Lens families or series

[swmcl]

Hello again,

I was ponderin' ...

In a lens series or family like say, Grandagons, Sironars or Super Angulon XLs etc. there is a certain formula. X number of elements in a certain arrangement. Surely it follows that there will be one or two focal lengths in the series that are best suited to the formula ?

The word on the street might be that the 150mm Sironar is the best Sironar. It would then follow that the Sironar lenses that are most different from the 150mm are the worst examples in the Sironar range no?!?

It depends on the formula obviously. For example, I would expect that the more you try to bend (refract) light the worse will be the result. So in Grandagons the Grandagon with the largest shutter opening and longest focal length might be the best for that series ... ?

I realise the manufacturer will try to make the lenses across the series perform similarly and I might be clutching at straws a bit here but my question still haunts me. Is there a best lens in a series and is that 'best' because of the formula - the glasses used, the curvature required etc. etc.

Cheers,

Steve

[Dan Fromm]

Prescriptions are scaled up/down by changing all dimensions -- surfaces' radii, elements' thickness and diameter, spacing -- proportionally. To double the focal length, double all dimensions. The same glasses -- refractive index, Abbe number -- are used for all focal lengths.

If you visit www.dioptrique.info, for example, you'll see lens prescriptions scaled -- usually -- for a focal length of 100 mm. All members of the "family" are derived from that.

That said, aberrations also scale with focal length so, for example, a long member of the family will have worse chromatic aberration than a short one. For aberrations that are affected by aperture, one cure is to make the longer version of the lens with a smaller maximum aperture. Another is to recalculate. But that changes the prescription. There's no optimal focal length for a prescription; the shorter the sharper.

And that brings up the point that to some extent what you think of as a family is a marketer's concept, not an optical engineer's concept. From the engineer's perspective, every prescription defines a family. From the marketer's, the trade name defines a family; member's of marketing family can have quite different prescriptions.

I know what one manufacturer, Boyer, did. They made each design in a range of focal lengths and for a design used the same prescription. But, and this is a big but, within trade name they sometimes changed prescription. So, for example, the longer f/1.9 Saphirs are to a different prescription than the shorter ones and have much less coverage, relative to focal length. They also have different internodal distances, relative to focal length.

[swmcl]

So effectively Dan, each lens should be considered separately. Well I have learned something. You can see I have not studied lens designs eh!

Thanks.

[Dan Fromm]

Not exactly, Steve.

For example, most f/6.3 Zeiss Tessars were made to the same prescription, although after WWII a few focal lengths were recomputed. All of these lenses should shoot more or less equally well, understanding that long ones won't be as sharp as short ones at the same aperture.

But I agree with you that each lens -- I don't mean make and model, I mean each individual example -- should be considered on its own. For me, acceptance testing is necessary, and is needed for new as well as for used lenses. There've been many laments here about brand new lenses from highly respected makers that weren't as good as they should have been.

Cheers,

Dan