Different measurements with spot vs incident metering on same target?

DREW WILEY · Mar 3, 2016

The quality control on most gray cards is miserable. And meters are NOT color neutral. Different types of photocells see the spectrum differently; so
even specific hues of gray between one card or another will affect the reading. I once took a stack of about twenty gray cards and scanned them with
a high-end spectrophotometer. Not a single one was the gray value alleged, and none were neutral gray. If you spend enough, there are some good
ones out there; but these can fade or get dirty. One more reason to get to know your meter through sheer practice. As they say, Better the devil you
know than the one you don't.

xya · Mar 4, 2016

OMG, 40 years of professional life as a photographer and I NEVER considered one of these problems. every time I change one of the components in my workflow (camera, lens, meter, flash...), I shoot a roll, process it and have an hour with a magnifier under my enlarger. that's it. so if I change my meter, I shoot some difficult subjects, including bracketing, and I will know. I know that it's kind of brute, but for me it works.

Old-N-Feeble · Mar 4, 2016

xya said:
OMG, 40 years of professional life as a photographer and I NEVER considered one of these problems. every time I change one of the components in my workflow (camera, lens, meter, flash...), I shoot a roll, process it and have an hour with a magnifier under my enlarger. that's it. so if I change my meter, I shoot some difficult subjects, including bracketing, and I will know. I know that it's kind of brute, but for me it works.

Oh... please stop being so pragmatic and logical.

Arvee · Mar 4, 2016

xya said:
OMG, 40 years of professional life as a photographer and I NEVER considered one of these problems. every time I change one of the components in my workflow (camera, lens, meter, flash...), I shoot a roll, process it and have an hour with a magnifier under my enlarger. that's it. so if I change my meter, I shoot some difficult subjects, including bracketing, and I will know. I know that it's kind of brute, but for me it works.

Correct answer! A gold star for you!

There are a huge number of variables to be considered in this discussion: individual mfrs. calibration protocol, calibration drift due to aging of the meter/dome, gray card QC, how the person holds the gray card to minimize specular reflections just to name a few.

frank · Mar 4, 2016

Use of a spot meter takes a bit more understanding and experience. Dead easy once you're there, but it takes a while to get there. A spot meter does not suffer fools lightly.

RobC · Mar 4, 2016

frank said:
Use of a spot meter takes a bit more understanding and experience. Dead easy once you're there, but it takes a while to get there. A spot meter does not suffer fools lightly.

Very true. And really no better than an incident meter if you don't validate that shadow, AND very importantly, highlight values actually end up on a print where you would expect them when using normal print grade of 2 to 3. If that doesn't happen then a spot meter is a waste of time.

wiltw · Mar 7, 2016

MattKing said:
An 18% grey card isn't the same reflectance as an average scene, so a straight reading off one should usually vary from an incident reading. In addition, there is a particular technique that you should use with a grey card (a particular angle you should use).

An 18% grey card is useful, because it provides a standardized reflectance.

An 18% reflectance card represents the MIDDLE of the range of tones (between Black at one extreme and White at the other extreme), so 'proper exposure' places the 18% grey card 'in the middle' of a print's or transparency's range of values when the image represents that tone 'at its inherent brightness'.

It is said that a reflected light reading actually needs to be taken of a 12% grey tone in order for an 18% grey tone to end up 'in the middle'. And that same 12% density happens to correspond to the same value in calibration of a reflected light meter, according to the ISO standard equations for incident and for reflected meter calibration.

Here is what the histogram of an 18% grey target, when the same 18% grey is also used to meter for reflected reading.

In comparison, where is an 18% grey card showing exactly in the middle of the Tone Curve histogram (right side, half way down), when the incident reading is used

Lastly, consider that the ISO standard equations include a VARIABLE which the meter manufacturer gets to CHOOSE to use for their meter calibration. And that is why you can compare two brand new meters and have them give you somewhat different results in the same light...the value of the variable which each chose to use in the equation is not the same.

tomfrh · Mar 7, 2016

RobC said:
Because an 18% grey card is not the middle and is 1 stop lighter than the middle

I thought 18% was typically considered middle grey?

