Basic newbie question on cmos sensor and optics

I have a very silly question, I suppose... please forgive me, but I'm really
just an electronics guy.

I really would appreciate a word of wisdom!

Suppose you have two (very) different cmos sensors, from the same
manufacturer (Micron, in this case):

Sensor A (MT9V111):

1/4 inch
Active area: 3.58mm(H) x 2.69mm(V) (9.63 mm^2)
Resolution: 640x480
Pixel size: 5.6um x 5.6um
Sensitivity: 1.9 Volts / lux*sec at 550 nm

Sensor B (MT9M001):

1/2 inch
Active area: total 6.65x5.32; region of interest really used: 5.32x3.99
mm (21.25 mm^2)
Resolution: 1280x1024, region of interest really used: 1024x768
Pixel size: 5.2um x 5.2um
Sensitivity: 1.8 Volts / lux*sec at 550 nm

Pixel size is similar, so I can undestand why sensitivity is similar, also.

Given the same target illumination, and given the same area of view (roughly
11.8 x 8.8 mm at 8 mm of target distance from external surface, position of
best focus, with a wide field of view of roughly 85 degrees), and given the
same perceived depth of field (I suppose I need to state "at same percent
of loss of MTF" ?).... how will be sensitivity ?

Obviously using two different custom designed optics.

FIRST THOUGHT:

Sensitivity is similar, so I can reach 1024x768 with similar output signal,
so similar noise, given the same (constrained) exposure (I need live video,
30 fps). Of course, it would be more costly...

SECOND THOUGHT:

But now I'm asking myself.... if the light I drop on the target is the same,
the light that comes back must be spread on a much wider area on sensor B,
so the single pixels will receive a much smaller light flux... Used,
"illuminated" area of B would be 21.25 / 9.63 = 2.2 times bigger

What signal I'll get from B ?

Just 1.8 / 1.9 = 0.947 times the signal of A as stated from sensitivity ?
This would be okay...

Or will B output 0.947 times * (1 / 2.2) = 0.430 times the signal of A ?

In other words... if the two different optics are made to fit each one
active area, and target size, distance, light will be the same, do I need to
factor in also the bigger area of B ?

It's counter-intuitive to me why a much bigger, more costly sensor, with
similar pixel size, given the same target image and illumination, would give
me back a more noisy image...

I'm trying to learn optics by myself, but It's not easy...





.

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