Let's start at the beginning with a simple definition of f-stop. F-stop (Focal stop) is a terminology used for the specific measurement of light loss or gain in relationship to lenses. One f-stop gain is equivalent to a one hundred percent (100%) gain in light passage in a lens. One f-stop loss is equivalent to fifty percent (50%) light loss. To help explain how we can have a 100% gain or 50% loss think of the following scale of numbers:
1 2 4 8
What is the relationship of one to two? 100% gain. The relationship of two to four? 100% gain. Four to eight? 100% gain. On the retrospect, what is the relationship of eight to four? 50% decrease. Four to two? 50% decrease, et cetera. This explains how we can have a 100% increase or a 50% decrease in any factor scale.
The next step is to understand the f-stop scale. We start (under normal lens conditions) with f1.4 and go up from there. The larger the number, the more light loss we will have within a lens. The scale is easy to build if you remember the first two numbers of f1.4 and f2. From this point it is a matter of doubling each number in alternating progression.
f-Stop Scale I:
f-1.4 f-2 f-2.8 f-4 f-5.6 f-8 f-11 f-16 f-22
Ok, now let's take a level of light and work it up and down the scale.... 8 foot candles (fc) at f-2 will become 4fc at f-2.8, 2fc at f4, 1fc at f-5.6, .5fc at f-8, et cetera..... Going the opposite way, up the scale, it's 100% gain.
As of about nine or ten years ago, Panasonic developed the Aspherical lens. This lens was originally developed for the purpose of a non-distorted wide angle image. In the process however they developed a lens that added new dimension to light passage. Consequently, we had to build the f- stop scale up from f1.4. So we now have a scale as below:
f-Stop Scale II
f-.7 f-1 f-1.4 f-2 f-2.8 f-4 f-5.6 f-8 f-11 f-16 f-22
At long last, let's look at what all of this has to do with the effect of changing lenses on cameras. Cameras are selected based upon their 1) sensitivity 2) resolution 3) features. Sensitivity refers to how little light a camera will perform in. Often times, the cost difference between two cameras can be extreme based upon sensitivity. That is Camera A has a sensitivity of .01fc while Camera B has a sensitivity of .005fc. This means that Camera A will stop producing a suitable video image somewhere around dusk while Camera B will continue to produce an image at twilight. Camera A costs $500.00 while Camera B costs $1,000.00. Wouldn't it be nice if we could find a way to make Camera A work under twilight and save $500.00 by not having to buy Camera B? Absolutely.
By checking the specification sheets on both cameras, we find that both cameras were tested for sensitivity using f1.4 lens with 75% reflectance charts. This means that both cameras were tested under equal circumstances. Now, if we can find a lens for camera A that had the ability to pass light at a rate better than f1.4, we would technically increase the sensitivity of the camera. By how much? By what ever f-stop rating the new lens had as equated to the increase over the test lens. As an example, let's say we could find a lens for camera A that had an f-stop rating of f.7. This would be a full two f-stop improvement over the lens that was used to test Camera A. Therefore the sensitivity of Camera A would go from .01fc to .005fc (one f-stop improvement {.01 / 2}) to .0025fc (second f-stop improvement {.005 / 2}). What this means is that for the cost of a slightly better lens (perhaps one or two hundred dollars), we are able to increase the overall sensitivity of Camera A to better than Camera B.
To take your exact example, if I have a camera that has a sensitivity of .1fc as tested at f1.4 at 75% and I start with a lens in the field rated for f1.2, my actual sensitivity of the camera would be about .075fc or « f-stop improvement. Now, you want to change the current lens on the camera from an f1.2 to an f1.8. This would represent about a 4/5 f-stop increase. Therefore the new field sensitivity of the camera would drop from .075fc to about .135fc (((.075 / 5) x 4)+.075).
I sincerely hope that I have given you enough information to explain to your technicians the relations of f-stops and light loss and camera sensitivity. If you have any other questions, please feel free to email again... I am in your service.
