Focus pulling through a Low Angle Prism and Hi-Speed Focus Pulling

[Taylor Jackson]

Hi All,

 

I wonder if anyone could help me or provide me with any tips on focus pulling when using a Wide Angle Inclining Prism and sharping at high speeds such as 300fps and above?

 

When I used a Prism once before, I noticed there was a slight difference between what I measured to the subject, and then by getting eye-sharps? Is this because the Prism is acting as a mirror?

 

Also, when filming at high speeds such as on the PhantomFlex 4K oe even Red Epic is it a matter of luck and hit-and-miss as to whether you get the timing right to anticipate the action? For me, it is as if you have to second-guess the action or time the pull depending on what speed you are at.

 

Any help would be appreciated. Thanks.

 

 

Tay

[Cem Ali]

As for the prism, your subject could move without you realizing or your tape measure can extend overtime.

 

So double checking is always good.

 

I had a shoot that the bananas were being dropped as well as making a circle and going down at the end.

 

I had several opportunities to try and the best approach is to try to sync your hand rotation to the bananas droppings. It took me 3 takes to find this and were good through the whole day.

 

Regards,

[Satsuki Murashige]

I'm not familiar with the wide angle inclining prism, it is the same thing as a low angle prism? For focus, you would still measure from the focal plane to the mirror, and then from the mirror to the subject. Probably the simplest thing to do is to determine the focal plane to mirror distance which should be fixed, and then for all subsequent measurements just measure from the mirror to the subject and add the offset.

 

Pulling for high speed can be tricky. The worst part is watching playback when you have buzzed the focus as it is soft for what feels like an eternity with everyone huddled around the monitor judging you. On the other hand, at least you can see when it is soft so you can do another take! No tips really, just be on top of your game. Everything happens so quick during a take that you really only have time for one mark at the start anyway.

[Dom Jaeger]

When I used a Prism once before, I noticed there was a slight difference between what I measured to the subject, and then by getting eye-sharps? Is this because the Prism is acting as a mirror?

 

A prism acts like a mirror yes, so you can't just measure the distance from the focal plane to subject. But it's not quite as simple as measuring to a mirror surface, because these sorts of prisms have multiple internal reflective surfaces. Since you need to adjust the prism position for different physical length lenses, or to fit more filters in, there is no fixed or definitive focal-plane-to-front-surface-of-prism distance. You would need to do some testing to work out what offset you'd need to add to the lens markings for each lens set-up if you want to do critical focus work with a tape, and measure from the front surface of the prism. I suspect most of the time people just focus by eye with this sort of tool.