Help with making a correction diopter lens for Holga Instant Back

Hello all! I'm making an Instax film back for Holga, following the example of this DIY one: https://www.flickr.com/photos/116526163@N02/12380551873/in/photostream/ I got in contact with the maker of this and asked him about how he corrected for the increased film plane distance. He told me he just used a corrective diopter lens from a different Holga polaroid back and it worked great. Unfortunately I cant find one of these corrective diopter lenses by itself for sale anywhere, and don't wanna spend $100+ for a polaroid back. All I know is that it's a "0.3x corrective lens". In Holga spirit I've been looking at various optics suppliers for diopter lenses, but I just don't really know what that '0.3x' stands for, I've only found lens specs for magnification and diopter values. Does anyone have any idea how I could go about replicating that correction lens, or perhaps even have one they could send a few pictures of?

There's a standard formula for converting between diopters and diameter magnification. Presuming this "0.3x" is intended to mean "3/10x" that should be equal to a negative lens of 10/3 - 1 = 7/3 diopter, or a focal length (for a negative lens) of a bit less than -420 mm. Remember, all it needs to do is set the focus plane back by something like 3/4 of an inch past the usual 60mm, and you can check my math by measuyring the required new focal length and convert that and the 60mm stock lens to diopters to get the diopter strength needed for the corrector.

there was a ready made solution for a manual instax back. it came with a lens to put in front of the holga lens. I have 2 of these. have a look at https://www.plastikcam.com/holga_instant_.htm and contact me, if there are further questions.

xya said:

there was a ready made solution for a manual instax back. it came with a lens to put in front of the holga lens. I have 2 of these. have a look at https://www.plastikcam.com/holga_instant_.htm and contact me, if there are further questions.

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I had seen those but haven't been able to find one for sale, do you by chance know of any listings?

no, I don't know of any listings. but it should be easy to dermine the value...if we hadn't corona lockdown. in germany opticians are closed. I will have it measured when they are open again and I will send you a message. might be a month or three..