Calculating EI with an Incident meter?

chiller · Jan 9, 2018

Does anyone have a methodology they use, or know off for film testing EI [correct exposure and correct highlight development] using an incident meter? I don't need to have the reflected light method explained. I have used and understand using a reflected light meter to do this task but wonder if an alternate method can use an incident meter. My Spectra IV incident meter and my Pentax spotmeter give extremely close readings based on a grey card.
Suggestions as to how you would approach this will be helpful.

Sirius Glass · Jan 9, 2018

Yes, shoot box speed.

Bill Burk · Jan 9, 2018

BTZS is like the Zone System but uses incident meters. You could look into that.

Meanwhile use the reflected light meter together with the incident meter to confirm the gray card is being held at the correct angle.

chriscrawfordphoto · Jan 9, 2018

Use the same speed you calculated when you used your spotmeter. In light without harsh shadows, give one stop more exposure than the meter indicates. If there are deep shadows (like in bright sunlight) meter in the shadowed area and give one stop LESS than the meter indicates if you intend to develop normally. Or, if you are going to do N-1 developing, give the exposure indicated by the meter in the shadow without changing it and give your shorter N-1 develop time.

John Koehrer · Jan 10, 2018

Bill Burk said:
BTZS is like the Zone System but uses incident meters. You could look into that.

Meanwhile use the reflected light meter together with the incident meter to confirm the gray card is being held at the correct angle.

BTZS was Fred Pickers site(I think!) and it'll give you step by step instructions.

ic-racer · Jan 10, 2018

How will you use the meter? If you have a barn with two walls showing. One wall is in shadow, illuminated by the expanse of the sky. The other wall is in direct sun. Where are you going to meter? You question can't be answered without that information. If you put the incident meter in the shadow, you will probably be OK shooting at box speed. If you have the meter in the sun, then you will lose shadow detail if you expose at box speed (though if you are doing copy work, with the paper in the sun, you will be OK exposed at box speed.)

Did you incident meter come with a manual? Some manuals give detailed information on how to use the meter.

Poisson Du Jour · Jan 10, 2018

ic-racer said:
How will you use the meter? If you have a barn with two walls showing. One wall is in shadow, illuminated by the expanse of the sky. The other wall is in direct sun. Where are you going to meter?

Reading that, my reflex action is to use a spot meter, not an incident meter.
EI can only be determined by examining images critically and understanding the difference a change will make. This is particularly critical when using transparency film.

Sirius Glass · Jan 10, 2018

ic-racer said:
How will you use the meter? If you have a barn with two walls showing. One wall is in shadow, illuminated by the expanse of the sky. The other wall is in direct sun. Where are you going to meter? You question can't be answered without that information. If you put the incident meter in the shadow, you will probably be OK shooting at box speed. If you have the meter in the sun, then you will lose shadow detail if you expose at box speed (though if you are doing copy work, with the paper in the sun, you will be OK exposed at box speed.)

Did you incident meter come with a manual? Some manuals give detailed information on how to use the meter.

If you read the manual, you would know that incident meters are aimed at the camera and not the subject. Reflectance meters are aimed at the subject.

MattKing · Jan 10, 2018

BTZS is a system created by the late Phil Davis.
It makes use of an incident meter, but you do not use the meter in the same way Sirius is talking about.
If I understand it correctly, in a BTZS approach you meter the intensities of both the main illumination hitting the subject and the indirect (often shadow side) fill information hitting the subject, and use those two readings to determine both exposure and development.
You don't point the incident meter at the camera - you point it at the sources of light - and you interpret the results in a very different way.
To great effect too!

Ozxplorer · Jan 10, 2018

Steve, I’m not sure if this is where you want to go? Link to late Phil Davis’ site - http://www.btzs.org/Articles/Sensitometry Part 5.pdf

chiller · Jan 11, 2018

Just to clarify. I have and do use the complete BTZS system with both incident and spot metering for sheet film. My darkroom is set up for BTZS film testing and have done so with many 5x4 films and developers since 2005. I have and use a densitometer and have also used the Fred Picker visual system for determining film speed with a roll film. I have used an incident meter for over 50 years both commercially and for my private photography and understand the advantages and limitations of both my Incident and spot meters.

I'm attaching a small worksheet that I have put together for roll film testing using an incident meter and a grey card. I will test my theory this afternoon but we are in the midst of our summer and it is just on 100f at present so not the most pleasant in my darkroom.

It has one entry field to enter the box speed of your film. This may or may not work but it is a possible solution to my original question.

Sirius Glass · Jan 11, 2018

MattKing said:
BTZS is a system created by the late Phil Davis.
It makes use of an incident meter, but you do not use the meter in the same way Sirius is talking about.
If I understand it correctly, in a BTZS approach you meter the intensities of both the main illumination hitting the subject and the indirect (often shadow side) fill information hitting the subject, and use those two readings to determine both exposure and development.
You don't point the incident meter at the camera - you point it at the sources of light - and you interpret the results in a very different way.
To great effect too!

