Figuring out Latitude from the Characteristics Curve

[The Evil Mariachi]

Hey Guys,

Maybe I'm a little dense, but I'm not exactly sure how to figure out a film stock's latitude from its characteristics curve. I've been trying to read Kodak's sensiometry guide, but all those numbers, logs and what not have got my head spinning. Can anyone explain this a little clearer?

Thanks in advance, guys.

[Ryan Patrick O'Hara]

I would be glad to help.

The latitude of the stock is usually determined by length of the straight line between the curve of the shoulder and toe.

First of all, do you have the characteristic curve given from Kodak? Or are you testing the latitude yourself?

Thanks.

[Ryan Patrick O'Hara]

Examples:
http://www.kodak.com/US/images/en/motion/products/v3/5219/5219_char.gif
http://www.kodak.com/global/images/en/motion/products/reversal/85sensi.gif



The first curve is KODAK VISION3 500T Color Negative Film 5219 / 7219.

The second curve is KODAK EKTACHROME 100D Color Reversal Film 5285/7285. Reversal film is known for having more contrast and less latitude. Notice the more 'steep' rush between the shoulder and toe.

This is how I learned to read the curves. So looking at the two I would say that the Vision3 500 5219 would be about 4 stops under and 6 stops over, for a total of 10 stops.

The reversal film, on the other hand I would read at 3 under and 2 over (?) for a grand total of 5 stops? Reversal is tricky to read, because it is high in contrast and punchy. It might pull 6 stops, you would be best to run a test regardless but I think you get the idea of reading these charts.

It's been awhile since I've brushed up on my chart reading so if anyone has any comments for me, please do.

-Ryan

[Ryan Patrick O'Hara]

Uh, Bump?

I guess the person found the answer else where. I was all excited to brush up on the topic.

oh well

[bwest]

All dressed-uped and nowhere to go

[The Evil Mariachi]

My apologies to the artist formerly known as Prodigy. I've been away, but not really away, y'know?

I've got me Krasnogorsk, on it's way from mother Russia, and I'm excited to start shooting on film.

Which brings us to the characteristics curve...I get the concept of the steepness of the curve and its relation to latitude, but I just don't see how you figured out the over/under in the exposure.

[Ryan Patrick O'Hara]

Congrats on the film camera.

Are you asking how I figured out the amount of stops over and under?

Let's say I have a gray card to which I am setting my exposure to. I would first set up my lighting and then take my calibrated light meter to the gray card. If it read an F/4 I would set my lens to an f/4.

Now I am at what is considered 'N' or '0' on the charts. Looking at the steepness of the latitude and where the line curves at the shoulder and toe, I can figure out how many stops of latitude I have below and above my proper exposure.

Add them together and you have the total amount of latitude. The reason for knowing the 'under/over' is because some stocks handle shadows better and some handle brighter zones better. Now you will know where your shadows will turn to black and where your highlights will clip.

Not sure if I explained what you were asking.

[The Evil Mariachi]

Alright,

I think I'm beginning to understand. But, what about reading a chart like this one:

Ihttp://kodak.com/global/images/en/motion/products/bw/ti7266chaR.gif

How do the log numbers translate to f stops? What's my baseline number for figuring out the under/over?

I really appreciate you taking the time.

Thanks again.

[Ryan Patrick O'Hara]

I'll have to get back to you on that. I know that LUX can easily be converted to footcandles (LUX vs Footcandles is kinda like the metric system vs feet and inches)


1 lux = 0.0929030436 footcandles

Once I have footcandles, I think I can figure out stops.


Every time an f-stop closes down one stop you need twice the amount of footcandles to expose. Every time you open up one f-stop you need half the amount of footcandles to expose.

Example FROM kodak website on the 7266:......
Exposure Table For Tungsten Light

This table is based on EI-80 (tungsten) and reversal processing with a shutter speed of approximately1/50 second, 24 frames per second (fps), and 180-degree shutter opening:
Lens Aperture_____________f/1.4____f/2____f/2.8____f/4____f/5.6____f/8
Footcandles Required[1]_____32_____64_____125____250____500____1000
[1] At 18 frames per second, use 3/4 of the footcandles (fc) shown. When the film is used as a negative material, the values specified should be doubled. Lighting Contrast

The recommended ratio of key-light-plus-fill-light to fill light is 2:1 or 3:1. However, you may use 4:1 or greater when a particular look is desired.



I'll try to figure this out soon. Right now it's not adding up at all. I think it has to do with (LUX-seconds)....

-Ryan