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Getting to know the F55; Inaccurate ISO rating?

sony f55 iso

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#1 Markus A Ljungberg

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Posted 01 October 2013 - 04:13 AM

Hi All,

 

I'm a couple of days into an LB indie-feature shooting on Sony F55 with Angenieux zooms. With budget and time constraints we never had a chance to properly test the camera, and I now have a couple of questions perhaps someone enlightened might help me out with?

 

I find that on native ISO (1250) the camera need 1 to 2 stops more light then what my light reader says. We are shooting in SR Codec so the picture I'm looking at is flat profile, and when shooting in SR it appears there is no way to get a Rec709 preview in the EVF/Monitor. At first I thought this was what was throwing me of, but even after the DIT applied a LUT it still feels like the camera does need at least one more stop of light the I'd expect on ISO 1250. Has anyone else had this experience? Has anyone done any tests under more controlled circumstances?

 

The exposure assist in camera is terrible, there is the waveform which is fine, but that's the waveform before LUT is applied so it's not telling me what my luminance will be in the final picture. There's no false colour etc. only zebra which says very little about the information in the picture. 

 

Using the waveform at least I know I'm not crushing blacks or clipping whites, but I feel like I don't have the same control over exposure as I have on film, Alexa, Red or even on my magic lantern'ed 5D!

 

The picture looks great though. And I must say that the low light capabilities are great, and the low weight appreciated. Though when fully kited up it becomes a very long camera.

 

I'm also looking for a good comparison between SR codec to XAVC. The "low" bitrate of XAVC scares me a bit, but it seems to be a very sophisticated codec. Does anyone know how it holds up in grade? And how it holds up for keying?

 

I will try to find the time to do some more controlled tests to post here for everyone. In the meantime I'm rating my light meter to +1 stop for F55.

 

Thanks, 

 


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#2 David Mullen ASC

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Posted 01 October 2013 - 10:25 AM

This is being discussed on CML right now too --- it seems a number of people are setting the camera to 1250 ISO but exposing one-stop slower, like at 640 ISO.  They feel they have enough headroom to do this and want more shadow detail.

 

I'd be curious what a waveform monitor reads when pointing at an 11-step grey scale in S-Log at 1250 ISO.  Panalog is something like 70% for white and 10% for black, and Arri Log-C is even flatter, like 65% for white and 15% for black. Which means that if you monitor in log, you'd naturally expect to see an image with dim highlights and milky blacks.  You don't want to expose a log image so that highlights look normal because then you're defeating one of the benefits of log, which is extended overexposure range.

 

Also, you don't know if the LUT you are applying for designed to convert S-Log in the F55 to correct Rec.709 levels, it may have been designed for some other log or camera.

 

The Canons have this problem too, which I find annoying, the lack of ability to record in log but send a gamma-corrected Rec.709 picture to the viewfinder and monitors.  

 

Juan Martinez at Sony just posted this on CML: "Based on this concern, at the camera's nominal ISO: F65 ISO800; F55 ISO1250, F5 ISO2000, S-Log2 provides eight stops above and six stops below middle gray. S-Log2 +8, -7 weighing has better SN; a major consideration for compressed recording. However, as long as the master gain goes untouched, changing EI+/- allows increasing headroom or increasing detail in the shadow without affecting exposure latitude."

 

 

Sounds to me like you'd be OK rating the camera at 640 ISO or something if that's true.


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#3 Markus A Ljungberg

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Posted 01 October 2013 - 04:52 PM

Thank you David that's very helpful. I'll check out that CML thread.

 

I'll try to shoot a greyscale to figure out what the S-Log levels are. Those Panalog and Log-C numbers are good references though, and thanks for passing on Juan Matinez post I was looking for those numbers.

 

If I learn anything else on the matter I'll post it here.

 

Thanks


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#4 Markus A Ljungberg

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Posted 05 October 2013 - 02:04 AM

So with the new firmware that just came out you can get a LUT ie 709 on your output whilst recording in S-log. And there is a false colour update for the EVF but you need to take your EVF to Sony for it :)

Anyway. Rating the camera to 640 for the last couple of days and happy with the results. Haven't gotten around to shooting any step charts unfortunately.
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