Perkil8r Posted June 19, 2012 Share Posted June 19, 2012 I have stumbled across a lens cell on eBay, 6.5inch dia, double lens. I am thinking, how would you find the focal length of an unknown lens? I am tempted to take a punt on it if it goes for a song, with the intention of making a DIY scope. What's the worst that can happen Link to comment Share on other sites More sharing options...
philjay Posted June 19, 2012 Share Posted June 19, 2012 If its this one Mike beware http://www.ebay.co.uk/itm/6-5-Diameter-Telescope-Lens-Vintage-/251088977030?pt=LH_DefaultDomain_3&hash=item3a7611bc86 as it looks to be a projector lens. The title says telescope lens but then the write up says telescopic lens hmmm If its a projector lens it will be designed to project a point source from a short folcal length of a few inches onto a wall therefore wont really be suitable for a scope However it could be a ww11 reccy camera lens of which there were loads around at one time and folks used to convert them into astrographs but the cell design doesnt look to be that style it looks too early circa 1900-1920 so could be a projector lens It may pay you to ask a few questions. The way to roughly check focal length is to hold the lens up to a wall opposite a window during the day. Then slowly move the lens away from the wall untill an in-focus image (upside down) appears on the wall. That distance is the focal length. Phil Link to comment Share on other sites More sharing options...
Perkil8r Posted June 19, 2012 Author Share Posted June 19, 2012 Thanks Phil, that helps quite a bit. It is that one yes, I'm not sure what sort of question to ask to try and suss it out really, since the seller doesn't appear to know too much, hence the lack of description. Or maybe it's a case of they expect people to know what it is because they themselves know if you follow? Any ideas on what or how to ask to get some idea of if it might be suitable? Would a projector lens be double element though? Link to comment Share on other sites More sharing options...
+Graham Posted June 19, 2012 Share Posted June 19, 2012 That looks like the lens from an old magic lantern. Link to comment Share on other sites More sharing options...
Perkil8r Posted June 19, 2012 Author Share Posted June 19, 2012 I've asked if there is any more info and asked if they can measure the focal length for me. If you don't ask you don't get Could make for an interesting cloudy season project if it's anything half decent. I'm only going to be interested if it's over 1000mm fl Link to comment Share on other sites More sharing options...
philjay Posted June 20, 2012 Share Posted June 20, 2012 Yeah Mick thats about all you can ask, the FL is the critical thing but Im pretty sure its a projector lens because of the curvature of the front element Link to comment Share on other sites More sharing options...
Perkil8r Posted June 20, 2012 Author Share Posted June 20, 2012 Ah yes, I hadn't looked at that too closely, but now you mention it.... Hmmmm... I want a big lens now lol. Wonder if Anchor supplies have any...... Link to comment Share on other sites More sharing options...
Perkil8r Posted June 25, 2012 Author Share Posted June 25, 2012 Turns out it's a fl of 10.5 inches, so about f1.6 which sounds like a projector to me. Link to comment Share on other sites More sharing options...
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