Full stop, half stop... bus stop?

[luke]

Hi everyone,

Great forums. I've already picked up a lot of info just reading, now I thought I might post.

I picked up a 400D w/18-55 on the weekend, which has been a lot of fun so far. No keepers as yet really, but I'm still enjoying it.

I also picked up an EF 50mm 1.8II, which is also great.

Now, to the question (finally!).

When I cycle through the apertures (with the 50mm), it goes:

1.8 2.0 2.2 2.5 2.8 3.2 etc.

Are those each a full stop? ie. 1.8 -> 2 or 2.5 -> 2.8

Can someone refresh my memory. I've kinda forgotten whole 'stop' business, it's been a long time since I've used an SLR. Close to 10 years I'd say. Anyway, thanks for the help.

cheers,

Luke

[rooboy]

2, 2.8, 4, 5.6, 8, 11, 16, 22...

These are full stops. The numbers you quoted are 1/3 of a stop increments.

Have fun with the new toy

[Andyt]

Hi! Luke,

Welcome and congrats on your new "toy"

You may find this of use: http://en.wikipedia.org/wiki/Aperture

I am not faimiliar with the 400D but there would be provision for you to "push" or adjust the selected aperture setting in + or - in 1/3 increments (refer manual). This in effect allows you to fine tune the exposure.

It may also be usefull for you to Bracket your pics. i.e. say F5.6 with a + 1/3 push. With the camera set like this and a continuous shooting mode, you will be taking 3 x pics when the shutter is released, one will be F5.6 less 1/3 of a stop, one right on F5.6 and one at F5.6 plus one third of a stop.
This "spread" will give you three differently exposed pics, and having this range the chances of a "keeper" that is correctly exposed is greater.

Practice, Practice and then Practice some more!!

Look forward to seeing your pics!!

[luke]

Cool thanks guys.

It actually came to my mind when I was playing with the bracketing.

I was wondering how it could bracket +-1/3 if they were full stops.

Should've been obvious I suppose.

Thanks again.

cheers,

Luke

[Antsl]

Hi Luke,

The f numbers are mathmatically derived numbers; they are a ratio of the front element of a lens divided into the focal length of the lens. With this in mind a 50mm lens with a 50mm wide piece of lens element on the front will have an maximum aperture of f1 while a 50mm lens with a 25mm element on the front will have a maximum aperture of f2. Because a 25mm diameter lens also has a quarter of the surface area of the 50mm lens it will also only let in quarter of the amount of light.

This is where the f1.4 comes into the equation.... a 35.7mm wide front optic would let in half the amount of light into the camera as the 50mm wide f1 lens but twice as much light as the 25mm wide f2 lens.

This is where this set of numbers comes from. If you have a look at the full f stops on the lens you will notice they are all multiples of 1.4 (1, 1.4, 2, 2.8, 4 etc.). 1.4 is also the approximate square root of 2. Every time you increase the f stop by a factor of 1.4 you are letting half the amount of light into the camera however you are also giving yourself more depth of field. Decrease the f-stop value by 1.4 and you are letting twice as much light into the camera and decreasing the depth of field.

All lenses, no matter who makes them or what camera they are for will have a common set of whole f-stop numbers. Learn these numbers and what they do and you will be half way to understanding how to drive any camera, anytime, anywhere.

[luke]

Excellent.

Great explanation Antsl

I was aware it was the relationship between focal length and aperture diameter, although that solidifies a few previously hazy points.

It's all a lot clearer now I know what the full stops are.

It appears as though the lens performs better at full stops, although that may just be a fluke. When stopped down, it is definitely sharper, and it also seemed to like being at the full stops. This is just from some test shooting I was doing in my back yard. I guess characteristics are different for each individual lens, when it comes to thing like that though.

thanks again.

cheers,

Luke

[Yi-P]

As all being said above,

Regarding the numbers, all you need to remember are two numbers:

a) 1 (common factor of all real numbers)
b) 1.4 (the square-root of 2)

The rest are just power of 2 multiplication(2^0, 2^1, 2^2...). Then apply the long lost primary school math.

1x1 = 1
1x1.4 = 1.4
2x1 = 2
2x1.4 = 2.8
4x1 = 4
4x1.4 = 5.6
8x1 = 8
8x1.4 = ~11
16x1 = 16
16x1.4 = ~22
32x1 = 32
32x1.4 = ~45

etc... etc.... you will see the relationship and pattern there...

[johnd]

Antsl, your explanation is probably the best I have ever read that explains the relationship between f stop and amount of light. Thanks for helping to make it clearer to me.

Mods, I think this should get "Explanation of the Week" recognition.

Regards
John

[norbs]

Hi Luke,

The f numbers are mathmatically derived numbers; they are a ratio of the front element of a lens divided into the focal length of the lens. With this in mind a 50mm lens with a 50mm wide piece of lens element on the front will have an maximum aperture of f1 while a 50mm lens with a 25mm element on the front will have a maximum aperture of f2. Because a 25mm diameter lens also has a quarter of the surface area of the 50mm lens it will also only let in quarter of the amount of light.

This is where the f1.4 comes into the equation.... a 35.7mm wide front optic would let in half the amount of light into the camera as the 50mm wide f1 lens but twice as much light as the 25mm wide f2 lens.

This is where this set of numbers comes from. If you have a look at the full f stops on the lens you will notice they are all multiples of 1.4 (1, 1.4, 2, 2.8, 4 etc.). 1.4 is also the approximate square root of 2. Every time you increase the f stop by a factor of 1.4 you are letting half the amount of light into the camera however you are also giving yourself more depth of field. Decrease the f-stop value by 1.4 and you are letting twice as much light into the camera and decreasing the depth of field.

All lenses, no matter who makes them or what camera they are for will have a common set of whole f-stop numbers. Learn these numbers and what they do and you will be half way to understanding how to drive any camera, anytime, anywhere.

Thats needs a special place on the internet. I have read quite a few explanations, but none as easy to understand as that. Well done.