All Things Color for Film and Digital Cinema
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Posts Tagged ‘LAD’

iRiS – Photochemical Color Timing Reference Image

Here is a color reference image I generated for quickly visualising Color Density Readings. You should ask your lab to write down their density readings of the LAD / 445 CV grey patch at the head of each Roll on each Box/Can of print film you receive. With this information you can immediately determine if the print is a “little magenta” or a “little dark” and pre warn clients before a screening that the neg is OK but this particular print out of that particular days processing bath is a biased in that direction.  

Click the LINK HERE for a downloadable version.

 

iRiS - Lab timing reference

iRiS - Photochemical Lab Timing Reference Image

 

 

The printer light settings on the rotary dials are arbitary 25-25-25 settings. More than likely each lab changes these settings each day after calibrating themselves to the processing bath using a sensitometric wedge. I have personally found it is better to talk to your Lab consultant in terms of printer point changes      IE +1 red -2 Green +1 Blue      rather than actual printer lights, 26 23 26, as this can cause confusion. 

For example if your print had a LAD density reading of 1.03 1.06 1.03 you could deduct that the print was a little Red and a bit lighter. IF though the print LAD was “on AIM” ie 1.09 1.06 1.03 but you still thought the print was a little red you could ask in subsequent prints for a trim of -1 Cyan (the complimentary opposite of RED) which would result in a print that was less red and a fraction darker. The numbering seems a little counter intuitive but referencing the numerics on the RGB rotary wheels you can see that subtracting one point on the red ‘wheel’ you would be pushing it up (making 25 become 24) which in turn is letting less red light through. This in turn makes the Green and Blue ‘lights’ more visually dominant, hence Cyan. 

 

 Coming very soon is my visualization Application for MacOSX that is pretty much the above user interface but provides realtime color feedback to the image of your selection with adjusted printer lights using your preferred input 3DLUT. Very useful for dealing with your film LAB of Choice when making and timing Answer Prints.

If you would like to know any more about this application please feel free to email me.

 

 

 

 

Adrian Hauser


18 Percent Grey …. “Middle Grey” and Magic Numbers

 

    For a while I have been pondering the function of the 18 percent grey card. Why 18%, what is its historical reference…ect.

    After a lot of reference reading I came up with the following ….

    Traditionally 18 % refers to the statistical average reflectance of a photographed scene. ‘Normally’ exposed skin tones also generate an average incident light reading of about 18%.

    Photographically, if you were to make a set of 11 patches starting with 100% reflectivity and each subsequent patch was halved in reflectance, you would end up with a logarithmic scale where the light intensity is being halved each patch. The seventh step, middle grey, would yield a photographic Status M Neg density value of 0.7

    Using our Density math described in previous posts we know Transmission = 1/10^Density   

    Therefor    T=1/10^0.7= 0.18      AND       Transmission is directly proportionate to Luminance L*. Not boring you with the math the result is 49.496.

   So    0.7 Density = 0.18 Transmission = 50% Luminance(L*).

 

 

Fig.A - Status M against Status A Density

Fig.A - Status M against Status A Density

 

 

  Interestingly If we map our 21 step sensitometry readings of a 21 strip grey scale test wedge over Neg and Print densities we see that 0.7D is the cross over point !   See Fig.A

 

 

Fig.C

Fig.B

 

 

 

 

Looking at the image below, Fig.C, you will notice that this point of around 18 % grey is the mid point at which the cineon Log file is expanded when overlayed with a Print emulation 3DLookUp table. Both Mid Grey LAD Patch’s are almost identical as seen on the corresponding waveform Representations. 

 

 

18percent

Fig.C

 

Looking at the graph in Fig.C one can also see the reference point of 18 % on each of the mapped targets averaging around 50% luminance. Interestingly with this chart I have mapped Cineon Log levels against the 2.2 and 2.5 video gamma transfer functions and CIE Luminace L* values. They are all relatively close to each other in their Logarithmic encoding. I can see im going to have to rewrite this as its going to get messy from about here on in, although quite fascinating. 2.5 gamma looks like its the best match for CIE L* but somehow we got stuck with 2.2. Near enough is good enough I guess. It was decided back in the 8Bit video days that to help save on visual data that video/TV could also have a perceptual gamma encoding, once again mimicking the eyes response to nature. A gamma of 2.2 was decided apon.  

 

 

Adrian


China Girls 2

Back in 2005 Julie Buck and Karin Segal put on an exhibition in New York called “Girls on Film”. 

Julie was manager of the Harvard Film Archive and had the task of sorting and preserving the Film Archive Collection. The exhibition was a tribute to the many anonymous women who worked in the film industry.


China Girls .

 

China_girl7

China_girl7

For a bit of fun and curiosity I will update this post over time with images of China girls. China Girls are used to adjust and check printer density.

They provide subjective and objective colour and gray scale patches.

These patches can be read on Densitometers to confirm the process. They are a standard laboratory test film usually incoporating a face and a greyscale; used for printer line-up.

 

 

 

1_11


Density and the Film Lab – Density Pt.4

 

Lab Aim Density / China Girl

Lab Aim Density / China Girl

 

China Girls are used to adjust and check printer density.

They provide subjective and objective colour and gray scale patches.

These patches can be read on Densitometers to confirm the process. They are a standard laboratory test film usually incoporating a face and a greyscale; used for printer line-up.

See the post on China Girls for a look at some of the older examples of these majestic images.


Protected: Densitometry Pt2 “the 21 step test strip”

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Protected: From neg to print via digits – DI – the round trip

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Protected: Excel application for visualizing Print Densitometry readings

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