Exposing for the RAW format

[Gregor Hagey]

but where do you set your t-stop?

but where do you set your t-stop?

It's important to test, test, test so you can confidently set your iris though. The ISO might be just metadata but you can't change your mind later about the T-Stop!

One confusing aspect of shooting/metering at ISO 320 is that a middle grey card actually reads at 40% on the waveform instead of 50%. You need to overexpose the card by 2/3 stop to get it to 50%.

I think the most important thing to understand though is that it's not important to get middle grey or skin tone in the right place, just expose as bright as you can without clipping white. This means each scene needs to be evaluated holistically. A simple and easy tool for this is the histogram. A lit white object should have a slope at the right of your histogram not a straight line. If you are monitoring at ISO 320 you can trust that when the histogram is clipping the sensor is clipping too. First the blue channel, then green and finally the red channel will clip. Of course don't let specular highlights throw you off (they will clip no matter what you do). Dulling spray and polarizers are the best way to deal with them or power window a blur effect on them in post.

When the .r3d's are processed you can darken your picture at this stage to set the mood you want. This is where prep is important so you can create Red Alert or Redcine presets(LUTs) so the digital lab processing the .r3ds knows what your intentions are.

Of course a meter is still indispensable for balancing lights. Also, you should stick to one exposure level for a scene or your grain/noise could change with each shot.

 

[Hans von Sonntag]

One confusing aspect of shooting/metering at ISO 320 is that a middle grey card actually reads at 40% on the waveform instead of 50%. You need to overexpose the card by 2/3 stop to get it to 50%.

I think the most important thing to understand though is that it's not important to get middle grey or skin tone in the right place, just expose as bright as you can without clipping white. This means each scene needs to be evaluated holistically. A simple and easy tool for this is the histogram. A lit white object should have a slope at the right of your histogram not a straight line. If you are monitoring at ISO 320 you can trust that when the histogram is clipping the sensor is clipping too. .

With the current LUT implemented whites on the sensor don't clip when they clip on the monitor. There is a 0.40 EV offset in the monitor out LUT which gives a bit headroom on the whites. The historgram shows only the values of the LUT not of the sensor. Hence for really accurate exposure the histogram is not useful, though safe (which is good, thanks Graeme).

Hans

 

[Hans von Sonntag]

Of course don't let specular highlights throw you off (they will clip no matter what you do). Dulling spray and polarizers are the best way to deal with them or power window a blur effect on them in post.

When the .r3d's are processed you can darken your picture at this stage to set the mood you want. This is where prep is important so you can create Red Alert or Redcine presets(LUTs) so the digital lab processing the .r3ds knows what your intentions are.

Of course the whites clip when they are off. Nothing bad with this. In a high contrast scene unavoidable. The beauty about film is the nice shoulder (roll off) it creates in the whites. It's one of the secrets of a great colour correction to mimic with digital footage this pleasing film shoulder. For this some headroom in the footage is nice to have.

The goal of RC is to create the best base to grade off. Better than using RC is grading directly off the r3d files with a dedicated grading tool.


Hans

 

[Dj Joofa]

The beauty about film is the nice shoulder (roll off) it creates in the whites. It's one of the secrets of a great colour correction to mimic with digital footage this pleasing film shoulder. For this some headroom in the footage is nice to have.

Hans

To get to the latitude the film is providing a digital camera must have at least 17-bit ADC, where even the least significant bit is measuring signal and not noise. When digital cameras get to that level then film's latitude and shoulder won't be missed that much.

 

[Mitch Deoudes]

Of course a meter is still indispensable for balancing lights.

This is one thing that always confused me about "digital" guys lighting without meters... How are they setting their contrast ratios? I've not shot as extensively as some of the folks around here, but we were taught that CR was the most important aspect of matching shots.

Also, you should stick to one exposure level for a scene or your grain/noise could change with each shot.

Another interesting point - film is probably a lot more forgiving in this area, as well. So much for not waiting for that cloud to pass before shooting...

 

[lordtangent]

To get to the latitude the film is providing a digital camera must have at least 17-bit ADC, where even the least significant bit is measuring signal and not noise. When digital cameras get to that level then film's latitude and shoulder won't be missed that much.

