I have been doing remedial research on zoom and magnification. Here are some of my conclusions ("sensor" here assumes a digital camera):
- The zoom-x number of a zoom lens is the maximum focal length ÷ the minimum focal length.
If I am a fair distance away from an object (say 40 to 300 feet,) the size of the image projected on the sensor is nearly in direct proportion to the focal length.
- EDIT: If I am a fair distance away from an object (say 40 to 300 feet,) the size of the image projected on the sensor when focused is a function of the focal length and the distance of the object. Thus, for a given distance of the object from the lens, the in-focus size of the image projected on to the sensor is in (nearly) direct proportion to the focal length.
- Thus, if I am a fair distance away from an object (say 40 to 300 feet) using a zoom lens, the size of the image projected on the sensor at maximum zoom ÷ the size of the image at minimum zoom will equal the zoom factor of the lens.
Ie, if I have a 70mm/210mm zoom lens, the zoom factor is 3x. So if I am viewing an object, the image projected on the sensor will be 3x larger when zoomed all the way in than when zoomed all the way out.
Say I have a 3x zoom lens on a digital camera. I take a photo of an object which is 40 feet away at minimum zoom, then take a photo of the same object from the same position at maximum zoom. Then I open both images in Photoshop. Using my select tool I carefully measure the pixel height of the object in each image. I would assume that if I take the pixel height of the object in the max zoomed image, and divide it by the pixel height of the object in the min zoomed image, the ratio would be nearly equal to the zoom factor of the lens.
Does this all seem correct?
I am asking this question because I have purchased a surveillance camera which advertises 5x zoom, however when I test it the way I described above (pixel height of object in max zoomed image ÷ pixel height of object in min zoomed image,) it comes out to only about 3x. When questioning the manufacturer about this, they replied "The zoom number is not equal to the magnification," with a link to this SO question: How do zoom, magnification, and focal length relate?. However I find nothing in the answers to that question which contradict what I have described above.