Re: Realist vs. 4"x5"

[P3D John Bercovitz <bercov@xxxxxxxxxxx>]

BobH writes:

> I think John B. has it backwards..the large format has poor 
> resolution of lenses and depends on its fine definition and 
> gradation on little enlargement of the film..near contact print. 

Actually, the problem of lower angular MTF of large format lenses 
was what I was trying to talk about when I said:

> PS: The plate resolution of a lens is limited by physical 
> optics (that's wave interference and stuff like that there) to 
> somewhere around 1400 to 1800/(f/no) for the number of lines 
> per mm.  But there are other problems which I'm not completely 
> clear on that make it more difficult to approach this limit as 
> the lens is scaled up.  So it's not all gravy for the lens 
> designer if he  moves up to 4x5 or 8x10 format.

BobH writes:

> The Achilles heel of large format is the need for depth of field 
> that forces small f/stops (large number) and these are never 
> much resolution at f/16 or 22. 

Is that the whole story or is there more to it?  Even flat to flat 
I think a large lens is behind a small lens in angular MTF.  I 
think it's the film MTF dropping out of the equation which gives 
large format its advantage, angularly speaking.

I'd like to check out the curves to be more sure of the first 
statement in the above paragraph but as far as I know, miniature 
format lens makers don't publish MTF curves.  I based that 
statement on my recollection of medium format versus large format 
curves.  I will check them out tonight if I have time.

> Of course we now know that apparent sharpness is a contrast 
> thing. 

Right; an edge contrast thing.  I think technically sharpness is now 
called "acutance" because sharpness is a word which is so multi-valued.

> MTF is modulation transfer function and can be done for film, 
> lens, or both. It was discovered by Schrade of RCA when 
> investigation why lenses that tested lousy gave better TV images 
> than renowned lenses.  (All the surplus Ektra lenses were 
> remounted for TV turrets on the first TV studio cameras!)

_Interesting_ bit of history.  I hadn't heard what the impetus 
was.

John B


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