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Speed Ramps & HMI's
>Anyone out there done any hi speed ramping with flicker free HMIs?
>I have a big night exterior to shoot which kinda needs HMIs, however I also have to ramp the camera up to 150fps on the 435ES.
>I hope I have a chance to shoot tests but maybe not.
>Everyone says it should be OK.
>Yes, I've tried HMI & Kino ramps.
>No, you cannot do it! They will/do flicker.
>I tried 24fps ramped to 60fps [both HMI safe speeds] using a Moviecam Compact set at 180 degree shutter. We shot the ramp both as a fast ramp [2 second total speed shift] and a slow ramp [8 second speed shift i believe] and yes, there was a very bad flicker. We used square-wave ballasts on the HMIs and standard 4' Kino bulbs. I even aimed the camera directly into the HMI fresnel and also shot another fixture into a bounce card [where flicker always seems to be more noticeable] and really saw the flicker more in the 'fall-off' of the bounce card.
>When the camera ramped through the 40fps mark [a safe HMI speed] I noticed a slight drop in the flicker but once it went past 40fps there certainly was an increase in the flicker.
>Seeing this flicker at these lower frame rates leads me to believe that at the higher speeds you require will lead to vast amounts of flicker.
>It's great that these cameras can now ramp their speeds, it's just too bad the technology is not there yet with the lighting units. But, you would be safe [of course] if you shot with daylight or tungsten.
>Shot your own tests, I would love for somebody to prove me wrong. But my 2,500' of 5279 really gave me a warning.
>I've ramped on the 435ES with 4K flicker free pars with no problem at all.
>Double check with Arri if you're concerned.
>If you're scared of flicker, and you're outside, try shooting with arc lights instead of HMI's.
>If this involved the Moviecam's 'Moviespeed' or a similar iris control like the Preston Speed Aperture Computer I don't think it was a valid test.
>With the shutter locked at 180, you would go from a shutter speed of 1/48 sec. (safe) to 1/120 sec (also safe) but pass through a whole range of in-between frame rates *and exposure times* which are not safe....just like if you spun the speed control at random. Naturally you get flicker.
>The Arri RCU or LCC in conjunction with the internal shutter control on the 535A and the 435ES (but not the 'B' --have I got that right Marc?) would give you 24fps at a 45 degree shutter angle ramping/changing to 60fps with a 180 shutter, *maintaining 1/48 shutter speed the whole time*. The only question is whether the accuracy of the unit is sufficient to keep flicker out...it would have to be in the hundredths of a degree I think. A little slippage might only result in a minor exposure change, but a horrendous flicker.
>Whether the strobing effect of the 45 degree shutter at 24fps is acceptable is a whole other question.
>Alan 'close that window!' Thatcher
>That is correct. The 535A and 435ES have an electronic mirror shutter that can change its open angle on the fly, while the camera is running. This is useful for a variety of occasions, but mostly for speed/exposure ramps, where the exposure compensation is performed by the mirror shutter.
>Remember, the exposure time for each frame is a value derived from the fps AND the shutter angle. The 535A and 435ES can keep the exposure time for each frame constant, by having the shutter counter the exposure change resulting from the speed change.
>If you start at 24 fps and 180 degree shutter, your exposure time per frame is 1/48th.
>While ramping to 12 fps, the mirror shutter would close down, and you end up at 12 fps and 90 degrees. Note that the exposure time per frame is still 1/48th, and in fact has remained 1/48th for every single frame throughout the whole ramp.
>The LCC has a calculator built in that can show you not only the exposure time for each frame for a given ramp, but also tell you what your fps range is that can still be compensated for by the 11.2 to 180 degree range of the shutter (4 stops). On the 435ES, for instance, the extremes are about this:10 fps to 150 fps, or 1 fps to 16 fps, and of course anything in between.
>Take a look at this when you have a chance, it is useful even if you don't use the LCC for anything else.
>More information on this topic can be found on the CSC web site, go to the "technical info" page http://www.cameraservice.com.
>Now on the topic of HMIs, consensus here at Arri is that THEORETICALLY you should be able to use the 535A or 435ES with a speed/shutter ramp and be OK if you use flicker free HMIs. BUT since there are so many variables that we have no control over, I must URGE you to shoot tests to confirm this for any given shooting situation.
>Please also note that this does not apply to ramps where you use the ICU to compensate for the expsoure change (possible with all Arri cameras). When using the ICU, the exposure time for each frame does change, but the ICU will change the iris accordingly to keep the amount of light per frame constant. Since exposure time changes, HMIs are going to be unhappy.
>Marc Shipman-Mueller, Technical Representative
>Arriflex Corporation; 1646 N. Oakley Ave, Suite #2, Chicago, IL 60647-5319, USA
>True, but you should note that 1/48 is not actually safe with 60 Hz line frequency (or 50 Hz, for that matter). It only works at 24 fps because (in theory) any shutter angle is safe at 24 fps. To do a ramp under HMIs you should use a shutter time of 1/60; ie. start at 144 degrees. If you were ramping up from 24 fps you would probably need a shutter time of 1/120 to get a feasible shutter angle at the fast end, so you would start with an angle of 72 degrees.
>No problems whatsoever.
>Always with flicker free HMI's, I don't use anything else, speed changes at various rates but every speed from 3 fps to 150 fps.
>I guess 25 to 75 is the most common major change, although I have done 25 to 150 without problems. Most common ramp is 25 to 30/32 just to take the edge off something or 25 to 18/20 to speed up a part of a shot.
>Alan 'little slow in math' Thatcher wrote:
>>would give you 24fps at a 45 degree shutter angle ramping/changing to 60fps with a >180 shutter, *maintaining 1/48 shutter speed the whole time*.
>Obviously this isn't right at all...the principle is correct but where did I get those figures???. Maybe I shouldn't post late at night.
>The shutter speed at 60fps would be 1/120 sec. with the 180 shutter, and the shutter angle to give the same shutter speed at 24fps would be 72 degrees.
>24 = 72
>When I'm working things like this out on the set I always write it down. It's amazing how much clearer things look in front of my eyes than behind them.