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[Luigi_tadiotto]

Is it possibile to change the shutterspeed  with120fps mode enable?. I had some flickering problems an i had to change the speed to 100 fps, i can't set the shutter angle down to 45 or 22,5 fps or better i hadn't effect.
Any suggestion?

[Michael Borenstein]

Hello Luigi,

You should be able to change the shutter angle from 5 to 356 deg while in HS mode.  You can also vary the frame rate from 60 - 120 FPS. 

You'll probably want to increase the shutter angle to reduce what you're perceiving as flicker, depending on the cause of it's origination.  This will increase the sensor integration time.  One by-product will be increased motion blur but less flickering, I image.

Hope this helps.

[Guenter N.]

Luigi,

there is an app for the I-phone or iPad called "flicker free"
it is a simple calculator to show you what frame rate and shutter combination will be flicker free and work best under specific lighting conditions - either at 50 or 60 Hz.

[Luigi_tadiotto]

Thanks, but the problem on the set was that it wasnt possible setting the right shutter speed...it didn't work! we tried changing every .5 step around 45 ° and 22.5° and didn't perceived differences...
It seemed to me that changing shutter angle didn't change the lightness perceived on the EVF (we were at 24FPS at 120fps hi speed mode, 50hz flickering).
I thought it could be an override in the Shutter angle caused by Hispeedmode software, at this time i haven't had that kind of problem (it was a night shot).. i was, an i'm am afraid it could be a software problem/bug.
The problem is this:
when i set the camera to 120 FPS the camera told me 172,5 °, it's impossible because there were no motion blur image and there was a lack of light, so i deducted the shutter was changed but it still displayed 172,5... Any suggestion?

[Michael Borenstein]

Luigi,

If you're shooting with 50hz lights you'll have to use a frame rate that is a multilier of the light freq, which is why the flicker was gone when shooting 100fps.  You're best bet would be to shoot with 60hz lights if you need to shoot at 120fps.

Regards
Mike

[Ants Martin Vahur]

Dear Luigi!

First of all- 172,5° shutter doesn't make any sense. If you are shooting in Europe or anywhere else with 50Hz electricity frequency then your shutter to go with 24fps project would be 172,8°. That makes the exposure for one frame exactly 0.04 seconds or 1Hz, thus not introducing any flicker.

The shutter angle / frame rate ratio is the easiest to calculate because it is linear.

So if 24fps is 172,8° then:
120fps would need theoretically! 864° shutter angle. As this shutter angle is not possible with Alexa (it is with some cameras but it would show double exposure in consecutive frames) you should start dividing by 2. So..
First option is 432°. Still out of Alexas range. Let's go with the 216°. Motion blur is almost perfect but now the lamps flicker? Then you probably have lamps that do not let you shoot with this frame rate.
Which means that you either have to change the lamps,
or shoot 60fps with 216° shutter angle.

With some lamps you are only able to shoot up to 50fps in 25fps project with 180° shutter.
(That's the same as 48fps in 24fps project with 172.8° shutter).

This is as simple it is.

Hope this helps.

Ants Martin Vahur
Cinematographer / Steadicam operator
www.widescreen.ee
Tallinn, Estonia