Quote Originally Posted by Tony Lakin View Post
....if single Protar lens cells should be mounted in front of or behind the iris.
I've compared the performance of the Zeiss Protar VII cells, the B&L Protar VII cells, the Wollensak Ia cells, the Turner-Reich (another Protar VII derivative) cells, as well as many other double-anastigmat cells mounted at the back a well as at the front of their barrels. Nearly all of them loose a lot in sharpness if put in front of the iris - except the convertible Sironar front that was intended for that very usage and the Dagor halves that turned out to be nearly indifferent to their position. While a Protar VII mounted behind the aperture can be stopped down at least a full stop less than a Half-Dagor to show a similar level of sharpness, the same Protar VII becomes no better than the Half-Dagor when mounted at the front. As front mounted double-anastigmat cells need far less bellows draw (working as slightly telephoto lenses actually), I enjoy the 840mm front half of my 480mm Dagor on my Calumet C-1 and the 290mm half of my 165mm Dagor on my old compact and lightweight 9x12cm folder. Normal lenses of the same focal lengths would be practically impossible to focus on those cameras except at the very infinity.

And yes all those single cells suffer from the chromatic aberration badly, and a yellow filter improves their sharpness a lot. Though myself I like the blue-sensitive film look way better....

As for the spherical aberration which is negative and thus very good for the out of focus background rendition in all those lenses well stopped down, it is of the same character but different in its amount. The complete Double-Protar VIIa (as well as the Wollensak Ia and the Tuner-Reich) has less SA and correspondingly makes less smooth background defocusing than the complete Dagor; the single Protar VII (and the Wollensak Ia and the Tuner-Reich) cells have more SA and more pleasant OOF background rendering than the complete Dagor, and the Dagor halves have still more SA and the smoothest OOF background rendering of all them. Though the difference between the 'really good' and 'still better' is probably not worth loosing sharpness - which is unavoidable with single cells instead of complete double anastigmats. For me, the complete Dagor is still the best of both worlds - if only I don't need a lens longer than my 480mm (I don't own a 600mm.... and nobody owns anything like a 840mm).

I also often prefer to put a simple negative supplementary lens on a complete Dagor to using any lens halves. Those combinations give much sharper results than any single lens cells, and are especially better in their chromatic aberration so the yellow filter is not needed.