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Forum: Pentax SLR Lens Discussion 11-05-2014, 09:56 PM  
How is F-ratio calculated?
Posted By Dom543
Replies: 3
Views: 1,105
Hi All,

I am a new member and this is a conceptual question. I am not sure, if this is the right forum to ask it.

We learned in school in the old days that f-ratio is focal length divided by aperture. It can also be considered as a measure of the angle-width of the light cone at focus.
Since camera lenses always have their focal lengths and f-ratios specified, their aperture can be calculated from the above formula as aperture = focal length / f-ratio.
With the simple mind of the old-time schoolboy, one would expect that the diameter of the front lens would be then the number that we got as aperture in the previous sentence.

I have just looked at my Takumar collection and made the following interesting observation.

For lenses up to about 1970 (Takumars and Super-Takumars) the diameter of the front lens is close to the aperture number obtained from the above formula.
But then, at about the time of the introduction of the SMC Takumars, the front lenses suddenly became dramatically smaller.

Could someone please explain me what happened? Or more importantly, how should I update my knowledge? What is meant by the f-ratio these days? How is it related to the geometry of the optics?

Thank you,
--Dom
Forum: Welcomes and Introductions 11-05-2014, 04:11 PM  
New member from Seattle/Boston
Posted By Dom543
Replies: 8
Views: 573
Thank you all for the welcome!

The post above was my very first one on Pentax Forums and needed to be approved by a moderator. I didn't want to risk instant denial by admitting that I own only Pentax optics but no Pentax cameras. But my very first 'serious' camera was a 35mm Practika from Pentacon, Dresden. I believe that it was an East German copy of a Pentax camera. It had a 42mm screwmount and the two Pentacon lenses that I still have ( 50mm and 35mm primes) work perfectly well in M42 adapters. They may have been subconscious factors predestining me on the Takumars...

Cheers!
--Dom
Forum: Welcomes and Introductions 11-04-2014, 09:05 PM  
New member from Seattle/Boston
Posted By Dom543
Replies: 8
Views: 573
Hi,


My name is Dom. I live in Seattle and work in Boston.
I have a small collection of Takumar lenses and a couple of Pentax eyepieces.
I have been visiting this website for useful information about lenses and have just signed up officially.


Cheers!
--Dom
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