Aperture values and F Stops

CDS will figure it out soon. We had slipped a random key generator into his keyboard long ago. But then the way he types, he can overwhelm it and his text comes out readable.

---Bob Gross---

It may be worth adding here just as a way of clarifying the issue mathematically, that the number following the f/ is the effective diameter of the lens as a fraction of the focal length so at f/4 the aperture would be 1/4 the focal length. Since the amount of light reaching the CD or film varies as the square of the diameter of the aperture (you could think of it simply as the area of the aperture) and one stop halves or doubles the exposure, you can see that each next stop should be a diameter ratio of approximately 1.414 (square root of 2) times the diameter of the previous one. I guess this means that by common agreement over many years, "STOP" is actually a mathematical term rather than a mechanical term. I can see this may be clear as mud but it's the best I can do.

This explains it all:

A Tedious Explanation of the f/stop

Who brought maths into photography?


I just know that x amount of light requires z amount of shutter speed, and y appature to get correct exposure.
I set them, take a test, check the histogram, adjust if needed, then set either apature or shutter speed to suit whats more important and adjust the other the corrosponding amount.
More apature, faster shutter and vice versa.

And I always shoot manual, unless Im to drunk.


Basicly understanding the Maths isn't important for understanding exposure.

Then someone comes along and introduces i, also known as ISO.

Every scene is lit by light of a certain exposure value. And for that particular exposure value, there are combinations of apertures and shutter speeds that will give you the correct exposure for a particular ISO.

For any given aperture, there is a specific shutter speed that needs to be used to get the proper exposure. And vice versa, for a given shutter speed, there is a specific aperture that needs to be used.

If either the shutter speed or the aperture is not right for the effect you want in the picture, then you have to either change the ISO so that you can get the right combination of aperture and shutter speed or you have to change the exposure value with the use of artificial lighting, if too dark, or neutral density filters, if too bright.

So I take it no-one can just walk out of their house, look at the sky, look at their subject and know exactly which exposure settings to use ?

Gary.

Well, there's always the "Sunny 16" rule - that'll get you close, but there are a lot of variables that you might want to consider when shooting that can raise or lower your required exposure.

Why do you say that? Pros who's life is photography, especailly nature and landscape photographers, usually have a good idea of exposure for a particular subject because they've been doing it so long. Someone (I forget who off the top of my head) posted a thread recently that pointed to a site which contains a table of EVs and the "typical" scenarios representative of that EV. Take a look at it. It's interesting stuff.

http://www.fredparker.​com/ultexp1.htm

The chart is towards the bottom of the page.

quickben wrote:
Thanks alot, Scottes.

I had an idea, just wanted to clarify it. Although I didn't know that one stop either way can halve or double the light. I still can't use the camera in fully manual mode. I'm scared of it How do you gauge which settings to use ?

Cheers.

Gary.

I flip a three sided coin.

And then there's always the changing amount of water vapor in the air; this is a constantly changing value, as amateur astronomers know. So one day the sky may be a clear dark blue, other days it may be much whiter in hue even though technically clear. Also the illumination of the sun is a distinct factor when at lower angular altitudes. And then if you're in the shadow of a mountain, ... Hmm, it's not so simple!

(Never bring beer to a star party!)