focal length maths question

[simonSE15]

hi all

I was looking to devise a maths activity to do with photography and was thinking about doing it on focal length.

I got the idea from here:
http://www.mrmartinweb.com/photomath.html

It's the formula I'm interested in: f-stop =focal length / diameter.

In theory easy enough to understand but a quick look at my Zuiko 24mm/2.8, 50mm/1.4 and 85mm/f2 suggest that aperture diameter of 85mm is not wider than the 50mm at same f-stop (e.g both at f4 85mm diameter is smaller).

Why is this?

thanks,

Simon.

 

[ferider]

Maybe because you look at the aperture through front or back elements that distort the real dimensions ?

Best,

Roland.

 

[simonSE15]

ok now I feel really stupid 😱

 

[ferider]

Don't - it was a good and non-obvious question, IMO.

Some great photos on your flickr site.

Cheers,

Roland.

 

[Harold Gough]

You will find lots of such discussion here, but much will mainly be relevant only to close-ups/macro.

http://www.photomacrography.net/forum/index.php