Originally posted by dtmateojr You do not understand f-stop at all. It's very basic physics. F-stop controls the amount of light hitting the sensels. Sensor size is immaterial.
---------- Post added 07-15-15 at 09:30 ----------
For the sake of everyone, allow me to explain the very basic concept of f-stop. This is the sort of explanation that even kids should be able to understand. After this, I will stop responding to this thread. Buckle up...
Assume that you are shooting a uniform light source and comparing m43 and FF sensors. To get the same angle of view, we need to use 25mm and 50mm lenses respectively. Assume for now that the lens openings are the same for both.
You mix a lot of thing trying to prove you point.
Quote: The effect of increasing focal length is magnification. Longer focal lengths project larger images on the focal plane. If a subject is projected as a 1x1 square at 25mm it will be projected as a 2x2 square at 50mm.
The projected size of the image is not affected by focal length, it depends on the image circle of the lens. a 50mm lens for m43 is projected on a smaller area at the focal plane than a 25mm for FF. If the light source is uniform, focal length do not affect exposure, only f-stop does.
Quote: The same amount of light coming from the source is therefore spread more thinly over a wider area by the longer lens. This effectively reduces the amount of light PER UNIT AREA hitting the focal plane (sensor). Every sensel behind the longer lens will receive four times LESS the amount of light compared to the smaller sensor. This results in underexposure of the larger sensor. It's like spreading a spoonful of peanut butter on different sizes of loaves. The smaller loaf will have more peanut butter per bite.
If using the same source of light for different sensor sizes (same f-stop) the illumination will be the same on both and the larger sensor will capture more total amount of light.
Using different focal length with same aperture size (different f-stop) as in your example is not using the same source of light.
Fi using 25mm f/2 for m43 has the same aperture size as 50mm f/4 on FF.
Quote: To compensate for the unfair spread of incoming light, we need to
1) increase the opening of the longer lens, or,
2) reduce the opening of the shorter lens
What is usually done is to increase ISO with two stops on FF compared to m43 in you example,
On m43 - 25mm f/2 @ 1/100s @ ISO100
On FF - 50mm f/4 @ 1/100s @ ISO400
Quote: The ratio between the focal length and lens opening is your f-stop. That's why for the same f-stop, each sensel receives exactly the same amount of light. Sensor size is immaterial. This is the physics of photography in its entirety.
Only true if assuming that the sensel size is the same of different sensor size. If sensor size was immaterial, then why are not everyone using smartphones?