Incident light meter 101 please.

[fahim mohammed]

How does one use an incident light meter?

Thanks.

 

[Doug Kerr]

Hi, Fahim,

How does one use an incident light meter?

Basically, one places the meter at a location what we would expect to receive the incident illumination that would be experienced by the most important subject (hopefully, experienced by everything in the scene!) and with the receptor oriented parallel to the most important surface of the subject (if there is no clear meaning to that, then toward the camera is the best bet - you will hear other recommendations from others).

The exposure index (usually just labeled "ISO) should be set to the rated ISO sensitivity ("ISO speed") of the camera in the mode in which it will be used.

You may find that doing this consistently yields either a greater or lesser exposure than you find desirable. In that case, adjust the exposure index setting to taste (lesser values for greater exposure). (Many digital cameras assign ISO sensitivity ratings in a way that is not consistent with what the meter expects, for various "strategic" reasons.)

Then, use the meter output (however it works) to choose a pair of aperture and shutter speed settings, and set them into the camera in "M' mode or equivalent.

Then just point and shoot!

 

[fahim mohammed]

Doug, thanks for the lucid explanation. No 18% ( 12%) compensation? This is my confusion.

Best.

 

[Gary Ayala]

Hey Fahim,

An Incident meter reads the light source ... not the light the light reflected from the subject. So one meters at or near the subject with the light sensitive element of the meter aimed towards where the photog will be standing (the white bulb).

Light meters measure the light (either reflective or incident) and tell you what exposure is required to attain an 18% or 12% gray. This gray is midway between white and black. There is a on-going arguement whether light meters are calibrated to 18% reflective or 12% reflective (12% is a shade darker than 18%).

The incident meter is pretty close to using a gray card when metering.

Gary

 

[Doug Kerr]

Doug, thanks for the lucid explanation. No 18% ( 12%) compensation? This is my confusion.

The role of the numbers 18% and 12.8% is complicated, and widely misunderstood. I won't try and give a quick discussion of it here.

In any case, there is no "compensation" required in connection with the use of an incident light meter.

Incident light meters are "permitted", by the applicable ISO standard, to have a rather wide range of "calibration factors". And digital cameras have a rather wide range of how ISO sensitivity is assessed (notwithstanding the fact that this is clearly prescribed by an ISO standard).

As a result, in incidental light metered exposure, "your mileage may vary". But this should not be thought of as a matter of "compensation", and doesn't in any direct way involve the numbers 18% or 12.8% (which do figure into the overall story of exposure metering, but not in this part of it).

Best regards,

Doug

 

[Doug Kerr]

Hi, Gary,

There is a on-going arguement whether light meters are calibrated to 18% reflective or 12% reflective. . .

Unfortunately, that phrase doesn't really describe any specific relationship, so we don't know what property perhaps has the value 18% or 12%.

I commend you to this discussion of that very point:

http://www.openphotographyforums.com/forums/showthread.php?t=8189

Best regards,


Doug

 

[Doug Kerr]

Hi, Gary,

The incident meter is pretty close to using a gray card when metering.

Another way to say that is that we can make, with a reflected light meter, the same measurement we make with an incident light meter, by letting the reflected light meter regard a gray card of known reflectance bathed in the incident light of interest.

The ISO standard for both incident and reflected light exposure meters allow a generous (read, "sloppy") range of their respective "calibration factors".

Unfortunately, few exposure meter manufacturers tell us the values of the calibration constant they shoot for in their instruments. (In some cases, one can reverse-engineer it from various tables given in the instrument manual.)

If we take the values of the two calibration constants used as a premise for the tables of exposure factors under APEX as being the "preferred" values for the respective kinds of meters, then the exposure recommendations would be the same when the gray card mentioned above has a reflectance of about 16%.

Note that for automatic exposure systems, the ISO standard prescribes a single value for the calibration factor (but because of the way the standard is expressed, you can't directly tell what that value is).

Best regards,

Doug

 

[fahim mohammed]

Gary, Doug

Thanks for your responses. I will just measure and set the indicated exp. in the camera and see.

Regards.

 

[Alain Briot]

Gary, Doug

Thanks for your responses. I will just measure and set the indicated exp. in the camera and see.

Regards.

