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  • Hey all, just changed over the backend after 15 years I figured time to give it a bit of an update, its probably gonna be a bit weird for most of you and i am sure there is a few bugs to work out but it should kinda work the same as before... hopefully :)

Shooting with 80A and 80B Filter on Red

I use a 80C a lot, it's a single stop and it gets the camera very close to balanced.
 
I have 80C and 80D. So far I've only used the 80C on jobs. Tomorrow I'm going to shoot a few simple tests with the 80C & 80D prepping for a job. What is the formula to calculate the K shift, I assume it is not linear?

2900k w 80C = ?
3200k w 80C = ?
2900k w 80D = ?
3200k w 80D = ?
 
I'm not sure but i think thats the formula:
T2=1/(1/T1+F)
Which is:
T1=source light color temperature (MK)
T2=resulting color temperature
F =Filter shift value in (MK-1)
please correct if i'm rung
 
Four points

Four points

1) Any mixed light on the subject will make off color shadows, by the degree of difference. If you need to shoot with mixed lights, flag off one from the other so that they do not mix on the subjects, unless you want "bi-color" effects lighting.

2) Boosting the voltage to tungsten lamps reduces the ratio of blue to IR, whereas using a blue filter can increase the IR to green ratio. Its better to boost the lights since the IR goes up a little and Blue goes up very much.

3) Every stop of unbalance reduces the dynamic range by one bit or cuts it in half. If the sensor is 12bits balanced, the de-mosaic must cut that to 10 bits if the blue is 2 stops under-expsoed relative to the green or red. If the blue is 3 stops under-exposed relative to the green or red you are only getting 9 bits of dynamic range, and so on. To get the full 12bits you paid for the light going to the sensor must be balanced (with pink and blue filters as needed, or yellowish for twilight skylight, etc.).

4) You can add a color tint to the 48bpp (16bitx3) frames later by adjusting the red/blue ratio of gammas to get any warm or cool tint without the noise problems you have if you shoot off balance in the camera. The reason for this is that you have more bits in the color correction software, and the digital images are not subject to noise like the analog parts of the sensor. (Do not rotate the color vectors using Hue (sometimes mislabled Tint) in your software, use the red/blue gammas to keep white and black where they should be and to keep the primary colors in their right places.)
 
Michael Lindsay said:
I believe:

a 80c takes 3200k to 4348k

.a 80c has a given mired shift but its kelvin shift depends on the starting temp.

regards

Michael L

PS having said that I do agree the 80c is a good compromise.
Click to expand...

Not 4329˚K???...:thumbsup:

Michael is [annoyingly, as ever :cuss:...] correct.....

I'm glad I'm not the only one getting it wrong though :rolleyes::

http://www.reduser.net/forum/showthread.php?t=24355&page=2

I think maybe part of the confusion is that a lot of the literature will tell you that an 80C will correct 3800˚K light to 5500˚K (none of these figures are particularly helpful in the real word though!!)...but if you start with 3200˚K light you won't end up with 4900˚K light but with more like 4329˚K (or maybe 4348˚K :thumbsup:), which is where the dark arts of Mired shifts comes in: if I understand correctly 3200˚K tungsten light has a Mired value of 312 and the Red sensor (5000˚K) has a value of 200 Mireds, so to correct tungsten to the Red you need a Mired shift of -112 (which conveniently is what the 80B offers...but with a rather painful 1 & 2/3 stop loss).

Alternatively an 80C has a Mired Shift of -81 which should get you more than half way there with only a one stop light loss (the rest you can safely colour correct in post without too many problems).
 
80C is a mired shift of -81.
80D is a mired shift of -56.
So,
2900K + 80C= 3790K ish
2900K + 80D= 3460K ish
3200K + 80C= 4320K ish
3200K + 80D= 3830K ish
I think... using a simple Excel calculator (Roscoe has one online; so does AV Net) Hope I'm not adding noise or spreading misinformation.
Cheers,
Harry
 
Shot a white card test on Friday. 80D looks to be getting me up to 4700k, much higher than expected. Light source is four 1k open face Micky Moles through a 6x6 1/2 gridcloth.

The bottom of the white card has green spill from the floor. Based on spot meter tests the 80D eats a little over 1/2 stop, 80C a bit more than 1 stop.

3500577773_7e445395cf_b.jpg
 
Interesting.
I only own the 80D, mainly because it was only 2/3 stop loss and "felt" right on some quick tests a year ago. By the numbers, it should not correct quite so much but I felt that it was "close enough". David's test seems to indicate that it is indeed a good choice.
Cheers,
Harry
 
to david
impressive...
also because you might consider that gridcloth has warmed up a tiny bit your 1k. Did you use a color meter to know what was the effective color temperature during the shoot?

P.S. I think you used the "Pick WB" function on RA to determine that 4700K. where did exactly measure it?
 
Hello Marco,

To locate proper WB I adjusted the "Kelvin" slider in Red Alert until the peak "red, green, blue" in the histogram matched. The example shot is of a white card, so the peaks of the histogram represent "white."

I did not have a color meter with me on set. I'm sure the lamps were a but warmer than 3200k, however I'm not sure how much color bias is in the lite gridcloth, anyone know?

Anyone want to buy a basically new 80C Panavision size filter? :)
 
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