f/1.8 on m4/3, ASP-C, FF; are they the same?

Started Nov 1, 2010 | Discussions thread
Gao Gao Contributing Member • Posts: 561
Because DXO uses a strange way to define ISO

The actual standard gives several options and leaves the choice pretty much open to the manufacturers. So far they have adopted the rule that at the same rated ISO, the final output pictures look similar (w.r.t. middle gray). In my opinion, since we are talking about photographic equivalents, it's how the pictures look that matters.

DXO's definition for ISO is different - they measure the saturation level. The lower the saturation level, the higher the ISO they rate. However, we know that the mid-gray points need to more or less match in the final pictures for the same manufacturer ISO. Therefore, a lower saturation level at the same manufacturer ISO simply means lower highlight headroom, not higher sensitivity.

In my opinion, saturation-based ISO rating can only be used within one specific product across different ISO speed settings, and is not applicable across products. And this is one of the several limitations of DXO mark.

Actually, in the ISO 12232 document, there is another definition for ISO speed, which is Signal-to-Noise Ratio. This is another justification for the equivalent values mentioned in my earlier post.

Here's how ISO speed is defined at DXO:


Everdog wrote:

Gao Gao wrote:

Go to the compare sensors page on DXO mark...

But when you look at the REAL ISO values you get things like..
the GH1 at ISO 1126 has a DR of 9.32EV...
the Canon 5D mkII at ISO 1093 has a DR of 10 EV

Not the big difference you are claiming. Why in your examples are you NOT using the actual ISOs???

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