bug99
Pulling my weight
- Joined
- Dec 27, 2016
- Messages
- 397
- Reaction score
- 154
Does auto focus work well on a slow moving object? If I zoom in (max zoom) to 12mm on a varifocal camera like the HFW8232E-Z and start from a couple of hundred feet out and then approach the camera (I think @nayr did something like this while checking it out), do I stay in focus all the way in (to a few hundred mm), or does the focus stay at infinity?
It occurs to me ( I am sure something like this has occurred to lots of others), that in many situations, the farther the camera can be from the subject the better, so long as there is enough lens to zoom. By being a long way away and zooming all the way in, the resultant depth of field that is in focus is far greater and there are very few useless pixels to the sides as the beam angle is very narrow. As an extreme example I ran some numbers for the 25X PTZ starlight. If I move the camera far enough away to get only 70 PPF at the far end of what I care about at max zoom, that line at the far end of the narrow (2.4 deg) triangle is over 650 feet away! If I move closer to ½ of that distance I will have 140PPF at 325 ft, likely all still in perfect focus even if the focus is fixed.
While I am not saying that this will work for most situations (likely very few), it does point out a great difference. I first noticed this at my front door. I could not move the camera away from the person at the mat. Even with a 2.8mm lens, unless they stepped back two extra feet, I would miss faces or packages. With a 2.8mm lens, detail is useless at about 15 ft and by that time I was wasting most of the pixels on the sides. A similar set of calculations with the 12mm lens on the varifocal above has the far side of the normal road I live on with too few PPF yet the near side is fine, yet the road is only 35 ft wide.
Here is something to keep in mind: 70PPF = 27.4 ft with 1920 pixels (or 19ft at 100PPF). It makes no difference what the zoom is. More covered area is always better, but getting more beam angle in most situations only throws away pixels. So as long as focus can keep up, depth is free. In other words the usable ground area is the triangle area from where the triangle is 27ft wide - back to the camera. If it is 650 ft away, it is a lot more area than if it is 10 ft away (h*b/2 = 650*27/2 vs 10*27/2 or 65 times more usable area).
I suspect that focus cannot keep up and somewhere in the middle is where truth and usability lies.
It occurs to me ( I am sure something like this has occurred to lots of others), that in many situations, the farther the camera can be from the subject the better, so long as there is enough lens to zoom. By being a long way away and zooming all the way in, the resultant depth of field that is in focus is far greater and there are very few useless pixels to the sides as the beam angle is very narrow. As an extreme example I ran some numbers for the 25X PTZ starlight. If I move the camera far enough away to get only 70 PPF at the far end of what I care about at max zoom, that line at the far end of the narrow (2.4 deg) triangle is over 650 feet away! If I move closer to ½ of that distance I will have 140PPF at 325 ft, likely all still in perfect focus even if the focus is fixed.
While I am not saying that this will work for most situations (likely very few), it does point out a great difference. I first noticed this at my front door. I could not move the camera away from the person at the mat. Even with a 2.8mm lens, unless they stepped back two extra feet, I would miss faces or packages. With a 2.8mm lens, detail is useless at about 15 ft and by that time I was wasting most of the pixels on the sides. A similar set of calculations with the 12mm lens on the varifocal above has the far side of the normal road I live on with too few PPF yet the near side is fine, yet the road is only 35 ft wide.
Here is something to keep in mind: 70PPF = 27.4 ft with 1920 pixels (or 19ft at 100PPF). It makes no difference what the zoom is. More covered area is always better, but getting more beam angle in most situations only throws away pixels. So as long as focus can keep up, depth is free. In other words the usable ground area is the triangle area from where the triangle is 27ft wide - back to the camera. If it is 650 ft away, it is a lot more area than if it is 10 ft away (h*b/2 = 650*27/2 vs 10*27/2 or 65 times more usable area).
I suspect that focus cannot keep up and somewhere in the middle is where truth and usability lies.