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I don't know what your budget is but you can get an Industar enlarger lens for $10 on ebay and an m42 adapter to put on the enlarger so you can screw the lens in for maybe another $7. You can also use a normal camera lens, you don't need to do anything special but you might have problems because camera lenses are heavier. You will need a lens that can do f3.5 at 70mm or so. For the app - it is probably easier if you just use the enfojer app. When I made this project, there was no enfojer app so I had to make my own. You can find my laser cutter templates here: https://1drv.ms/f/s!AlfMITAXF6-GoZM41upW62zXywPN5Q Unfortunately I haven't written instructions but let me know if you have questions.
I used a $10 Industar enlarger lens off ebay. I mounted it by gluing an adapter ring that is typically used to mount Russian lenses to canon cameras onto the enlarger housing. In the mean-time I've found that this version doesn't work very well, I made a new and improved one that is a little better: https://sway.com/AC1I6UdXtDE9AyjT let me know if you'd like the laser cutting templates, android app or anything else.
I made a better version of this since then: https://sway.com/AC1I6UdXtDE9AyjT also sorry about the delay in reply. Let me know if you have more questions :)
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Darkroom Enlarger for PhoneView Instructable »