Focused at night, blurry in day?

[Smith 0]

We are having some issues with some of our field cameras with focus shift.

 

We get the predicted loss of focus when a camera is focused at daytime and then is out of focus at night (reduced depth of field, which I understand).

 

BUT, what we also sometimes get is that when a camera is focused at night, it is out of focus in the daylight. This I do not understand, especially as we are using IR filters on the camera which (as I understand it) should actually improve the day-night focus situation vs. no filter.

 

Info: we are using zoom lenses on b/w IR sensitive camera with a 750 IR filter, and IR illumination at night.

 

Do I have some false assumptions? Any help appreciated.

 

We are also getting some glare issues in the day, but I might delay that question to another post

 

Many thanks.

[CameraGimp 0]

Your theory sounds right. The 750nm filter, can you clarify how you use it for me.

[Smith 0]

Your theory sounds right. The 750nm filter, can you clarify how you use it for me.

 

The IR filter is permanently fitted to the camera (in front of the zoom lens). The filters are mainly there to reduce glare from headlights etc.

 

I hope that is what you wanted to know.

 

btw if anyone knows of a way to fit a smaller filter between the back of the lens and the camera's sensor that would be very interesting (smaller filters are cheaper!).

[CameraGimp 0]

btw if anyone knows of a way to fit a smaller filter between the back of the lens and the camera's sensor that would be very interesting (smaller filters are cheaper!).

 

It is easy enough to fit a filter between the CCD and the lens on a black and white camera if you can get the parts. All colour cameras already have one fitted and as most manufacturers use the same mechanics for colour and black and white cameras you should be able to fit one in the same manner as they would on the colour models in the range.

 

fyi the filter on a colour camera is called an OLPF and IR cut filter. The Optical Low Pass Ffilter gets rid of barbers pole (colour fringes) on edges and the IR cut filter stops IR getting to the CCD and effecting the colour reproduction.

[CameraGimp 0]

Forgot to add that putting a filter between the CCD and lens will effect the back focus distance.

[SBTVideoman 0]

Is the lens an "IR-corrected" lens (also known as a true day/night lens)?

 

It sounds like when you focus the camera at night, then it's actually focussed on the IR wavelength.

 

But during the daytime the lens is working primarily with visible wavelenghts. A non-IR-corrected lens cannot focus IR wavelenghts and visible wavelenghts to the same spot.

 

I tried this once with a IR-sensitive B&W camera and two light sources - fluorescent (no IR component) and a dedicated LED-type IR illuminator.

 

If I fitted a normal lens to the camera, I could have good focus EITHER with the fluoro visible light OR the IR illuminator but not both, no matter which one I used to focus the lens.

 

If I then fitted the IR-corrected lens I had good focus on both light sources.

 

Hope that helps.

[Smith 0]

Is the lens an "IR-corrected" lens (also known as a true day/night lens)?

 

It sounds like when you focus the camera at night, then it's actually focussed on the IR wavelength.

 

But during the daytime the lens is working primarily with visible wavelenghts. A non-IR-corrected lens cannot focus IR wavelenghts and visible wavelenghts to the same spot.

 

No the lenses are not IR-corrected types, but they are fitted with a IR pass filter that cuts out anything below about 750nm.

So yes, when it's focused at night it is focused on IR wavelength, but it should stay working on the IR wavelength even during the day (I think!).

Because the daylight is often direct sunlight, I wonder if the filter is just not cutting enough light?? I don't know, I was hoping there was an "obvious" answer.

Can backfocus affect the day/night focus shift??

[CameraGimp 0]

I come from manufacturing, not installation so my knowledge is technical theory rather than installer real world experience so I may well be talking absolute rubbish but that's never stopped me. Take this with a dose of mmmm.

 

You are seeing some of your cameras go out of focus during the day rather than at night and as you are aware this is arse about face to convention. We would expect to see a camera go out of focus at night as this is when the lens would normally be fully open and depth of field is at its smallest. I have two suggestions.

 

1. Could the cameras be set up incorrectly? Specifically electronic iris or auto exposure turned on with motorised iris lenses. I think I have seen this cause soft focus in bright light in the past as it usually causes a lens to open fully.

2. If you are using an IR pass filter I wonder if your cameras are seeing less light during the day than at night when your lamps are on? In which case you would have your worse depth of field during the day. If this is the case you should focus the camera during the day rather than at night.

 

I don't think back focus is the issue. It may need adjusting to get you in focus but I don't see how it would cause the drift between night and day unless the shift is temperature related, so the mechanics move due to changes in temperature, you're not in some extreme location are you?.

[Smith 0]

1. Could the cameras be set up incorrectly? Specifically electronic iris or auto exposure turned on with motorised iris lenses. I think I have seen this cause soft focus in bright light in the past as it usually causes a lens to open fully.

 

We are using a fixed electronic shutter speed so I don't think so but I will check some of the camera settings in case some dip switches are not set right.

 

2. If you are using an IR pass filter I wonder if your cameras are seeing less light during the day than at night when your lamps are on? In which case you would have your worse depth of field during the day. If this is the case you should focus the camera during the day rather than at night.

 

I don't think that's possible, the sun puts out a lot more IR than our lamps ever could!

 

I don't think back focus is the issue. It may need adjusting to get you in focus but I don't see how it would cause the drift between night and day unless the shift is temperature related, so the mechanics move due to changes in temperature, you're not in some extreme location are you?.

 

I would not say 'extreme', but the systems are completely exposed to weather 24hrs, so they can go from quite cold at night to hot in the day (due to being in direct sunlight). I had not considered temperature change, that might be possible. Also some electronics can be temperature sensitive so they could be drifting a bit. Either way, if it IS a thermal issue I can't see any solution.

 

Many thanks for your help.

[rory 0]

Ive been told of this method before, tried it down here with some 850nm filters, it didnt work, so threw the idea in the bin. In fact, I could barely see through the filter in the day. Also tried polarizing filters, those were useless.

 

But, I dont think the sun will be out all day (its not even down here), so are you certain this is not happening when clouds are going over, and there is not full sunlight?

 

Also, you mentioned a Zoom lens, is this a manual zoom lens (Varifocal) or Auto Zoom - if the later, that maybe the problem, have they been backfocused properly?

 

BTW if you use fixed lenses you should not have this problem, but then ofcourse you wont be able to zoom

 

Also, what cameras and what lenses, and is it happening on all of them?