Focal length 35mm equivalent, but not F-stop?

Started Jul 24, 2014 | Discussions thread
Lee Jay Forum Pro • Posts: 55,139
Re: Focal length 35mm equivalent, but not F-stop?

eyeswideshut wrote:

I take it you see my point then? A manufacturer - as well as any run of the mill photographer - using fl as short hand for angle of view simply need not worry about all the ramifications of creating equivalent images.

That depends.  If you're like me and commonly shoot with many different formats, sometimes all at the same time, it's a good concept to keep handy especially if you want a consistent "look" (noise, mostly) from several cameras shot at the same time in the same conditions but with different sensor sizes.

And the beauty of f-stops is that they are already expressed as a ratio.

Which is relatively meaningless now, since we have in-camera metering, the ability to adjust ISO real-time, and more people using more available formats.  When cameras were manual and everyone used 35mm film or just one medium format, keeping f-stops in mind was much more important than it is now.

Only if manufacturers were to suggest that say a bridge camera with a small sensor and some wild super zoom could create equivalent images to some significantly larger sensored camera would they actually mislead.

That's exactly what they do when they say stuff like "25-600mm f/2.8 lens".

But the fact is they don't.

Yes, they do.

Equivalists simply impute that nasty intention - usually out of a samaritan impulse to save a newby from dire straits

I've had people tell me straight up that their 400mm/2.8 lens on their hyperzoom was just as good as a $10,000 400/2.8 at only $400 and with a camera included.  Of course, their hyperzoom doesn't have a 400mm/2.8, it has a 72mm/2.8.

Regarding telescopes I have no experience with them. 11" pretty nearly equals 2800mm


so I don't know which you call it - but it seems to be the same thing.

No, one is focal length, one is aperture.

Equally, I don't know what typical apertures are on telescopes, but if it is indeed f/10 then a diameter of 2800/10 = 280mm = 2,8 cm should follow.


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