Focusing IR vs visible light.

[Bitslizer]

I have been diving into the old threads in this lpr forum a lot recently. Many have already discussed the issue of day vs night focus issue. Although I suspect many of you know or have thought about this, but I don't think anyone have explicitly stated the cause here. I also ran into this issue with my ptz where if I pre dial the day time focus it would be a mushed plate at night.

Excerpt from Google

What is the difference between visible light and infrared?

Near IR light has longer waves than visible light, so it focuses at a different point. Infrared photography requires a shift to achieve good focus, so it's necessary to use IR focus marks or have the focus calibrated, which is explained in this chapter. Infrared focus marks on zoom lenses.

Think chromatic aberration in regular photography but exaggerated even more with IR

 

[wittaj]

Here is a great older post that goes into detail how the focus for daytime and nighttime can be different because of IR. For some they are the same, but for a lot of people, it is different:

Infrared focus shift - explanation

While reading the forum's threads, I found some posts with complains about soft image while the camera is going from color to IR or the opposite. In this Testing roundup IPC-HDBW4431EP-AS versus IPC-HDBW5431E-Z thread, TVT73 is complaining about soft focus in IPC-HDBW4431EP-AS camera. Here Any...

ipcamtalk.com

 

[Bitslizer]

Here is a great older post that goes into detail how the focus for daytime and nighttime can be different because of IR. For some they are the same, but for a lot of people, it is different:

Infrared focus shift - explanation

While reading the forum's threads, I found some posts with complains about soft image while the camera is going from color to IR or the opposite. In this Testing roundup IPC-HDBW4431EP-AS versus IPC-HDBW5431E-Z thread, TVT73 is complaining about soft focus in IPC-HDBW4431EP-AS camera. Here Any...

ipcamtalk.com

Guess I didn't go deep enough into the rabbit hole

-- edit

Rather I needed to dive down a different rabbit hole as that post was in the dahua sub forum instead of this lpr one I have been reading up on

 

[richardgohth]

1. Some lenses will only focus visible light and not IR light
2. Some lenses are able to focus both but at different focus points ( day/night lens?)
3. Some lenses are able to focus both at the same focus point ( focus once ) ( IR corrected lens?)

 

[wittaj]

I have an LPR cam that is 1.5 years old and it is starting to struggle holding a focus. I guess a year and a half of changing twice per day it is wearing out.

Initially the focus steps were 32 steps different between day and night. Recently they started getting closer. For awhile they were the same and I thought that would keep it in check, but tonight the focus was way off, and now the steps are increasing again.

I use the sunrise/sunset utility that bp2008 wrote, so I know it is forcing it to what I want and can confirm the step numbers are accurate, but anything mechanical will wear out.