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Exposure and middle gray

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#1 Tom Yanowitz

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Posted 30 January 2015 - 06:04 PM

Hello everyone,

 

I was doing some tests at my school with an Alexa. I wanted to do graphics with the different EI : Luma value of a Kodak grey card = f(stops from -10 to +10) basically.

I wanted to do this both in Rec 709 and Log C but only did the 709 tests as of yet.

 

 

For each EI I placed my "+0" by spot metering the grey card, but noticed two things.

1/ As I kind of suspected, it wasn't falling on the 50% IRE (512,512,512 when shooting Rec 709 in 10 bit)

2/ My Sekonic gives me half a stop of difference between spot metering of that 18% grey and incident reading (when the spot was reading 5.6, the incident gave 4 1/2)

 

So I kind of have three different possibilties for my zero.

Which one should I use ? Is that true that Sekonics are set to read 12.5% grey card instead of 18 (which I guess would explain the weird difference)

When a cinematographer says he places a caucasian face on +1, which one of the three 'zeros' is he refering to ?

Ultimately, do we place the grey card anywhere we want between say 30 and 50 as some people suggest ?

 

That's all I have to ask for now, hope the questions make some sens !

And I hope it hasn't been asked and answered a thousand time.

 

Tom Yanowitz, film student


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#2 David Mullen ASC

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Posted 30 January 2015 - 08:42 PM

I seem to recall that on a waveform, black falls at 15%, white at 65%, and middle grey at 45% with ARRI Log-C.

 

You should read this article by Art Adams:

http://provideocoali...w_you_use_it/P2


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#3 Tom Yanowitz

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Posted 31 January 2015 - 04:47 AM

Thanks David, Art Adams is a great read, I learned a lot through his posts !

 

I think maximum white in Log C depends on the EI. To quote Art :

the MAXIMUM WHITE VALUE changes in LogC. As the ISO drops, the MAXIMUM WHITE VALUE lowers. At ISO 200 it’s 85%. At 1600 it’s 100%. At 800 it’s 95%. At 400 it’s 905. Why is that? I don’t know. That’s just the way LogC works.

 

As for my Rec 709 tests I got my clipped blacks at 1.5%, whites at 100%, and my mid-grey... well that depends :

 

The value of my grey card drops slightly depending on the EI : with a spot meter, usually 40.5%, less (38.5) when we reach ISO 3200.

If I set my f-stop to the incident meter reading, the grey card falls roughly between 45 and 48%.

So I guess that on set I should trust the incident meter more than the spot because of this 1/2 stop difference ?

 

 

The reason I wanted to do it in Rec 709 as well is because.. some projects at school are set like that (rather unfortunately)


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#4 Dylan Sunshine Saliba

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Posted 31 January 2015 - 07:44 AM

because.. some projects at school are set like that (rather unfortunately)

I've learned in the real world, sometimes you need to shoot it all rec.709 in the can, just because of nothing more than preference, time, or budget. They probably do this to simulate changing project delivery terms. I'm not a teacher, so I don't know.

I like your tests and thank you for sharing your results.
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#5 Bruce Greene

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Posted 31 January 2015 - 02:09 PM

Tom, it's great that you're doing these tests to understand the camera!

A couple things:

A kodak grey card is not the reflectance value for 50%. I would suggest that you try to get something like a DSC greyscale test chart to compare it to. You will see that the grey card matches the grey in the test chart one or maybe two steps below the middle grey square which one might set at 50% on a waveform monitor in Rec709 mode.

Second, the REC709 mode on an Alexa is Arri's interpretation of a pleasant image in REC709. It does not match what one would expect of a standardized broadcast video camera. And this is what you should discover from your testing.

So, get yourself a standard video test chart with the two greyscale ramps going from dark to light on the top, and light to dark on the bottom.

Light the chart evenly and adjust the iris so that the middle square is at 50% (for starters). Look at your waveform monitor. You should see something resembling an "x" pattern. In a standard broadcast setup this would look like an "x" with straight lines. What does yours look like?

I know from doing this test that an Alexa, in REC709 mode will show an "x" with curves at the top and bottom, rolling off highlights and shadows. Play with your iris to see what happens as you get close to and beyond maximum white and black (note: light the chart so that you begin at f5.6!)

