Doing the Maths of Exposure Quckly

maths exposure T-stop ISO fps

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#1 Alexandros Angelopoulos Apostolos

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Posted 27 June 2016 - 02:55 AM

I was reading the Oscillating the ISO on Alexa thread yesterday and in it David says

An ISO change by less than a stop isn't going to change the look much, but it can help you out now and then, like when you've lit the set to f/2.8 and suddenly you have to put an f/3.5 zoom on the camera.  Or maybe you've added enough light to shoot at 48 fps wide-open on the lens and then the director wants to go to 60 fps.

so I was wondering is there an easy way do to a calculation of this sort on the spot without any aid from a gadget or an exposure calculator: What is the difference in exposure between ƒ/4.5, 24 fps, ISO 640 and ƒ/2.8, 60 fps, ISO 800?

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#2 Miguel Angel

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Posted 27 June 2016 - 06:30 AM

Well,

If we consider the following as the "normal" set up of the camera

- T4 1/2, 24Fps, ISO 640

And then you want to go to:

- T2.8, 60Fps, ISO 800

You have to take into consideration what you get and what you lose.

First of all, the T STOP:

From T4 1/2 (which is the equivalent to F4.5 without starting talking about F and T differences) to T2.8 you gain 1 1/2 stops of light because of the way T stops work.

T2 >> T2 1/3 > T2 1/2 > T2 2/3 >> T2.8 > T2.8 1/3 > T2.8 1/2 > T2.8 2/3 >> T4 > T4 1/3 > T4 1/2 > T4 2/3 >> T5.6, etc

So from T4 1/2 to T2.8 you have to open up the T Stop 1 1/2 times.

Hence you have, already, more light coming into the camera.

Second thing, FPS:

From 24Fps to 60Fps you lose around 1 1/2 stops of light, because from 24fps to 48fps there is 1 stop of difference and then from 48Fps to 60Fps is about 1/3 or 1/2 stop (more or less)

Then you are almost even, the amount of light that you got by opening the T Stop in the lens has been sucked up by the change on the speed, so the only thing that you can change in camera if you want to get a bit more of light is the ASA.

Third, ASA:

From 640ASA to 800ASA there is 1/3 of difference (the difference between these ASA numbers is 1 stop 100 > 200 > 400 > 800 > 1600)

Take a look at the following link to see the ASA Speeds and the differences between all the ASA speeds.

https://en.wikipedia...wiki/Film_speed

Hence, you end up with a setup which will give you around 1/3 or 1/2 stop more of light when you go from - T4 1/2, 24Fps, ISO 640 to - T2.8, 60Fps, ISO 800.

Is there an easy way to make this calculation? I suppose that it all comes with practice and knowing how T stops, FPS and ASA work together.

(Now I hope I haven't messed up with my calculations haha)

Have a good day.

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#3 David Mullen ASC

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Posted 27 June 2016 - 11:10 AM

I was reading the Oscillating the ISO on Alexa thread yesterday and in it David says

so I was wondering is there an easy way do to a calculation of this sort on the spot without any aid from a gadget or an exposure calculator: What is the difference in exposure between ƒ/4.5, 24 fps, ISO 640 and ƒ/2.8, 60 fps, ISO 800?

The math isn't that hard, you just have to break it down.  Now I've always found the in between f/stop numbers like "f/4.5" on lenses to be a bit confusing but I'm going to say that it means f/4.0 + 1/3, not an f/4.0 - 5.6 split.

So exposure-wise...

You went from f/4.5 to f/2.8 so you gained 1 1/3-stops

You went from 24 fps to 60 fps so you lost 1 1/3-stops

You went from ISO 640 to ISO 800 so you gained 1/3-stop

You now you're 1/3-stop of a stop brighter than before.

Even with a calculator, you'd have to convert one thing at a time.

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#4 Miguel Angel

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Posted 27 June 2016 - 05:22 PM

Now, that's why David is a fantastic teacher and I'm not!

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#5 Patrick Anton Saefkow Seaman_64263

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Posted 12 July 2016 - 10:22 PM

For shutter speed you can calculate the difference using the simple formula of a/b/2. 60/24/2=1.25 or 1 1/3 stop

Memorize the difference of common frame rates like 24, 48, 60, 120, 240 and so on, but you never know when you'll stumble upon something weird or want something specific.

Just memorize the fstop scale and you're good as far as those are concerned. The math for figuring it out is too complex for my little brain. Just know that 8 is two more than 4, and 11 is one less than 16. Not as many integers as framerates, and most shots are in the T2-T8 range anyhow. I guess magic of knowing = 1 1/3 stop

Iso is easy. Divide one unto the other. 800/640=1.25 or 1 1/3 stop

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