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Digital Sensor Rating


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#1 Ashim

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Posted 10 October 2006 - 07:18 AM

This is one facet of digital camera thats eluding my understanding for quite some while now.
What I'd want to know is the way digital cameras are rated and why?
For eg: f/8@2000lux for 90% white??
Does this mean that digital cameras are calibrated for white just like films are calibrated for 18% gray.
I know I am getting it quite muddled up,but if the above is true then why 90% white??

Also how is it possible to use your lightmeter for digital shoots(except for measuring the units of light).
Light meters are basically for film use.
If I expose my window according to the light meter readings at f/8 but I want my window to be 1 stop
brighter, I will expose at f/5.6.
But how do I apply the same logic with digital shoots.

A kind appeal to all the members of our Brotherhood to Enlighten me on the above.
Thank You
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#2 Stephen Williams

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Posted 10 October 2006 - 01:07 PM

,but if the above is true then why 90% white??


Hi,

So they seem at least 2 stops faster than they are!

Stephen
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#3 Chad Stockfleth

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Posted 10 October 2006 - 01:23 PM

If I expose my window according to the light meter readings at f/8 but I want my window to be 1 stop
brighter, I will expose at f/5.6.
But how do I apply the same logic with digital shoots.


The exact same way. You determine an ASA rating for the digital camera you are using, and then use your meter accordingly, bearing in mind that you will have much reduced latitude(again, depending on the camera).

Of course, you can use a monitor and forgoe the meter, although they can be helpful.
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#4 Ashim

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Posted 11 October 2006 - 01:58 AM

The exact same way. You determine an ASA rating for the digital camera you are using, and then use your meter accordingly, bearing in mind that you will have much reduced latitude(again, depending on the camera).

Of course, you can use a monitor and forgoe the meter, although they can be helpful.


Hello

But the methodology of rating one's digital camera for a particular ASA doesnt seem logical to me.
Film has a gamma of 0.6, its midpoint is 18% gray, has a far greater dynamic range.
Digital has a gamma... linear or variable if I'm right, and the rating of digital cameras on technical manual
(f/8 @2000 lux for 90%white) seems alien to me.
Why then do we use a gray card and correspond it with around 45-55 IRE rating for around mid gray and then rate our sensor accordingly,
when we shld be using a white card with a corresponding IRE rating and sensor rating...

Help... I need somebody Help
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#5 Ashim

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Posted 16 November 2006 - 02:06 AM

I know the literature above may seem obtuse but I'd be thankful if you could kindly lemme know where I am going wrong here.

Thank You
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#6 Brian Drysdale

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Posted 16 November 2006 - 07:54 AM

I know the literature above may seem obtuse but I'd be thankful if you could kindly lemme know where I am going wrong here.

Thank You


These are just two different references. Film uses the 18% mid grey using an exposure meter and video engineers use the 90% level which they use with a waveform monitor.

Some grey scale charts for film use have a 90% level instead of a 100% white, so that you can set up levels in the telecine for transfer from film to video.

If you correctly exposure a video camera on a chart with a 90% level on a waveform monitor, you can use an incident light exposure meter to work out the ASA of the video camera by comparing the light falling on the chart and the stop that you've set on the camera.
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#7 Ashim

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Posted 17 November 2006 - 12:25 AM

If you correctly exposure a video camera on a chart with a 90% level on a waveform monitor, you can use an incident light exposure meter to work out the ASA of the video camera by comparing the light falling on the chart and the stop that you've set on the camera.


And at what IRE levels shld the luminance of the chart with a 90% reflection be set?

Thanks

If possible cld u gimme some links to this topic.
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#8 Brian Drysdale

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Posted 17 November 2006 - 10:25 AM

And at what IRE levels shld the luminance of the chart with a 90% reflection be set?

Thanks

If possible cld u gimme some links to this topic.


Since an IRE unit is 1% of the difference between peak white and the blanking level, it should be set at 90.

If you're using this to give your camera an ASA rating be careful if your camera has a manual knee setup that comes in below 90%.
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#9 Brian Drysdale

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Posted 17 November 2006 - 01:33 PM

Since an IRE unit is 1% of the difference between peak white and the blanking level, it should be set at 90.

If you're using this to give your camera an ASA rating be careful if your camera has a manual knee setup that comes in below 90%.


I should have mentioned that the grey scale is logarithmic, which is why the 18% mid grey is usually set on 55. 90% is the taken to be the highest non glossy white, so I'll correct what I've said to 100 on the waveform monitor.
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