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Asa Rating of Digital Camera

Asa digital camera rate rating

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#1 Matthew Levi Rowley

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Posted 17 December 2015 - 05:41 PM

Could someone walk me through the process of rating the Asa of a digital camera?
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#2 David Mullen ASC

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Posted 17 December 2015 - 08:52 PM

Depends on if you are picking an ASA for maximum dynamic range, or one that gives you an equal number of stops above and below middle grey, or one that gives you a noise level you can live with.  With some cameras like the Alexa, different ASA settings, within reason, give you the same 14.5 stops of dynamic range but just change how many stops over middle grey or under middle grey you get -- at 800 ISO, it is an equal split.  But some people prefer the noise at 400 ISO, even though it means a little less headroom and a little bit more shadow detail.  Other cameras, by the way they boost the signal, may be giving you less dynamic range at certain ISO ratings.

 

The other thing to consider is that some cameras employ noise reduction once you start using higher ISO settings.

 

If you can get ahold of a Xyla chart, that can help:

http://dsclabs.com/d.../rear-lit/xyla/

 

Or just do things manually, shoot a grey card at normal exposure for the ISO setting you pick, and then do over and under exposures in 1-stop increments to check the range (it would help to have a white and a black patch near the grey in order to see at what point the grey card becomes indistinguishable between black or white.)

 

Truth is that I would first figure out what camera I was using and then go online and see what users have discovered regarding the optimal ISO settings before I did my own tests, just to get into the ballpark in testing, otherwise you could spends days and weeks shooting tests.

 

You basically want to figure out your clip and crush point and what the base noise level is for different ISO settings.


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#3 Matthew Levi Rowley

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Posted 17 December 2015 - 10:13 PM

David, that answered my question. Thanks!

I'd always read in American Cinematographer people rating cameras at specific ASA's. I was always curious what process they were going through to arrive at the number.

Thanks, again!
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#4 David Mullen ASC

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Posted 17 December 2015 - 11:50 PM

Often the manufacturer will have a recommended rating as a starting point.

 

It's funny how people who shoot digital stills never talk about the "native" ISO of the camera.  I shoot a lot with a Nikon D600 and being a 24MP full-frame sensor, it is fairly low-noise at higher ISO's though I rarely go above 1000 ISO.

 

But in terms of color-correcting from raw still files, I find that it tends to work better to shoot at low ISO's and underexpose because there is so much latitude in the shadows at the low ISO ratings and you will preserve more detail in bright areas.

 

Many of today's sensors are actually not higher in sensitivity, they are just lower in noise, which gives them more dynamic range and more flexibility to be rated at high ISO's.


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#5 Matthew Levi Rowley

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Posted 18 December 2015 - 12:32 AM

Yeah, I own an A7s and a Fs700. The A7s is one of those cameras you're referring to. 12MP sensor. When shooting slog2 the lowest it will go is 3200. If I expose correctly, middle grey at 32%, per Sony's suggestion, then the low amount of noise turns out quite nice.

The Fs700 is much different with a base iso of 680. I can usually live with the noise at 1250, but I don't like pushing it past that point. The noise from the Fs700 isn't as visually appealing, in my opinion, as the noise from the A7s sensor.
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