You may or may not like this look, and you may decide that even in REC709 that you will need to grade the image in post production.

Lastly, set the iris so that the middle of the "x" is at 50%. Then hold up your grey card in front of the chart and see if the suggested exposure matches the iris and ISO you've set on the camera. It should be pretty close.

Have fun!
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#6 Tom Yanowitz

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Posted 31 January 2015 - 02:32 PM

Thanks a lot Bruce and Dylan !

 

I will try this stuff out when I get a chance !

And then hopefully study Log curves too !

 

My end goal is more creative than technical. Art Adams talks about 'the sweet spot' of the dynamic range. Which implies (as for Zone System in film) that we should try to expose everything in the frame (or at least everything where we want details) in the zone of the response curve where the slope is the highest, instead of using on-set the full dynamic range from black clip to white clip. I think this also applies to Log C (at least of the blacks)

 

I got interested in this both via Art Adams' posts and this zone system advice video.

A lot of people say 'expose closest to the final look' but this video suggests we can actually better contrasty/low noise blacks by placing them in an unpleasant-looking way at -3 for the Alexa for example, and then getting them to the desired level in grading.

 

What do you guys think about this way of doing ?

Cause I've suggested the idea on a couples of projects at my school and it's considered 'too safe, without any risk taken' which I guess means anti-artistic.


Edited by Tom Yanowitz, 31 January 2015 - 02:36 PM.

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#7 John E Clark

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Posted 31 January 2015 - 04:49 PM

Thanks a lot Bruce and Dylan !

 

I will try this stuff out when I get a chance !

And then hopefully study Log curves too !

 

My end goal is more creative than technical. Art Adams talks about 'the sweet spot' of the dynamic range. Which implies (as for Zone System in film) that we should try to expose everything in the frame (or at least everything where we want details) in the zone of the response curve where the slope is the highest, instead of using on-set the full dynamic range from black clip to white clip. I think this also applies to Log C (at least of the blacks)

 

I got interested in this both via Art Adams' posts and this zone system advice video.

A lot of people say 'expose closest to the final look' but this video suggests we can actually better contrasty/low noise blacks by placing them in an unpleasant-looking way at -3 for the Alexa for example, and then getting them to the desired level in grading.

 

What do you guys think about this way of doing ?

Cause I've suggested the idea on a couples of projects at my school and it's considered 'too safe, without any risk taken' which I guess means anti-artistic.

 

A couple of things... While the DSC charts are very good... they are also very expensive...

 

As mentioned the 18% Kodak (or equivalent) does not yield a 50% on the IRE scale. Some camera makers will mention what it is, some do not and one has to glean from Internet posts an blogs to find such numbers for most cameras.

 

In the case of the Blackmagic Pocket camera I have been testing most recently, from a company insider it seems that for the 'log' format, 38.4 % is the target value for an 18% grey card... and naturally... getting sub-percent accuracy with an ordinary NLE is... uh... not to be...

 

So, while 38.4% may be the the most accurate... I'll be using 40%...

 

On my GH-1, I find that I prefer 18% yielding a 50% but then I only have about 2 stops above that for 'brightness'... Probably 45% would be 'better'...

 

If broadcast is not a goal, one can 'fudge' on these values, and one can find personal values for ISO that yield known values on 'your' screen... there is the issue of what monitor you use for evaluating the actual look, and for most people using a variety of digital display devices, none of which are 'broadcast' calibrated monitors, what others may see on their devices...

 

Theater presentation goals have it far more easy to get a consistent look, unless the theater owner has cheaped out on calibration of their projection systems...

 

But I think the Adams articles are good for understanding things, and also not that one can not be absolutely 'slavish' to the Zone System for moving pictures.


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#8 David Mullen ASC

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Posted 31 January 2015 - 05:30 PM

All that matters are results, not how you got there, in terms of artistic merit.

 

But I would say that personally, I try to not be a slave to metering, I do a lot of stuff by eye and by a well-calibrated monitor just to free me from thinking technically about lighting and exposure.  But it doesn't hurt to keep your dark scenes more in the middle and print down so to speak in post.


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