Pentax DAL 50mm F1.8 : incomming

Started May 21, 2012 | Discussions thread
awaldram Forum Pro • Posts: 12,828
Re: Pentax DAL 50mm F1.8 : incomming

Marc Sabatella wrote:

awaldram wrote:

Cideway wrote:

They must be looking to knock down the vignetting as this lens on those specs has a larger filter ring the the FA 50 f1.4

The filter ring has no input to vignetting.!

Well, perhaps not the ring itself, but surely the potential for vignetting is a major concern in deciding what size to make the filter ring? And since most other Pentax 50mm lenses take 49mm filters, the fact that this one doesn't - but physically could - might be evidence that in fact the front element on this in fact slightly larger than it might have been with previous designs of similar maximum aperture. That is, if the lens did in fact have a smaller maximum aperture than it should for a 50/1.8, it would indeed vignette as much as or more than other lenses, but then, it should be able to take 49mm filters without vignetting any further, just as is the case for other Pentax 50mm lenses. So why wouldn't they have given it a 49mm filter ring?

I think 49mm is more historical than logical.

indeed most of the more recent designs have required a stepped barrel to get down to 49mm.

It easier to fit a filter thread matching the barrel dimensions which in all mi zooms outsizes any optical requirements (focal divided by aperture).

i.e a 50mm f.18 needs a front element of 28mm !!!

so 49mm or 52mm is a design decision not an optical one.

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