How do you deal with out of focus issues when the Aperture opens (during the evening). ?
typically our efforts at focusing are done during daylight conditions, but camera goes out of focus at night
Camera Focusing Tutorial
Published Oct 14, 2019 14:21 PM
Camera focus is fundamental to quality imaging. Mistakes can significantly reduce details, making cameras less effective.
In this guide, we explain focus issues and proper procedure, covering:
- Fixed vs. Varifocal Cameras
- Auto vs. Manual Focus
- Two Fundamental Lens Controls
- How to Focus
- Using Autofocus
- Potential Autofocus Issues
- Manufacturer Focus Control Differences - Avigilon, Axis, Hikvision
- Manual Varifocal Lenses
- Manual Iris Lenses
Fixed ***** ****** *******
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Autofocus *** **** ******
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How ** *****
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Using **** *****/********* ******
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Manufacturer ****/***** ******** ****
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Comments (15)
UI
Undisclosed Integrator #1
Aug 09, 2018UM
Undisclosed Manufacturer #3
Aug 09, 2018I am surprised there was no mention of neutral density filters being used as part of the focusing process.
When you focus in daylight, the iris is relatively closed which maximizes depth of field (the amount of distances in focus).
When you hold a neutral density filter (they're dark) in front of the lens, it "tricks" the lens into thinking it's nighttime, the iris opens creating the worst case for depth of field. You can then focus on the primary target distance. There still may be distances out of focus but you will have done the best you can.
Also, note the f-number of the lens you're using (or integrated) with the camera. The lower (or "faster") the f-number of the lens, the better you can achieve low light performance as well as increased depth of field compared to a "slower" lens.
As I recall from many years ago, 1 f-stop "slower" represents a 50% loss of light passed. For example, an f1.4 lens passes 50% less light than an f1.0. Optical physicists, kindly correct me if that is mis-stated.
The other potential problem is if the camera drops to B/W and relies on IR to illuminate the scene. When a camera goes from color to B/W with IR, there is a focal shift which can cause the focus to go soft. I think most lenses are IR corrected to solve that problem but not necessarily.
Luca Fogliati
Aug 10, 2018Instead of filters i'd rather use a simulated night scene (if the focusing is done in day light conditions) by adjusting the shutter speed at 1/1000 or 1/5000.Camera goes in black and white mode, the iris opens as if it's night and you can fine focus in a way that works both for day and night conditions .Still there is no way to simulate the IR different focal plane but to focus at true night time with IR on
Corey Vavra
Oct 15, 2019I had never heard of a neutral density filter, but after reading it sounds like how I was taught to focus cameras (back in the day), with the the iris open. We used a welding helmet glass lens. $3.50 at the hardware store. A little clumsy to hold the glass against the lens but it worked. Made the image look just like night and would get a great focus each time.
TT
Tan Tharp
Oct 27, 2020I've never given much thought to cameras focusing differently at night-time due to IR before this article. This comment is fantastic as it provides an easy and cheap solution to testing IR focus levels. Thank you for the tip!
UI
Undisclosed Integrator #4
Aug 09, 2018#2 welders glass in front of the lens will simulate night to allow for a proper night focus. You will still likely have a little offset if using IR to illuminate the scene.
Years ago, a manufacturer passed those out at an ISC show.
Howard Kohnstamm
Aug 10, 2018I believe they were #5 lenses for welding googles. I've passed them out at my training classes for years. They're getting harder to find as even welding has gone electronic with googles/hoods that darken automatically.
Ethan Ace
Aug 10, 2018Are you seeing this often? What cameras are you using?
It used to be common practice to use filters or welder's glass during focusing to simulate night time, but in our testing and based on comments from others, we have not found it to be a major problem in 2018.
Additionally, many (possibly most) autofocus models will automatically refocus the camera when switching from day to night, which reduces these issues further.
It's possible that there may be fine focus issues in specific cases, and I have seen autofocus fail sometimes, but in the vast majority of tests I've personally performed in the past couple of years, focus has been fine moving from day to night.
UM
Undisclosed Manufacturer #2
Aug 09, 2018Great material. One comment I wanted to make is that I know of at least 2 brands that allow you to perform an auto-focus on a specified area of the scene. So, if the auto-focus is confused and tried to focus on the wrong part of the scene, you can draw a box, and then press the auto-focus button and the camera will focus on that part of the scene.
A question I have - how long do different camera take to perform an auto-focus? What is the range you have seen?
Ethan Ace
Aug 10, 2018We mention the focus area in the second video, actually. The one thing I'd point out is that I've seen that focus area box be effectively useless on a number of cameras. It may make a slight difference, but ultimately if focus is so far out that it's having trouble on the full screen, I haven't seen the focus area change that.
As far as focus time, I'd say it ranges from about 2-3 seconds to maybe 10. I'd say "average" is probably 5-7. A full focus scan on some cameras may take longer, as well, but that's generally only recommended under specific circumstances or when re-initializing/re-calibrating a lens by the manufacturers which support it.
Luca Fogliati
Aug 10, 2018About autofocus it would be interesting to mention the mode that the camera uses to determine when the image is in focus.I guess that the most used (maybe the only?)is working with contrast so the more contrasted image=focus. But mayeb there are other modes that i don't know so it would be interesting to compare
CH
Charng Haw Guo
Oct 30, 2018Newer information about the fundamental control of the auto / manual focus setting / adjustment. Interesting!!
U
Undisclosed #5
Nov 19, 2018Great article.
JS
Joshua Stanley
Feb 05, 2019Good article. I notice sometimes some of the 36x rooftop PTZs we use struggle to get in focus when zoomed to their maximize zoom (36x). I can usually correct the issue by zooming out just a bit and then back in. I will try the fine focus now.
Chuck Vivian
Jul 24, 2020Great detail in the reading assignment.