'

Or just use LinLog?

like Gregorios, polarizers have been a favorite trick of mine for handling spec hits for quite some time. It really helps take the "video curse" off it.

 

[Gregor Hagey]

how do you know when you're really clipping?

how do you know when you're really clipping?

With the current LUT implemented whites on the sensor don't clip when they clip on the monitor. There is a 0.40 EV offset in the monitor out LUT which gives a bit headroom on the whites. The historgram shows only the values of the LUT not of the sensor. Hence for really accurate exposure the histogram is not useful, though safe (which is good, thanks Graeme).

Hans

Not completely true by the tests we've been running in Toronto with firmware 13. Yes the histogram will show more clipping then what is happening on the sensor at 320 ISO, but it gives the most accurate reading of what's happening on the sensor.
At 1 2/3 stops overexposure the blue channel on a lit white surface will start to clip. At 320 ISO the histogram will show this clipping (both blue and green; the green isn't actually clipping yet though), but monitoring at 250 ISO the clipping is not apparent at all. The blue channel will look like a spike, but it won't be near the right edge of the histogram and could mislead you into thinking you're safe. It's not until 2 stops overexposure (when green and blue channels of a lit white surface are clipping) that the 250 ISO monitoring will show clipping on the histogram. I would always monitor at 320 ISO for this reason.

 

[Gregor Hagey]

This is one thing that always confused me about "digital" guys lighting without meters... How are they setting their contrast ratios? I've not shot as extensively as some of the folks around here, but we were taught that CR was the most important aspect of matching shots.



Another interesting point - film is probably a lot more forgiving in this area, as well. So much for not waiting for that cloud to pass before shooting...

I think setting fill and back lights by eye is something you need to acquire with experience in a given format (and testing!). I still use my meter all the time whatever I'm shooting, but my eyes have the final word.

Don't forget there's an iris you can change to keep the S/N consistent when clouds pass or use NDs (watch out for the IR though!).

 

[Mitch Deoudes]

gregorios - slightly confused by your right-hand histo. If the blue channel were clipping, wouldn't the spike have a harder right-hand edge, and no detail farther to the right? (I'm seeing a small bump at the extreme right that tails off toward the edge.)

 

[Gregor Hagey]

this is what an unclipped blue channel looks like

this is what an unclipped blue channel looks like

The blue channel should have the same shape as the red and green 'hills'. The attached file is a histogram of an un-clipped blue channel from the same test (the picture is a model with a grey scale). Lit white is at 1 1/3 stops over.

Sorry having trouble getting file to upload.

 

[Gregor Hagey]

added attachment

added attachment

Having trouble getting the attachment to work. Here it is again.

 

[Mitch Deoudes]

Ah - ok. I missed the part about it being a white card. Makes sense now, thanks.

 

[Hans von Sonntag]

Hi Gregor,

I'm just curious: In your histogram red, blue and green don't overlap. Did you change Kelvin in camera for a test, was ist possible to get an even histo in this regard, and if so, did the blue channel clip as well?

Next thing: Did the histo in RC at 5000K look the same as the histo in camera? Did the blue clip as well? I found that although the camera is balanced to 5000 Kelvin a neutral white in a 5600 Kelivin scene will be accomplished at 6500ish in REDCine (meaning that the Kelvins in RC/RED are non-accurate numbers - probably...)

And finally: Did you light your test chart with tungsten and set the camera to 3200K?

I'm asking because as to many shooting and exposing with the RED is new to me and there is a lot to learn.

Hans

 

[Gregor Hagey]

Yes the colours don't align for the white patch on the right. I've attached a series of corrected histograms. The channels align when the WB is set to 9500 Kelvins and 4 Tint. I've also pulled back by 1/3 stop of exposure in Red Alert so the right edge is more clearly visible. At 1 1/3 over red, green and blue are the same, but at 1 2/3 over the blue channel is becoming slightly narrower and forming a peak. At 2 over there is obvious clipping with the blue channel compared to red and green. We shot with Daylight Kinos and I didn't note their colour temperature (I was assuming they were 5600K). I'm curious to shoot some charts with different colour temperatures to see if this changes.

