|
Author
|
Topic: How many lux from GS1200? A value list...
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Maurizio Di Cintio
Jedi Master Film Handler
Posts: 977
From: Ortona, Italy
Registered: Jan 2004
|
posted January 11, 2004 04:49 AM
Sorry Ugo ![[Embarrassed]](redface.gif) : though I agree with you 100% about the fact that the three to two blade shutter modification does not adversely affect flicker, I do not think your explanation thereof really addresses the question.
First off a foreword might be necessary and I beg everyone’s patience for the lenghtiness of this post. The SMPTE S/8 test film is meant to assess the following in projectors’ performance:
a) image steadiness (both horizontally and vertically)
b) frame edges coverage and alignment
c) lens definition and contrast handling capability
d) combined lens/mechanics definition
e) film flatness in the gate
f) lens’ flatness of focus
g) light output evenness
The RP 32 features several elements to evaluate all of these factors. But the “Travelling Ghost” you mentioned stands on its own in that its real function is to reveal even the slightest lack of syncronization between shutter blades and claw intermittent movement. If film transport is carried out when the lamp is not yet totally covered by the shutter, then some sort of vertical trail will be displayed by the travelling ghost’s squares, as if someone had smudged the white part of the image against the black background.
Now such a circumstance is impossible to happen on a S/8 machine, because of the S/8 inherent intermittent movement design, unless, of course, it is really poorly engineered (and despite all the machines I tested, I have never stumbled in a case like this, even with bottom of the line projectors). In any case, it wouldn’t be a problem of ageing or wear. It is possible to see a good example of trail from TG by running the SMPTE RP 32 S/8 test film in reverse on a Eumig Mark S 804. Because of its really thin claw, when running in reverse not only does the frame line shows well above the gate mask, but the portion of the image under the frame line features highlights that are ‘smudged’ like a ghost image. This happens because of claw thinness and the fact that in reverse the shutter covers the lamp a little bit too late after film transport has already started, rather than the other way around. Of course I am not implying the Eumig is poorly engineered (forward projection is always excellent); this is just the kind of compromise ona has to accept on a dual gauge machine like this. Nonetheless it is a good example of what a TG is. By the same token the higher-end Bauer’s and Silma’s are equipped with variable width shutter blades: in reverse mode, the blades grow a little wider and the problem experienced on the Eumig is eliminated (of course the image is a little bit darker).
In short in my opinion the TG is directly derived from 35 mm test film, a gauge with which a problem like the one I described may occur, because the sync between film transport (Maltese cross) and shutter movement are assured by mechanical organs that are not phisically in contact: indeed they are often linked to each other by means of a toothed belt (rather than gears, as was the case with older machines), and if the gears the belt has to go through, are not properly oriented, the sync between the Maltese cross sprocket drum and the shutter blades will be adversely affected. Also the TG on a S/8 test film may be of use on printers, but on S/8 projectors, as you see, it is probably not that meaningful (save for what you said in your latest post, Ugo), and surely it does not say anything as regards flicker.
Flicker in fact is the outcome of alternation between dark and bright moments on the screen in case that alternation drops below a certain threshold, i.e. it is not fast enough for the human eye to be perceived as a seamless illumination or, for that matter, as a sequence of photographs blending into a seamless movement. This threshold lies about a frequency of 40 (more or less) combined moments of light and dark on the screen in a second, which is way below the actual rate of a film screened at 24 fps with a two bladed shutter (= actual frequency: 48/second). Obviously if you project at a frame rate of 18 fps, you wil get an actual frame rate of 36/second which would result in flickering (but that would be bothersome only for the brighter areas of the image). Not to forget: for the purpose to just move frames in the gate, one blade only would suffice: the other two (for 18 fps) or one (for 24 fps) are just meant to compensate flicker.
That’s why S/8 projectors have three blade shutters as standard, so they can handle both speeds with no viewing stress. But if one just uses 24, then not only is the third blade useless, it is ‘detrimental’ to light output performance, as it steals some 30-40% therof, depending on blade width/shutter’s cam design for the purpose of claw movement fastness (the faster the film transport of each individual frame, the smaller the width of the blades required).
In conclusion, like Ugo (who succesfully modified my Bauer T610) I would say go for it to whomever wants to screen at 24 fps with blade modifications, because for said reasons, there is nothing to fear in terms of flicker (though the RP 32 won’t be able to witness that).
Happy screening to everyone.
--------------------
Maurizio
| IP: Logged
|
|
|
|
|
|
|
|
|
UBBFriend: Email this page to someone!
![[Smile]](smile.gif)
Kev.
Printer-friendly view of this topic
