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Matching "T" stop with changes in FPS


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#1 stephen defilippi

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Posted 18 April 2007 - 07:49 PM

HI i hope this isnt confusing.

Im planning a shot and wonder if ive got this right. Im filming a short bit of identical action which repeats 3 times (in this case the actor sits onto the floor in a certain way. The movement takes 3 seconds).

The first time she sits the camera is set at 24fps and lense is at T 4 (for instance)

The second (identical action) is filmed at 48fps and so I though to change the lense to T2.8

The third (also identical action) is filmed at 72fps and the lense would be at T2.4

When I cut this, i will place the three scenes close to each other, the first is interrupted by the second which is interrupted finally by the third which is the only cut that concludes the sitting down action.

Have i calculated the T stop correctly? If I double the frame speed do I open the aperture 1 stop do double the light??

Thanks

stephen
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#2 Michael Collier

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Posted 18 April 2007 - 08:54 PM

yeah you got the concept. double the frame rate, cuts the expose time in half and you'd iris up one stop. You also have the option to reset your light meter to the frame rate and check that way, though with math done I don't see why it would be necissary.

one aditional thing to consider. as you increase your fps your depth of feild will crush. You may want to consider using some ND to get the shot you want. start with and ND 9 (24fps) then an ND 6 (48 fps) then maybe an ND 4 or 5 (72fps) that way your iris remains constant and so too does the depth of feild/lens sharpness.

Edited by Michael Collier, 18 April 2007 - 08:56 PM.

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#3 stephen defilippi

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Posted 19 April 2007 - 04:03 PM

yeah you got the concept. double the frame rate, cuts the expose time in half and you'd iris up one stop. You also have the option to reset your light meter to the frame rate and check that way, though with math done I don't see why it would be necissary.

one aditional thing to consider. as you increase your fps your depth of feild will crush. You may want to consider using some ND to get the shot you want. start with and ND 9 (24fps) then an ND 6 (48 fps) then maybe an ND 4 or 5 (72fps) that way your iris remains constant and so too does the depth of feild/lens sharpness.



yeah you got the concept. double the frame rate, cuts the expose time in half and you'd iris up one stop. You also have the option to reset your light meter to the frame rate and check that way, though with math done I don't see why it would be necissary.

one aditional thing to consider. as you increase your fps your depth of feild will crush. You may want to consider using some ND to get the shot you want. start with and ND 9 (24fps) then an ND 6 (48 fps) then maybe an ND 4 or 5 (72fps) that way your iris remains constant and so too does the depth of feild/lens sharpness.


Oh yeah, i didnt consider DOF. thanks that all makes sense

stephen
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#4 Phil Gerke

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Posted 25 April 2007 - 06:01 PM

I thought standard practice with over cranking would be to increase your shutter angle? Could you not do that in this situation? That would save the ND and maintain a constant DOF.

Thanks!
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#5 Nick Mulder

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Posted 25 April 2007 - 06:35 PM

I thought standard practice with over cranking would be to increase your shutter angle? Could you not do that in this situation? That would save the ND and maintain a constant DOF.

Thanks!


Shutter angle changes could be used but youd get changes in strobiness in the shots - probably not noticable to many people but if they are over and undercranking then its probably becuse there is motion in the shot which makes it easier to spot (at least subliminally) - ND is the most transparent solution (snorter, giggle, heh) ...
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#6 stephen defilippi

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Posted 25 April 2007 - 07:10 PM

I thought standard practice with over cranking would be to increase your shutter angle? Could you not do that in this situation? That would save the ND and maintain a constant DOF.

Thanks!



i cant in this case, my arriflex has a fixed 180 degree angle. stephen
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#7 Michael Nash

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Posted 25 April 2007 - 11:31 PM

I thought standard practice with over cranking would be to increase your shutter angle?


Film cameras usually use a 180 degree shutter at 24fps, and can't go much wider than that in order for the shutter to be closed during the pulldown. Some cameras can go as much as 200 degrees, but there's still a limit to give adequate "closed" time for the intermittent movement. So no, it's not common practice to increase shutter angle because usually you can't.

It is however common practice to light the scene to the highest light levels necessary for overcranking, and then add scrims to the lights or ND's to the camera for normal-speed shots. That way you don't have to relight for the overcranked stuff.
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#8 Phil Gerke

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Posted 11 May 2007 - 03:38 PM

Shutter angle changes could be used but youd get changes in strobiness in the shots - probably not noticable to many people but if they are over and undercranking then its probably becuse there is motion in the shot which makes it easier to spot (at least subliminally) - ND is the most transparent solution (snorter, giggle, heh) ...



Forgot to setup up notification for this thread. Delayed response I know, bare with me.

Now in this situation the shutter angle is fixed, as it is in many circumstances due to the physical limitations of the camera and its need for closed time. This had not occured to me and makes total sense, thanks Michael. So the recomendation to light for the fastest speed and ND for the slowers seems the best course of action. However Nick, your comment put a flag up for me. You speak of strobiness being greater when using shutter angle to achieve consistent exposure. Now correct me if I am wrong, but by increasing your shutter angle proportionate to your frame rate increase you should still be exposing the negative for the same amount of time. Is that right? I guess I am unclear as to what would cause the strobiness, what am I missing?

Thanks a lot!
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