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How to figure out dynamic range shooting at wide f-stop?


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#1 Mark Coger

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Posted 01 January 2017 - 06:26 PM

I just had a question about trying to figure out the dynamic range of a scene when filming at a wide f-stop. If I'm shooting at a f-2.8 or 4.0 for my base exposure how much latitude would I have in the shadows if I'm using a camera such as the c300 that has about 6 stops of latitude on the shadows. The reason I ask is that there are no more stops that the camera can open to past f-2.0 or whatever the limitation of the lens. Should I be thinking in foot-candles to figure out if there will be details in the shadows?


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#2 David Mullen ASC

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Posted 01 January 2017 - 08:49 PM

Saying that you're shooting at f/2.8 doesn't tell us anything about how much light the scene itself has.  Out in a field under the real moon and stars, you might be dozens stops underexposed at f/2.8 and outside in sunlight you might be several stops overexposed at f/2.8.

 

If your camera has six stops of shadow detail but you underexpose the subject by six stops, then in theory you wouldn't have any detail in your shadows.

 

As for figuring out the amount of light you have, you can work in stops combined with an ASA rating, or you can work in foot-candles.

 

A simple test would be to set the camera at the desired ASA rating, shutter speed, and frame rate, and then shoot a test subject and grey card at normal exposure, and under and overexposed in one-stop increments as well and then try to correct each shot back to normal in post to see where you start running into problems.


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#3 Michael Rodin

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Posted 02 January 2017 - 07:46 AM

Guess we should assume T2,8 brings a 18% (or more likely 11%) grey card to a "middle grey" IRE specific to the camera and the gamma curve. So we're "correctly exposed" and the whole "6 stop shadow latitude" thing starts to make sense.

Then "6 stops underxposure latitude/range/whatever they advertise" means something like: what meters at T0,3 will be completely buried in noise, what reads T0,5 will show a little low-contrast detail, T0,7 clearer detail but desaturated and so on. All at the same T2,8 aperture.

But this is what the manufacturer thinks their camera can deliver. How deep can you go into the shadows in practise, you find out through tests.


Edited by Michael Rodin, 02 January 2017 - 07:48 AM.

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#4 David Mullen ASC

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Posted 02 January 2017 - 01:00 PM

It reminds of when I was in film school and someone brought their 16mm footage in to show me and the cinematography professor because it "looked bad".  We saw that it was very grainy and milky from being underexposed.  The student said "but I shot it as wide open as the lens would go!"  

 

If your meter tells you you need to shoot at f/1.4 but your lens only goes to f/2.8 and you shoot at that, then you'll be underexposing at 2-stops.  Your choice is to either add more light or do whatever you need to get more exposure (increase shutter time maybe) or live with the 2-stops underexposure and the results of that.  Maybe you'll find that the scene should look about 1-stop under for effect and you only need to brighten it up by 1-stop, not 2-stops.


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#5 Igor Trajkovski

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Posted 02 January 2017 - 02:48 PM

Determining of the dynamic range of a scene has nothing to do with your ability to open wider.

If you end up at f2.0, ISO800 and 1/50,  and start spotmetering your scene,
everywhere the meter shows f2.0 will (should) render as mid (18% grey) value.

Areas where the meter shows higher stops, (2.8, 4.0 ...) are lighter,
and lower stops (1.4, 1.0) means are darker. 

 

In order to see lower values then 1.0 which in this example is -2 stops darker,

you enter the brightness difference function of the spot meter.

Meter till you find the f2.0, enter brightness difference function and as you
hold the meter button the display will show you the difference in stops (-/+) of the areas you point your meter on.

When the meter shows -6, you are at the lower limit of the camera,
and that area will render either as still textured black or featureless jet black,
whatever is meant by that -6 by Canon.

 


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#6 Mark Coger

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Posted 02 January 2017 - 04:49 PM

Thank you so much for the tips. In terms of trying to figure out dynamic range in the shadows I was in the mindset of thinking how many f-stops wider the lens could open too.

 

I just bought a light meter and learning how to use it. I see that it can calculate the EV difference which makes a lot more sense now.


Edited by Mark Coger, 02 January 2017 - 04:50 PM.

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#7 John E Clark

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Posted 04 January 2017 - 12:26 PM

Thank you so much for the tips. In terms of trying to figure out dynamic range in the shadows I was in the mindset of thinking how many f-stops wider the lens could open too.

 

I just bought a light meter and learning how to use it. I see that it can calculate the EV difference which makes a lot more sense now.

 

The scene brightness values set the 'dynamic range of the scene'.

 

The F-stop setting of the lens will not change that range of values.

 

The sensor will limit how much of that dynamic range is transformed into usable electronic signals which are then digitized and recorded.


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#8 Robin R Probyn

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Posted 04 January 2017 - 07:21 PM

Is not the DR of a scene simply from the brightest/highest reading to the darkest/lowest reading.. 


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#9 John E Clark

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Posted 04 January 2017 - 07:29 PM

Is not the DR of a scene simply from the brightest/highest reading to the darkest/lowest reading.. 

 

Yes, it is a ratio of 'high' to 'low', and that is unaffected by the f-stop setting on a lens. The sensor/film stock has its dynamic range and threshold of usable recording, aka 'the speed' of the sensor/film.

 

All is well if the sensor/film has a larger dynamic range than the scene in question. And the lowest light level is above the minimum threshold of the sensor/film.

 

Then there's the dynamic range of the display/output/print... which is typically much lower than ether the sensor/film recording DR, or the scene DR.


Edited by John E Clark, 04 January 2017 - 07:40 PM.

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#10 Robin R Probyn

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Posted 04 January 2017 - 07:35 PM

 

Yes, it is a ratio of 'high' to 'low', and that is unaffected by the f-stop setting on a lens. The sensor/film stock has its dynamic range and threshold of usable recording, aka 'the speed' of the sensor/film.

 

All is well if the sensor/film has a larger dynamic range than the scene in question.

 

Then there's the dynamic range of the display/output/print... which is typically much lower than ether the sensor/film recording DR, or the scene DR.

 

 

Elegantly put sir.. I only work in TV .. so all that 14 stop Log still has to pushed back into 7 stops of REC709.. although REC 2020 will hopefully change that in the near ish future.. but even movie theater screens are about the same 7 stops DR I read..?


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#11 Stephen Baldassarre

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Posted 06 January 2017 - 02:53 PM

The actual contrast ratio (different from dynamic range) is determined by a lot of factors from which projectors (or film) is being used to the type of screen to ambient light in the theater.  It's completely possible to shoot a movie with a 12-stop range and see that full range on the big screen, but a spot meter will likely show a 200:1 ratio.  The mid-tones can still be pretty proportionate to what was on-scene, it's just that the contrast of the shadows has been reduced by light pollution, maybe the highlights have been compressed by the LCD etc.  Needless to say, input never equals output in terms of dynamic range or color regardless of what system you use.


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