How big would a 250mm f/2 stabilized lens be for a CX formfactor?

Started Oct 20, 2012 | Questions thread
rmpossible
Forum MemberPosts: 81
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Re: Do you mean a real one?
In reply to Leonard Migliore, Oct 22, 2012

Leonard Migliore wrote:

A 250mm f/2 is going to be the same size if it covers FX, DX or CX. You don't save anything by limiting it's coverage. It's going to have a front element with a diameter over 125mm and a bunch of heavy glass behind it. It will, in fact, look like a 200mm f/2 only bigger. This is known as big and heavy. It will also cost like a 200mm f/2 only more. This is known as horribly expensive.

A 90mm f/2 is going to be the same size as the 85mm f/1.8G, again regardless of the field that it's going to cover.

I am indeed looking for a CX with an actual focal length of 250mm.   I have been arould long enough to know the expensive and heavy part.  I have been around photography for over 35 years(arghhh, has it been that long?) and have shot the 400, 500 and 600mm Nikon offerings on everything from a D50 to D300s to D3s.

Leonard, I mean no criticism nor insult, but I wonder if such an absolute number works.  Maybe I am missing something   DX lenses are quite a bit smaller than FX lenses.  Just take a look at what we know,  a 55-300mmVR DX f/4.5-5.6 lens is 4.8" long and around 19oz and a 58mm filter size; while a 70-300mmVR FX f/4.5-5.6 is 5.6" long and weighs 26.3oz. with a 67mm filter size.  So, maybe I am presuming a bit too much, but the front eliment is significantly smaller on the 55-300 than it is on the 70-300.  The difference is about 13% give or take a bit.

I know there are a lot of variables with zooms that I don't understand, but I was under the understanding that the front element needs to be at least big enough to support the maximum focal length.

We gauge everything by the 35mm field of view; but with DX, 4/3 and CX we have smaller and smaller fields of view which negates the need for the larger eliments.  Near as I can tell, a 250mm f/2 lens would give me a field of view equivalent of 675 and a maximum f/5.6 give or take a bit. (225mm probably would be OK as that is closer to the FOV of 600mm).  Point is, the lens doesn't quite need all that extra glass.  I know this lens would be big, but I am wondering if it would be as big as so many folks claim.

Of course this is theoretical, because I seriously doubt that this lens will ever be made.  But I suspect that if made, the lens would not be as big as a 125mm front element.  Anybody out there, I would be interested as to why or why not this would work.

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Roger
(old DPreview ID: rmcovering)

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