Convertible Lenses

[cowanw]

I am following Ari's thread about the Cooke convertible lens with interest, as are others, I am sure.
The difficulty in utilizing the 25.5 inch option diminishes it's utility. While the combination of two lenses increases the utility of the lens with 10.75 and 14.5 inch options.
This got me thinking about the Velostigmat/Raptor 13 inch Series 1a, or other convertibles.
Has anyone had occasion to mix two such lenses and achieving about 11 inch and 15 inch combinations?
Or is that asking two much of an older lens?

[Kevin Crisp]

Many convertibles can be freely assembled in different combinations. The classic Protars, for example, can be assembled into a wide range of focal lengths if you have enough separate elements to do that. The seldom discussed drawback to that, despite the cost and the years it takes to put all that together, is that for all those combinations, many of them are so slightly different that the utility of doing so is arguable. For example, with a Series VII protar, how many times do you find that a combined 180mm won't do it, so you go with your 162mm combination? Or 162 is too long so you go with the 158mm combination? Don't get me wrong, I love my Protars but the practicality of the different combinations is often overstated. This was certainly true of the short-lived Wisner set I used for awhile. Lots of not-very-different choices. And on the Wisner set I found the single focal lengths unacceptable outside the center.

There are a number of protar combinations that will get you in the 11 to 15 inch range. So this is not asking too much of an older lens set. But if the photographer was looking for a basic "normal" 13" lens for 8X10 the two elements are going to on the long side with the protars. The 13" Wollensak lens is the most common convertible 1A I have seen, by far, but there were many other focal lengths available for a time. The Wollensak approach was to make triple convertibles for a particular film size, without any emphasis on going beyond that by adding a cell or two to the basic set. There may be issues with the diameters not matching the mount if you try that with the Wolly. It should work, as the Wolly does appear to be a Protar based formula with 4 elements and three glued surfaces. The catalogs over at Camerecentric.com can answer a lot of questions on older convertible sets.

[karl french]

What's the difficulty in using the 25.5 option of the Cooke XVa? Is it the bellows extension? The 646mm behind the shutter does need about 28 inches of extension to focus as infinity. Many classic and modern 8x10 cameras have more than enough bellows to handle the extension needed. The 8x10 Canham Traditional I have for sale, for example :-) does a fine job in such a situation. I've mostly used my Cooke XVa with my 8x10 Deardorff, which easily handles the 646mm behind the shutter. I think you'd be out of luck with a Chamonix 8x10.

Kevin raises very valid points in terms of having a bunch of different convertible cells.

[cowanw]

Thanks Kevin.
Yes, Karl, just the extension.

[Bill_1856]

Do convertible lenses have a serious focus shift when stopping down?

[Kevin Crisp]

For single cells, the protars certainly do. I did not notice it with the Wollensak ones I've had.

[8x10 user]

Half a dagor? If you like soft lenses the Hyperion is a triple convertible. Half an Eidoscope also works.

[8x10 user]

If they are made from a low refractive index (crown) glass then the answer is yes, unless it has been aspherically corrected. Some like the look of focus lenses that have focus shift (spherical aberration). Some of these lenses are soft others appear sharp but have enough spherical aberration to help with the apodization of the circle of confusion, I.E. it smooths the background and creates "better" "bokeh".

Do convertible lenses have a serious focus shift when stopping down?

[karl french]

I have a Protar VIIa which doesn't really seem to have a focus shift issue. I think someone recemented the front group it looks so nice. The Raptor 1A I used to have really had a problem with it with the 25.5" group. The 20" Raptar 1A group seemed not to have any significant focus shift stopped down. I think a lot of the focus shift problem relates to individual lenses and how well the individual components are centered within the whole group.

Maybe I'm just not that critical, but I think the whole Dagor/Protar focus shift issue is overstated. It happens, but not with every lens.

[Peter Collins]

I agree with Kevin--

Lots of not-very-different choices.