BTL Question

[Sung Rae Cho]

Starting a movie this week and since we're shooting on Panavision, DP wants to take advantage of behind the lens filters. Hell yes. 2nd assistant cut up N3/6/9, 85&85N3/6/9 and 81EF.

 

Now DP asking me if we can slide non Kodak wratten filters in occasionally, to compensate the overall colors, like 1/4&1/8 CTB, CTO, Straw, as well as Cyan 15 you can find on Lee Filter Book to be used on grip frames.

 

I've used most available Kodak wratten filters but only because those are made to be used in front or behind the lens. I can't find above shades online or at the store, rental houses, etc in NY. B&H is backordered on Cyan 15.

 

Would it be safe to just cut up Lee filter book and make them to be slided? Look and feel safe as long as each gels are clean.

 

Any thoughts? Thanks!

[Stephen Williams]

Hi,

 

Lighting filters are not of optical quality, also they are very thick so they will put the FDD out & you will will have some out of focus shots.

 

Stephen

 

Would it be safe to just cut up Lee filter book and make them to be slided? Look and feel safe as long as each gels are clean.

 

Any thoughts? Thanks!

[Sung Rae Cho]

Hi,

 

Lighting filters are not of optical quality, also they are very thick so they will put the FDD out & you will will have some out of focus shots.

 

Stephen

 

Oh,

 

I'm comparing side by side of Kodak wratten cyan series (I've used many times as behind the lens) and those from Lee Filter book. To my eye and fingers they look and feel relatively the same. I would say that those gels in grip frames are poorer qualities but rather thinner no?

Yes I am aware of the fact that BTL changes the flange focal depth by 1.5/1000 of an inch. Completely negligible.

 

Thanks for the note regardless. I will consult with DP tomorrow and figure out what to do.

[Stephen Williams]

Yes I am aware of the fact that BTL changes the flange focal depth by 1.5/1000 of an inch. Completely negligible.

 

Hi,

 

The change on the FFD is 1/3 of the thickness of the gels in use, using a wide angle lens @T1.4 it can be a real issue, which is why I mentioned it.

Ideally the camera should be set up with a clear gel in place.

 

Stephen

[Tim Carroll]

Yes I am aware of the fact that BTL changes the flange focal depth by 1.5/1000 of an inch. Completely negligible.

 

That's 38 microns, which for FFD is far from negligible. I certainly wouldn't shoot with a camera that had a FFD off by 38 microns.

 

Best,

-Tim

[Chris Keth]

Oh,

 

I'm comparing side by side of Kodak wratten cyan series (I've used many times as behind the lens) and those from Lee Filter book. To my eye and fingers they look and feel relatively the same. I would say that those gels in grip frames are poorer qualities but rather thinner no?

Yes I am aware of the fact that BTL changes the flange focal depth by 1.5/1000 of an inch. Completely negligible.

 

Thanks for the note regardless. I will consult with DP tomorrow and figure out what to do.

 

That's not a negligible amount for a camera's flange focal depth to be out. You would notice it with wide lenses on wide stops.

 

If there's a possibility of using behind the lens filtration, the camera should be set up with a clear filter in place and there should always be a filter in that slot, whether it's clear or something else.

 

As for being the same thickness, it's not just a matter of physical thickness. It's also a matter of refractive index of the material. Lighting gel is made of much tougher stuff than wratten filters and will have a very different refractive index and optical quality to it. I did what you are describing with a bolex when I was in school. It turned out OK but shots with the filters were noticeably softer than shots without. I wouldn't condone it for professional work.