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.
