How close can the new High Quality Camera focus with the 16 mm C-mount lens? As close as 25 cm?

Question

I have an application that needs to image something in the range of about 25-35 cm from the front of the lens. I'm looking at the new High Quality Camera with the 16 mm C-mount lens. The lens itself has a focus adjustment and it seems that the camera itself has an additional adjustable back focus:

• Back focus: Adjustable (12.5 mm–22.4 mm)

Question: Right now I just want to know if this combination can focus on things 25 to 35 cm in front of the lens or not, and produce reasonably good quality images.

Using extension tubes to get images closer than the closest design focus of a lens can degrade image quality in some cases, and a bit of that is not a problem. Perhaps an additional extension is possible, but for the purposes of this question I just need to know if these two items off the shelf will allow me to focus anywhere within 25 to 35 cm from the front of the lens.

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Answer 1

It is likely able to focus on things 25 to 35 cm in front of the lens. Note that this answer is inferring that from documents on the web only. The author of this answer has not been able to test it and can neither comment on image quality.

Multiple vendors of the "Raspberry Pi HQ 16 mm C-Mount Telephoto lens" list it as model: PT3611614M10MP (hereinafter referred to as "PT-Lens"). They also seem to link to the same datasheet, e.g. www.pollin.de and www.elektor.de; or present the specs copied from the datasheet directly on their page, e.g. www.pbtech.co.nz. The listed value for M.O.D., the minimum object distance between the front of the lens and an object the lens is able to focus on, is 0.2 m. So an object at a distance of 0.25 to 0.35 m to the front of the PT-Lens should be ok.

To put this lens into perspective consider the "Techspec 16mm C Series Fixed Focal Length Lens" (Edmund Optics) with a minimal working distance of 0.1 m (further referred to as "EO-Lens") that is somewhat comparable (though about as six times as expensive as the PT-Lens, so the comparison might as well be apples and oranges here). The PT-Lens boasts with a distortion spec of 1"(-0.7%); 1/2"(-0.5%); 1/3"(-0.15%), whereas the EO-Lens is listed with better than 1.25%. The EO-Lens offers broadband antireflection coating in the VIS range, while the PT-Lens does not make any claims about coatings at all. So under certain conditions the PT-Lens might be more prone to spurious reflection, ghost images and glare.

Answer 2

@Ghanima nailed it! I got about 27.5 cm from the end of the lens to the surface of my laptop screen with the aperture wide open for minimum depth of field.

I then added a second "6 mm" spacer bought at a local security camera store (equivalent to the "extension tube" concept mentioned in the question) and the distance shortened to 33 mm. Thus we now have a macro lens, though you need stop down the aperture to get a clear image.

I don't recommend repeating this because I found it frightening to see how dirty my laptop's screen is! :-)

I didn't bother to make the camera parallel to the screen or play with any settings, just confirm the optics. If f = 16 mm and the lens' entrance pupil is 2.5 cm behind the front of the lens (looks roughly right) then the focus mechanism can move out to almost 16.9 mm to focus close range.

If we add a second 6 mm to the lens distance to make it 21.9 to the sensor, then the thin lens equation, then the distance to the object should be 59 - 25 = 34 mm, which nicely agrees with my 33 mm.

Physics works! :-)