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Re: [OM] 4/3'rds, why no "standard" lens?

Subject: Re: [OM] 4/3'rds, why no "standard" lens?
From: Moose <olymoose@xxxxxxxxxxxxx>
Date: Sat, 15 Mar 2003 23:18:43 -0800
You've got the direction right now. But the math isn't quite as simple as you say. The formula for dist. from film to subject (D) is:

D = F*(1/R + R + 2), where F = focal length and R = repro ratio as a fraction, 1:4 = 0.25.

For simplicity, all these calculations and comparisons use the simple assumption that the sensor size of a 4/3 system is 1/2 the size of 35mm film. I don't know that, but it seems to be what Oly and everybody else is using.

The calculated D for a 50mm lens at 1:4 is 12.3" (which pretty much agrees with the 50/3.5 markings).
The calculated D for a 100mm lens at 1:2 is 17.7".
So in this sense, the working distance for the 100mm @ 1:2 is about 5" or 440nger than a 50mm lens @ 1:2

However, 100mm lenses are longer than 50mm lenses, to the difference in practical working distance measured from the front of the lens to the subject will be smaller than that.

The distance from the front of 50/3.5 to the subject at 1:4 is about 8.25"
The distance from the front of a Kiron 105/2.8 to the subject at 1:2 is about 9.75" (Sorry, the only 100mm Zuiko I have is the 2/8, which doesn't focus that close.) The distance from the front of a Tamron90/2.5 to the subject at 1:2 is about 9", so it looks like a 100mm would be about 9.5"

So the answer is that you are roughly right from the standpoint of distance from front of lens to subject, as one would expect you to be from your experience as a practical photographer, but the 50mm macro on a 4/3 system will have somewhat less practical working distance than a 100mm on 35mm film. Looks like the difference will be about 1.25", depending on specifics of lens design.

Does that answer everybody's questions?

Moose

Jan Steinman wrote:

<snip> Let's work backwards. Say you want a flower to fill the frame. In 35mm, 
it has to fill a larger area -- a smaller reproduction ratio is required. In 4/3, it 
needs to fill a smaller area, requiring a larger reproduction ratio. Thus the working 
distance is proportional to the field of view, NOT the reproduction ratio, and will 
be identical on a 50mm 4/3 lens as on a 100mm 35mm lens.

This becomes quite clear if you've done any large format work. The 150mm 
"normal" lens for 4x5 work produces the same image size, relative to the film, 
at the same working distance as a 50mm lens does for 35mm work. (If course, the 
reproduction ratio is smaller, though.)

I do a lot of macro in various formats, and feel quite confident of this, but I 
guess I'm having some trouble explaining it.

Did that help, or did I confuse things even more?





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