The difference between "f-ratio" and "aperture"

jrtrent said:

In modern days, it's still appropriate to compare formats using
factors other than equivalence in image-making

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My reply to that is here:

http://www.josephjamesphotography.com/equivalence/#purpose

jrtrent said:

but we've also reached a point where commonly-used formats are no longer
capable of being equivalent.

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Also answered in the above link.

jrtrent said:

It has only been in the digital age that I've found a use for
aperture diameter. With the absence of usable distance and depth of
field scales on the lenses, I've been turning more to Harold
Merklinger's approach to achieving the needed depth of field in an
image, and his system is based on aperture diameter.
http://www.trenholm.org/hmmerk/DOFR.html

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Since I've never shot film, my experience has only been with digital. So, perhaps with film, people did not need the use of "aperture", but, as you say, in the modern digital world, the concept of "aperture" has much relevance, and, in fact, is central.

--
--joe

http://www.josephjamesphotography.com
http://www.pbase.com/joemama/

JoePhoto said:

Does anyone know for sure what was "unique" about that lens that
allowed them to get a f/stop BELOW f/1 ??? ( f/.95 )

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I happened to come across this post purely by accident. I normally hide out in the Sony forum (have an R1).

You'll also have to excuse me for not having any theoretical optics background, I'm just a stupid truck driver. All my information comes from reading and/or using the equipment. Took my first pic in 1950.

Firstly, if I interpret this post correctly, I don't believe JoePhoto is correct in stating that f/0.95 is a stop (am assuming he means full f stop) below f/1. If you take a look at the progression of normal f stop numbers starting at say, f/4, the next one down (wider aperture in my book) is f2.8, then f/2, then f/1.4. Each second stop is half the numerical value of the previous one. Therefore the progression would continue with f1.0, f/0.7 and the theoretical limit in air of f/0.5.

As to the F0.95 lens itself, I happen to own one and have a few theories.

Canon designed that lens for a rangefinder camera called the Canon 7s. I suspect they ended up working backwards with several restrictions in order to come up with the fastest possible (standard focal length) lens for the 7s, the f/0.95. It was already going to be a fairly expensive proposition to produce so I suspect they didn't want to increase the cost (and weight) by adding an in-between reflex housing. So the lens had to work properly, and very precisely, with the cam focusing rangefinder mechanism of the 7s. That then automatically restricted the physical dimensions of the rear element. I think the restriction of the rear element also determined the physical size of the front element which has a pretty direct relationship with what maximum aperture one can achieve in the first place.

Second restriction was to make the lens physically small enough so it would not block the windows of the viewfinder and rangefinder (the matt white windows with ribbing in the first shot below were used for the highlighted outer frame and the highlighted central focusing square). They just, just made it with the rangefinder so it would be useable, but the lens actually blocked off a fair portion of the viewfinder, making composing a lot easier by using an add-on viewfinder in the accessory shoe.

The first shot I'm including shows the lens mounted on the camera.



This is the rear end of the lens. It's the only rear element of any lens I'm aware of where they had to physically remove the top portion of the glass in order to fit the brass finger that mates to the cam mechanism of the rangefinder.



Here's the relative size to two other lenses, a 35mm f1.4 Summilux for an M4 Leica, an 18mm f/4 Distagon for a 35mm Zeiss Contarex (SLR) and a Canadian penny (same dimensions as a US penny).



Cheers,
Jerry