RobC · Mar 7, 2016

It's middle of Munsell grey scale. But reflection meter uses 8% of what is metered to place in middle of film curve. The Formula says that based on the K factor of 12.5.
But the point that everyone is missing because its so ingrained in everyone from what is written is that NO film density equates directly to any reflection density in a print. There is no standard for it. It's totally arbitrary depending on how you print it. It's all ball park. So yes 18% is middle grey of munsell system but it is not middle of film standard 7 1/3 stop scale. The middle of that is 8% which is 3 2/3 stops less than what is metered. There is a disconnect between grey card and middle of film curve. If the subject happens to be 10 stops in range then the middle is 3.125%. i.e. the middle of any range is variable depending on the range. As far as film is concerend we generally want to put a middle grey in the middle of the film curve but the middle of film curve isn't same thing as middle grey for usual B+W and colour films. Your reflection meter is calibrated for 8% of what is metered as being the middle of the film curve except it isn't unless the subject brightness range matches the 7 1/3 stop range of the average subject that your sytem is calibrated to.
18% is always 2 1/2 stops less than 100% regardless of whether the Subject Brightness Range is 10,12, 100, 5 or any other range of stops.
So place your grey card in a 10 stop SBR then its effectively zone 7 1/2. Place it in a 4 stop range and its effectively zone 1.5. But it will always be in the middle of the film curve which is not the same thing.

Probably best you don't worry about it becasue what happens is you will meter however you meter and will adjust to get what you want when you print. And/or you will adjust your EI to get what you want based on how you meter. So in the long run it really doesn't matter unless you are trying to get a grey card to print with 18% reflection from standard G2 with set print time every time. Are you doing that? Are you checking print reflection density with a densitometer? If not then whats the point of worrying about it. If you are using colour film and want accurate colour reproduction then use an incident meter and calibrate EI to get what you want and pray the lab know what they are doing.

Sirius Glass · Mar 7, 2016

Better yet, shoot box speed and trust your light meter [built-in TTL, hand held reflectance, hand held incident, spot meter] and be done with it, instead of wasting time doing endless, meaningless testing and wasting internet bandwidth.

RobC · Mar 7, 2016

Sirius Glass said:
Better yet, shoot box speed and trust your light meter [built-in TTL, hand held reflectance, hand held incident, spot meter] and be done with it, instead of wasting time doing endless, meaningless testing and wasting internet bandwidth.

Well yes except enquiring minds like to know about these things. But gaining deep enough undertsanding to get a real handle on it takes more than reading a web forum and yes you can (and I have) waste a huge amount of time on it. My conclusion is: Forget about grey cards and don't use one.
I calibrate my dev without one so as to capture a 10 stop range which prints directly to G2 paper based on the density of a zone 1 neg. That way I can meter any zone between 0 and 10 and set my exposure for it in the knowledge it will be recorded on film and reproduce correctly. I tend to use a zone 7 for setting exposure which gives me better neg density consistancy for similar print times and adjust contrast to get my shadows where I want them. No grey card needed. 18% ? Who cares. I certainy don't.

RobC · Mar 7, 2016

Just to confuse the hell out of every grey card user. A grey card reflects 18% and according to what you might interpret in Adams zone system it equal to zone V, a middle grey. Now if 18% is zone V then zone VI would be twice as bright and be 36% reflectance if in the same light. And zone VII would be twice as bright again and be 72% reflectance. And zone VIII would be twice as bright again and be 144% reflectance. Hang on a second, how is it possible to have 144% reflectance from the light falling on a subject. Well of course it isn't so how can 18% be the middle of Adams zone 0 thru zone 10 system? Simple, it can't be. Mull that one over... Or to make it easy just forget about grey cards and learn how to use meter properly without one.

Sirius Glass · Mar 7, 2016

I never had a problem using my meters and box speed, but I have done several deep dives into the readings of incident meters and reflective meters with and without 18% gray cards, MacBeth cards, ... , 8%, 12.5% ... testing and after getting a good understanding of photographic metrology I ended up going back to just trusting my meters. I use spot meters and reflectance meters most of the time, but I will use an incident meter if the composition is either very dark or very light, and when the SBR is very large.

RobC · Mar 9, 2016

Yes, for manufacturers recommended film and dev at film ISO speed I rekon an 8% reflectance card would be close to optimal when using a reflection/spot meter. That way you could just meter the card and use that reading without adjusting it. But it isn't necessary to use any card. I think we agree on that.

tomfrh · Mar 9, 2016

I'm totally confused. I think I'll stick to pointing the meter at stuff and making the needle go where I know it should.

RobC · Mar 9, 2016

tomfrh said:
I'm totally confused. I think I'll stick to pointing the meter at stuff and making the needle go where I know it should.

good policy

k.hendrik · Mar 9, 2016

take 3 shots for every picture; one under / one right / one over. No meter, no gray card; just you and your experience. I learned this in the 70' from another (then)old fart

RobC · Mar 9, 2016

k.hendrik said:
take 3 shots for every picture; one under / one right / one over. No meter, no gray card; just you and your experience. I learned this in the 70' from another (then)old fart

expensive in film though. But film is cheap so also a good policy if you ain't sure.

