Sunday, August 11, 2019

Doing the math on pixels per inch at a distance for your cam

Seeing a lot of posts where people are talking up 1080p cams with wide angle. Let's do the math on that. There is a simple calculator online to get the width of the area being viewed by a cam with a given view angle. For example let's take my old work hours the Foscam 9800 series that is 720p and 70 deg view angle.

So at 20 feet the view width is 24.4 feet or 293 inches. 720p resolution is actually 1,280 pixels so when we divide the pixels by the view width you get  4.3686 pixels per inch. So a face, about 6 inches wide will be 26 pixels across at 20 feet with this cam. 40 is considered to be the min. So even with the best lens focused for 20 feet this is still not going to cut it.

For convenience I worked up the charts below from my compare sheet. Note these charts assume the camera is focused for the target distance and not the upscaled resolution some cams advertise. Despite claims, outdoor fixed focus cams seem to be focused at around 20 to 30 feet. You might get a "decent 2X screen grab" of targets within +- 10 feet of that. Of course for a large range of  depth you really should look at a camera with autofocus like the Reolink RLC-411WS I've tried. Though it has had the occasional glitch as well. For long distance something adjustable like the Microseven 6-22mm 3MP Manual Zoom Varifocal Len HD 1080P.







Not those are best case numbers under perfect conditions. While 7 pixels per inch is bare min 14 is the more accepted min. If I change the wanted pixels per inch to 14 the last chart becomes

Note 12 degrees is a fairly extreme telephoto lens in the range of 22mm
70 is standard though up to 90
between 90 and 130 is often called wide angle
180 to 360 are often called 360 view cameras

For a more indepth info look here.

No comments:

Post a Comment