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-   -   F stops (https://e-group.uk.net/forum/showthread.php?t=46360)

 MJ224 17th October 2017 05:27 PM

F stops

Following a thread here by Paul, I was always of the opinion that f stops were arithmetical.......ie f5.6 was double or half of f2.8

When evaluating a lens, this is obviously very important knowledge......

Need educating........:)

 IainMacD 17th October 2017 05:36 PM

Re: F stops

I thought f2.8 allowed in twice the light of f4 and 4 times f5.6, but it has been a long day and I could be confused?

 pdk42 17th October 2017 05:43 PM

Re: F stops

Quote:
 Originally Posted by MJ224 (Post 429973) Following a thread here by Paul, I was always of the opinion that f stops were arithmetical.......ie f5.6 was double or half of f2.8 When evaluating a lens, this is obviously very important knowledge...... Need educating........:)
OK - I'll adopt my teacher stance and try!

The key thing to do is not to confuse f-stop and f-number.

The f-number of a lens is the inverse ratio of aperture diameter to focal length. That is, if you have an aperture of 25mm wide on a 50mm lens, then it's f2:
- f-number = reciprocal of 25/50 = 2
So, if you double that ratio - e.g. f2.8 to f5.6 then it means that you've halved the aperture diameter.

Now, the amount of light passing through the aperture is proportional to the area of the opening. Since Area = pi * r^2, if you half the aperture width then you reduce the area of the opening by a factor of four. That means that the light intensity is reduced by a factor of four too.

Now, photographic stop, or f-stop represents a halving (or doubling if you go the other way) of light. So, going from f2.8 to f5.6 means a four fold reduction in light intensity which equates to halving twice - i.e. 2 f-stops.

The progression of aperture f-numbers from f1 downwards in f-stops is thus:

f1, f1.4, f2, f2,8, f4, f5.6, f8, f11, f16

You'll see that each doubling of f-number is actually 2 f-stops.

 MJ224 17th October 2017 07:18 PM

Re: F stops

Thanks teacher Paul..:):)

Sounds a bit logarithmic to me........Will study your figures later.

Actually interesting to compare the f2.8 40-150mm to the Pany 100-400mm and to understand how much faster the Oly lens is.

Many thanks for the enlightenment........

*chr

 Wreckdiver 17th October 2017 07:55 PM

Re: F stops

The difference in the amount of light admitted (for a given exposure time) for an aperture change of 1 stop is a factor of 2 i.e half or double the amount. That means the surface area of the aperture is also double or half, which is logical: double the surface area and you let in twice the amount of light. Halve the surface area and you admit half the light. If you now do the sums you will see that for the surface area to double or half the diameter will change by a factor of the square root of 2 (1.414).

If you now look at the whole stop changes, give or take a bit, you will see that the aperture diameters change in increments or divisions of 1.414.

i.e. 2.0 x 1.414 ≈ 2.8, 2.8 x 1.414 ≈ 4, 4 x 1.414 ≈ 5.6, 5.6 x 1.414 ≈ 8.0 ....... and so on.

Steve

 OM USer 18th October 2017 01:07 PM

Re: F stops

1.414 of course being the square root of 2 and as the square root of 2 multiplied by the square root of 2 is 2, then multiplying twice by the square root of 2 is the same as multiplying by 2.

 RobEW 18th October 2017 02:05 PM

Re: F stops

Yes, if you want to let in twice as much light, the three common ways are:

Expose for twice as long a duration
Stop down by one stop (e.g. from f4 to f2.8, or f2.8 to f2, or f2 to f1.4 etc)
Double the ISO speed (e.g from 100 to 200, or 200 to 400, or 400 to 800 etc)

Comparing lenses with different focal lenghts is a rather different matter ...

 MJ224 18th October 2017 03:53 PM

Re: F stops

Quote:
 Originally Posted by OM USer (Post 430031) 1.414 of course being the square root of 2 and as the square root of 2 multiplied by the square root of 2 is 2, then multiplying twice by the square root of 2 is the same as multiplying by 2.
Can you put that another way........*chr

:D:D:D:D

 Gwyver 18th October 2017 05:31 PM

Re: F stops

Quote:
 Originally Posted by RobEW (Post 430038) Yes, if you want to let in twice as much light, the three common ways are: Expose for twice as long a duration Stop down by one stop (e.g. from f4 to f2.8, or f2.8 to f2, or f2 to f1.4 etc) Double the ISO speed (e.g from 100 to 200, or 200 to 400, or 400 to 800 etc) Comparing lenses with different focal lenghts is a rather different matter ...
Almost, but not quite. Just to be pedantic, whilst the 1st two methods capture twice as much light, doubling the ISO speed merely increases the sensor amplification - hence also increasing the noise being captured.

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