The "reverse Tessar" may be an optical trick to increase the maximum aperture without needing a larger shutter. At least my f:3.5 Xenar Typ D is a reverse Tessar, with the rear cell having slightly negative focal length. This puts maximum "loupe power" in front of the aperture, increasing the optical aperture with the same physical aperture that would give f:4.5 in a more symmetrical lens.
Ole, I fear you may have been tripped up by R. Kingslake's extremely idiosyncratic english.
The rear group of a regular tessar has a biconcave element facing the diaphragm cemented to a biconvex element facing the film. In Kingslake-speak, a reverse tessar has a biconvex element facing the diaphragm cemented to a biconcave element facing the film. In other words, per Kingslake a reverse tessar is a tessar with the rear group turned around.
When most of us say reversed tessar, we mean a normal tessar turned around, i.e., with the cemented doublet facing the subject and the pair of singlets facing the film.
Cheers,
Dan
Dan, that's what I mean too. And that's what a Xenar Typ D f:3.5 is, too.
Ole, pardon my slowness. Are you using Kingslake-speak or normal English?
Dan, in this case it's "normal English" - to the best of my ability.
The Xenar Typ D has a cemented pair in front of the aperture, with a shorter focal length than the entire lens assembly. Behind the aperture there is an airspaced pair, with a negative focal length. In other words the Xenar Typ D is slightly telephoto, and has the cells exchanged compared to a "normal Tessar-type Xenar".
I can't remember what the Vade Mecum says about the Typ D, but I do remember that my lens doesn't agree with that.
Thanks, Ole, for the explanation.
The Xenar Typ D isn't the only reversed tessar type around, there are a number of macro lenses with that design. I've had 48 and 75 mm B&L MicroTessars, 35/4.5, 50/4.5, and 75/4.5 Tominons, 90/6.3 Mikrotar and 100/6.3 Neupolars. All reversed tessars. What's interesting is that the Mikrotar and Neupolar are quite good down to at least 1:4. In fact, the Neupolar is great at infinity, but because of mechanical vignetting covers at most 6x6 at infinity.
Here's what the VM says about the Typ D: "Xenar f3.5 There may have been a f3.5 Q15 type lens in short foci for movie work, but the well known f3.5 was the next item. The small version for Handkameras was called Type D in 1926, and this was noted as engraved on a prewar F3.5 Xenar for about 150mm. But note Sc005 which is a reversed Q15 type layout. This would explain the 'new' f3.5 Xenar of conventional Q15 layout announced in 1935 below. But note the next item.
Xenar Type D f3.5 There seems to have been an uncemented 3-glass Xenar for Portrait work, of excellent quality and able to stand comparison with the 4-glass. This type was for small cameras only."
Typical VM muddle and lack of copy editing.
Cheers,
Dan
Posting Permissions
Reply With Quote
Bookmarks