F-stop numbering? Wide f-number range images?

I understand f-numbers (though far from having an intuitive grasp of what settings to use). However, I don't really understand the numbering scheme.

My A620 has f2.8, 3.2, 3.5, 4.0, 4.5, 5.0, 5.6, 6.3, 7.1 and 8.0

2.8 means the focal length is 2.8 times the aperture,right? Assuming the above are "full" stops (or was the term "whole"?), how does that work? I would think full f-numbers would be: 1.0, 2.0, 3.0 etc.

Oh, perhaps it is not the ratio that is being judged full or whole. Is it the amount of light? 2.8 is twice the amount of light as the next full f-number? This is just a guess based on the assumption that it really does make sense somehow, just not to me as of yet.

Also, terminology: f-stop, f-number, whole, full etc. I am unclear on the correct terms (in case you didn't notice already). I'd appreciate any clarification.

Lastly, I've read about blending photographs that are exposed to differently so that you can capture details in both bright and dark areas. Makes sense. However, how is the range determined? How would I measure this? Do I just use spot metering and point it at the brightest and darkest areas to determine the range the scene covers?

Thank you.

Well, the last thing you talked about is called "HDR," and that is used by using different shutter speeds, not apertures. The best way is to bracket the scene, and then merge the bracketed files in an HDR program.

This page will answer all of your questions about the f-stop and more:

http://www.uscoles.com​/fstop.htm

enjoy!

Nice link Bob!

For someone who has spent the last 10 years using 35mm compact and digital p & s it's a good refresher.

I even got my old fully manual SLR out the attic and had a wee practice!!

Can't wait for my insurance policy to mature to get my hands on dSLR.

Thanks

The site Bob A linked is very good and I look forward to checking it out in detail, but he goes into a technical explanation that zips right past the reason f-stops are approximately multiples of 1.4; that is because as pointed out in the link the area of the aperture is based on the square of the radius and you want to double or halve the light with each stop in order to match up with the ISO and shutter speeds, so the aperture progesses in increments of the square root of 2, or approx. 1.4

The f-stop series on your A-620 is in one-third stops. The standard f-stop series is 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32, 45, 64, 96, 128, and so on, with each value representing half the light transmission of the value preceding it.

The explanation for this series isn't complicated. The amount of light transmitted is controlled by the area of the opening, but the F-number refers to the diameter of the opening. To get the area, you divide the diameter by 2, square it, and multiply the result by pi. Pi is a constant and so is 2, so they affect all settings equally and don't affect comparisons as you move along the scale. The only part of that relationship that changes as you move along the scale is the power of 2.

Thus, when you double the area, the diameter increases by the square root of 2. When you halve the area, the diameter decreases by the same factor. The square root of 2 is 1.4, or thereabouts. When you multiply the diameter by 2, (say, to go from f/8 to f/4, or from f/2.8 to f/1.4), you change the area by 2 to the power of 2, which equals 4.

For example, f/4 times 1.4 is f/5.6, near enough. Thus, f/4 has twice the area of f/5.6, which is why it admits twice the light (i.e., one stop). Going 2 stops, as from f/4 to f/8, halves the area twice. 4 times 1.4^2 is the same as 4 times 2, which equals 8. So, doubling the diameter increases light transmission by a factor of four, which is two stops.

There is more fun to be had. Since light transmission is controlled by the area of illumination, the f-stop series works for lighting distances, too. If I have a studio light that is too bright by a stop, I can move it 1.4 times the distance away from the subject. If it happens to be 4 feet away from the subject, I can move it to 5.6 feet away and it will put a stop less light on the subject. Moving it 8 feet away puts two stops less light on the subject. If I want a 2:1 ratio between main and fill lights, I just make sure the fill light is farther away by one notch on the common f-stop scale. That way, I don't have to multiply by 1.4 in my head.

Rick "fun with geometric progressions" Denney

rdenney wrote in post #2167912
The f-stop series.....

.....Rick "fun with geometric progressions" Denney

Fantastic answer This should be put in to a sticky somewhere.

Wonderful answers!
I'm now off to read that link...
Thanks