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Video S/N & optical color temp filters


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#1 Peter J DeCrescenzo

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Posted 07 August 2005 - 07:39 PM

Apologies to those for whom this info is old hat, but I just had a "Duh!" moment. :)

Simon W. kindly pointed me to this Sony document which contains more than 70 pages of info about how video cameras work:
http://www.sonybiz.n...ologyBasics.pdf

The following section caught my attention:

===
"... All color cameras are designed to operate at a certain color
temperature. For example, Sony professional video cameras
are designed to be color balanced at 3200 K (white balance:
refer to ?White Balance? ). This is the color temperature for
indoor shooting when using common halogen lamps. However,
the camera must also provide the ability to shoot under
color temperatures other than 3200 K. For this reason, a
number of selectable color conversion filters are placed
before the prism (refer to ?Prism? ) system. These filters optically
convert the spectrum distribution of the ambient color
temperature (illuminant) to that of 3200 K, the camera's operating
temperature. For example, when shooting under an illuminant
of 5600 K, a 5600 K color conversion filter is used to
convert the incoming light's spectrum distribution to that of
approximately 3200 K.

Your question now may be, "why do we need color conversion
filters if we can correct the change of color temperature
electrically (white balance)?". The answer is quite simple.
White balance (refer to ?White Balance? ) electrically adjusts
the amplitudes of the red ® and blue (B) signals to be
equally balanced to the green (G) by use of video amplifiers.
We must keep in mind that using electrical amplification will
result in degradation of signal-to-noise ratio. Although it may
be possible to balance the camera for all color temperatures
using the R/G/B amplifier gains, this is not practical from a
signal-to-noise ratio point of view, especially when large gain
up (refer to ?Gain? ) is required. The color conversion filters
reduce the gain adjustments required to achieve correct
white balance. ..."
===

My "slap to the forehead" moment was realizing that electronic color correction done in a video camera comes at a price, however slight. I knew color correction in video post-production can add varying amounts of noise depending on how its handled, but I hadn't realized until I read the above that the S/N performance of a video camera with all-electronic color temperature compensation might be adversely affected if the light source isn't 3200 K.

I suppose an add-on, appropriately-colored (85, 85B or 85C?) glass filter on a video camera which features all-electronic color temperature compensation (such as my new DSR-450WS and many popular cams) might result in slightly lower-noise video when shooting under >3200 K light (especially >5600 K)?

However, I don't know if this a case of making a mountain out of a molehill. 5600 K lighting instruments (HMI, KinoFlo, etc.) are more popular than ever, and outdoor daylight shooting is obviously quite common. But maybe modern cams are generally "quiet" enough that this needn't be a cause for concern?

As I said, this is probably old news to most of you, but it was a small but pleasant realization for me. Thanks again, Simon!

- Peter DeCrescenzo
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#2 David Mullen ASC

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Posted 07 August 2005 - 09:58 PM

Pro video cameras have internal color-correction filters -- on many Sony cameras, the "B" filter for 3200K is actually clear, the "C" filter is a half correction for daylight (like an 81EF) and the "D" filter is a little warmer than an 85B. On the /3 version of the Sony F90, the star filter in the "A" slot was replaced by a true 85B filter for correction of 5500K light to 3200K, which is what the camera is naturally balanced for. The "D" filter gives a slightly warmer look in daylight, something of a "Sony look".
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#3 Peter J DeCrescenzo

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Posted 07 August 2005 - 11:24 PM

Thanks for the additional info, David. Much appreciated.

I knew high-end video cams typically have optical color temp compensation filters, but I wasn't familiar with their actual values/equivalents.

Sony's new DSR-4xx series cams have a 4-position ND filter wheel, but none of these filters do color temp compenstation. There's a clear, 1/4ND, 1/16ND and 1/64ND. The cam can do electronic auto tracing white balance, manually-triggered white balance, or you can manually adjust the white balance, etc.

It was the concept of using an optical color correction filter (built-in or external) as a way to potentially prevent avoidable video noise that got my attention.

I have no idea if it can significantly affect a cam's S/N performance, but it got my attention.

All the best,

- Peter DeCrescenzo
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#4 David Mullen ASC

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Posted 08 August 2005 - 12:35 AM

It would be easy enough to get an 85B filter and shoot a MacBeth color chart in daylight, once corrected optically (with the filter) and once with no filter, just with the white balance alone, and look at the noise levels in the various color patches.
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#5 Simon Wyndham

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Posted 08 August 2005 - 05:10 AM

I am currently working out how I want to shoot an upcoming production in Sept. The whole white balance issue and post adjustment is something that has been bothering me. I want to retain the cleanest signal possible. Hue shifts and other artefacts resulting from white balancing the camera were something that I wanted to avoid. The Sony document added the issue of reduced signal/noise ratio to that concern.

In contrast to an article I wrote recently I am shifting more and more towards the idea of shooting the production like a film before Digital Intermediates came into being. I won't be going for any extreme looks, and so, as Peter has also mentioned, I am considering using correction filters in the matte box as much as possible and then only performing very, very, basic adjustment in post to compensate for the FLM gamma setting.

Peter, I thought that it was just the 400 that only had electronic correction?
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#6 Peter J DeCrescenzo

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Posted 08 August 2005 - 11:35 AM

... Peter, I thought that it was just the 400 that only had electronic correction?

<{POST_SNAPBACK}>

To the best of my knowledge, based on reading the user manual and working with the camera, the DSR-450WS and DSR-400 do not have any user-selectable, built-in optical (glass) color temp compenstaion filters.

For example, on page 7 of the manual it says:

"... Electric CC filter: The CC filter that was previously paired with the ND filter has been eliminated, so the ND filter is now the camcorder's only optical filter and this makes operation easier. With the wide-band white balance amplifier of this camcorder, the white balance can be adjusted in a wide range of color temperatures without an optical CC filter. ..."

The cameras do have a "5600 K" button (page 10 in the manual). When I press it I don't hear any "clunk" of an optical filter being dropped into place, so I assume it triggers an electronic CC only.

It'll be interesting to do a test using a color chip chart per David's suggestion. I have a MacBeth chart I can use. I'll have to give some thought about how I'll monitor/view the results to best determine if there's a meaningful difference in noise level, with and without an add-on CC filter.

- Peter
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#7 Simon Wyndham

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Posted 08 August 2005 - 04:19 PM

Sounds interesting. It may be that somehow they have found a way of optimising the electronic 5600k filter for minimal noise and hue shifts.
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#8 David Mullen ASC

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Posted 08 August 2005 - 11:04 PM

Either vew the tests on a really large monitor or import frames into PhotoShop or something and look at frame enlargements, or both.
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