Why don't reflective meter reading of zone V card and an incident reading match?

The grey card method is prone to mistakes mainly due to the fact that you have to keep it a 45° between the light source and your subject, and you don't have to project your shadow or the camera shadow over it (very easy to project a camera shadow), and that you should avoid flare on the card, and that, I suppose, also on the vertical plane the card should be at 45° between light source and subject.

You basically need three or four arms. It's extremely unlikely you have more than 2 arms.

A grey card is a cheap substitute or an emergency substitute for an incident light meter.

Nowadays one can find relatively cheap and good incident light meters so grey cards are something that I learned to disregard since years.

Chan Tran said:

The dome isn't an average measurement of the scene but rather a measurement of light falling on the dome ...

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That is a great description Chan Tran. Thank you.

In practice, the dome is used to integrate the light illuminating the scene. It is normally held somewhere in the scene and pointed toward the camera. Used in this manner it will integrate the the light coming from the light source (sun, flash, etc.) and the light reflected off of the environment in front of the scene, all which illuminate the scene. The meter reads this integrated light and gives you a camera setting that, based on a given ISO should place an 18% gray card in the scene, or any light level, at its proper place on the curve. This is a general description and actual results may be affected by where exactly the meter is pointed but pointing it at the camera is known to give good results.

mr.datsun said:

Bill, your comment about Tungsten speed rating is interesting. I keep asking myself why Tri-X Super 8 Reversal is rated at 160ASA for Tungsten and 200ASA for daylight. I never thought about b&w Tri X 35mm having two different ratings as I thought it was panchromatic. Is this related to the selenium meter sensitivity that you mention (which I guess many Super 8 cameras, like mine have) or another factor?

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I'd known of the different speed ratings in darkroom dataguides for years, and never thought too deeply of why until recently...

I picked up a (new to me, but I'd had one before) Weston Master II recently, and noticed the needle really jumps when aimed at a regular light bulb. At the same time I was reading LP Clerc. Section 402. Daylight and Artificial Light Speeds. Explained several factors: "spectral distribution of the artificial light, colour sensitivity of the film and spectral response of the exposure meter." "Photo-sensitive -- and especially photo-conductive -- cells tend to respond more to the longer wavelength bands than an average panchromatic emulsion." "This tungsten speed rating is thus an equivalent rating rather than an actual emulsion speed"

(p.s. CdS cells are the photo-conductive cells).

Some time in the 1960s the color temperature of the calibration light source was changed from 2800 K to 4700 K. It was suggested that the change would allow the discontinuation of requiring a daylight and tungsten EI for films.

The spectral sensitivity of the photo cell has always played an important role with meter accuracy. Two meters could disagree simply because of the difference in sensitivity to the color temperature of the light source. Also, most photo cells are sensitive to infrared. If the material reflects a lot of infrared it will effect the reflective meter while the incident meter is unaffected. A simple test for infrared sensitivity can be done with any remote. Point it at the meter and monitor any changes.

Chan Tran was right about how a reflective exposure meter has a greater chance of matching an incident meter when the incident meter is used with the disk. They should be about 1/2 stop difference when using the dome.

1. The ISO standard for calibration of reflected meter has a manufacturer chosen Constant -- essentially this is a VARIABLE value, selected by the meter manufacturer.

2. The ISO standard for calibration of incident meter has a manufacturer chosen Constant -- essentially this is a VARIABLE value, selected by the meter manufacturer.


...so the fact that meter #1 does not match meter #2 is inherent to the ISO standards for the two meters!

3. Although 18% grey tone is 'middle of the tonal range', one can determine that really one needs to meter 12-13% grey card, for #1 to truly match #2 in reading.

A very detailed explanation of the constants can be found in the tread "Is the K factor relevant to me or should I cancel it out?" Basically, the exposure constants K and C do two things. Hand held meters are separate from the camera's optical system. The light loss caused by the camera lens needs to be factored into the exposure calculation, and because the meter isn't reading through the camera lens, the actual value must be averaged. Second, the exposure calculation needs to factor out the influence of the physicality of the meter, ie spectral sensitivity. There is a rather involved equation for determining K. The following is a page from the K factor thread which shows the equation for K.

What's considered the average reflectance comes from K / C. 1.16*pi / 30 = 0.12.

In which decade was your meter last calibrated?

Kirk Keyes said:

In which decade was your meter last calibrated?

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Kirk. Never. Is it expensive and where would one go in the UK, for example?

I have no ideas where to go for the UK. But most good camera repair shops should have a calibrated light soure to check your meter with.

"Never"
But that's kind of my point, how can one expect a measuring tool like a light meter that more than 50 years old, to be accurate? Add to that all issues Steve Benskin points out above...

Even when calibrated, meters are only "accurate" under certain conditions. Not that bad if you consider the actual purpose of the meter. These are exposure meters and not light meters.

Kirk Keyes said:

I have no ideas where to go for the UK. But most good camera repair shops should have a calibrated light soure to check your meter with.

"Never"
But that's kind of my point, how can one expect a measuring tool like a light meter that more than 50 years old, to be accurate? Add to that all issues Steve Benskin points out above...

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You may be correct. The thought that they didn't last for ever never cross my mind until now. Perhaps it's time to get it serviced.

But. It does have user-calibration setting on the back. This is used to zero-set it.

And then. If it is out of calibration then that does not necessarily explain why the two readings differ. The sensor is the same one but in one case uses a diffusion cone and in the other not. Would this meter be sophisticated enough to have two separate calibrations, one for each mode of use?

Many meters are calibrated against a 12 percent reflectance standard, not 18 percent. That probably accounts for the one zone difference. You will probably find that your reflectance and incident light readings pretty well agree for an average landscape, although there can be differences depending on the landscape.

It's possible too that the position of the dome leaves it partly in shadow, so it gets a lower reading than the incident meter, which is averaging a planar card.

getting back to the OP, the instructions from the grey card itself reads:
"Meter readings of the gray card cshould be adjusted as follows- 1) For subjects of normal reflectance increase the indicated exposure by 1/2 stop. 2) For light subjects use the indicated exposure; for very light subjects decrease exposure by 1/2 stop 3) If the subject is dark to very dark increase the indicated exposure by 1 to 1.5 stops"

So to answr your question, the grey card is actually designed to be lighter than 'middle grey'.