stupid numbers

[Josh Bass]

Ok, according to some photography article online, these are the fstop numbers, expressed in whole stops? f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45 and f/64.

So, I have an XL1s. My widest aperature, at least with the manual 16x lens, is 1.6. I guess that's my version of the 1.4? And of course, no 1.

So these in between numbers, like 1.8, 2.2, 2.4, 2.6 etc. . .what are those, exactly? 1/3 stops? 1/2 stops? After you get to 2.0, there seem to be three "tweenies" between each successive whole stop. Of course, it closes down completely after f16.

I want to seem less ignorant about all this stuff. "Open up a stop Josh. Close down half a stop, Josh." "What the hell do you mean?"

[Jeff Donald]

The whole stops are exactly what you said:

f/1, f/1.4, f/2, f/2.8, f/4, f/5.6, f/8, f/11, f/16, f/22, f/32, f/45 and f/64.


1/2 stops are:

f/1.2, f/1.7, f/2.4, f/3.5, f/4.9, f/6.9, f/9.8, f/13.8, f/19, f/27, f/38, f/54

1/3 stops are:

f1.1, f1.3, f1.6, f1.8, f2.2, f/2.5, f3.2, f/3.5, f/4.5, f/5, f6.3, f/9, f/10, f/13.5

f/1.6 is f1.4 plus 1/3 of a stop. The plus here gets very confusing, because it usually means you're adding something (well you are, 1/3 of a stop), but remember, the larger the numerical number, the smaller the aperture and the less light. So by adding stops your making the scene darker and by subtracting stops the scene is getting lighter.

Open up a stop is letting one stop more light in. In other words, "Josh, change the aperture from f/4 to f2.8." You have opened the aperture one stop and doubled the amount of light passing through the lens.

[Josh Bass]

Still seems confusing. Suppose I'll just have to memorize, unless I want to do logarithmic calculations in my head on the spot. Thanks.

Also, what's the deal with the XL1s, then? I have no 3.5, but I do have a 3.6? no 1.7, but a 1.6?

[Jeff Donald]

There are different methods for rounding. Some numbers have been used historically even though they are slightly off the true value.

[Richard Alvarez]

Josh,

It might be a good idea to take a basic still photography course. There you will learn the basics of exposure controll and the relative benefits of different shutter/aperture combinations. These factors are as important in video as in still or motion picture photograpy.

Besides, it's a lot of fun!

[David Mintzer]

Josh--memorize!

[Josh Bass]

Thanks, guys.

[Aaron Koolen]

Also Josh, at the risk of explaining the bleeding obvious, you'll notice that some fstops stick to simple maths - every second number is double the one two numbers before it.

1, ?, 2, ?, 4, ?, 8, ?, 16, ?, 32 etc etc (Every 2 stops increase quadruples the light)


Aaron

[Josh Bass]

And each single increment: 1 to 1.4 for example, halves/doubles it?

[Aaron Koolen]

Yes - well sort of ;)

As Jeff said, they are not necessarily that accurate so I'd imagine you'd have to memorise them but if you look closely you'll see that they use is a root system. (I'm new to this myself, so hope I can explain accurately here)

You'll notice that to find the values between the whole numbers (every second stop) you are in effect multiplying the square root of 2 (1.414 rounded) by the fstop before it.

So to find the fstop after f1 you go
square root of 2 (1.414) * 1 which is obviously 1.414

To find the fstop after 4 you go
1.414 * 4 which is the 5.6 (Well 5.656 etc)

And so on like: 1.414 * 16 = 22.6 (22)


If you want to find partial stops then you just take the 4th root (For 1/2 stops) and 6th root for 1/3 stops etc. Example:

4th root of 2 is 1.189, so the 1/2 stop above the full ones (Or in fact any stop you already know) are..

1.189 * 1 = 1.189 = half stop above f1
1.189 * 2 = 2.378 = half stop above f2
1.189 * 8 = 9.514 = half stop above f8



Hope this was clear enough (And accurate enough ;) )

Aaron

[Josh Bass]

Makes more sense as I learn more.

[Norm Couture]

Thanks Aaron!
I learned something here. Again.

[Dennis Vogel]

<<<-- Originally posted by Richard Alvarez :It might be a good idea to take a basic still photography course. There you will learn the basics of exposure controll and the relative benefits of different shutter/aperture combinations. These factors are as important in video as in still or motion picture photograpy. -->>>

Yes! Yes! I recommend this to people often. I came up with 35mm still photography and much of what I learned is, indeed, applicable to digital video.

Go to your local library and pull out a dusty volume and sit and read. Even if it was written 50 or more years ago, the physics of what is happening is still valid. You'll learn a lot that you can employ as you shoot digital (or whatever is next!) video.

Good luck.

Dennis Vogel

[Barry Green]

There are really only two numbers you need to remember: 1.0 and 1.4. All the stops are basically derived from those two, by doubling the numbers.

So you have:
f/1.0 f/1.4
f/2.0 f/2.8
f/4.0 f/5.6
f/8.0 f/11.0 (rounded off from what would have been 11.2)
f/16.0 f/22.0


Each new f-stop represents half the light transmission. So going from f/4.0 to f/5.6 means the lens is letting in half as much light (and that's referred to as "stopping down"). Going from f/2.8 to f/2.0 would let in twice as much light (and is called "opening up").

The numbers also correlate to the inverse square rule of light intensity, which basically says that if you double the distance from a light to a subject, the light intensity falls to 1/4 of what it was. So to get a light to be half as intense, you move it about 40% further from the subject. That 40% = 1/2 ratio corresponds to the f/1.0 to f/1.4 relationship. Each f-stop number is about 40% bigger than the previous one (f/1.4 plus 40% = 1.96, which rounds off to 2.0, etc).