Technical question: shutter speeds

Hello everyone.

From what I understand, the larger the photodiodes, the more photons they accumulate in a given time, thus a smaller required exposure time or slower ISO speed than those achieved by using a smaller sensor with smaller photodiodes. That seems pretty logical.

So I wonder, if the XTi has 10 MP, the XT has 8 MP and the same sensor size, the XT has larger photodiodes. Noise control issues aside, XT's sensor theoretically accumulates more light then the XTi in a given time. That means the XT could achieve shorter exposure times for any given shot. Is that right?

not_this_punk wrote in post #4873084
Hello everyone.

From what I understand, the larger the photodiodes, the more photons they accumulate in a given time, thus a smaller required exposure time or slower ISO speed than those achieved by using a smaller sensor with smaller photodiodes. That seems pretty logical.

So I wonder, if the XTi has 10 MP, the XT has 8 MP and the same sensor size, the XT has larger photodiodes. Noise control issues aside, XT's sensor theoretically accumulates more light then the XTi in a given time. That means the XT could achieve shorter exposure times for any given shot. Is that right?

I don't think that's the case. Digital ISO speeds are standardized based on exposure from a shutterspeed/aperature combination. Even if it were true that the XT sensors accumulate more light than the XTi, the exposure level would be standardized to the respective ISO. So if you dialed ISO 100 on each camer you will have the same or very similar shutter/aperture combination

Though NOISE is usually affected by the size of the photodiodes, so in theory larger photo sites produce less noise if all things are equal. In the case of the XTi vs XT, I believe the arrangement of photosites in the XTi allows for similar sensitivity and noise levels as the XT.

ISO is standardized and the gain of the sensor's is managed to this standard so that for a given ISO/Shutter Speed/Aperture/Light on subject combination the exposure is essentially the same (allowing for some minor variations).

If that were the case, light meters would have to have inputs for sensor size and number of megapixels.