PDA

View Full Version : Darlot Petzval front element as Meniscus

GSX4
26-Jan-2011, 11:54
Okay, after seeing some of the amazing work posted in the image sharing section regarding meniscus lenses, I decided to try a few things with some smaller petzvals I currently have....

I have a 9" F4 darlot and a 5" F4 darlot. Both are lantern lenses, but they exhibit some interesting effects if I just use the front elements. So far so good. I realize that the focal length is approximately doubled as a result, but does that also apply to the effective aperture? Will they both approximately be F8 lenses? I'm looking forward to trying some of these combinations out when the weather gets a little warmer here!

Mark Sawyer
26-Jan-2011, 12:17
Okay, after seeing some of the amazing work posted in the image sharing section regarding meniscus lenses, I decided to try a few things with some smaller petzvals I currently have....

I have a 9" F4 darlot and a 5" F4 darlot... the focal length is approximately doubled as a result, but does that also apply to the effective aperture? Will they both approximately be F8 lenses?

Yes! Just measure the bellows extension and divide by the diameter of the lens to find your true f/stop at any given focal length. If at infinity, your 9" f/4 lens now focuses at 18", you divide the 2.25" diameter into 18 instead of 9, which gives 8 instead of 4. Piece of cake! The same formula is an easy way to figure the true f/stop after bellows extension too, much simpler than "the square of this divided by the square of that..."

Steven Tribe
27-Jan-2011, 06:59
"I realize that the focal length is approximately doubled as a result".
In some cases, perhaps.
But the Petzval design is not dependent on the efl of the two halves being the same. In fact, I believe that the front achromat is always shorter than the rear doublet.

An example: I have a projection Busch petzval that has a combined efl of 4.5". The achromat gives an approximate efl of 6.25" whilst the rear set have 13.5" efl. This matches well with the usual approximation 1/x + 1/y = 1/combination efl.
This one of the reasons the achromat from a Petzval is so useful. The efl doesn't get as long as half a single rectilinear does and the f value is quite high.