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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 :)

ASA rating and meters

J

Jesper Holmström

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During our tests we have noticed that when you set the camera to 320 ASA
and expose a graycard/grayscale using falsecolor or histogram our light meter reads 160-200 ASA.
When I look at the preview signal it looks a little to crushed in the shadows compared to the R3D file.
If I adjust contrast in camera to match what I see in the R3D file (-0.5) the Camera meters are affected and my light meter now reads 320 ASA.

How are we going to be able to use false color and zebras together with a look if they affect the reading?

Is the default setting of brightness and contrast in camera really my "flat" image I should be measuring from?

Jesper Holmstrom
Dagsljus Filmequipment
Stockholm, Sweden
 
Do you think what you are seeing could be contribute to macgregor's "Issues" thread?
 
I believe that there is a built in .4 stops of 'headroom'. So, if you take an ISO rating of 320 and multiple it by -.4 stops, you will get ISO 243. This is closer to your findings.

320 * 2^-.4 = 320 * .758 = 243

You can either take the 'headroom' into account in your metering methodology or not as you see fit, but knowledge of its presence can be useful.
 
fergus said:
Do you think what you are seeing could be contribute to macgregor's "Issues" thread?
Click to expand...

Very possible. See my posts in that thread if your interested.

Kevin
 
Unless you are exposing grey scales and chip charts in flat lighting, there isn't really a right or wrong exposure for most subjects, particularly one shot at night -- there is some leeway in either direction depending on the look you want. Histograms and false color information are more about giving you ideas about where to place an exposure, not so much about getting the perfect exposure.
 
David Mullen ASC said:
Unless you are exposing grey scales and chip charts in flat lighting, there isn't really a right or wrong exposure for most subjects, particularly one shot at night -- there is some leeway in either direction depending on the look you want. Histograms and false color information are more about giving you ideas about where to place an exposure, not so much about getting the perfect exposure.
Click to expand...

I fully agree, I never intended to convey that there is but one 'correct' exposure for any given scene. The shooter is always allowed to place their image anywhere within the exposure range that a given system can capture, but if your unaware of the 'headroom' you might not realize that you are under by this factor. In many cases, an inexperienced shooter is better off not to know about the headroom as it will protect them (to a degree) from their own choices. However, in the case where someone is using a metering methodology, knowledge of this 'bias' could be useful information.

Kevin
 
18% gray in 709 gamma corresponds to 41 IRE. If I follow that rule I rate the camera 160-200 ASA.
If in fact the default contrast in the camera is to high and I adjust the contrast to reflect that, I rate the camera 320 ASA.
If the preview signal is corrected to look good on a monitor then that affects histogram, zebras and false colors as well.
What I'm looking for is to find the correct "base" for the camera and a way to accurately measure the signal not the look.
After that it is of course a creative decision on where to let your exposure fall.
Jesper Holmström
Dagsljus Filmequipment
Stockholm, Sweden
 
Hello Jesper,

Please feel free to PM me if you would like to discuss this in detail.

Kevin
 
Jesper Holmström said:
If the preview signal is corrected to look good on a monitor then that affects histogram, zebras and false colors as well.
Click to expand...

You may as well discuss it here Kmuse because if the above statement is accurate then that seems a little scary to me. I would think the histogram/false/zebra should be reading before any preview monitor correction since those are just XML changes anyhow and the sensor isn't changing.
 
If you guys exchanged further on that topic thru PM; can you share your conclusions here on the forum with everybody.
I think a lot of people are indeed wondering how to set their exposure since we know that the signal from the monitor is not a direct result of the image being recorded thru the redcode raw compression.

Thanks

Arno
 
joelnet said:
You may as well discuss it here Kmuse because if the above statement is accurate then that seems a little scary to me. I would think the histogram/false/zebra should be reading before any preview monitor correction since those are just XML changes anyhow and the sensor isn't changing.
Click to expand...

Hello joelnet,

I will be happy to continue the discussion here if you wish. Please clarify your exact area of interest/concern and any questions/comments that you might have.


Kevin
 
khmuse said:
I will be happy to continue the discussion here if you wish. Please clarify your exact area of interest/concern and any questions/comments that you might have.
Click to expand...

I guess my concern is if you changed the ASA to something high just to get a good look at a scene on your monitor that false color might indicate the wrong exposure since the real exposure is always based somewhere around 320.

Or to rephrase - if you raise the ASA in camera does false color indicate highlights are blowing out (at assumed Redcine ASA) when in fact the camera is capturing those highlights (actual ASA of around 320)?

Might 'cause you to drop exposure when you didn't need to.
 
The last testing that I did was at the end of September and this behavior is certainly under the control of the firmware, so the question is somewhat 'build' dependent. I will check with an early camera owner and confirm if this is the current behavior. Give me a day or so and I will get back with a confirmation.
 
I just have a hard time believing that the false color and zebras would be affected by changes applied to the monitoring (contrast, brightness…).
This wouldn't make sens, since those settings are not baked in but your exposure is.
Anyone could confirm this behavior?

Emmanuel
 
(Thinking out loud)

This means that the in camera "exposure lookup" is based on the signal to be sent for output? So, if you were to load a LUT in camera, that would affect how the zebras and false colors react since the metering is done after processing.

Then, the only way to get a very precise exposure rating would be to load the footage into say redalert (or redcine) and check against log gamma (to see your absolute position to the total camera dynamic range), then taking into account the size of your usable dynamic range depending on the iso you plan on applying in post (since there's no in-camera actual signal gain)... taking your final format.

(Did I get this right?)

We're really back to the film educated guess workflow... I like it.
 
David Mullen ASC said:
You'd think that the clip point is the clip point no matter what ASA setting you use, i.e. a false color reading showing clipped areas must always be accurate.
Click to expand...

Exactly
This is precisely the question that needs answering, are false color and zebras affected by changes made to monitoring settings?
And clearly it shouldn't be so.:help:

Emmanuel
 
khmuse said:
The last testing that I did was at the end of September and this behavior is certainly under the control of the firmware, so the question is somewhat 'build' dependent. I will check with an early camera owner and confirm if this is the current behavior. Give me a day or so and I will get back with a confirmation.
Click to expand...

Hey Khmuse,

It sounds like you need a camera to test with. I would be happy to donate mine to your cause for a couple of days if you like. It looks like we are both in OC so it shouldn't be any hassle.

PM me and I will send you my info.
 
The monitor output corrections does affect zebras, false color and histogram, and maybe it should, especially for ASA settings, as long as there is a quick way to revert to a flat technical image for measuring.

Jesper Holmström
Dagsljus Filmequipment
Stockholm, Sweden
 
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