I never looked much into BTZS.

Bill Burk · Jan 11, 2018

I really think you should be careful trying to correlate a gray card with an incident meter.

Because the angle that you hold the card changes the apparent brightness of the card, the same incident light could result in different negative densities at the same camera shutter/lens aperture settings.

It's why I would say to use a spotmeter to confirm the brightness of the card.

At the least you might want to try to have the spotmeter and incident meter read the same.

Now it may be entertaining to try to ferret out once and for all... the whole 18% gray thing.

To calibrate the incident meter to the gray card "properly" to 18% gray, there might actually be a specific expected difference between the two readings... and there might actually be a specific expected deviation from the density at the metered point.

Or you can angle the card just right to get it to read the same as the incident meter and make everything simple for the tests you are trying to do.

wiltw · Jan 11, 2018

Bill Burk said:
I really think you should be careful trying to correlate a gray card with an incident meter.
Because the angle that you hold the card changes the apparent brightness of the card, the same incident light could result in different negative densities at the same camera shutter/lens aperture settings.
It's why I would say to use a spotmeter to confirm the brightness of the card.

...Or you can angle the card just right to get it to read the same as the incident meter and make everything simple for the tests you are trying to do.

The color photos in this post of mine illustrate the variability of the reflectance from an '18% gray' card

https://www.photrio.com/forum/threads/light-meter-calibration.147889/page-2#post-1936615

chiller · Jan 11, 2018

Hi Bill and thanks for your considered points. My Pentax spot meter and the Spectra IV incident meter read exactly the same when compared using the gray card. In fact all of my meters are within .1 f stop reflected or incident. I appreciate the angle of reflectance with the grey card and it can be significant with a reflected meter and much like using a flat disk on the incident meter. A change in angle can vary the reading a fair amount.

Bill Burk said:
Now it may be entertaining to try to ferret out once and for all... the whole 18% gray thing.

I've always found this debate interesting in theory but for me personally over at least 40 years my incident and spot meters have always agreed with the grey card and I blissfully knew no better Possibly our Australian sunlight knows how to respond correctly to a reflected and incident meter.

RalphLambrecht · Jan 11, 2018

Sirius Glass said:
Yes, shoot box speed.

interesting; I always end up shooting box speed minus 2/3 even after a ton of testing; seems like I could skip all testing and just use godspeed-2/3 at all times.

chiller · Jan 11, 2018

While you are there Ralph, thanks for the excellent reference in Way beyond monochrome. I particularly like the chart on page 288. Very handy.
I refer to 2 books quite often - Way beyond monochrome and the superb Beyond the zone system by the late Phil Davis.

Doremus Scudder · Jan 11, 2018

I'm not sure I understand why you asked this question. It seems to me that you have enough mastery of BTZS, the Zone System, metering techniques, etc. as well as proper equipment (e.g., densitometer) to be able to answer this yourself.

Anyway, back to the beginning: Film manufacturers determine ISO using rigorous and very controlled testing including exposure, development and sensitometry. An E.I. is a personal thing that deviates from this for one basic reason: to get the desired shadow detail. Finding your E.I. doesn't have to be as rigorous a process as ISO testing. Many of us end up tweaking our E.I. based simply on the old Kodak advice, "If your negatives are consistently too thin, give more exposure..."

So, if I were using an incident meter to determine my personal E.I., I would simply find a scene with both important shaded and lit subjects and with lighting I would call "normal" (you can use your incident meter to measure the difference between light and shade and use the BTZS formula to find the spread...). Then, I would meter the light falling on the shaded area (open sky or clouds or whatever) and base my exposure on that. I'd shoot an exposure at box speed and then one at 2/3 stop over and one at 1 1/3 stop over. I'd develop and print on my standard paper, making a Fred Picker "proper proof" and pick the one I liked best regarding shadow rendering. This would be my E.I. If the rendering I liked best fell between two of the prints made, I'd choose the 1/3-stop intermediate value as my E.I. Done and done.

Then, I'd keep careful field notes and adjust if necessary.

Best,

Doremus

Sirius Glass · Jan 11, 2018

Most people who use an EI adjust the ISO about half an f/stop. So why bother to do all the testing when that is where one will end up any way?

Maris · Jan 11, 2018

I do this once for each different film. First set box speed on the incident meter, take a reading, and then shoot a bracketed set of exposures. Develop the film and pick the best negative. The exposure record for this negative indicates the correct exposure.
( Light meters may lie about correct exposure but film doesn't.) Adjust the film speed input on the incident meter so the meter would have indicated this correct exposure: calibration done. Now to test the next film.

Calculating EI with an Incident meter?

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