Use standard, cheap M12 lenses with your GoPro Hero3!

What is it?

This is a replacement lens mount for GoPro Hero3 Black and GoPro Hero3+ cameras. It has a longer threaded portion, which allows for standard M12 lenses to be used and focused. It also has space for an internal IR/UV filter, although I ended up cutting my own to fit instead of waiting for one from China. If you can find an 11mm square filter it will work.

Lenses that need this mount

Obviously the standard GoPro lens does not need this mount. Lenses with a focal length of 6mm or less do not need this mount, as they need to be close enough to the sensor that they have good thread engagement with the original lens mount.
Lenses with focal lengths longer than 12mm need this mount to be held securely and focused. I have not tested lenses between 6mm and 12mm, and as there are a variety of vendors for M12 lenses, it's possible that there are longer focal length lenses that work fine with the standard mount, or lenses with a shorter than 6mm focal length that require this mount.

Testing

This has been tested on GoPro Hero 3 Black, GoPro Hero 3+ Silver, and GoPro Hero 3+ Black cameras. I don't know if it will work on Hero 3 Silver or Hero 3 White cameras, and I don't know if it will work on Hero 4 cameras. However, the stock replacement part for these cameras appears to be the same, so it's possible this lens mount works with all 3 and 3+ cameras. This may work with GoPro 3 clones as well; YMMV.

Why?

M12 lenses are super cheap ($10-30 USD) and the Hero3 Black is a great little camera that can be found very inexpensively on Ebay (often under $50 for one in good condition). Phone cameras are fantastic, but the Hero3 Black is still a great basic camera that can be used for entry-level cinematography. It's also very small, lightweight, and designed to handle being tossed around. By using longer focal length lenses, you can achieve standard FOVs.

However, longer focal length M12 lenses need to be held further from the sensor than the standard GoPro lens, and the threads on the factory lens mount do not extend far enough to make this possible for the lenses I tested. The easiest fix is to replicate the part with the threads extended to the lip of the lens mount.

How?

This is a drop-in replacement piece for the existing lens mount, so you will need to remove 12 screws and avoid damaging a ribbon cable to install this mount. Once installed, the camera is reassembled and you're ready to use standard M12 lenses with your GoPro.

Lens FOV Conversion

The Hero3 Black (and Hero3+ Black) have 1/2.3 sensors. This can be plugged into any online focal length comparison calculator (e.g. https://www.omnicalculator.com/other/crop-factor) along with the lens focal length to determine the equivalent full-frame focal length.

For basic M12 focal lengths, the following conversions can be used:

• 2.8mm M12 on GoPro ≈ 16mm full-frame ≈ 24mm APS-C
• 3.6mm M12 on GoPro ≈ 20mm full-frame ≈ 30mm APS-C
• 6mm M12 on GoPro ≈ 34mm full-frame ≈ 51mm APS-C
• 8mm M12 on GoPro ≈ 45mm full-frame ≈ 68mm APS-C
• 12mm M12 on GoPro ≈ 68mm full-frame ≈ 102mm APS-C
• 16mm M12 on GoPro ≈ 90mm full-frame ≈ 135mm APS-C
• 25mm M12 on GoPro ≈ 140mm full-frame ≈ 210mm APS-C
• 35mm M12 on GoPro ≈ 200mm full-frame ≈ 300mm APS-C
• 50mm M12 on GoPro ≈ 280mm full-frame ≈ 420mm APS-C

Other Considerations

• This mod causes standard GoPro lenses to be unusable, so if you want that fisheye, you will need to swap the old lens mount back in OR modify your GoPro lens to allow it to thread far enough in.
• This is a reversible mod, but will require disassembly which shouldn't be done in the field.
• These lenses typically do not have adjustable irises, so they are fixed aperture. There are multiple aperture options for most focal lengths, however, so some amount of customizing is possible.
• The GoPro sensor is pretty amazing, but it's still super tiny compared to full-frame cameras; that means that it needs a lot more light than a full-frame or APS-C sensor. However, it has a pretty high dynamic range, which helps a lot.