How to translate IREs to stops?

[Sttratos]

How do you translate IREs to F-stops?

For example, in the picture bellow, how many stops difference between the top and the bottom waveform readings? They are from the same scene.

Thanks.

 

[Cassius]

One full stop halves or doubles the light from the previous stop, depending on which direction you're going. It's relative.

This looks like one full stop to me, maybe slightly more.

 

[Barry_Green]

You cannot judge stops by IRE values, there is no formula.

The IRE scale ranges from 0 to 110. If your camera has five stops of latitude, then you'll see the the lowest stop registering right above 0, and the highest stop registering right below 110.

If, on the other hand, your camera is capable of 12 stops of latitude, in that case you'll see the lowest stop registering right above 0, and the highest stop registering right below 110.

See the problem?

Furthermore, the stops are not an equal interval apart, in IRE value. The difference between the darkest stop and the next-to-darkest might only be three or four IRE points. And the difference between the brightest and the next-to-brightest might be 20 or more IRE points.

So you simply cannot translate IRE into F-stops, it's not possible.

What you can do, however, is shoot a grayscale chart or, better yet, a Stouffer chart, and count the steps on the waveform.