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naavt
08-16-2013, 07:59 PM
I've been searching the web for some information about photometrics. To ensure that you have sufficient light in your set and don't go bankrupt renting lights that you will not use photometrics is an essential wisdom (or so I think). That's why I would like to understand it from within.

I have the basic notion that everybody knows: 100fc/100ISO=f2.8 but from that to understand why it works that way is another question.

If anyone could enlighten this for me and/or point me in the right direction to some written information I would be very grateful. I would really like to know how to read a photometric chart...

Thanks all in advance.

rze
08-18-2013, 09:51 AM
Theres few different small concepts you need to know about to understand a photometric chart. There stops of light, aperture stops, the inverse square law, and footcandles. Which concept do you want to know more about?

naavt
08-18-2013, 11:30 AM
Hi rze. Thanks for your reply.

I understand them all quite well. What I would like to know is:

1. How does one reach to the main formula 100fc divided by 100ISO equals to f2.8? Why not f4 or f2? There must be an explanation to it.
2. With that in mind, how can I take a photometric chart from a light manufacturer and see what aperture will I need with that specific light unit.

razz16mm
08-18-2013, 02:25 PM
Hi rze. Thanks for your reply.

I understand them all quite well. What I would like to know is:

1. How does one reach to the main formula 100fc divided by 100ISO equals to f2.8? Why not f4 or f2? There must be an explanation to it.
2. With that in mind, how can I take a photometric chart from a light manufacturer and see what aperture will I need with that specific light unit.

One has to include shutter speed in that formula at some point. Are we assuming 24fps 180 degree shutter, I.E. 1/48sec exposure time for this "rule"?

Assuming this is the case, f/2.8 under the rule should produce a middle gray exposure equal to Zone 5 of a 10 step Adams grayscale range, or 50-60 IRE on a video waveform monitor from an 18% gray card.

An change of one photometric stop, say from f/2.8 to f/4 represents a doubling of scene illuminance in foot candles, from 100fc to 200fc, to get the same exposure at the smaller aperture. 1 f/stop less exposure = -10 IRE= -6dB=ISO/2= -1 Adams Zone= -1 EV.

Best way to incorporate this into your work is to use an incident light meter that reads directly in FC and profile it to your camera so that an 18% gray card shot will produce a scope reading in the 50-60 IRE range. The meter ISO setting that produces the correct middle gray range exposure for a given aperture and shutter speed is your true exposure ISO, regardless of what the camera setting may tell you. Exposure ISO's can be affected by any lut applied, like log curves,or any optical filter attached to the lens. Depending on LUTS and how they are graded for finish, middle gray can fall between 40-60 IRE.

naavt
08-19-2013, 05:27 PM
Hi razz,

I think that shutter speed doesn't do much here. It only does if you're after some special imagery beyond a 180 degree shutter because you will need more or less light accordingly.

Anyway, even if I understood what you have said it doesn't relate to my question. You are talking about exposing with Ansel Adams zone system in mind. I was talking about photometric charts that light manufacturers release with their systems.

What I want to know is how to read those charts to preview how many lights do I have to bring with me to light a shot after I decided what look I'm after.

Is a question of power (where beam spread, fc/lumens, and the inverse square rule applies), rather a question of exposure. Learning to read those charts has more to do with pre-production than a production issue.

rze
08-21-2013, 10:27 AM
Hi razz,

I think that shutter speed doesn't do much here. It only does if you're after some special imagery beyond a 180 degree shutter because you will need more or less light accordingly.

Anyway, even if I understood what you have said it doesn't relate to my question. You are talking about exposing with Ansel Adams zone system in mind. I was talking about photometric charts that light manufacturers release with their systems.

What I want to know is how to read those charts to preview how many lights do I have to bring with me to light a shot after I decided what look I'm after.

Is a question of power (where beam spread, fc/lumens, and the inverse square rule applies), rather a question of exposure. Learning to read those charts has more to do with pre-production than a production issue.

Its pretty simple during your tech scout take your light meter with you. Get a reading of the footcandles, and check the charts to make sure your fixture selections have enough punch with compensation for diffusion. Daylight is 8000-10000fc shade is around 1000, overcast is around 1000, indoor can be a a wide range between 0-300ish. Wheres the issue?

razz16mm
08-21-2013, 01:52 PM
Hi razz,

I think that shutter speed doesn't do much here. It only does if you're after some special imagery beyond a 180 degree shutter because you will need more or less light accordingly.

Anyway, even if I understood what you have said it doesn't relate to my question. You are talking about exposing with Ansel Adams zone system in mind. I was talking about photometric charts that light manufacturers release with their systems.

What I want to know is how to read those charts to preview how many lights do I have to bring with me to light a shot after I decided what look I'm after.

Is a question of power (where beam spread, fc/lumens, and the inverse square rule applies), rather a question of exposure. Learning to read those charts has more to do with pre-production than a production issue.

Ok. The charts typically give you a reference level in lumens output or FC at a known distance and coverage angle. In order to know how much light you need for a particular setup, you need to know what your middle gray FC level needs to be for the planned camera exposure setting.
Take the rated Lumen output of the light for your planned coverage and divide it by the square of the distance between the light and your subject to get FC. For more than one light aimed at the subject from the same direction, add the lumen values first, then do the inverse square law calculation. Leave yourself at least a 30% reserve margin to account for factors like lamp aging and difference in angles of incidence between lights in a setup.
The reverse works too. If you measure a light at a known distance with a FC meter, multiply the value by the square of the distance between the meter and the light to get the output in lumens. Useful if you are planning to use gels or diffusion, that reduce the light output. You can test that before hand. Fall off with distance can be more than inverse square law prediction with heavy diffusion.