher than our current source material I believe. Whether or not film should be our point of reference is another matter, and some people say the aim is reality, so having things at cinema levels rather than somewhat brighter can be a personal preference rather than an aim (though currently it's my preference but things can change. ).

MTF in the display itself plays a big part in visible resolution, and I would think DLP has a higher MTF than other technologies providing it has good optics (I think the HD2K was measured at around 75% which seems pretty impressive), so that combined with any temporal or spatial dithering may be another reason for preferring lower reflectance levels. Any thoughts on that Alan?

Gary

 

[Alan Mac]

I think cinema picture luminance levels were mainly dictated by the practical limitations of the projector light source rather than anything more fundamental. This luminance limitation was also the original reason for the darkened cinema rather than any great desire to achieve good black levels. A darkened room does however help the viewer to “suspend dis-belief” and in that sense contributes to the “cinema experience”.

The MTF of a projected 35 mm cinema release print at a typical cinema is inferior to that of a BD or HD-DVD negative-sourced copy of the film projected by a good 1080p home-cinema projector.

So perhaps the commercial cinema may not necessarily be the best model to follow when it comes to HD home-cinema picture luminance levels.


Can you elaborate on what was measured at around 75%, Gary? e.g.. the MTf was 75% at what frequency?


Alan

 

[Peter Parker]

Hi Alan,

IIRC, flicker was one of the reasons why a lower FL number was determined (higher luminace made the flicker more noticeable), though that was back in the days of 24fps. With double shuttering to 48hz the flicker was further reduced but many cinemas still run at around 7 to 9FL to save lamp life and running costs.

The MTF of a projected 35 mm cinema release print at a typical cinema is inferior to that of a BD or HD-DVD negative-sourced copy of the film projected by a good 1080p home-cinema projector.

Thanks for the info.

So perhaps the commercial cinema may not necessarily be the best model to follow when it comes to HD home-cinema picture luminance levels.

Yes indeed. Most of my viewing is currently SD so I set up to that accordingly, but with HD that may be a different matter. I find that higher levels may start to look less cinematic and more like video, but I'll have to experiment with that at sometime and see if it still holds true with HD.

Can you elaborate on what was measured at around 75%, Gary? e.g.. the MTf was 75% at what frequency?

Alan

Yes, sorry, I believe the 75% figure was derived from horizontal or vertical alternating single pixel lines, at the native resolution of the display, and the results measured via hardware. I don't know any more than that. Just seemed pretty good for DILA especially where CRTs perform poorly in that respect IIRC. I'd like to see how other technologies perform under the same test.

Gary

 

[Timbo21]

I would say I perhaps like things a tad brighter than cinema levels.

But when watching in the dark you have to say there is definitely a cut off point when it comes to brightness. for example, I hate watching TV in a completely dark room, it's just too intense. Likewise, over a certain brightness on a projector will be over the top, especially for a large screen. You don't want to be reaching for the sun glasses on bright scenes. I have to say, however, that for me, an ND2 takes the vibrance away a bit too much from the colours, but I like the amount of brightness an FL-Day reduces by.

I think ND2 does give a more 'filmic' image, and, as Gary says, reduces artefacts, but it just takes a bit too much punch away for me, but of course each setup is different.

T.

 

[KelvinS1965]

I have been using a 'square' Cokin ND2 (A152) filter and last night I noticed a strange effect during the end titles of a film. When the white titles were scrolling down the screen (against a black background) I could see a smaller 'ghost' image of the titles going up the screen to the left of the original. It is almost as if the light is somehow bouncing of the filter (possibly being reflected at an ange to the screen by an adjacent patio door). This makes me wonder if the intra scene contrast is being erroded by this effect occuring with larger blocks of light colours 'ghosting' onto darker parts of the image. My light coloured room doesn't help, but that is soon to be fixed with a dark 'chocolate' screen wall and 'Mocha' coloured side walls.

I have blue tacked the square filter across the front of the lens surround, but it doesn't completely cover the front of the whole lens surround ( the main lens itself is covered though). Would you think getting a slightly smaller round (67mm I think) filter and putting it right up to the lens would elleviate this problem? Only the round filters I have seen are much more expensive than my £8.99 Cokin so if they have the same effect then it would be a waste of money.

Without the filter in place the image is obviously brighter, but I find my eyes soon adjust to it, yet I still notice the black level is higher, including the 'black' bars that in my case (2.35:1 screen setup) are projecting on the lighter coloured wall behind the screen.

I have to say I am slightly disappointed with the black levels of my AE1000, but I suffer RBE and as I'm prone to headaches with too much PC work, I figured I won't get over RBE. I'll have to buy a 3 chip 1080 DLP when they come down to £2K. I do wonder how much improvement is possible with a darker room, but a 'batcave' is out of the question and an ND4 filter will probably make the image so dim as to be unwatchable. Even when I turn off the PJ the screen is clearly visible, so that is the best 'black' I could theoretically get with a 'perfect' PJ in my existing room.

