Apature value & f/stop

[Bluelens]

Ok, this one has me scratching my head.

What is the difference between apature value and f/stop? I thought that they were one in the same but after a recent testing of my lens it doesn't appear so.

When looking at the EXIF info on some pictures I took last night I noticed that the Apature Value was always less then the f/stop. The pictures in this thread (http://photography-on-the.net/forum/showthread.php?t=33549) shows what I am talking about.

For example, I noticed that on a lens I had an f/32 yet the Apature Value was 10 on one lens ansd 10.3 on another. f/29 the Apature Value was 9.7 and on f/5.6 it was 5.0

What am I not understanding?

[PacAce]

Ok, this one has me scratching my head.

What is the difference between apature value and f/stop? I thought that they were one in the same but after a recent testing of my lens it doesn't appear so.

When looking at the EXIF info on some pictures I took last night I noticed that the Apature Value was always less then the f/stop. The pictures in this thread (http://photography-on-the.net/forum/showthread.php?t=33549) shows what I am talking about.

For example, I noticed that on a lens I had an f/32 yet the Apature Value was 10 on one lens ansd 10.3 on another. f/29 the Apature Value was 9.7 and on f/5.6 it was 5.0

What am I not understanding?

Well, aperture value and f-stop are very closely related but, as you discovered, they're not exactly the same although a lot of people, including myself, mistakenly use the two interchangeably.

Aperture value (Av) is actually an indication of the amount of light that passes through a lens. The f-stop, on the other hand, indicated the size of the opening, relative to the focal length of the lens, of the aperture that is used to control the amount of light (Av) that goes through the lens.

And, like I said earlier, there is a direct relationship between the f-stop and the aperture value as indicated by the following fomula:

Av = log2(f-number**2)

where f-number is the f-stop number such as 2.8 or 5.6.

Or, in words, the aperture value is equal to the log base 2 of the f-stop number squared.

And hence the reason f/32 has an Av of 10 and f/5.6 has an Av of 5.

And, to complete the picture, time value (Tv) is given by the following formula:

Tv = log2(1/t)

where t is the shutter speed in seconds.

And Exposure value (Ev) = Av + Tv

Hope that helped and not confused you even more.

[CyberDyneSystems]

... /stuff I did npot know...

We use the f/stop almost exclusively.. and I THINK this is why...

The F/stop takes into account the other physics of the lens.. (ie focal length)

So.. when we compare f/stops from one lens to another we have apples to apples as far as working out exposure... if we use AV instead.. the exposure numbers become specific to the lenses focal lenght?

So the f/stop number is much more handy to work with and remember ...

Is this correct?

[maderito]

Is this correct?
No.

Perhaps more intuitive is this formula:

f-stop = (sqrt of 2)**N (where N = aperture value)

that is, the f-stop is the square root of 2 raised to the Nth power, where N is the aperture value. This is equivalent to the formula given by PacAce.

The focal length of the lens only comes in when you are computing the actual size of the aperture for a given lens.

[SoCal69]

I know I get all confused when someone starts talking formulas. However, the simple explanation of why the aperature value shows up as "10" at f/32 is simple to explain:

The "Aperature Value" is simpy the number of times the aperature area was halved. In other words, if your lens had actual full f-stop clicks, how many clicks from f/1 to f/32. Going from 1 to 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 is 10 clicks or 10 full stops. Hence your aperature value is 10 for f/32. Seems to me that it isn't really all that helpful when the f-stop is already given, but there it is.

Hope this helps.

[Tom W]

I know I get all confused when someone starts talking formulas. However, the simple explanation of why the aperature value shows up as "10" at f/32 is simple to explain:

The "Aperature Value" is simpy the number of times the aperature area was halved. In other words, if your lens had actual full f-stop clicks, how many clicks from f/1 to f/32. Going from 1 to 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 is 10 clicks or 10 full stops. Hence your aperature value is 10 for f/32. Seems to me that it isn't really all that helpful when the f-stop is already given, but there it is.

Hope this helps.

I don't mind formulas, but I hate ASCII characters when they're used to show formulas. I'd rather write it down and then take a picture of it.

BTW, nice explanation, Chuck. Frankly, I prefer the f-stop.

[PacAce]

I know I get all confused when someone starts talking formulas. However, the simple explanation of why the aperature value shows up as "10" at f/32 is simple to explain:

The "Aperature Value" is simpy the number of times the aperature area was halved. In other words, if your lens had actual full f-stop clicks, how many clicks from f/1 to f/32. Going from 1 to 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 is 10 clicks or 10 full stops. Hence your aperature value is 10 for f/32. Seems to me that it isn't really all that helpful when the f-stop is already given, but there it is.

Hope this helps.

Yes, a very good point, Chuck when working with full stops. Unfortunately, it doesn't help much when working with stops in 1/2 and 1/3 increments such as f/4.5 or f/7.1.

[stopbath]

I know I get all confused when someone starts talking formulas. However, the simple explanation of why the aperature value shows up as "10" at f/32 is simple to explain:

The "Aperature Value" is simpy the number of times the aperature area was halved. In other words, if your lens had actual full f-stop clicks, how many clicks from f/1 to f/32. Going from 1 to 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 is 10 clicks or 10 full stops. Hence your aperature value is 10 for f/32. Seems to me that it isn't really all that helpful when the f-stop is already given, but there it is.

Hope this helps.

Yes, a very good point, Chuck when working with full stops. Unfortunately, it doesn't help much when working with stops in 1/2 and 1/3 increments such as f/4.5 or f/7.1.
Still helps... For 1/3 stop use .3, for 1/2 stop use .5, for 2/3 stop use .7... thus 9.7 = f29 (1/3 less than f32)..

[SoCal69]

And that is why I ignore the "aperature value" that is in the EXIF info and just stick with the f-stop value. I was just trying to explain where the value of "10" came from.

[Bluelens]

Wow, well this has been quite an educational posting for me. Thanks a lot everyone. Now tell me again why I should go out and buy books when I have such a resource here :wink:

[stopbath]

Wow, well this has been quite an educational posting for me. Thanks a lot everyone. Now tell me again why I should go out and buy books when I have such a resource here :wink:
Cuz they look pretty on the book shelf, and people get impressed... And... and... ah...

[Bluelens]

Wow, well this has been quite an educational posting for me. Thanks a lot everyone. Now tell me again why I should go out and buy books when I have such a resource here :wink:
Cuz they look pretty on the book shelf, and people get impressed... And... and... ah...

HAHA, well ok you have a point there. :lol: