About what IRE value is "correct" exposure?

J

John Kary

Active member
This question is relative to HVX/video shooting, but is applying film techniques...

If you are shooting 35mm and use a light meter, you dial-in the stock speed and shutter angle/speed and then meter your subject. Let's say your subject is a person with white skintone and you meter at f5.6, so obviously you set your lens aperture to 5.6.

Is the meter saying "if you set aperture to 5.6 your subject will come out in the center of the film's latitude" ? Where in the stock's latitude will your resulting image be? Does this value transfer reliably to an equivalent IRE?

Does what I'm asking make sense?
 
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LighthouseMedia said:
See here hopefully this will answer your questions:
http://www.freshdv.com/2007/07/calibrating-a-light-meter-for-digital-video.html
Click to expand...

Kendal,
Thanks for pointing out your video. You calibrate a 18% gray chip to 50 IRE in-camera, and set the light meter speed accordingly so their meter reads the same as the camera's aperture. While all of this makes sense as to calibrating the rating of a camera, it doesn't fully answer my question...

The video doesn't really comment on what 50 IRE is good for in this instance. You say in the end of the video that your face reads 5.6 using as an incident meter, and that you would set the aperture to 5.6 to be "properly exposed." This would result in your face coming off at 50 IRE, right? Is 50 IRE where you always want your skintones to fall? I would think no, since you want to draw attention to the faces of a scene, and could do this by exposing them at a stop or so above your meter reading.
 
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For decent/conventional interview exposure you'll want your facial skin highlights to fall in the 70-80 IRE range. So if you see face zebras at 80 then stop it down/back off the light. Nice mid tones should fall in around 60-70 IRE. Determine your contrast ratio from there at your discretion.

On another note...
White information is actually stored up to 109 IRE and you can 'pull' it out in post.
Black information is stored down to 0-7 IRE but don't quote me on this.

Can someone elaborate on this, confirm these numbers, or argue against my IRE proclomations. ;)
 
jonnyh1968 said:
On another note...
White information is actually stored up to 109 IRE and you can 'pull' it out in post.
Black information is stored down to 0-7 IRE but don't quote me on this.
Click to expand...


Yeah I use the 100+ IRE information all the time, though the color fidelity can be erratic for that stuff.

On the original question- much of what constitutes proper exposure is really a personal preference. There are many variables to consider such as the camera's scene settings, how much (if any) noise is tolerable, etc. And I find the camera performs very differently in sunlight vs. artifficial light... So the best thing is to just do lots of test shots under different conditions and with different settings to figure out what works for you.
 
jonnyh1968 said:
On another note...
White information is actually stored up to 109 IRE and you can 'pull' it out in post.
Black information is stored down to 0-7 IRE but don't quote me on this.
Click to expand...


Yeah I use the 100+ IRE information all the time, though the color fidelity can be erratic for that stuff. So if I've got a hot spot on a wall that is just over 100 IRE I don't worry about it, but if it's highlights on someone's face I stop down.

On the original question- much of what constitutes proper exposure is really a personal preference. There are many variables to consider such as the camera's scene settings, how much (if any) noise is tolerable, etc. And I find the camera performs very differently in sunlight vs. artifficial light. So the best thing is to just do lots of test shots under different conditions and with different settings to figure out what works for you.
 
Hey John,
For a reflected meter reading of 18% Gray Chip in a given lighting situation the corresponding IRE is 50%. I think part of the problem is you are dealing with two separate ways of measuring light the IRE scale measure a range of capturable data by any given system. F-stops has no DIRECT correlation to IRE for a given camera UNLESS you use a meter specifically calibrated to your camera. They are two totally independent ways of measuring light and as such have separate scales and units of measurement. Maybe I'm miss understanding your question could you try and re-phrase it?
 
LighthouseMedia said:
F-stops has no DIRECT correlation to IRE for a given camera UNLESS you use a meter specifically calibrated to your camera. They are two totally independent ways of measuring light and as such have separate scales and units of measurement. Maybe I'm miss understanding your question could you try and re-phrase it?
Click to expand...
Actually I think this is making a bit more sense now. Different stocks/cameras have differing dynamic range, like 5 stops over on an HVX would result in blowout of highlights, but some film stocks would still hold that detail. Thanks to everyone for clarifying.
 
Correct different cameras have different dynamic ranges and yet those ranges are represented as 0-100% ire, so that scale on one camera may be 9 Stops and on another it maybe 5 stops so correlating a given F-stop to a given ire level can quickly become convoluted. Better to use a waveform hand in hand with a calibrated light meter setup to your particular camera.
 
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