If I could place the RX100 sensor in A500 for bird shooting, .........

I can see that I have really stirred things up. Almost everybody thinks that I am wrong. I am not going to reply to every single post, so here are instead two replies to all of you.

It seems that my opponents in this thread can be divided into two groups:

1. Those who do not agree that a 2.7x crop sensor behind an f/5.6 lens will receive the same total amount of light as a full frame sensor behind an f/15 lens.

2. Those who understand that my claim about the total amount of light is correct, but who also thinks it is irrelevant.

In order to keep some structure in the discussion, I will write one separate reply to each of those two groups. If you are in group #2, please reply to my other post.

This is my reply to those in group #1:

I assume that we can all agree that a sensor behind an f/15 lens will require a 7.3x higher ISO than a sensor behind an f/5.6 lens if scene lighting and shutter speed are the same.

The different definitions of ISO are based on light quantity per unit of sensor area. There is a direct (but reciprocal) relation between ISO and exposure measured in seconds * lux or seconds * lumen / m².

This means that at a 7.3x higher ISO, the necessary amount of light per sensor area will be 7.3x smaller, no matter which sensor size you look at.

So, the full frame sensor behind the f/15 lens will...:

• have a 7.3x larger area
• receive a 7.3x smaller amount of light per unit of sensor area.
  

End result: Same total amount of light.

If you agree with me now on this point, but think it is irrelevant, please reply to my other post.

If you still do not agree with me on this point, please feel free to reply to this post.