Cooke Speed 25mm question

[Saba mazloum]

Hey lads,

Quick question, whats the difference between a series 2 25mm Cooke SP and a series 3 25mm Cooke SP?

Thanks!

 

[Ignacio Aguilar]

If I remember well, Series 3 was a redesign (circa 1954) of the 18mm and 25mm lenses to improve the coverage and probably the sharpness (edge to edge) of the lenses due to the new widescreen ratios (1.66:1-1.85:1) that appeared in the early 50's.

 

[Tommaso Alvisi]

What Ignacio said, plus the newer design is much much smaller and lighter.

 

[Ilya O.]

Ser.III were made to decrease bulkyness of inverted telephoto design so that the usage on turret cameras was okay.

18mm and 25mm had 1 ASPHERIC surface!

Well, let's give COOKE speak for themselves:

...The new Speed Panchro Series III lenses represent a major advance in two respects in the design of inverted telephoto lenses. Namely, a considerable reduction in bulk and length of the lens and a high aberrational correction over the whole of the wide angular field achieved by the use of non-spherical surfaces.
The reduction in size is very closely related to the greater correction of aberration yielded by the use of an aspheric surface. Furthermore, it has been possible to reduce the complexity of construction by the elimination of two elements from the previous designs. This reduction in the number of elements decreases the light losses due to both absorption and surface reflection, such that at f/2.0 the lenses achieve T2.2. Compared with the 25 mm. f/1.8 (T/2.2) Series II, the new Series III f/2.0 (T/2.2) lens has an increased depth of field...

The text above is from the overview article '18mm and 25mm COOKE SPEED PANCHRO LENS SERIES III - f/2.0, T/2.2' from 'MOTION PICTURE LEAFLET LEN. 51'.
I scanned it to pdf with text layer for ya, use it plz and feel free to share:
https://goo.gl/Hg0vjo

 

[Ilya O.]

Here we can see the difference between 'old' ser.II (upper) and 'new' ser.II (lower) 18 and 25:

What is interesting, It seems that 18 and 25mm have the same optical scheme in both series.