Hey everyone, I'm back after having made many, many, many changes and I want to share some of what I have learned which I believe is NOT intuitive. I'm still not 100% happy so I continue to try different things, but I've learned a lot.
In summary, sharpness, saturation, and iris settings all have a dramatic impact on motion blur in low-light situations with license plates that are a small part of the overall image. When I say, "motion blur", please note that I am not talking about the result of classic slow shutter speed. I've been using 1/2000th of a second (or faster) and I'm talking about the "blur" that I showed at 0:18 in the YouTube video in post #15 of this thread, with a vehicle moving no faster than a human can walk.
Sharpness as a contributor is something that should be somewhat obvious, and I credit
@wittaj for sharing his settings with a sharpness=2. I've found that sharpness=0 works even better for me much of the time (but not all!). The
Dahua wiki documentation on settings also clearly states that sharpness, "can blur or define details in an image", which I believe shouldn't surprise anyone who understands how that works.
Having said that, here is the next one which I didn't expect. That same
Dahua wiki page on settings says, "Also if the picture is over-saturated then it will also cause blurry or artifacting pixels." As I showed in my very first post, I left the saturation alone with the default value of 50, but I've tested and have significantly reduced my "motion blur" with a saturation setting of only 20. I want to do some more exhaustive testing to determine whether I'd be better off with a different value, but my testing shows a dramatic reduction in motion blur with a value of 20 (vs. 50). At a minimum, I did not expect a default saturation setting, on an image which is overwhelmingly black, to have such a negative impact on my motion blur.
Now for the final one, which completely caught me by surprise. A high iris setting creates blur. I'm an old photography guy, very familiar with manually setting an aperture on still/non-video cameras. I've always known the iris to be a physical restriction that controls the amount of light, but I now believe that Dahua also treats the iris setting a bit like "brightness" or "gain", using software/firmware to further adjust the image. In "old school" photography, the primary downside of a wide-open iris was that the depth of field would be reduced, but I've found that with this Dahua camera, a high iris setting contributes to my "motion blur" in low-light situations at distance.
For what it's worth, I discovered the problem with the high iris setting because during the day, I was using shutter priority with otherwise default picture settings. I wanted to ensure a fast shutter speed, thinking I didn't care about the iris/aperture. All my LPR images during the core of daylight were fantastic, but I noticed that around sunrise or sunset, when the camera just flipped to day/color mode or just before the camera would flip over to night (B&W) mode, I would see a similar license plate blur with motion. All the other settings were fixed, so I assumed the cause was a combination of less light and the auto-changing iris.
I want to also mention that when the license plates are larger or brighter, the motion blur is not nearly as significant or at least not enough to prevent reading the plate. Instead, the edges of the license plates digits may be fuzzy, but still readable. But when the plates are further away, so each digit is smaller and the light is less, the same "motion blur" makes it unreadable. If the license plates in your images are simply larger, you may not even realize that you have some of this motion blur occurring, thinking only that your image might be a little "soft". FYI, I'm trying to capture plates 175-200 feet out. This wasn't a critical issue at 150 feet, so I believe I'm also approaching or at the limits of this camera.
With that in mind, I did buy one of the
led "laser" lights off AliExpress and it's very nice (for $37 US). It allowed me to experiment with even faster shutter speeds to ensure even 1/2000th of a second was absolutely not the root cause of what I was seeing. There are other threads in these forums about the use of such lights, and it definitely makes a difference, but they have a very, very strong red glow which does draw people's attention to them, unlike the camera's built-in LEDs which have only faint glow.
I continue to tweak/test, and eventually would love to convince my wife to drive back & forth numerous times while I change settings to capture "before & after" clips with each setting. My testing definitely hasn't been scientific, as I'd often change multiple settings at once, out of frustration, but I am confident that individual reductions in sharpness, saturation, and iris have each significantly reduced my "motion blur" at distance with low light.