Miy 15mm does feature the 4 feet mark and the front lens diameter is 69 mm (measured with a caliper). It's a K SMC PENTAX 1:3.5/15, sn 5068171. Ciao Fabio ----- Original Message ----- From: "Andre Langevin" <[EMAIL PROTECTED]> To: <[EMAIL PROTECTED]> Sent: Thursday, May 20, 2004 6:50 PM Subject: Re: A 15mm (ASP and non-ASP)
> Dario wrote: > > >Interesting to me, Andre. You were sure such a topic will have raised my > >ears, weren't you? > > I was checking your ears and as predicted... > > But don't you already know this group? > > >You can also notice different name lettering on lenses. smc (lower case) > >followed by wider PENTAX is obviously a late type. However, how can we be > >assured that 3 insted of 4 and/or smc instead of SMC will mean > >non-aspherical instead of aspherical? There were several cases when Pentax > >applied step-by step changes during the manufacturing period of a product, > >with no sure combinations between different features (the LX is a case > >history in this field). > > Indeed, I wouldn't bet on these differences as a sure way to know > what we have. From your article in Spotmatic, we know that very few > K lenses were aspherical. Probably a lot more were smc (small type) > AND non-aspherical. > > > > But there could be a better way to know which lens is ASP: > >> > >> http://www.ucatv.ne.jp/~tweety/Report/Comparison15mm/Comparison15mm.htm > > > >This lens reflection proof is related to lenses, hence more reliable to me. > >The problem could be how to repeatedly produce proper reflections, useful > >for on-field tests. > > It would be easier with both lenses side by side. But it looks like > some reflections are ovalish instead of roundish... So there may be > a better way than what we tried in september 2002: > > > > >Andre, > >> My "SMC Takumar 1:3.5/15" front element inside the retaining ring > >>measures about 69.85mm. The distance from the tulip hood to the front > >>element measures about 5.56mm. Hope this helps. > >> > >>Bob Rapp > > > >Now that's interesting. The few measures we had until now were > >around 68,5mm, but Bob seems to have a Takumar with a different > >diameter. It looks like you might have the aspheric lens, Bob... > > > > > >Andre (SMC-T 8014040) : 68mm (approx.) > >Stephen (SMC-K 7368xxx) between 68 & 69mm > >Rod (SMC-A) 68,6mm (with a caliper) > > > >Antti-Pekka (SMC-K 505xxxx) ? > >Vic (SMC-K 7367862) ? > > > >Bob (SMC-T 8013862) 69.85mm > > > >For the moment, our best hypothesis is: > >Diameter of front element of non-aspheric lens: 68,6mm > >Diameter of front element of aspheric lens: 69,85mm > > > > > > > >Previous discussion: > > > >>are you all sure there are too versions of this > >>lens? > > > >It has been ascertained not long ago. There is an article in > >Spotmatic about it. There are two versions of the 15mm design. Only > >400 lenses has the aspheric element: 300 Takumar (out of 900) and 100 > >K-series Pentax. No A-series has the aspherical element. > > > >>Wouldnt the change from aspheric to non aspheric cause > >>the need for a total design of the optics? > >>JCO > > > >The way I understand it, the aspheric element was used to get zero > >distorsion on an otherwise low-distorsion lens. Asahi indeed > >modified the lens once they decided to do without the aspherical > >element, but (again, if I understand) the only VISIBLE difference is > >in the front element diameter and curvature. The diameter is easy to > >calculate. The curvature ? Well, Bob proposed to measure the > >distance from the top of the glass to the border of the hood (we > >assume both versions have the same hood. > > > >If the un-aspherical lens is known to have very low distorsion it > >could be because the original design was basically a low-distorsion > >one and also because they tweaked the design a bit to do without the > >aspherical element. No need for a complete redesign of the lens. > >The aspherical element probably had the sole benefit of cutting what > >remained of distortion. But I might be wrong. >
