Help understanding gray card use
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Isn't the point of light meters and a question such as the OP asked to have a couple or s to have read his post and answer his actual question accurately and boringly while most want a more exciting debate?
pentaxuser
...which was done in the responses in posts 2,4,5 ...the rest was discussions/elaboration
Thanks all, learning interesting "culture" of creative minds on this forum - I didn't intend to spark a debate that veered off as much and became unrelated to the question asked 
. Not unusual though, and often useful, so again, thanks to all contributed. I *think* I understand what people are saying, but wanted to confirm in simple terms. Please stop me (and elaborate if you don't mind) when you spot a mistake in the following:
In essence, a gray card represents (to a meter) imagined very generic scene, where all the colors of a "collective" subject(s) represented about equally - averaged in entire field of view it's not too red, too yellow etc; e.g. it is nature color-balanced. Yes a forest could be too green in average, a body of water - too blue etc, but since a light meter electrically is really a black-and white sensing device, an every day typical average object/subject (there's no such thing, but it's assumed for metering purposes) can be reduced to a uniform field with equal light intensity reflected by it and hitting the meter's sensor and that uniform field's reflection intensity happen to be equal to a real scene's light intensity when the field is 18% gray. In other words, if I look, say, at a city street with my eye [through a 30 degree cone if my meter's FOV is 30 deg.) I will perceive the scene as bright as if the entire visible it's replaced with gigantic 18% gray surface. Am I close? (Let's put aside spectrum densities for now e.g. how much brighter 5000 Lux yelow light looks to an eye/sensor than same 5000 Lux blue light, etc, I get that part) So this surface reflects as much sun's light (or whatever source of lightis) as real average city street would. So much *averaging* happen, that it blures to a total gray. BUT, for metering purposes, if I know the LUX intencity hitting my sensor and all I need to calibrate the gain of the circuit so that I get, say, 100uA current through the galvanometer coil and a needle deflecion such that it points to a correct EV exposure number which results in not too dark or too light (e.g. well balanced) B/W negative, I can get 5000 lux off of ANY surface - not gray and not 18% reflective. It can be direct sun, can be any visible light of a typical scene. Cannot be monochromatic say, red light only - even if I get exactly 5000 Lux of pure red light emitted toward my [passive analog] meter and calibrate needle deflection to point to an EV number such that photographing that light source will give me perfect gray picture (as, say, a red sunset on a B/W film - well balanced gray), photographing real life objects with colors other than red will give me an error. So the light for calibration must not only be a certain number of Lux, but contain all the colors (to the sensor - color temp. intensities) my human eye likely to encounter. And the gray represents this spectrum as it contains some of red, some of blue, green, yellow, etc - a mixture closely representing real color objects. Is thius why *gray* is chosen for calibration? E.g. any other color could be chosen just to calibrate the sensor's output, but it will skew typical real life's colors distribution. Yes, any sensor will react differently to the same Lux intensity of different colors as they have very different spectral sensitivety curves - some react to UV or infrared unlike human eye, thus for metering purposes they are produced with these curves as closely resembling human eyes as psossible. Yes, I'm more of an EE than a photographer, but want to get to the bottom of fundamentals many experts here are so confidently talking about. Thanks in advance!