Sirius Glass · Mar 9, 2016

Old farts get younger? Now I am really confused.

Ian Grant · Mar 9, 2016

I remember sitting looking at the Bottalack Tin Mines (Cornwall) a few years ago on an APUG meet up. 4 of us compared our metering and exposures. I used a Gossen Incident meter and a Minolta Spotmeter. What surprised us all was the exposures we cam to were within a third o half a stop (allowing for differen films peeds)

That didn't mean our meter read the same, rather that we knew our meters and how best to interpret the resuslt.

Ian

Sirius Glass · Mar 9, 2016

And if the readings are within a third to half a stop that is good enough for photography. It also allows for aiming the meters in slightly different directions.

RobC · Mar 10, 2016

I'm looking for feedback on the article I just wrote which is related to this here topic so please feel free to do some proof reading

(there was a url link here which no longer exists)

wiltw · Mar 10, 2016

RobC said:
To OP. Please try the following:

Take your grey card and orient it so it faces directly at the primary light source(not the camera). Then take your incident meter and using the flat disc (NOT Dome), put it infront of grey card and point it directly at the primary source(not the camera) and take a reading.
Now take your spot meter and with the grey card still facing directly at the primary light source, meter the grey card with the spot meter lens axis perpendicular to the grey card. i.e. straight in front of it without casting a shadow on it.

Now tell us how the readings differ.

And anyone else with a spot and incident meter can try the same and report back too and then maybe we can get a definitive answer about what's going on.

And yes I know everyone says point incident meter at camera but for this test please point incident meter with flat disc(not dome) at the primary light source.

In diffuse light (window light, indoors)

a Minolta Autometer Vf with flat disk reads ISO 400, 1/8 f/2.8 +0.1
a Minolta Spotmeter F reads ISO 400, 1/8 f/2.8 +0.4

...a difference of 0.3EV

In diffuse light outdoors with 18% grey target oriented at a slight angle upward (not parallel to the wall nor ground)

a Minolta Autometer Vf with flat disk reads ISO 400, 1/8 f/16 +0.7
a Minolta Spotmeter F reads ISO 400, 1/8 f/22 +0.1

...a difference of 0.4EV

which is about what I expect, as the meters were calibrated so that they return the same result when the reflected light meter targets about 12.5%, not 18% target.

In diffuse light outdoors, but with 18% target laid on the ground pointing upward to the sky...

a Minolta Autometer Vf with flat disk reads ISO 400, 1/8 f/32 +0.1
a Minolta Spotmeter F reads ISO 400, 1/8 f/32 +0.5

...a difference of 0.4EV

Lastly,
In diffuse light (CFL in 12" white reflective glass overhead about 30 degree angle from metering target)

a Minolta Autometer Vf with hemisphere reads ISO 400, 1/8 f/4 +0.4
a Minolta Autometer Vf with flat disk reads ISO 400, 1/8 f/4 +0.6
a Minolta Spotmeter F reads ISO 400, 1/8 f/4 +0.6

...the hemisphere differs from the flat disk, but the reflected light spot matches the flat disk. So the reflected differs from the incident with hemisphere by 0.2EV and also from the flat disk reading by the same amount under the test condition.

So what have we learned?.."somewhere around 0.3EV" is the norm for incident to differ from reflected. But "somewhere around 0.3EV" is also the difference between hemisphere and flat disk. Differences vary, and there is no such thing as a 'standard difference'

DREW WILEY · Mar 10, 2016

Can't believe I'm wading through all this. I make life easy on myself. Every meter that I've used in the last 35 years has been either a Pentax or Minolta spotmeter, and every one of them has read absolutely identical to all the others over their whole range. If not, time to recalibrate. Black and white or color work, indoor or out, no surprises. Now I've got one of them almost totally worn out and held together with electrical tape, but I also just stumbled onto another barely-used Pentax at a very good price, which itself reads identically to my unused reference meter. That should last me the
duration, and spare me any learning curve. But this discussion has been interesting in its own right.

foc · Mar 10, 2016

I shot film professionally for 20 years and I used reflected reading for all B&W and C41 colour (spot for some tricky situations), and incident reading for transparency film. Worked fine for me and my clients then and I'm still is business now.

Different measurements with spot vs incident metering on same target?

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