Or just take a photo with your camera and if too dark overexpose and too light underexpose ;-)

 

[Doug Kerr]

For what it's worth, a detailed discussion of this area is contained in my tutorial article, "Exposure Meter Calibration", available here:

http://doug.kerr.home.att.net/pumpkin/index.htm#ExpsureMeterCalibration

Best regards,

Doug

 

[Ken Tanaka]

While responses by Doug and Gary are illustrative, Alain's suggestion is the practical solution in today's photography terms.

I own and use two Sekonic meters which are very good devices. But I only really use them to set exposures for studio strobes and with my 50 year-old Rolleiflex TLR which has no meter. Otherwise, I've found my digital cameras to be excellent light meters.

The heyday of handheld light meters was the film era when you could not immediately see the result of an exposure. They're still occasionally handy but digital photography has greatly marginalized their utility in practical terms.

 

[Doug Kerr]

The heyday of handheld light meters was the film era when you could not immediately see the result of an exposure. They're still occasionally handy but digital photography has greatly marginalized their utility in practical terms.

Well said.

(Art by Aaron Johnson.)

Best regards,

Doug

 

[Jack_Flesher]

Or just take a photo with your camera and if too dark overexpose and too light underexpose ;-)

Excellent! LOLOLOLOL!!! I was going to say, "or you could look at the histogram."

 

[Alain Briot]

While responses by Doug and Gary are illustrative, Alain's suggestion is the practical solution in today's photography terms.

I own and use two Sekonic meters which are very good devices. But I only really use them to set exposures for studio strobes and with my 50 year-old Rolleiflex TLR which has no meter. Otherwise, I've found my digital cameras to be excellent light meters.

The heyday of handheld light meters was the film era when you could not immediately see the result of an exposure. They're still occasionally handy but digital photography has greatly marginalized their utility in practical terms.

Thanks Ken! I appreciate this. I went and checked on this thread today because I was concerned my post may be misinterpreted as "curt" so I am pleased you understand where I come from.

As you may know, I currently use a P45 (Phase one back) with a Hasselblad V and no meter of any sort. I expose by guessing (after many years of doing photography my guesses are fairly accurate), checking the histogram to see what the exposure looks like, and then correcting by under or overexposing for the amount I am off. I usually get within a stop of the correct exposure with my first guess and I get to the proper exposure by my second capture.

The Histogram is my best friend!

Alain

 

[Alain Briot]

Excellent! LOLOLOLOL!!! I was going to say, "or you could look at the histogram."

Thanks Jack. I did mean it as tongue in cheek. But again, humor doesn't always come across on the web!

I wish the poster had explained why he wants to learn how to use an incident light meter and whether he is using film or digital. A reflective light meter is way more useful. I may not have it all in mind right now, but the only time I would use an incident meter would be to measure flash output...

Since I was puzzled I pointed to the simplest ( and most practical ) approach, using a digital camera.

 

[Jack_Flesher]

Thanks Jack. I did mean it as tongue in cheek. But again, humor doesn't always come across on the web!

I wish the poster had explained why he wants to learn how to use an incident light meter and whether he is using film or digital. A reflective light meter is way more useful. I may not have it all in mind right now, but the only time I would use an incident meter would be to measure flash output...

Since I was puzzled I pointed to the simplest ( and most practical ) approach, using a digital camera.

The tongue and cheek tone came across loud and clear, no worries! I asume the OP is using it for film, but was poking a bit of fun too since he didn't specify.

Re the reflected meter I agree, at least for landscape; a properly calibrated spot meter and knowing how to use it are tough to beat! However for outdoor people shoots, especially when dealing with a mix of skin tones, the incident meter can be invaluable. Of course if you don't have one, you can always meter a gray card with your spot meter

Cheers,

 

[fahim mohammed]

Alain, Ken,Jack... thank you and all the others for their responses.

Alain...Rest assured your ' tongue in cheek ' response was taken as humor and not at all 'curt'.

I was asking the question for use with a film camera and to clear up some ambiguity that I have had re:
incident light measurements and the role of the zone v relationship.

I mostly use a digital camera ( with a histogram to check ) but I find I am at peace with my cam and my hobby when I am using film..strange but that's how I feel. I like the anticipation to see what I captured as opposed to the instant display of my photographic attempts.

May the light be always right for you all.