Sony A7s: waveform IRE and f-stops: do we know the correlation???

N

Nicholas Natteau

Veteran
I decided to start this thread after reading this post on dvxuser:
http://www.dvxuser.com/V6/showthread.php?96197-IRE-and-Stop-correlation
According to that thread, there is no general rule for f-stop and IRE correlation, and it depends on the latitude capacity of the camera in question.
If that's the case, does anyone know what the correlation would be between f-stops and IRE increments on the Sony A7s?

Sony claims the A7s to have up to 14 stops of dynamic range, but allegedly only 12 are usable. To determine what this would mean in terms of IRE increments on a 0-100% waveform monitor, should we be dividing 100/12? Would it be correct for example to assume that a stop would equal 8% or 9% on a waveform monitor?

For those of us using waveform monitors, it would be very helpful to know what stops correlate to in IRE terms on the A7s.

If for example middle grey is 42% in S-Log2, and we wanted to expose two stops over middle grey, should we therefore be exposing at 58%-60% IRE??? (42% + 16% or +18%)
 
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The theoretical relation is in this white paper. In practice, this chart shows how your signal changes when you change exposure by a third of a stop each time, using slog2 on the a7s:

waveform_slog2.jpg
 
is there any official LUT from Sony , for the Sony A7s and Slog2 - Sgamut pricture profile ?
I try to load some other slog LUT ins Davinchi but the results are not nice....
 
Hey guys,

Thanks very much for the feedback. Sorry I was wrong, yes 32% is middle grey.

I did read Chapman's very detailed study and found it very informative. For skin tones he doesn't recommend overexposing beyond 2 stops as you begin to lose texture or detail at that point.

Another good link to check out is this equally detailed tutorial (which includes a 59min video) by Sareesh Sudhakaran who claims that the optimum exposure for skin tones when shooting with the A7s is to overexpose by 3 (rather than 2) stops:
http://wolfcrow.com/blog/how-to-expose-and-grade-s-log2-for-the-sony-a7s/

For me the only problem with that technique is that the A7s metering mode doesn't display "3.0" when going over 2 stops. It just says "2.0+"
So how do we know that we're at exactly 3 stops and not more when looking at the A7s monitor?
 
overexposing by 3 stops is not the same as setting your skin 3 stops above the 0 reading, because usually skin is brighter than mid grey
(as we already discussed on this thread)
 
He suggests using the Zebra Setting at 70%, that way you know you have the Skin Tones properly exposed when the Zebra Shows on Skin......
 
Samuel, can you post a larger screen grab of that very helpful waveform? I checked out the white paper but it doesn't appear there. Thank you in advance.
 
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There you go. You'll notice I did this in two passes, for mids+highs and mids+lows, and the match was not perfect. But it should be close enough.
waveform_slog2_big.jpg
 
Thank you Samuel, that's very helpful. From the graph it looks between IRE 30 and 90, there are three 1/3 stops for every 10 IRE, correct? And between IRE 20 and IRE 10, it looks like 9 one third stops or 3 full stops.
 
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