View Full Version : 26" lens

9-Nov-2017, 09:35
Hope someone can help me out with this question and if it's a daft one, I apologise:)

I have just bought a 26" Charles Beseler projection lens and wanted to know if it says 26" does that mean it's a 660mm lens. Also how will I figure out what aperture it is and the image circle.

I have tried to google the answers I need but I can't find what I am looking for.

I paid 120 for it and I'm hoping I didn't pay to much for it as I couldn't find any similar lens for sale at the time.

Steven Tribe
9-Nov-2017, 14:34
Yes, nearly all projection lenses have the EFL written as you describe. Beseler made Petzvals early on, but went over to the Cooke triplet design with shorter barrels and only 3 lenses to manufacture!

The early Petzval projection lenses always have a arrow pointing in the direct of the projector - or, these days, camera!

9-Nov-2017, 19:11
For determining the aperture, I think you just hold the lens with a ruler across the front and measure the actual illuminated area you see as you are looking thru the front of the lens. If it is 75mm, and your focal length is 660mm - 660/75 = 8.8, so you could say your lens aperture is f/8 (rounded down). If it measures 110mm, 660/110 = f/6. I think that is a "fairly" accurate way to determine fixed aperture. Not sure of an easy way to determine image circle.