. Not unusual though, and often useful, so again, thanks to all contributed. I *think* I understand what people are saying, but wanted to confirm in simple terms. Please stop me (and elaborate if you don't mind) when you spot a mistake in the following:In essence, a gray card represents (to a meter) imagined very generic scene, where all the colors of a "collective" subject(s) represented about equally - averaged in entire field of view it's not too red, too yellow etc; e.g. it is nature color-balanced. Yes a forest could be too green in average, a body of water - too blue etc, but since a light meter electrically is really a black-and white sensing device, an every day typical average object/subject (there's no such thing, but it's assumed for metering purposes) can be reduced to a uniform field with equal light intensity reflected by it and hitting the meter's sensor and that uniform field's reflection intensity happen to be equal to a real scene's light intensity when the field is 18% gray. In other words, if I look, say, at a city street with my eye [through a 30 degree cone if my meter's FOV is 30 deg.) I will perceive the scene as bright as if the entire visible it's replaced with gigantic 18% gray surface. Am I close? (Let's put aside spectrum densities for now e.g. how much brighter 5000 Lux yelow light looks to an eye/sensor than same 5000 Lux blue light, etc, I get that part) So this surface reflects as much sun's light (or whatever source of lightis) as real average city street would. So much *averaging* happen, that it blures to a total gray. BUT, for metering purposes, if I know the LUX intencity hitting my sensor and all I need to calibrate the gain of the circuit so that I get, say, 100uA current through the galvanometer coil and a needle deflecion such that it points to a correct EV exposure number which results in not too dark or too light (e.g. well balanced) B/W negative, I can get 5000 lux off of ANY surface - not gray and not 18% reflective. It can be direct sun, can be any visible light of a typical scene. Cannot be monochromatic say, red light only - even if I get exactly 5000 Lux of pure red light emitted toward my [passive analog] meter and calibrate needle deflection to point to an EV number such that photographing that light source will give me perfect gray picture (as, say, a red sunset on a B/W film - well balanced gray), photographing real life objects with colors other than red will give me an error. So the light for calibration must not only be a certain number of Lux, but contain all the colors (to the sensor - color temp. intensities) my human eye likely to encounter. And the gray represents this spectrum as it contains some of red, some of blue, green, yellow, etc - a mixture closely representing real color objects. Is thius why *gray* is chosen for calibration? E.g. any other color could be chosen just to calibrate the sensor's output, but it will skew typical real life's colors distribution. Yes, any sensor will react differently to the same Lux intensity of different colors as they have very different spectral sensitivety curves - some react to UV or infrared unlike human eye, thus for metering purposes they are produced with these curves as closely resembling human eyes as psossible. Yes, I'm more of an EE than a photographer, but want to get to the bottom of fundamentals many experts here are so confidently talking about. Thanks in advance!
MattKing
Moderator
That is too complex, I'm afraid.
A grey card is, quite simply, something with a standard reflectance.
If you were to use a brick wall as your standard reflectance target, you would have to carry that wall around with you if you wanted a repeatable standard target.
Grey cards are way more portable.
In days of yore, there were very precisely made and checked grey cards around - the sort that might have been used on catalogue shooting sites or movie sets - as well as less expensive grey cards that were still useful.
If you have successfully obtained a very good exposure of something - say an exposure that worked perfectly in your Sears Roebuck catalogue - you would then take a reading off of the grey card, held in a very particular way, under the exact lighting setup that gave you that very good exposure. Then you could shut down for the weekend, come back on Monday, set everything up again, take a reading exactly the same way off of the grey card again and start the new week with that very same good exposure. The fact that the catalogue item was different on the Friday before from the catalogue item on the Monday wouldn't matter.
Meters measure the light that hits them. You can either use them in incident mode - which means that the meter measures the light that hits the subject - or reflective mode - which means the meter measures the light bouncing off the subject toward the meter.
If you substitute a grey card for a subject and take a reading off of the card, as long as you use the correct technique and do so in a repeatable way, you can convert the reading off of the card to give you the equivalent results of an incident reading.
Grey is used, because it does a fairly good job reflecting back a range of colours that roughly corresponds what one might encounter in natural light situations. The quality of the card may have an effect on that.
18% was chosen because:
1) something in the middle of the range of reflectance available had to be chosen; and
2) in some measures, 18% is in the rough middle, and with the appropriate built in configuration, meters can be set up to give the correct exposure of a statistically "average" subject if the reading is taken the right way, off of that 18% reflective surface, held at the right angle.
With all that being said though, there is nothing magical about an 18% reflective grey card. It is just that it is a repeatable and reusable target.
You cannot calibrate a meter using just a grey card. You could calibrate a meter using a grey card and a calibrated light source.
A grey card is, quite simply, something with a standard reflectance.
If you were to use a brick wall as your standard reflectance target, you would have to carry that wall around with you if you wanted a repeatable standard target.
Grey cards are way more portable.
In days of yore, there were very precisely made and checked grey cards around - the sort that might have been used on catalogue shooting sites or movie sets - as well as less expensive grey cards that were still useful.
If you have successfully obtained a very good exposure of something - say an exposure that worked perfectly in your Sears Roebuck catalogue - you would then take a reading off of the grey card, held in a very particular way, under the exact lighting setup that gave you that very good exposure. Then you could shut down for the weekend, come back on Monday, set everything up again, take a reading exactly the same way off of the grey card again and start the new week with that very same good exposure. The fact that the catalogue item was different on the Friday before from the catalogue item on the Monday wouldn't matter.
