how is it different?, the more FOV you try to fit in the screen, the more distorted the image will get, and the farther away the objects will be perceived, try this in practice mode and you'll see.
This zoom is to correct that effect when needed, which is ironsighted and controlled breathing, about missing out information... there are peripheral view indicators for that reason right?
The thing is that the fov distortion only describes the effect of flattening a round image. The only way to effectively remove the fov distortion effect is to either go for a curved monitor or making the fov infinitely small.
The fov zoom effect is something different.
If A is shorter then B then everything in the game will look smaller than it would in real life. If A is longer then B then everything in the game will look bigger than it would in real life. If A is equal to B then objects look to be of the same size as in real life.
If you have a monitor with a width of say .5 meters and you sit at a distance of .5 meter to your monitor. Then calculating the fov the monitor takes up from your field of vision would be:
The fov you percieve = 2*arctan((w/2)/B))
2*arctan(.25/.5) = 53 degrees.
The ingame fov by default is 85 degrees, or backwards calculating
A = (w/2) / tan(ingamefov/2)
.25/tan(85/2) = 0,27 meters.
So we have B = .5 meters while A=.27 meters.
This means that in this case everything on the screen appears (B/A =) 1,8 times smaller than it should be. In order to get a 6x zoom sniper scope you would need to end up with a value of A and B such that A/B = 6
In order to make enemies appear at the same size as in reality the in game fov would need to be changed to 53 degrees.
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You can feel free to test this, measure the distance from your eye to your monitor and measure the horizontal width of your monitor (not diagonal or vertical). Then using 2*arctan((w/2)/B)) calculate the fov in degrees your monitor takes up for you.
If you fill that amount in Red Orchestra everything will be at a 1:1 scale with reality. However if you move your head halfway closer to your screen everything will appear 2 times bigger than in reality. Or move your head double as far from your screen everything will seem 2 times smaller than in reality.