# measuring and setting f stop

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### #1 Quinten Densak

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Posted 17 September 2007 - 08:17 AM

Hi

I am confused about f stop scales and how to set your stop on the barrel. How do you understand the math for 1/2, 1/3, 1/10 stops ( on an analog and digital meter)? How do you set the stop on the lens after the measurements have been made? So far ive been rounding things off. I want to start to get more accurate. Also, what is the correct way to communicate the readings? Could someone take the time to explain this?

Thanks
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### #2 Laurent Andrieux

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Posted 17 September 2007 - 08:54 AM

On an analog meter, 1/2 stop is half way between 2 stops etc. It's a linear scale.

On a digital one, stops are given by 1/10 th so you usually consider 0.3 is a third, 0.5 is a half etc. You read 2.8 3 for 2.8 + 1/3 for instance.

On the lens barrel, it's linear too, so you get the half stop half way between 2 stops etc.

The reading is just usually communicated as : "F + a third" or "and a half" for instance, or "F - 1/3" etc. Just easy.

Finding the proper math value is another story, but it's a bit useless, unless you work as an AC with a DP you gives you stops like "3.2", you've got to know it's 2.8 + 1/3, but, as a beginner, don't worry about that...

Hope it helped,

Regards.

EDIT Oh, and, change your profile location, you made a type mismatch, BTW
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### #3 Quinten Densak

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Posted 17 September 2007 - 09:31 AM

[quote name='Laurent Andrieux' date='Sep 17 2007, 05:54 AM' post='194048']
On an analog meter, 1/2 stop is half way between 2 stops etc. It's a linear scale.

On a digital one, stops are given by 1/10 th so you usually consider 0.3 is a third, 0.5 is a half etc. You read 2.8 3 for 2.8 + 1/3 for instance.

Thanks Laurent

Things are definately a bit clearer now.

just want to check:

If i m reading 2.8 3 ( 2.8 and decimal 3) on my meter that means a third over 2.8. On my barrel i would set the stop just above the 1/4 mark between 2.8 and 4.

Similarly if i read 2 8 ( 2 and decimal 8) on my meter i would set the stop at a bit below 4.

Do you think i have got it?

If the dp did say 3.2 would i be right at setting the stop at a 1/4 over 2.8?

I would really like to get a math lesson in this...Hope someone has the time for that....
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### #4 Laurent Andrieux

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Posted 17 September 2007 - 12:39 PM

If i m reading 2.8 3 ( 2.8 and decimal 3) on my meter that means a third over 2.8. On my barrel i would set the stop just above the 1/4 mark between 2.8 and 4.

Yes, I mean a bit over the 1/4 mark (do you have 1/4 stops marks on your lens ?) since a 1/3 is bigger than a 1/4.

Similarly if i read 2 8 ( 2 and decimal 8) on my meter i would set the stop at a bit below 4.

No. If you read 2 + 8 1/10th, it would be a bit below 2.8 (Mind that 2.8 is 2 + 10/10th, a full stop over 2, while 2 + 8 1/10th is a bit below 2.8...)

If the dp did say 3.2 would i be right at setting the stop at a 1/4 over 2.8?

No, as I said, it's 1/3 over 2.8, not a 1/4 (doesn't make that much a difference, BTW...)

I would really like to get a math lesson in this...Hope someone has the time for that....

As an attachment, you have an excel file I've done with the intermediate values, 1/3, 2/3 and 1/4, 1/2...

I'll come back later as to give you the math logical, I've got to make dinner for my daugther right now...

EDIT : Strangly I'm "not permitted to upload this type of file"... I uploaded on my site, then. Download it here
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### #5 Laurent Andrieux

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Posted 17 September 2007 - 03:57 PM

Ok, let's go for the math, then

1) The F stop number is defined by F=f/D where f is the focal length and D the diameter of the entry pupil, that is proportional to the focal length as well, so that whatever focal length you have, F is a given (geometrical) aperture. If you have a 50 mm lens or a 135 mm, if your meter says 4 it's 4 for all the lenses.

2) Hence this definition, for a given focal length, opening the iris (increasing D) will lower the stop number F.

3) They are organised as a scale so that the light is doubled at each step of the scale when you open up (lower the number) and resp. halfed when you close it (increase the number).

4) The ligh quantity is doubled when the surface (S) of the iris disk is doubled, so the Diameter must be multiplied by RMS 2 wich is close to 1.4 (S= PI.DÂ²/4). This is why the scale is 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22...

5) As to find out the intermedite values, you have to notice that these numbers are in a geometrical series. Each term is multiplied by the same number : RMS 2 eq. 1.4. As to get the value half between 2 full stops, you have to multiply by RMS 1.4 = 1.19. As to get the intermediate values by thirds, you must use the cubic root (^1/3) of 1.4 (someone helps for the translation please) wich is closed to 1.12.

Mind in the excel file that you have two "hidden" lines at the top (lines 1 and 2) where all the necessary values are calculated.

Hope I'm clear enough to understand...

Regards
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### #6 Quinten Densak

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Posted 18 September 2007 - 04:00 AM

Thanks!!!!!!! I think im getting a hang of this now...i really need to get my hands on the camera assistants manual!!

Thanks Again Laurent
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