View Full Version : Front doublet from Pentac as a soft focus portrait lens?

Ian Greenhalgh
11-Dec-2013, 10:36
Hi folks

While rooting in my boxes of junk I found the front portion of an Air Ministry Pentac 2.9/8". This is a positive achromatic doublet, right?

So, can I use it as a soft lens for portraits? Any idea what the focal length would roughly be? What is the aperture value likely to be?

Anyone done something similar to this and can show some results?

Dan Fromm
11-Dec-2013, 11:01
You have the lens, you can ask it what it can do yourself. Where's your spirit of adventure?

Mark Sawyer
11-Dec-2013, 12:40
The Pentac has a Heliar layout with a positive doublet front and rear, so the front element will work as a landscape lens.

The measurements you asked for are pretty basic. For focal length, focus an image of something far away on a wall (pointing the lens out the window works best for me), and measure from the center of the lens to the wall. That's your focal length.

Measure the diameter of the lens. That's your aperture.

Divide the focal length by the aperture. That's your f/stop.

Lots of us have played with achromatic doublets. Here's something probably semi-similar to what the Pentac front element will do:


Ian Greenhalgh
11-Dec-2013, 13:52
Thanks Mark, that's useful.

I just wanted to canvas some info before I broke out the tools and mounted it.

11-Dec-2013, 14:02
Let us know how it goes.

Ian Greenhalgh
11-Dec-2013, 15:08
I will Ed, cheers.