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Why is aperture backwards??

RobertCarter124 Avatar
26 Feb 2021 9:31AM
So, I get that larger f numbers like f/22 actually mean that it's a smaller opening and that smaller f number like f/2 is a big opening. I learned that thanks to a really helpful beginner video I just watched - this video. But my question is this...WHY is the size of the aperture backwards from the numbers. Shouldn't f/22 be a big aperture since it's a big number and f/2 should be a small aperture since it's a small number???
Cephus Avatar
Cephus 18 2.7k England
26 Feb 2021 9:40AM
Suppose we have a 50mm focal length lens. If we have a big size hole – a big aperture, it might measure 25mm. So 50 divided by 25 gives us 2: the f/number is 2, which we write as f/2.

If the aperture is smaller, say, 3mm in diameter, 50 divided by 3 gives us about 16: the f/number reads f/16. As the hole is smaller, less light gets through. So f/16 is said to be a small aperture or small f/number.

That’s why you could get confused if you read about an aperture of 16 being smaller than 2: that does not make sense and is, in fact, wrong. A photographic aperture is written as ‘f/number’: it means the focal length divided by the aperture diameter. So f/16 is indeed smaller than f/2

Google is my Friend. WinkWinkWink
mervyntattoo Avatar
mervyntattoo 14 1.4k Wales
26 Feb 2021 10:22AM
Think of it as a resistance like in an electric circuit.
rambler Avatar
rambler Plus
15 1.1k 17 England
26 Feb 2021 11:34AM
I like to of it by thinking the small f numbers give a narrow depth of field or focus, or large f numbers a large depth of field

mrswoolybill Avatar
mrswoolybill Plus
16 4.6k 2635 United Kingdom
26 Feb 2021 11:52AM
As Cephus says - the F/ number is the focal length divided by the diameter of the aperture. For this reason a particular F number does not indicate a fixed size of aperture, the size varies according to the lens.
thewilliam2 Avatar
thewilliam2 6 1.7k United Kingdom
26 Feb 2021 2:29PM
The F stop doesn't actually tell us how much light will reach the film or sensor, which is why the movie world uses T stops, where the f stop has been corrected to transmission losses.
pablophotographer Avatar
pablophotographer 12 2.2k 450
26 Feb 2021 7:12PM
F denotes a fraction calculation.
The larger the denominator the smaller the result of the division, thus smaller the iris hole.
Big Bri Avatar
Big Bri 22 16.7k United Kingdom
27 Feb 2021 10:50AM
As they said ^

In other words, 1/2 is larger than 1/4

thewilliam2 Avatar
thewilliam2 6 1.7k United Kingdom
27 Feb 2021 1:17PM

Quote:As they said ^

In other words, 1/2 is larger than 1/4

Those of us who have done any teaching will know that some students don't readily accept that fact and are only convinced by a demo of cake-cutting!
Tianshi_angie Avatar
27 Feb 2021 4:06PM
Reminded me of a brilliant illustration of that fact 'thewilliam2' I was trying to get a class to understand measuring and was using the illustration of 'hands. So I asked them to stretch their fingers out amd make a mark where the tip of their little finger came and where the tip of their thumb came and then to measure it with their rulers. I then asked them to get the measurement of two hands, which they did. But when I asked what the measurement of three hands would be there was silence until a little voice piped up 'I haven't got three hands miss'.
RobertCarter124 Avatar
Thank you so much
RobertCarter124 Avatar
"WOW, so many responses! You guys are awesome, I will review your responses when I get home today!"


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