[nater]

Zoom C-mounts are a different story, though. Do not make the mistake of thinking it will have the same functionality as a "real" zoom lens. The ones I have tried works more like a number of primes in the same barrel: whenever you change focal length (or F:stop) you will also have to refocus the lens.

A lens that changes focus point when zooming is called a varifocal lens (there are even some K-mount zoom lenses that are varifocal, though it is not that common). Most manual focus DSLR zoom lenses are parfocal, i.e. they retain the focus point while zooming. There is extra optical design work needed to make a lens parfocal, which is why it's less common with CCTV lenses (and some AF lenses, since it's not as important to think about retaining focus point while zooming in an AF lens).

Focus point changing when adjusting F-stop is known as focus shift, and is something that happens with lenses that let in lots of light (this is common in full-frame DSLR lenses that are f/1.4 and especially f/1.2).

[nater]

General advice regarding C-mount lenses is difficult, since I've found things can vary quite a bit from lens to lens since C-mount has been used for so many different applications. I'm price conscious, and I don't mind searching used markets and waiting for a good deal, and have found some great older lenses that I think were aimed perhaps for science and/or industrial imaging as much as for CCTV security. Modern C-mount lenses are often rated in terms of megapixels, so they'll say "2 megapixel" or "5 megapixel", which is a pretty clear indicator you shouldn't expect to get 12.4 megapixels of resolution out of those lenses fully open. It also wouldn't surprise me if C-mount lenses from the era when they might be used with film (even 16mm film) would be designed with more resolution capability than more recent lenses intended for standard-def CCTV applications.

[Erich_H]

A lens that changes focus point when zooming is called a varifocal lens (there are even some K-mount zoom lenses that are varifocal, though it is not that common). Most manual focus DSLR zoom lenses are parfocal, i.e. they retain the focus point while zooming. There is extra optical design work needed to make a lens parfocal, which is why it's less common with CCTV lenses (and some AF lenses, since it's not as important to think about retaining focus point while zooming in an AF lens).

Focus point changing when adjusting F-stop is known as focus shift, and is something that happens with lenses that let in lots of light (this is common in full-frame DSLR lenses that are f/1.4 and especially f/1.2).

Thanks Nater - I didn't know that the Chinese had a scientific alibi for "crappy".
I guess that'll be my quota for new stuf learnt for monday -
thanks/Erik

[Afterthought: I checked the listing for my 2.8-12 mm, and the Chinese did sell it as a varifocal lens...]

[Afterthought 2: "The ones I have tried works more like a number of primes in the same barrel"...so it really IS exactly like this!]

[GibbyTheMole]

The Fujian 35/1.7 is certainly worth the price, it's small enough to be nice on the Q.

Just want to heartily second that recommendation. Mine showed up the other day & I had a chance to take it out for a test run. It's quite sharp, especially for a $25 lens. Build quality is decent, it's a great focal length on the Q (200mm equiv.) and I don't notice much of the edge softness on the Q that the m4/3rd guys talk about. Probably because of the Q's sensor size. It also looks perfectly at home on the Q, if that sort of thing matters to you.

It might sound unlikely, but the Fujian 35 may have a hair better IQ than the 02 zoom. I think it's at least equal. Only thing is, you really have to nail the focus for best results (but don't you always? and MF focus assist is a BIG help.) and the supplied lens cap falls off. The Fujian 35 has 34mm filter threads, so I ordered a 34mm to 37mm step up ring and a proper 37mm snap cap for it. $4 total. No biggie.

Another potential plus is that because the front element is pretty deeply recessed, it may not need a hood.

For the price & the IQ, this one's certainly a big winner.

[interested_observer]

The camera - with the 01 lens came about 2 weeks ago. I have taken it out and its a fun little camera. Its a replacement for the Panasonic LX3 that my son took to school with him. I wanted something with an interchangeable lens that was relatively small, and this fit the bill. The LX3 was 24 to 60mm at f2 which worked out very well for me. So, I am looking for something in the same range, which leads me to my first question.

Since the Q has an equivalent crop factor of about 5.5, a 24mm equivalent focal length would be something around 4mm. I am seeing a lot of 6 to 12mm focal lengths here in the C mount. Has anyone come across something in the 3 to 4mm that has performed well?

[Erich_H]

Since the Q has an equivalent crop factor of about 5.5, a 24mm equivalent focal length would be something around 4mm. I am seeing a lot of 6 to 12mm focal lengths here in the C mount. Has anyone come across something in the 3 to 4mm that has performed well?

Hi! In this other thread "0.5x wide angel attachment? or C-mount wide angle?" you'll find some more;
try this link:
https://www.pentaxforums.com/forums/pentax-q/210949-0-5x-wide-angel-attachmen...ml#post2235194

Thanks/Erik