Which Sony ISO RatingS Are Correct?

460x150-Banner-Box Which Sony ISO RatingS Are Correct?

In the course of my tests with the FX3 and comparing it with the FX6 and FX9 I discovered a strange anomaly with the FX3 and A7SIII ISO ratings when compared to the FX6 and FX9. 


The FX3’s default picture profile is PP11 and S-Cinetone. If you have an FX6 or FX9 these cameras also default to S-Cinetone in SDR mode. In the FX6 and FX9 the base ISO for S-Cinetone is 320 ISO. Therefore you would assume that if you also set the A7SIII or the FX3 to 320 ISO and expose all the cameras the same, same aperture, shutter etc that the exposures would match.

BUT THE EXPOSURES DON’T MATCH!!

FX6-Exposure_1.1.1-600x338 Which Sony ISO RatingS Are Correct?
The FX6 at 320 ISO, 1/50th shutter, S-Cinetone.

 

FX3-Exposure_1.2.1-600x338 Which Sony ISO RatingS Are Correct?
The FX3 with the same 320 ISO, 1/50th shutter and S-Cinetone. It’s clearly brighter.



The FX3 and the A7SIIII are just over 1 stop brighter than the FX6 and FX9 when all the exposure settings are matched. I tested all the cameras with the same lens to ensure this wasn’t a lens issue, but it isn’t the lens.

300x250-ad-box1 Which Sony ISO RatingS Are Correct?
FX3-S-Cinetone-scopes-copy-1 Which Sony ISO RatingS Are Correct?
FX3 S-Cinetone is brighter compared to the FX6/FX9 and over exposed according to my light meter by just over 1 stop. The white of the white card should be at approx 83% and my skin tones are well into the highlight roll off and looking flat as a result.

 

FX6-s-cinetone-scopes-copy Which Sony ISO RatingS Are Correct?
The FX6’s exposure much more closely matches my light meter and is only a fraction of a stop under with the white card just touching the 83% exposure I would normally expect with S-Cinetone.

 

 

Screenshot-2021-02-26-at-13.16.16 Which Sony ISO RatingS Are Correct?
Here’s the FX3 again, the whites are much, much too bright when exposed against my light meter, even the middle grey is over 60%. The MM+0.7 indication means the camera thinks it is over exposed.

 

Screenshot-2021-02-26-at-13.16.49-copy Which Sony ISO RatingS Are Correct?
And the FX6 with exactly the same settings and the same lens matches my light meter very closely, white is around 83% and middle grey around 45%, as I would expect. Something odd is going on here, it’s not just my light meter it’s something else as the cameras should at least match, even if they don’t agree with the light meter.



I then went on to test other gamma/picture profile settings and I found a just over 1 stop difference between the FX3 and my FX6/FX9 in any similar combination EXCEPT S-LOG3!

Screenshot-2021-02-26-at-13.14.18 Which Sony ISO RatingS Are Correct?
The FX3 shooting S-Log3 now it matches my light meter very closely and the exposure is add I would expect.

 

Screenshot-2021-02-26-at-13.14.41 Which Sony ISO RatingS Are Correct?
This is the FX6 set exactly the same as the FX3 shooting S-Log3. Now they both match and now the both provide the same exposure and closely match my light meter.



When using Picture Profile 2 on the FX3 which is uses Sony’s “Still” gamma and then using the “Still” Profile on the FX6 there is a difference of around 1 stop. If I set the FX3 to PP3 (ITU-709) and the FX6 to ITU-709 then the difference is again around 1 stop, in every case the FX3 is brighter except when you select S-Log3 where the FX3 and the FX6/FX9 match almost perfectly!

I find this very strange. They should not be different. My light meter suggests to me that the FX6/FX9 are correct.

Comparing to my light meter I believe the FX6/FX9 ratings to be correct and the FX3 to be between 1 and 1.3 stops brighter than it should be when using gammas that are not S-Log3. What I really don’t understand is why the FX3/A7SIII match the FX6/FX9 when using S-Log3 but do not match when using the other profiles, normally I would expect to see a consistent offset. This further makes leads me to be sure this is not a problem with my light meter, but something else.

I would love to hear from anyone else that’s able to take a look at the ISO ratings of the A7SIII and compare it with an FX6 or FX9.

The bottom line is – DON’T EXPECT TO PUT THE SAME EXPOSURE SETTINGS INTO BOTH AN FX3 AND AN FX6/FX9 AND GET THE SAME RESULTS, because you won’t, unless you are using S-Log3, then they match. 

Also in the clip metadata I found that 0dB for S-Cinetone is 100 ISO, and whether this is a coincidence or not, if I set the FX3 to 100 ISO and the FX6 to 320 ISO and then match shutter speed and aperture then the exposures are very close.

This one has left me confused!!!!

6 thoughts on “Which Sony ISO RatingS Are Correct?”

    1. Sony have not published a definitive number. But if you expose correctly for a known gamma such as S-Log3 and then switch to an unknown gamma, such as S-Cinetone keeping the same ISO/Shutter/aperture, you can then see exactly where white and middle grey falls. The 83% value that I have measured also fits correctly with the charts Sony published in the S-Cinetone white paper where 100% input is a bit above 80%.

  1. I can’t seem to find my notes at the moment but I recall doing some tests comparing exposure on the A7iii and the Fs7 and noticing that at a number of different gammas the A7 always seemed much more sensitive than I would have expected given the same ISO settings. i doubt i compared them at Slog though and they weren’t identical gamma settings either ( comparing hypergammas to cinegammas or even the 709s on both cameras aren’t neccessarily the same), so it wasn’t a careful test . However it was striking and i just tossed it off to not trusting ISO ratings on video cameras. I mostly only meter to balance lighting not for exposure, though it does help on site survey. Your tests showing they match in sLog but not in s-Cinetone is odd. I’ve always thought there was something inherently arbitrary about those ISO ratings anyway but I’m no engineer.

  2. Alister I found the same oddity a while back when using my A7S3 along side my FX9, I Jett hadn’t gotten around to mentioning it on the FB user group.

    Thanks for all the tests and the confirmation.

    I’m wondering if this is a simple labelling mistake on Sony’s part? That their GUI is wrong seeing as its exactly 1 stop?

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