Cinematography Mailing List - CML

 

Search

 

HMI And Time Lapse

Published : 10th November 2005

I'm shooting an event and reducing 3 hours to 30 seconds. We have 6K HMI lighting.

Does anyone have any experience?

I'm worried about Flicker. Will Flicker Free HMI Make a difference?

Thanks

Steven Gladstone
New York Based Cinematographer
Gladstone Films
www.gladstonefilms.com
CML East Coast List Administrator


Issue really doesn't concern how many frames get shot per hour, really...it is more a matter of how long each exposure is. If you are using old fashioned (and less fussy) magnetic ballast HMI's, you need to shoot at exposures that would correspond to the "HMI safe" speeds that you can find on various charts and, of course, in Mr Eubanks EXCELLENT pCINE pda pgm (PDQ)

If you are shooting with "flicker free" HMI ballasts and they are actually set on "flicker free" rather than on"24fps" or "sound" then you should be fine at any frame rate, as the "flicker free" setting on these ballasts yields a high frequency squarish wave that is fast enough that one or two cycles more or less per frame make not enough difference to show up as a density change.

If you are shooting slow frame rates and nd'ing and stopping down to compensate, you are probably going to be OK...if you look at the "safe" speeds, the slower you go, the better it gets.

I hope this answer is somewhat useful.

WARNING - THIS ANSWER IS BASED ON THEORETICAL KNOWLEDGE AND RELATED EXPERIENCE - I HAVE NOT SHOT TIMELAPSE WITH HMI'S MYSELF...(THOUGH I HAVE SHOT MOTION CONTROL MINIATURES WITH HMI"S)

Mark Weignartner
LA BASED


Steven Gladstone wrote :

>I'm worried about Flicker. Will Flicker Free HMI Make a difference?

Shooting Film? What is the exposure time per frame. I'd doubt if there are going to be any issues if you have an exposure time of 1/15 sec per frame or greater.

Usually flicker arises when there are an un even amount of AC cycles per frame which would be more apparent as you go to higher frame rates/shorter exposure times.... which would reveal the small exposure difference in each frame.

Mark Smith
DP/ sifter of pixels
NYC


Mark H. Weingartner wrote:

>Issue really doesn't concern how many frames get shot per hour, >really...it is more a matter of how long each exposure is.

Sort of what I was getting to in my first post. If you are shooting 24fps with magnetic ballasts you shoot HMI safe speeds because you wan the same amount of AC discharges from the lamp on each frame...the difference between 2 and 4 discharge cycles would be a stop.

That all said, if you are shooting f**m and your minimum exposure is a 1/15 second( typical Norris) or a 1/4 second which is sort of a typical Aaton minimum exposure. the amount of discharge cycles over those durations may vary slightly but the longer your exposure time the less affect the number of discharge cycles captured in each frame has on individual frame exposures.

Or something like that.

Mark Smith
Oh Seven Films
143 Grand St
Jersey City, NJ 07302


Mark Smith pointed out the issue in a good way - the longer the exposure, the less difference a 1/120th of a sec light pulse more or less makes, so the slower the frame rate, the less percentage change difference and the therefore, the less density difference.

Now to the numbers :

With a 180 degree shutter, 1/15th sec (for example) would correlate to 7.5 frames/sec...which is an HMI safe speed.

1/4 sec exposure correlates to 2fps which is another safe speed...Once you get down below 8 fps, there are a LOT of safe speeds, and the exposure variation from frame to frame with the unsafe speeds between them makes less and less difference the slower you go...

Mark Weingartner
(the other Mark)
LA based (but used to work with the other Mark long ago in a galaxy far away)


Steven Gladstone wrote :

> I'm shooting an event and reducing 3 hours to 30 seconds. We have 6K >HMI lighting.

> Does anyone have any experience?

Yes.

> I'm worried about Flicker. Will Flicker Free HMI Make a difference?

No.

> Thanks

You're welcome

Brian Heller
IA 600 DP


Steve,

It depends on the length of exposure of each frame. If the HMI is not flicker free and the camera is not in sync but the exposure is long enough for several pulses then the difference between frames will be much less. However, you sometimes get a slight multiple exposure look off reflective objects that move through frame.

Test.

Paul Hicks


> I'm worried about Flicker. Will Flicker Free HMI Make a difference?

1/. An HMI light with a magnetic ballast flickers at twice the line frequency. In the U.S., that means 120 times per second. If the camera shutter is set to 180 degrees, licker occurs when the shutter rate is not a divisor of the flicker rate. 24fps is "safe" with a 180 shutter, because 5 X 24 = 120. If the shutter is not 180 degrees, the relationship for a safe
frame rate is more complicated.

2/. A "Flicker Free" HMI uses an electronic ballast that switches at over 100,000 times per second. It "flickers" at that high rate. Hence exposure is even regardless of the shutter rate.

The reasons why electronic ballasts are not the universal choice are :

1/. Rental houses have a huge legacy of magnetic ballasts.


2/. Electronic ballasts, like other electronic devices, can become non-functional due to component failure or abuse. Magnetic ballasts are more reliable.

Bob Morein
Indie filmmaker


Sponsored by

 

 

 

 

 

 

 

CML Home CML-Tests Home

© copyright CML - Cinematography Mailing List all rights reserved