Medium grey and luminance level : experts and scientists help me!

[superflash]

No need to 'correct for' color tempterature! 'Calibrate' your palm against an 18% gray tonality surface, viewing the difference in reflected light meter reading with your palm vs. with the gray card, it will be about 1EV difference (my palm is +1.3EV different, as measured by spotmeter with 0.1EV increements)

thank you again, wiltw; I understand this.

 

[benjiboy]

For get medium gray for slides. The exposure latitude is too narrow to mess around with EI and all the loony Zone System testing and "fun and games". Get the exposure correctly by not metering the sky only the subject.

I agree, in my experience, the easiest and best way to get the correct exposure in slide film is to use incidental light metering.to meter the light falling on the subject only.

 

[alanrockwood]

Leaving the technical aspect to the side for a moment, to me the terminology "middle gray" or "medium grey" or whatever one calls it does not match up with my perception. To my eye this tone is somewhat darker than what I perceive as a medium gray tone.

What is your perception of medium grey?

 

[Sirius Glass]

Leaving the technical aspect to the side for a moment, to me the terminology "middle gray" or "medium grey" or whatever one calls it does not match up with my perception. To my eye this tone is somewhat darker than what I perceive as a medium gray tone.

What is your perception of medium grey?

The color my beard is becoming.

 

[wiltw]

Leaving the technical aspect to the side for a moment, to me the terminology "middle gray" or "medium grey" or whatever one calls it does not match up with my perception. To my eye this tone is somewhat darker than what I perceive as a medium gray tone.

What is your perception of medium grey?

Looking at a tonal scale in a book about the Zone System, it is the center of the range of tones between white and black, what is referred to as Zone V in a range of zones from zero to 10. In luminance units Zone X has 512 luminance units while zone I has 1 luminance unit (zone 0 has 1/2 luminance unit) Zone V would have 16 luminance units. Each zone is said to be separated by 1EV difference from the adjacent zone, or double the previous zone's number of luminance units. The 'dynamic range' lies from Zone 1 thru IX, not 0 thru X.

 

[Sirius Glass]

I prefer medium or middle gray to medium or middle grey.

 

[superflash]

Good morning .
For my mind middle or medium grey is the part of the scene that reflect the same luminance that is measured on the slide; or in other words, the part of the scene that can be represented on the slide with maximum level of detail without become darker or lighter. And that's what I try to find ...

 

[markbarendt]

Good morning .
For my mind middle or medium grey is the part of the scene that reflect the same luminance that is measured on the slide; or in other words, the part of the scene that can be represented on the slide with maximum level of detail without become darker or lighter. And that's what I try to find ...

I don't think that is a workable definition.

Practically speaking along the straight line of the characteristic curve all the tones have the same amount of separation, so the amount of detail should be the same.

If you adjust the luminance of the light source that you view the slide with, that moves the mid-point/middle gray in the final result but does not change the detail available at either point.

 

[alanrockwood]

Here's an interesting thought about medium gray. Some people say it should be 18% reflectance. Some say it should be 12.5%. How does this map onto the zone system? If we put it 18% reflectance at zone 5, then we get zone six at 36%, zone 7 at 72%, and zone 8 at 144% reflectance. This means that (assuming one limits the discussion to diffuse reflections), the most you can get is zone 7.5 in a scene, and that would be for a perfectly white diffuse reflector. Anything higher is a physical impossibility, that is unless one brings specular reflections into the scene.

If one picks 12.5% as medium gray then in the absence of specular reflections the most one can get in a scene is zone 8 unless one brings in the possibility of specular reflections.

 

[wiltw]

Here's an interesting thought about medium gray. Some people say it should be 18% reflectance. Some say it should be 12.5%. How does this map onto the zone system? If we put it 18% reflectance at zone 5, then we get zone six at 36%, zone 7 at 72%, and zone 8 at 144% reflectance. This means that (assuming one limits the discussion to diffuse reflections), the most you can get is zone 7.5 in a scene, and that would be for a perfectly white diffuse reflector. Anything higher is a physical impossibility, that is unless one brings specular reflections into the scene.

If one picks 12.5% as medium gray then in the absence of specular reflections the most one can get in a scene is zone 8 unless one brings in the possibility of specular reflections.

Uh-oh, we have a blurring of two different concepts! 1) tonal range in reproduction vs. 2) density upon which reflected light meters are calibrated

1. In the Zone System, it is Zone V which is 'middle gray' which is the midpoint between absolute (detailess) white and absolute (detailless) black -- 18% density is the midpoint between 0% and 100%.
2. If one looks at the ANSI standards for calibration of reflected light meters vs. calibration of incident light meters, one can equate 12.5% as the reflectivity target which will result in 'same exposure' from either type of meter

But even THAT (#2) is debatable because the ANSI standards themselves allow the meter companies to choose a VARIABLE value to use in the equations. As a result, 'your results may vary'. For example, my Minolta Autometer Vf incident meter gives an identical reading to what i get if I aim my Minolta Spotmeter F or my Canon 40D at the same 18% gray card! So what happened to 12.5% here?!?!?!