Meters measure the light that hits them. You can either use them in incident mode - which means that the meter measures the light that hits the subject - or reflective mode - which means the meter measures the light bouncing off the subject toward the meter.
If you substitute a grey card for a subject and take a reading off of the card, as long as you use the correct technique and do so in a repeatable way, you can convert the reading off of the card to give you the equivalent results of an incident reading.
Grey is used, because it does a fairly good job reflecting back a range of colours that roughly corresponds what one might encounter in natural light situations. The quality of the card may have an effect on that.
18% was chosen because:
1) something in the middle of the range of reflectance available had to be chosen; and
2) in some measures, 18% is in the rough middle, and with the appropriate built in configuration, meters can be set up to give the correct exposure of a statistically "average" subject if the reading is taken the right way, off of that 18% reflective surface, held at the right angle.
With all that being said though, there is nothing magical about an 18% reflective grey card. It is just that it is a repeatable and reusable target.
You cannot calibrate a meter using just a grey card. You could calibrate a meter using a grey card and a calibrated light source.
I agree with Matt -- there's no need to over-complicate things, although "that's what we do here".
You want simple, and cheap? Find a plain, white, Styrofoam coffee cup. On a sunny day, set the ISO on your camera (or meter) to 125. Set the shutter speed to 1/125. Place the coffee cup over the lens (or sensor). Point it at the sun. The recommended f-stop should be f16.
For years, I've used the top of a WHITE L'EGGS pantyhose container -- MUCH more durable than a Styrofoam cup. Turns your reflectance meter into an incident meter -- FOR FREE.
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I’d do something with a trans-illuminated screen instead of trying to reflect light off any surface.
I could always send you my Luna Pro and then you can calibrate like-device for like-device, that would rule out a number of variables.
I could always send you my Luna Pro and then you can calibrate like-device for like-device, that would rule out a number of variables.
Besides Minolta any of the other brands meter have removable dome? I want to get one of the dome to built my own meter. The L'EGGS is too big for what I want.
Guys! Trying to read through large unbroken blocks of text on my screen is killing my eyes.
Can we please start a new paragraph every once in a while?
;-)
Can we please start a new paragraph every once in a while?
;-)
Nah, thanks a lot for the offer though. I've started messing with Luna Pro only because got into pinhole photography as casual hobby, and needed *something* to roughly measure light. I use B/W photo paper as negative and scan it in afterwards, compensating any kind of exposure errors in the software (within reasonable limits). I practically never view final photographs on paper, it's for viewing on screen only, though for fun I did try to make positive prints on paper by direct contact method.
As you can imagine, this type of photography is quite forgiving and there are several ways to fix wrong exposure to achieve very acceptable outcome: once you start developing the paper negative you immediately see if it was under- or overexposed, and can keep it in developer longer or pull and fix earlier. Lost contrast for short develop times of overexposed paper is easily compensated digitally later. Before I got my used Luna, I use to eye-ball the exposure time, and since it is typical 3-6 minutes (for paper's ISO = ~3) the judgement error of 15-20 sec does not really make such a great impact.
My point is several EVs error is OK, but I still make way too big errors more often than I'd like to admit, so needed any meter that is more sophisticated than my untrained eyes. AS such, mis-calibrated by a few EVs is totally OK as long as it's consistent - I just compensate by dialing different ISO after actual trial and errors, and now all prints come out very balanced, though I'm positive if I'd use your properly calibrated Luna Pro I'd get wrong exposure and would have to start my trials again. I'm not stuck, I can always carry around my Nikon D-200 DSLR as very well calibrated meter, it's just I want to fix the Luna Pro hardware I have on hand.
I just got my second Luna Pro off ebay just to compare to the first one or for parts. They show way different results, overexposing around 5 EVs on hight scale. But the first and older Luna Pro I calibrated agrees with the D-200, meaning the second one is way off - I opened it and found a sticker that it was serviced in 1982. The gray filter in it looks bad - little pieces of coating chipped off its surface so in average it looks more "transparent" as gray filtering material is missing from the filter - I'd say 25-30% is gone. So more light now hits the CdS sensor than is suppose to - something I can't fix unless find (or make) a piece of neutral filter 6 EV dense. Like a piece of negative film.
