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Old November 18th, 2004, 05:53 PM   #1
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F-stop , T-stop Conversion - Need Fast Help!

I'm shooting a 16mm School Project this weekend. I have a lens which is adjustable in T-stops, and a light meter which reads F-stops. Is there a table/method I can use to convert?

I gotta start shooting on Friday night, and there isn't any way to get other equipment. I'm also not sure if this is more suited to the lighting board, so MOD might need to move it.

If it matters, I'm shooting on 16mm B&W Reversal (Kodak 7266 Tri-X). ASA/EI/ISO = 200 Outdoor & 160 Indoor (Tungsten)

I'd be very appreciative toward any speedy replies on the topic. Thx guys!

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Old November 18th, 2004, 06:05 PM   #2
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I assumed that this relationship was inconeqeuential as processing, exposure readings, and emulsion speed would cancel out the small differences between the calibrated T and mathematical F-stops.

There is no chart because a T-stop is calibrated for individual lenses. I believe the only way to figure this out is through shooting the lenses and seeing the results from the lab.

However, if you have the AC Manual handy, there is a chart which gives you roughly the amount of light needed for a T-stop at a particular emulsion speed.
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Old November 18th, 2004, 09:00 PM   #3
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I just started reading up on T-stop, and now realize that any F-stop conversion table would be lens specific. Somebody told me the glass is roughly half a stop slower than the readings I'll get from the light meter.

I do have the Elkins' AC manual, but don't have that chart. Mine is from '91...although I doubt that would make a difference.

I'm using the Vario-Switar 100 POE lens on a Bolex Reflex H16 camera. Again Tri-X stock (ASA 160/200).

I guess unless I hear otherwise, I'll expose 1/2 stop open from what my light meter reads.
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Old November 18th, 2004, 09:30 PM   #4
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If it's any help, I took a picture of the page I was referring to in the AC Manual.

http://www.the-hegemony.com/tstop.jpg
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Old November 19th, 2004, 11:41 AM   #5
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Thanks so much for the picture, Xiaoli

So, according to this chart, when shooting on a film with an EI of 160 (200 outdoors), I look at the 1600 column, then if I measure the incident light on the subject to be an F4, I set the lens to a T2.2, or if I measure the light at an F8, I set the lens to a T3.2, and so on and so on...Is that right? It just seems like a pretty big discrepancy?
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Old November 19th, 2004, 11:49 AM   #6
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The numbers in the boxes represent the number of foot candles, not f-stops...the top row is the ASA (emulsion speed). So you look at 160, which is the second column on the right page, not 1600. Then you use your meter to determine the incident light in foot candles, which determines what t-stop you need to use.

What light meter are you using?
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Old November 19th, 2004, 12:00 PM   #7
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I'm using the Sekonic Studio Deluxe L-398. I'm gonna take a guess here. I take the reading and then use the numbers on the dial (0-640) without converting to F-stops?
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Old November 19th, 2004, 12:16 PM   #8
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Yep :)

http://homepage.newschool.edu/~schle...t_reading.html
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Old November 19th, 2004, 01:01 PM   #9
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Thanks for all your help Xiaoli. I would have been lost without you. I hope I can help you out some time!

I'll post back on Wednesday and tell you how the footage looks...
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Old November 19th, 2004, 02:15 PM   #10
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No problem Jesse. Excited to see the footage!
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Old November 23rd, 2004, 12:39 AM   #11
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if your lens only has T stops that is what you use !! if the meter says F 4 you set lens to T 4 ...

F stop is a mathmatical calculation based on the length and width of a lens .. if you have a 1 inch lens (long) that is 1 inch in diameter then that would be a F 1 lens. however it does not take into account light loss casued by the glass used in the lens so if there was a 1 stop light loss because of the glass then if you set the lens at F1 you would under expose it by 1 stop .. therefore they came up with T stops which measures the light that hits ( coming out the back of lens) the film ..

a light meter measures light levels falling on it's dome = T stops ( light that comes out of back of lens )

if you have BOTH F stop & T stops on lens -
therefore if your light meter says F2.8 you set the lens to T 2.8 while on the F stop scale (on lens )it says F2. you use the F2 to read the depth of field scales or D of Field charts ...

you will find 1/2 - 2 stops difference bewteen F and T stop scales on older lens usually by 5.6 F & T stops will match ( so your F8 would match T 8 - do not set to T3.2) ) ... most movie lens made in past 14 years show T stops only ... plus with todays technology you don't have all the glass socking up the light ...
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Old November 30th, 2004, 12:02 PM   #12
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Hey guys. Thanks for all your help again. The footage came out pretty good, just a touch darker than I would have liked. I was using a Bolex Reflex, which soaks up an extra stop when splitting the incoming light. Because the lens was supposed to be faster than my regular H16 lenses, I only opened about 1/2 stop past a reading. I used an older Bolex zoom lens on my second and third days, opening a full stop. I'm going to see that stuff tomorrow. Hopefully, I got it close enough to match.

Of course, if my lighting was a little more even that could have helped too. ;)

Shooting mostly video, and then jumping into B&W film has given me such an appreciation for the filmmakers of old. They had to know their stocks inside and out and be very talented to get nice separation and proper exposure all around.

While I love the look of film, I will be relieved to do some video shoots again for awhile.
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