 

[Peter Parker]

That's why I don't like using cheaper plastic or resin filters - they effect the image by reducing ANSI contrast (bright parts bleed into adjacent darker parst) and have too many unwanted reflections, so I always use Hoya HMC glass filters which don't have the same effects. The PRO versions are better still I beleive.

Gary

 

[KelvinS1965]

Thanks Gary,

Looks like I'll be spending the next few days trawling the web trying to find a deal on a Glass Hoya filter. (Un)luckily for me I'm going to be off work this week as I fell off a lader (the bottom rung at that!) yesterday and landed on a tin....I think I've cracked a rib and have an excellent bruise. I should have stayed indoors and done some work on the living room redecorating...now I can't do anything.

Any clues where to start looking....I'll be at home for delivery if I can get one off the web.

EDIT: I've ordered two Hoya HMC 72mm filters ND2 and ND4 total £54 from Tecno.co.uk

 

[KelvinS1965]

Just checking on the Projector Calculator and for my PJ, throw and screen size with a 1.8 gain screen, it says 17FL (theoretically). To get the reading down to 8FL I would need a screen gain of 0.8.

While I know this is all theoretical, what I'd like to know is what ND filter would bring my reading down to 8FL based on the above figures? As I would be reducing the FL by approx 50% would an ND4 give this effect?

 

[Peter Parker]

PJ Central use a 75% figure for lamp life, so it's based on a lamp with a few hours on it. A new lamp will be brighter so the ND filters may be useful. The ND2 will reduce lumens by around 50% and the ND4 by 75%. For you to percieve a drop of 50% the brightness will have to drop by around 82%, so the ND2 may not make the image look as dim as you thought it might.

If you can live with an ND2 for a few hundred hours, you can then remove it and get some brightness back.

Gary

 

[Alan Mac]

An ND2 filter will halve the projected luminous flux (lumens) and hence will halve the picture luminance:
e.g. it will reduce a 17 foot-lamberts (FL) picture luminance to 8.5 FL.

Similarly, an ND4 filter will quarter the picture luminance:
e.g. it will reduce a 17 FL picture luminance to 4.25 FL



Alan

 

[Alan Mac]

For you to percieve a drop of 50% the brightness will have to drop by around 82%, so the ND2 may not make the image look as dim as you thought it might.

Gary,

Should this read:

“For you to perceive a 50% drop in brightness, the luminance level will have to drop by around 82%.........”

(Brightness is subjective, luminance is objective)


Alan

 

[Peter Parker]

Yes it should.

I should brush up on my Poynton...

Gary

 

[sniffer66]

Useful info !

Ive just been on PJ Central and the calculator there tells me I have 15 FL. Given this is accurate on my 200 hr bulb then an ND2 should be giving me 7.5 fl ?. I watched Inside Man in HD on Saturday night with the filter on and it looked a tad dark to my eyes but very watcheable. I assume the only way to check how dim your lamp is over time is to use a light meter - how would screen again affect that reading ?

Edit: One other thought, my PJ has an auto iris (actually the HD72i has variable lamp voltage - Optomas Image AI) I have this on currently. This will have an affect on lumens output - Maybe permanent high lamp mode rather than AI on will give me slightly better(higher) levels, albeit at the expense of shorter lamp life,hmmm

 

[Alan Mac]

I assume the only way to check how dim your lamp is over time is to use a light meter - how would screen again affect that reading ?

The usual way to check the projector lamp output is to use an illuminance meter to measure the illuminance at the screen of a projected peak white area. This gives the screen illuminance in lumens per square metre (lux). Multiplying this illuminance by the screen area in square metres gives the projected luminous flux in lumens.


The screen luminance depends on both the illuminance and the screen “gain”.

The screen luminance (candelas per square metre) = screen gain x (1/Pi) x screen illuminance (lumens per square metre)


Alan

 

[KelvinS1965]

Just found out my Hoya filters are on 'back order'. I was hoping to get them this week to try out before I get started on the dusty DIY I've got planned.

I'll report back sometime later I guess......

 

[KelvinS1965]

I noticed that when using Projector Central's calculator it doesn't alter the FL reading when I 'move' the PJ closer to the screen and use maximum zoom. I understood from a review I read somewhere that at minimum zoom the brightness for a AE1000 is about 40% less than at max zoom.

Does anyone know what PJ central use for these figures, max, min or an average?

 

[Peter Parker]

If you click on the 'help' icon in the calculator, it tells you more about it, but I don't think they take into account the changes in lumens due to the change in the lens zoom.

Gary