Electrically calibrating out wrong physical light filtering error could do it, but is wrong approach. So, again, thanks for the generous offer, it jsut won't do me any good - I do have the reference I can check against. Any ideas what can I use as a piece of gray filter? It's got to be just around 10x10 mm square and as close to 6 EV dense as possible. Shooting a wall on a roll of B/W film while vary its illumination with small lighting increments so I can cut out suitable piece as filter after developing it is way too much effort to be worth it.
Sorry, let me edit my previous post and include spaces between paragraphs...
Not all of Minolta's meters had removable domes, and many meters have domes that simply slide in or out of position. Some meters require you to "flip a switch" when you make the change, others not.
Are there meters that go down to f500? I've never seen one. If you are shooting pinholes, any meter will work fine, but I'd use an incident meter -- or a L'EGGS pany hose holder on a D200. And you'll need a sheet of paper to convert f4 at 1/125 into f378 at ?????
Of course the spotmeter wouldn't have any dome but I don't know of any Minolta meter that has a fixed dome.
How many pinholes do you have? If you have only a few pinholes then convert the f/number to Av value. For example f/378 is Av17.12 so let round it down to 17.
Now f/4 at 1/125 is EV11 you can do it in your head, 11-17=-6 and also in your head Tv -6 is 60 second.
MattKing
Moderator
With pinhole, you meter at f/16 or f/22.
Then you convert by factoring in the fixed LV offset between the f/stop you metered at and the effective f/stop of the pinhole.
Then you convert that result to take into account any reciprocity failure compensation.
I made up a table that I used and re-used that corresponded to my pinhole and TMax 100 - my film of choice for pinhole.
Then I just read off the table - i.e. if the meter reading for f/16 was for X seconds, than I needed to use an exposure of Y seconds.
Then you convert by factoring in the fixed LV offset between the f/stop you metered at and the effective f/stop of the pinhole.
Then you convert that result to take into account any reciprocity failure compensation.
I made up a table that I used and re-used that corresponded to my pinhole and TMax 100 - my film of choice for pinhole.
Then I just read off the table - i.e. if the meter reading for f/16 was for X seconds, than I needed to use an exposure of Y seconds.
Minolta never made a meter with a fixed dome, but they made meters without domes.
I don't need any help converting numbers on my Minolta meters for use with my pinholes. Like Matt, I have a conversion table. My point was that while you can use a normal meter as a starting point, it is worthless to use directly -- you have to have a simple, easy, fast way to convert the meter reading for use with pinhole tiny f-stops.
I’ve used Matt’s method for years quite successfully. But the last couple of year’s WPPD festivities I’ve used Pinhole Assist on my iPhone for metering. It handles apertures to f/1024.
And then there’s possible reciprocity issues. So I usually go loose, as in “Good enough for pinhole!”
It’s difficult to operate intuitively when a half stop adjustment might be five minutes.
And then there’s possible reciprocity issues. So I usually go loose, as in “Good enough for pinhole!”
It’s difficult to operate intuitively when a half stop adjustment might be five minutes.
But my question is really. Minolta and even Kenko don't sell the meters or dome any more. Is there other brands of meters that have removeable dome so they would sell the domes? Does Sekonic or Gossen have removable dome meter?
I don't know about other meters, but I see Minolta domes on EBAY quite a bit. FYI, they came in different densities, ex. 2X ND.
You can buy a Minolta dome for $39.95 and get the whole meter with the dome for $49.95. Domes are removeable so you want can swap in a reflectance attachment with 5% or 10% spot depending on meter version.
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You can buy a Minolta dome for $39.95 and get the whole meter with the dome for $49.95. Domes are removeable so you want can swap in a reflectance attachment with 5% or 10% spot depending on meter version.
Minolta Dome for sale | eBay
Get the best deals for Minolta Dome at eBay.com. We have a great online selection at the lowest prices with Fast & Free shipping on many items!www.ebay.com
Yes that's why I didn't buy them because there are several reasons. Domes when they get old do yellowing a bit and do affect the accuracy of the meter. Kenko was selling brand new domes for $24 each and buying used on ebay for $39.95 hurts. I have enough meters to buy another one still.
I don't see any yellowing either but I have a Minolta flashmeter III which is old and compared to the much newer Flashmeter VI it reads 1/2 stop low. I thought that's because the meter drifts as it's old but swapping the diffuser the readings swapped. So I bought a new diffuser. Luckily at that time they still sell them new.
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