Focus problem. Samsung scb-2000N with sla-3580dn lens

[C8er 0]

Hi,

 

A couple of absolute CCTV newbie problems/questions. I have bought a used samsung scb-2000 cctv cam (also known as sdc-435 I think) , NTSC version, and it came with a samsung sla-3580dn 1/3" CS 3.5 = 8 mm f1:1.0 auto iris lens.

 

I have a focus problem, and a general question about auto iris lenses.

 

The question first. The auto iris lens has a manual focus lever and a manual zoom lever which I understand (am into photography and astronomy and have several digital photography cameras and telescopes, but this is my first cctv cam/lens), but as well as those two levers the whole body of the lens can also rotate 340 degrees. My questions is, what is that 340 degree rotation for and what does it adjust? The very basic lens manual calls it "slip state" but doesn't explain what it is for.

 

The problem now. I can power up the cam and can get a video stream showing just fine on both a TV screen, and to a laptop lcd screen with a dazzle usb-capture device, but I am so far unable to bring the camera/lens to focus, at any distance. I have tried releasing the two tiny lock screws and rotating the silver backfocus ring on the front of the cam to different places, with the focus lever on infinity and the cam/lens aimed at a hill in the distance, but no position of the backfocus ring gets me to focus that I can find (I presume the silver threaded ring on the front of the cam is for adjusting backfocus?). I see in the camera software menus there is a setting that says Focus Adjust but the camera manual doesn't actually say what that is for and how it might work, other than saying it is used for DC and "Video" type lenses. Do I have to use that somehow while setting the backfocus? I have the camera lens menu set to DC type lens which I think is correct for the SLA-3580DN lens.

 

Also, How can I adjust the backfocus ring while retaining a picture onscreen as to do that I have to rotate the lens on the camera, but the auto iris cable plugged into the cam stops me rotating the backfocus ring/lens more than one full circle at a time, and when I unplug the auto iris cable then the video stream goes black until I plug the cable in again.

 

Very frustrating so far. Since this is a used item I don't even know for sure if that SLA-3580DN lens is compatible with the scb-2000/sdc-435 cam. Does anyone know that, and if so can anyone suggest how I can bring the lens to focus?

 

Cheers,

Chris

[C8er 0]

Problem of focus now fixed. The previous owner had left another adapter, black plastic like the lens body, screwed in the back of the lens, that was preventing me getting the lens far enough inside the camera body to obtain correct backfocus. The adapter looked like it was part of the lens and I didn't even realise it was there, but eventually I worked out it wasn't a normal part of the lens from a pic of the lens on the web, and found that the adapter unscrewed. Without that adapter the lens back thread screws far enough into the silver backfous ring on the cam to obtain focus.

 

I still have the question about why the camera body can rotate 340 degrees once tightened up, and whether thaat does anything, or is just so you can position the auto iris cable and focus/zoom levers at a convenient spot?

 

Also the camera focus point is wildly different at different zoom positions. I.e If I set focus correctly, then move zoom lever, the focus then has to be completely reset for the new zoom postion. Is that normal for cctv cameras and lens?

 

Cheers, from a much less frustrated cctv newbie.

Chris

[Horizon 0]

Hi there C8er.

The screw ring you removed was most likely a C to CS mount adapter. A useful thing to have if you ever run into a C-mount lens (which has a 5mm shorter flange focal length than the more modern CS-mount).

 

The 340 degree rotation is for convenience/cosmetic purposes.

 

The change of focus when zooming will be because you have a varifocal lens. They are cheaper to make than zoom lenses, and are ideal for CCTV where the lens is only adjusted during installation.

[C8er 0]

Thanks for that Horizon. Very helpful answers. I have bought the cam to use for video astronomy attached to a telescope at night, so I will I think need that cs to c-mount adapter the old owner had attached to the lens. I have a c-mount to telescope adapter ordered.

 

Any idea what the FOCUS ADJ section in the camera software's LENS menu is for and how it works?

 

Cheers,

Chris

[Horizon 0]

Thanks - glad to be able to help. The focus adjust would be used to force the automatic iris fully open, so that you can set the focus correctly with a narrow depth of field.

[C8er 0]

Thanks once again horizon. That makes prefect sense about the focus adjust setting.

 

Cheers

[Numb-nuts 1]

Thanks once again horizon. That makes prefect sense about the focus adjust setting.

 

Cheers

 

I hadn't realized you bought a used camera and lens, it all makes sense now.

 

Thanks for this thread guys as I just recently learned about back-focus and

C / CS lenses and their distances to the sensor. Although I have been installing

VF DD AI lenses quite regularly I never really understood them properly.

[C8er 0]

I'm glad that thread and Horizon's answers were helpful to you too Numb. That certainly helped sort my lens problems out.

 

I've had the cam and lens trained on a birds nest in a climbing rose on our deck today, watching on an lcd monitor as the mother bird came and went all day.

 

Now I just have to wait for the ordered adapter that is on its way to connect the cctv cam to my telescopes for some live view video astronomy at night, which is what I actually bought the camera for. The lens that came with the cam was an unexpected bonus. I've removed the built in IR filter from the cam as for astronomy I will put a different bandwidth IR filter in the optical chain.

 

Cheers.

[the toss 0]

Have you re-set the backfocus ?

[C8er 0]

Yes. Didn't set the final back focus until after internal ir filter removed, but thanks for that. I understand that the change in back focus after filter removal is one third of the thickness of the filter

 

Also have had to set the white balance manually to fix subsequent red images after ir filter removal. Will use a more astronomy suited ir filter in an adapter once connected to telescope, to block ir but let through the hydrogen emission red light that is given off by many deep space objects, that the standard internal ir filter mostly blocks.

[Numb-nuts 1]

As I understand it

set the focus then do it again using an ND (nuetral density) filter to force the IR filter out and the iris open.

If you get a good focus with the ND filter in place you can rest assured in better lighting the focus will be good.

[C8er 0]

Thanks Numb. As Horizon explained, my particular cctv cam has a focus adjust software function in its settings menus, that can be used when the cam is fitted with an auto iris lens, which opens the iris fully so focus can be set at full iris opening, I think avoiding the need to use an ND filter. It's a Samsung scb-2000 cam, which also used to be called the sdc-435.

 

Cheers.