Originally posted by Photobill But this does not apply when exposing an image?
I dislike using water and buck analogy as it can be even as confusing to some.
Think of exposure in its context as the amount of light per unit area, we have 2 image capturing devices with 2 very different areas. if we have 2 very different areas that exposure (light per unit area) will put very different amounts of light into your final image.
Probably the best way to describe the difference is to think of exposure as a uniform layer of paint that you are applying to a surface, instead of thinking of light as per unit area you are using paint per unit area. Think of the shutter speed as the duration that you are spraying the paint onto the surface and the aperture as the density at which your spray gun applies paint.
Now if we are applying paint to both sensors using the same duration(shutter speed) and same density ( f-stop) both sensors will have the same thickness of paint applied to their surfaces but because there is a difference in size there is a difference in how much paint was collected and the larger surface area will have much more paint on it.
Now for you to place the same amount of paint onto the surface of the smaller sensor and you are using the same duration (shutter speed) you will need to adjust the density (f-stop) of paint being applied, this will put a thicker layer of paint ( larger exposure {light per unit area}) giving you the same net paint being applied to that smaller surface.
For the comparison between FF and cropped you will have to increase that density of light/paint by the size of their differences, for FF to cropped that is about 2.25 times, so to adjust the f/stop you will need to apply a 1.5 crop factor. For a f/2.8 on FF you will need 2.8/1.5=f/1.87 on cropped to put the same amount of light into that image.
Now with this knowledge you may think well why can I not capture the same amount of light with my cropped camera as my FF camera, the answer is yes but up to a point. There are 2 main factors 1. does the format have access to a lens with a large enough aperture to do this and if the answer is yes than you can. 2 is if your are not exceeding the maximum size of the exposure the camera can store then yes you can.
Think of iso as an exposure index and for iso 100 you can have very different settings of f-stop and shutter speed that will give you the same exposure size (they all will give you the same light per unit area). Now both camera formats that can capture the same size exposure at base iso 100 because of their size differences the smaller format would be unable to capture the same total amount of light/paint.
We can show this by going back to the spray paint . Just as shown above you will need to use different F/stop(density of paint) to apply the same amount of paint/light to the smaller sensor. But here lies the problem in order to do this you will be changing the size of the exposure and if you are unable to lower the iso in the cropped camera to a lower iso then you will be over exposing your image.
Think of it this way if I am shooting my FF camera at iso 100 150mm f/2.8 1/100sec and I am going to try and capture the same amount of light with the cropped sensor I will need to shoot using 100mm f/1.87 1/100sec. here is the problem at f/1.87 1/100 sec the image captured by the smaller sensor will be overexposed for the same scene. This will demonstrate why a smaller sensor is unable to capture the same amount of light as the larger sensor, If we are going to try and shoot that cropped body at f/1.87 we will have to change either the iso or the shutter speed as not to overexpose. Often times we are unable to lower the iso so we are stuck to using smaller duration on our shutter speed.
Now to keep the smaller sensor from being overexposed using f/1.87 we will need to decrease the shutter speed down to 1/200 sec cutting in half the amount of light reaching the sensor as not to overexpose. Half the duration half the amount of light collected.