Differences in board mount lenses

[bobgroger 0]

I have a couple Dahua 2MP dome cameras with the 2.8mm lens. The application I have requires a 6 or maybe 8mm lens. I bought some $7 M12 lenses from an ebay seller so I could test which one worked better. While the lenses I got have the appropriate field of view and the focus is sharp, there seems to be some major difference in the optical filtering or IR characteristics. The color with the new lenses is very washed out and everything is green or purple. What could the difference be, and what characteristics, brand, or seller am I looking for to get a decent performing lens. It doesn't have to be $7, but <$25 would be nice.

 

BTW, the original lens has a reflective purple coating on the inner lens, the replacement does not. I looked for IR Cut on the camera and could not find it.

Thanks!

 

Bob Groger

IT Guy: Yes

Surveillance guy: No

[buellwinkle 0]

Many of the cheap lenses you may be buying are not made for the larger sensor on the Dahua camera which I believe is 1/3" sensor, but your cheaper lens may be for VGA resolution and 1/4" sensor. The lens on the Dahua, since it's a day only camera has the IR cut filter coating right on the lens as you've already figured out. The lens you are buying are generic, so they assume the camera will have a mechanical IR cut filter or it will be a night only camera like Foscams and such.

 

Best bet is to contact the Dahua dealer you bought from and see if you can buy lenses from them.

 

Also, that may work better at night as you can use the new lens with IR illuminators. If the color bothers you, just switch the camera to b&w night mode.

[MaxIcon 0]

Yeah, it's a jungle out there. For instance, there are 2 kinds of IR coatings on lenses - one blocks IR, and the other corrects the IR focal point so that it's close to the white light focal point, avoiding a focus shift when you go to IR mode.

 

Sounds like your previous lens had IR blocking, and your new lens doesn't, assuming your Dahua isn't IR capable. If it is, well, it's the other way around! Aside for the 1/4" and 1/3" issue Buellwinkle mentioned, MP rated lenses are more likely to have good resolution on MP cams, especially around the edges, so that's something else to look for.

 

I've had mediocre luck with cheap board cam lenses - many of them have specs of dust or lint in the lens. I've had good luck with the vendor RageCams on ebay - they're helpful, and have a good selection of gear. The last few lenses I bought from them were in the $20-25 range.

[bobgroger 0]

OK, so here is what I have found out. This camera requires a lens with an IR-Cut filter. Kind of explains the poor low light performance. Anyway, most of the eBay sellers don't have a clue what they are selling, or if "IR" means IR pass or IR cut. I finally found a site that seems to have a handle on the difference and has a reasonable price considering how much I have spent researching this.

http://www.avcemporium.com/m12-x-0-5-lenses-with-optional-ir-cut-filters-for-1-3-cameras/

I have ordered a few and will report back.