Bought a FF Camera. Did I make a mistake?

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Chizuka
Chizuka Contributing Member • Posts: 868
Bought a FF Camera. Did I make a mistake?
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I have been using MFT cameras for a few years now and I recently bought a Full Frame Sony A7 III. I wanted to have more dynamic range for landscape photography and better image at higher ISO for Bird photography.

I have been doing some comparison and I don’t understand why I end up with a higher ISO for the Full Frame as I do for the mft Lumix G9. Doen't a bigger sensor allow to gather more light?  I have compared the G9 + PL 100-400 at 200mm to be equivalent to the Sony A7 III + Sigma 100-400 at 400mm.

In the following shot, the G9 with PL 100-400 at 200mm is at 1/640 f6.3 ISO 5000. The Sony A7 III with Sigma 100-400 at 400mm (or so) is at 1/640 f 6.3 ISO 8000. Why is that?

I know what Full Frame cameras do better with higher ISOs but I am disappointed to see that for a same subject, I use a higher ISO for the FF camera. I wonder if I made a mistake in purchasing a new FF + lenses?

I would like to understand better why that is.

Thank you.

Unprocessed jpeg, G9 + PL 100-400 ISO 5000

Unprocessed jpeg Sony A7 III + Sigma 100-400 ISO 8000

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 Chizuka's gear list:Chizuka's gear list
Olympus M.Zuiko Digital ED 60mm F2.8 Macro Olympus 12-40mm F2.8 Pro Sony RX10 IV Olympus OM-D E-M10 II Panasonic Lumix DC-G9 +4 more
Panasonic Leica 100-400mm F4.0-6.3 ASPH Panasonic Lumix DC-G9 Sigma 100-400mm F5-6.3 Sony a7 III
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