push-on filter for Agfa Isolette 3

[Neil Grant]

..here's a solution: a push-on to screw thread adaptor. It's made from two step-up rings. Lens side, pushing over the focus ring, is a 32.5 to 37mm with the throat opened up fractionally with a rat-tail file. Then attached to this is a 37 to 40.5mm.

 

[pentaxuser]

Neil, just so I have understood this correctly. You get a screw thread step-up 32.5 to 37 which is pushed over the focus ring. However you need to file the base thread of the 32.5 as it is slightly too tight a fit? How easy is it to file the 32.5 thread to get a tight friction fit? My worry is that it might be easy to file too much off. The second is a 37 to 40.5 mm. Is this chosen as it's the first step up that gets you to a size at which the full range of filters are easily obtained?

It sounds like a good solution if I have followed what you have done My only worry is getting the filing out done accurately
Thanks

pentaxuser

 

[Neil Grant]

...the 32.5mm step-up doesn't quite push OVER the focus ring - it's fractionally too tight. Forget about the threads! Metal needs to removed from the inside - away from the threaded surface, the throat of the step-up if you prefer. Filing away metal reasonably accurately, keeping the throat 'circular' is no problem. No special tools, beyond the 'rat tail' file are needed. Run the file around the 'throat' of the step-up in a circular motion using wrist -action and the 'wall' as a guide.The cutting is a circular one not a linear like normal filing (almost like you were strirring a cup of coffee with a spoon - but horizontally). Periodically turn the step-up by a quarter turn during filing so there is no bias - no ovality should be introduced. Trial fit the step-up over the focus ring and remove more metal until it fits to satisfaction. I think it took aboot 300 hundred revolutions of the file to remove about 1/10 mm. The 40.5 mm step was chosen because i have a few filters that size - but they are only single-coated, from an RB 67 fisheye. I may add a 40.5mm to 49mm for some MC 'contrast' filters i have - the Solinar 75mm lens is easliy good enough for the best.

 

[pentaxuser]

Thanks I hadn't heard the name rat tail but it looks like a normal round file. In the smaller sizes step-up rings do not seem to be that common in the U.K. One final question if I may. I take it that a big enough step-up from 32.5 to 40.5 does not exist hence the need for two rings?

pentaxuser

 

[Neil Grant]

...there are few choices in front thread from a 32.5mm starting point, 40.5mm wasn't one of them. I tried holding a 49mm filter in front of the lens - but the flare was severe. Non-image forming light was passing from behind the filter and then reflecting back into the lens. As it is, the stacked stepping rings look like a stumpy lens hood. The file was quite fine and about 3mm diameter - definitely NOT heavy duty.

 

[pentaxuser]

Thanks for the answers, Neil.

pentaxuser