In general though I find that colour temp meters don't agree at all with Red and at first I thought this was only a firmware issue. After a talk with an associate it was pointed out that colour temp meters are optimized for film sensitivity and not how silicon behaves. Maybe a colour temp meter for digital sensors will come out soon.

 

[Hans von Sonntag]

In general though I find that colour temp meters don't agree at all with Red and at first I thought this was only a firmware issue. After a talk with an associate it was pointed out that colour temp meters are optimized for film sensitivity and not how silicon behaves. Maybe a colour temp meter for digital sensors will come out soon.

Kelvin is Kelvin, no matter wether you measure for film or digital.

I suppose since changing Kelvins in camera does not change anything regarding the exposed RAWs the colour temperature settings are more or less just numbers. In the end the histo after WB in REDCine matters. I get over 6500K when creating an aligned histogram of a 5600K shot scene. By the way, WB does make a lot of sense, it does reduce clipping. The next clapper board we will get will contain a 18% grey section - great for WB in REDCine.

Hans

 

[Gregor Hagey]

Kelvin is Kelvin, no matter wether you measure for film or digital.

I think it's a little more complicated than that, but this is a subject for a new thread.

 

[David Mullen ASC]

He's right though -- a meter reading in Kelvin doesn't care what camera you are using, film or digital. Kelvin is Kelvin -- it's independent of the recording medium. What may need some adjusting or rethinking though is the recommended CC filters for dealing with green fluorescents, etc. due to differences in spectral response.

For example, the mix of dyes in a Tiffen FLB or FLD filter is based on film's spectral response -- otherwise you could just add some CC Magenta filters to whatever warm or cold filter needed. Just as when you compare a series of warming filters with the same MIRED shift value, whether Corals or DecaMired Yellows or 85 filters, etc. -- you can find filters that create the same shift in color temp even though they don't match each other in color (some are yellower or pinker, etc). This is because correction filters like the 85B are based on making 5500K daylight look correct for a spectral response of 3200K color film stock.

 

[Gregor Hagey]

Well I'm no physics major but this is how I understand it:

Tungsten and Daylight have a continuous spectrum whereas HMI, Fluorescents, LEDs, etc. are discontinuous and only simulate the visible spectrum (sometimes well, sometimes not so well). A colour meter will sample red, green and blue and give a Correlated Color Temperature for these discontinuous sources based on human perception of a black-body radiator but not necessarily how a CMOS sensor 'sees' it.

FYI A black-body radiator is a black object that is so black it absorbs all light that hits it (that's what I call a matte surface!). When this black object or 'body' is heated it first radiates Infrared and then as it gets hotter Visible light (moving from red to orange and so on). When it gets to a whopping 5, 300F (wow that's hot!) the colour it emits is what we know as Tungsten balance. In the Kelvin scale that temperature is 3, 200. The temperature of the Sun is almost 11,000F or 6,000K - which is the colour temperature of the Sun before it hits the Earth's atmosphere (and is changed). It's obvious that Fluorescents, LEDs and HMIs don't create light this way.

The practical reality is that a colour meter may give misleading data as to how the white point will look on your film stock, videotape or RAW file. Testing needs to be done to know how your camera behaves.

Maybe there's a colour scientist that can weigh in?

BTW I still think this deserves a new thread.

 

[David Mullen ASC]

The problem isn't that the color meter is reading the Kelvin "wrong" for digital cameras, Kelvin is Kelvin... it's just that the whole notion of Kelvin is based on a ideal black-body radiator, which discontinuous spectrum sources like LED's are far from being. So you need more information than a simple Kelvin reading to know how these lights would reproduce, whether for film or for digital.

Your point originally was that the Kelvin reading in a color temp meter was somehow optimized for film, which I don't see how that's possible. Either it reads the Kelvin of the source or it doesn't, regardless of the recording medium. How that source reproduces is another issue entirely. The only way I could see it being optimized for film is perhaps in the CC filter recommendations it makes.