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Last edited by Ken Lee; 22-Apr-2012 at 09:25.
Kingslake tells us (in his excellent paper on the History of photographic lenses ) that it was 'a detachment of eight Austrian bombadiers' ;o)Originally Posted by Mark Sawyer
You're right, Mark, that there are a lot of design parameters available, even in something like a Petzval or a Dagor ; however, in the original subject of the thread, we are lucky in that the manufacturer has helpfully 'frozen' almost all of the available degrees of freedom of the lens, by making it, hopefully close to design spec . What we have left is only one parameter - the spacing of these two groups .
We just have to be careful to realise that we need to evaluate the image quality over the extent of the design image circle , not just a check on focal length or the back-focus .
Now, looking back at the original post, I see that it may be even easier, given that the lens type and possible the shutter are known . There ought to be drawings or information out there ( someone can measure ) that would get the spacing pretty close. Maybe the older manufacturers would also use a variable shim in there , I'm not sure .
These days ( since 60's I guess ) everyone knows what the spacing of the reference faces of a Copal 0, Copal 1, Compur 1 etc. should be, and the manufacturers expect to screw in the lens cells without shimming . The tolerance is probably held to something pretty tight, like 0.025mm or so .
Well, there are two different takes on the question from the OP. My response was to the first:
...and I meant to point out that there isn't a simple formula leading to a defined perfect answer.
In post 16, the OP refers to a set of specific cells. Here your suggestion, "There ought to be drawings or information out there ( someone can measure ) that would get the spacing pretty close", would work. Even more likely, post the information on the lens and ask if anyone has a similar lens and what shutter is it in or what is the overall length, front to back.
On Petzval's mathematics resources, Kingslake wrote in A History of the Photographic Lens, "For help in his calculations he (Petzval) approached the Archduke Ludwig, Director General of Artillery in the Austrian Army, who ordered that Corporals Loschner and Haim and eight gunners skilled in computing be placed at his disposal." That's ten.(And this to help out someone who had already been appointed to professorships of higher mathematics at the Universitiy of Budapest and the University of Vienna!)
"I love my Verito lens, but I always have to sharpen everything in Photoshop..."
And Herr Schiempflug was.... an artillery officer. From Austria.
One man's Mede is another man's Persian.
Lens designers consider the air space between lens elements and groups as just another lens element.
Lynn
Correct. Therefore the airspaces, all of them, are an important part of the overall formula. The Plasmat was an improvement on the Dagor for this reason, it gave the designer an extra two "elements", and also made production a bit less costly.
One man's Mede is another man's Persian.
You've got me there, Mark. I hadn't checked who else was in the room !On Petzval's mathematics resources, Kingslake wrote in A History of the Photographic Lens, "For help in his calculations he (Petzval) approached the Archduke Ludwig, Director General of Artillery in the Austrian Army, who ordered that Corporals Loschner and Haim and eight gunners skilled in computing be placed at his disposal." That's ten.
There are various ways of looking at the air-gap / lens splitting option . One way to look at the various degrees of design freedom, is that each lens element has 'power' and 'bending' available ... unless it is cemented to something else. Releasing the inner positive elements from the others in a Dagor gives you the freedom to bend them independantly .
In the case the Dagor-to-Orthometer ( Plasmat ) development , it's actually more about higher-order aberrations - the ray angles in the small air gaps allow you to improve the correction of 5th-order spherical, which allows you to increase the speed retaining the same level of field correction .
The plasmat was a little ahead of its time, though, because it needed coatings to be developed before we could take advantage of the extra performance .
Actually the Plasmat is a big improvement over the Dagor in that the Dagor has quite a bit of spherical abberation wide-open, also shifts focus as you stop it down. The Plasmat commits neither of these sins, and is a bit faster. The Dagor scores in half the number of air to glass surfaces, an uncoated Dagor comparing very favorably to a single coated convertible Symmar.
One man's Mede is another man's Persian.
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