f stops - Please excuse my ignorance!!

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Dave
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I'm reading an article at the moment on portrait photography and it says ''for a lighting ratio of 4:1 you need to obtain an aperture of f/8 ie: two stops less than the f/16 provided by the key light''.
Now looking at my camera settings from f/8 I have f/9; f/10; f/13; f/14 etc
so what are the main f stops please as there are more than 2 f stops between f/8 and f/16
the answer to this first part may also answer my 2nd question - the article goes on to say when trying to get a white background...''meter the subject for say f/11 then aim to get the background at f/16 ie: the background receives one stop more light than the subject''.
Would really appreciate some clarification on the range of f stops and lighting ratios!!
Thanks
Dave
 
No, there are not more than two f stops between f8 and f16.

An f stop equates to a doublng or halving of the amount of light let through.

What you are looking at is 1/3 stops, figures in dark red are normal f stops:

f no: 1.0 1.1 1.2 1.4 1.6 1.8 2 2.2 2.5 2.8 3.2 3.5 4 4.5 5.0 5.6 6.3 7.1 8 9 10 11 13 14 16 18 20 22
 
The full f-stop series is: 1 / 1.4 / 2 / 2.8 / 4 / 5.6 / 8 / 11 / 16 / 22 / 32 / 45 / 64 / 90

As Ed says, anything between these is less than a full f-stop - e.g. f9 and f10 are thirds of a stop away from f8 and f11.
 
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The word stop comes from when camera lenses only had the "main" apertures.

The f stops i.e. f2, f5.6 etc are determined by dividing the focal length of the lens by the diameter of the actual stopped down hole so if you had a 50mm lens and a 50mm diameter hole you would have an f stop of 1. If you had a 50mm lens and a 25mm diameter hole you would have an f stop of 2.

However a 50mm diameter hole will let in 4 times the light as a 25mm hole as the area of the circle is increades by a factor of 4 when the diameter is doubled. Therefore the 25mm hole will give 2 stops less than the 50mm hole.

The main f stops therefore are: 1, 1.4, 2, 2.8, 4, 5.6, 8, 11, 16, 22, 32 etc.
The funny numbers are due to the equation for determining the area of a hole from its diameter.

You can see that each number is twice the number 2 places to its left and half the number 2 places to its right. By starting at 1 it is easy to get the series but I have never seen a lens with f1 !

Each movement one stop to the left doubles the area of the hole and lets in twice the light. Each movement one stop to the right halves the area of the hole and lets in half the light.

Modern lenses and cameras allow half and third stops which is why your camera may show f9, f13 etc.

Fortunately they also allow shutter speeds in between the standard ones. Just for completeness the shutter speed stops can also be worked out by starting at 1 sec.
You can double that to give 2 secs, then 4 secs, 8 secs etc.
Or halve the 1 sec to give 1/2 sec, then 1/4 sec, 1/8 sec, 1/15 sec, 1/30 sec etc.
Each change in this series obviously leaves the shutter open for twice or half the time.
 
Dave - have a look at the table on this page (scroll down).

http://www.uscoles.com/fstop.htm

The actual physical area of the aperture (shown in the right column) roughly doubles for each of the full f-stops in the series (left column).

NB. Table refers to a 50mm lens.
 
Edit: oops! Crossed post!

A 'stop' used to refer only to lenses, but has come to be known as any halving or doubling of the exposure. It's just easier to say than 'One Exposure Value'.

While the relationship between shutter speeds and ISO numbers is obvious, the relationship between lens stops - f/numbers - is the square root of two, ie 1.414. So f/8 to f/16 is two stops, quarter the light.

Your camera can be set to display third-stop or half-stop increments, when the intermediate numbers will change, but the full stops will remain as posted above by Fast Eddie and Slow Oz :)
 
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Thanks guys for the quick replies and really, really, appreciate the detailed explanations I'm going to copy and paste this info. for future use.
Will check out the link and thanks again everyone for your kind help.
Dave
 
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