Jump to content


Photo

Disparity between filter factor and meter reading


  • Please log in to reply
6 replies to this topic

#1 Brian Rose

Brian Rose
  • Basic Members
  • PipPipPipPip
  • 896 posts
  • Student
  • Kansas City area

Posted 23 February 2008 - 12:00 PM

All,

I'm doing a camera setup that involves various colored filters, and I noticed a funny thing with my Wratten Red 25. The stated filter factor is 8, or three stops. But, when I use my spotmeter, both with and without the red filter in front, the difference between readings was only a stop and a half. I repeated this test with my Spectra pro, as well as a third meter, and always the same results. Three meters can't all be wrong, but I can't believe that Kodak would be a stop and a half off in their filter factor, either? Anyone ever encountered this before, or have any idea why such a disparity?

Brian R.
  • 0

#2 David Mullen ASC

David Mullen ASC
  • Sustaining Members
  • 20068 posts
  • Cinematographer
  • Los Angeles

Posted 23 February 2008 - 12:47 PM

All,

I'm doing a camera setup that involves various colored filters, and I noticed a funny thing with my Wratten Red 25. The stated filter factor is 8, or three stops. But, when I use my spotmeter, both with and without the red filter in front, the difference between readings was only a stop and a half. I repeated this test with my Spectra pro, as well as a third meter, and always the same results. Three meters can't all be wrong, but I can't believe that Kodak would be a stop and a half off in their filter factor, either? Anyone ever encountered this before, or have any idea why such a disparity?

Brian R.


Maybe they are compensating for the loss of sensitivity of the film dealing with such a red image -- in other words, if you didn't compensate for the filter, you'd notice a grey scale ended up three-stops underexposed on film, even though the density of the filter was less.
  • 0

#3 Jonathan Bowerbank

Jonathan Bowerbank
  • Basic Members
  • PipPipPipPip
  • 2815 posts
  • 1st Assistant Camera
  • San Francisco, CA

Posted 23 February 2008 - 12:50 PM

We're you reading an 18% grey card? If not, could it be possible the surface you were reading might read higher on the grey scale once filtered red? Just as caucasian skin tones, when shooting b&w and using a red filter, will show up milkier and whiter.
  • 0

#4 Brian Rose

Brian Rose
  • Basic Members
  • PipPipPipPip
  • 896 posts
  • Student
  • Kansas City area

Posted 23 February 2008 - 01:08 PM

Thanks for mentioning that. I did use a graycard, and still the same reading. And when I use a Blue 47 and a Green 61 (I'm doing tricolor work), my reading versus the FF is just about dead on. It's only the 25A that is off, which leads me to suspect that what David mentioned is likely the source of the disparity. So if that's the case, should I ignore the meter reading, and stick with the manufacturer's recommendation? I'm doing test stuff right now, so I can probably just do both and see what happens.

Thanks for the great tips!
Best,
BR
  • 0

#5 Hal Smith

Hal Smith
  • Sustaining Members
  • 2280 posts
  • Cinematographer
  • OKC area

Posted 23 February 2008 - 02:17 PM

Just a thought here: Compare the filter's red spectral bandwidth graph with the red layer sensitometric data of the film you're using. If the filter is narrower than the film then with the filter less total red is getting through to the film. Kodak's filter factor may be addressing this concept.
  • 0

#6 Brian Rose

Brian Rose
  • Basic Members
  • PipPipPipPip
  • 896 posts
  • Student
  • Kansas City area

Posted 23 February 2008 - 03:35 PM

Thanks a lot. I should have done that sooner, but I did consult the tables for the particular film, and I think you and David are right. And since these are tests, I may just do a variety of exposures, and see which ones come out best. Thanks!
Best,
BR
  • 0

#7 Chris Keth

Chris Keth
  • Sustaining Members
  • 4427 posts
  • 1st Assistant Camera
  • Los Angeles

Posted 23 February 2008 - 11:38 PM

Lightmeters have a spectral response just light film does. If your meter is more sensitive to the blue end than the red end, a red filter would make a greater difference in its reading than if your meter's response was completely linear.
  • 0


Abel Cine

Glidecam

Ritter Battery

Rig Wheels Passport

The Slider

Tai Audio

FJS International, LLC

Metropolis Post

Willys Widgets

Aerial Filmworks

Paralinx LLC

Technodolly

CineLab

Media Blackout - Custom Cables and AKS

Gamma Ray Digital Inc

Visual Products

New Pro Video - New and Used Equipment

rebotnix Technologies

Wooden Camera

CineTape

Broadcast Solutions Inc

Visual Products

CineTape

Willys Widgets

Broadcast Solutions Inc

Aerial Filmworks

Technodolly

The Slider

Metropolis Post

Ritter Battery

Paralinx LLC

Rig Wheels Passport

Wooden Camera

rebotnix Technologies

FJS International, LLC

CineLab

Tai Audio

Glidecam

Abel Cine

Media Blackout - Custom Cables and AKS

Gamma Ray Digital Inc

New Pro Video - New and Used Equipment