21 Biogon... Distagon

[zanef2.8]

I bought a ZM 21 2.8 recently and have been enjoying it.

One day, I was cleaning the lens and found that it reads "Biogon" on the lens and I thought I have read somewhere that the Zeiss 21 2.8 lens is normally a "Distagon"

Not that I really care or worry, but just interested to know the difference and has there been a 21 Distagon made for ZM or is it for Contax only?

What's the significance really?

Ignore me if this is a stupid question, but thanks in advance for any answers.

 

[wintoid]

I'm wary of spreading regurgitated internet rumours, but I think I remember reading that a distagon is a retrofocus design, which is required for SLRs due to the mirror preventing the rear of the lens intruding too deeply into the body, whereas biogon is a superior (deeper) design which doesn't have to be retrofocus.

 

[zanef2.8]

I see!

So, is the Biogon better?

 

[fuwen]

I see!

So, is the Biogon better?

I am not a lens design expert and I think there are a few in this forum. But better or not is a tricky question.

For SLR due to the reflex mirror wide angle lens design has to be non-symmetrical to avoid the mirror hitting the lens, and thus the Distacon design. So from design point of view that creates a limitation for lens designer and the key compromise is the distorsion control.

For range finder there is no restriction on mirror so the designers are free to use a more symmetrical design so the Biogon. But even for Biogon the perfect symmetrical design will bring the rear elements too close to the film plane and thus creates problems for camera TTL metering so the ZM Biogon is still not the perfect symmetrical design but good enough to bring down the distorsion level to at least 50% of that in Distagon.

However, life is a compromise. The Biogon design will suffer higher light fall off, and the light fall off cannot be drastically reduce by stopping down the aperture. And in this case the Distagon design when adequately stop down the light fall off improves a lot.

But really in real life photography the compromises in both the Biogon and Distagon, IMHO, have been designed to have negligible effect.

Oh and of course, in terms of size vs performance, the ZM Biogon is such a sweet compact package compared to the Distagon.

 

[zanef2.8]

Right ... thanks a lot for the info. Well the compactness is more important for rangefinding i guess. thanks again. Zane