A question about ISO at m43 Locked

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Yannis1976
OP Yannis1976 Veteran Member • Posts: 4,929
Re: A question about ISO at m43

john isaacs wrote:

Yannis1976 wrote:

Hi,

if under a certain light condition with an f4 lens I get ISO800 (irrelevant of the sensor), do you know how much would the ISO increase at f5.6 and f6.3?

I am asking this to understand if a Sony RX10 IV at f4 needs an ISO800 at a certain light condition how much would a m5 III with f6.3 lens need? Have in mind that Fuji XT3 needed more than ISO1600 for the same exposure (ISO1600 plus increased exposure by 1.3 in LR).

Thanks

The answer is not irrelevant of sensor, or camera, or lens, unless you are using the same camera/lens combination. More about that later.

If you are using the same camera/lens, then increasing f-number is actually decreasing aperture size, so the ISO must increast proportionally. The change from f4 to f5.6 is one stop, and the corresponding change in ISO would be one stop (or double), so you would need ISO1600. The change from f5.6 to f6.3 is one-third stop, so the change in ISO would be one-third stop, or ISO2000.

Now if you change lenses (especially if you go from a prime to a zoom), you might get a change in transmissivity of the lens (how much light the lens actually lets through the system). The aperture may remain the same, but the amount of light changes, so the corresponding ISO changes. Often this is just a small fraction of a stop, but I have seen transmissivity changes of as much as one-third stop. This is, in fact, the way to determine the relative transmissivity of your lenses.

Similarly, if you change sensors, the transmissivity of the camera/lens combination may be affected. This can happen if one lens is more telecentric than the other, or if the sensor is sensitive to changes in telecentricity. To think of this, imaging one sensor has a grid that only allows light coming straight at the sensor to get to the surface and register, while another sensor does not. And then imagine one lens that brings light straight to the sensor, while another lens focuses light from many angles onto the surface. The sensor with the grid will cut out some light from the lens that does not bring light straight to it. This will cause a change in sensitivity in the camera/lens combination. It's not a common problem, but I have seen it.

Finally, we get to the "Elephant in the Room". The application of ISO is not a firm standard. In fact, the amount of light that one camera registers as ISO200 may actually be registered by another camera as ISO100. Maybe not so much between camera models by the same manufacturer, but quite commonly between cameras by different manufacurers.

What does that mean? Here is a simple example: I aim my Olympus E-M1 III out my window with a 25mm focal length, and I get exposure settings of 1/320, f4, ISO200. I then aim my Nikon Z7 out my window with a 50mm focal length (these are equivalent focal lengths) and I get exposure settings of 1/320, f4, ISO100. There is a one stop difference between the exposure settings on these cameras, for the same light condition.

Note that base ISO for the Olympus is ISO200, while base ISO for the Nikon is ISO64.

So you need to know the "real ISO" for each camera in order to extrapolate exposure settings from one to the other, as well as the "real transmissitivity" for each camera/lens combination.

To answer the question, you need to find out the "real ISO" of each camera, which may or may not be straightforward as the camera manufacturers don't tend publish that data.

From experience, I think Nikon tends to be pretty accurate, whereas Olympus is off by more than one stop. This may be some marketing tom-foolery to make their cameras appear to be good at high ISO. They aren't. Don't expect them to be.

Thank you for the very detailed explanation!

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