1 f-stop difference between incident and reflected metering?

Dear friends,

using the little Gossen Digiflash has always been fun, I have been happy with its results so far (b/w and colour negs, no slides), but one detail that I have no real explanation for worries me:

when measuring in incident and reflective mode (same situation, even light) I always get a full f-stop difference. I checked the manual, my use of the light meter seems to be ok (dome on for incident reading, dome off for reflected reading).

What am I missing, or is there a way to pin the cause of the difference down? Any help would be appreciated, thanks in advance,

Holger

It all depends on where you are taking the reflected reading from-Take one reading from hi-key, then one from a dark surface then average them. Might equal the incident reading, or at least be very close..

Rick

That reflective surface must have an 18% reflection (as a grey card).
Otherwise the comparison between incident and reflective measurement will give different readings.

AgX is right, you need to take reflective readings from a standard 18% grey card in order to get the same light reading as you would from an incident metering. Grass is a good substitute. Try that and see if you still get a 1-stop difference.

Which reading gives you the better exposures?

They should be different unless you're shooting 18% gray, but I wonder how do you get the "always 1 stop difference". According to Metz, the reading is only the same if the subject is %50 gray, hic.

I presume you were comparing the incident reading with a reading from a 18% gray card?
Perhaps you meter is calibrated to a 12% gray standard. Determining this and finding the K constant for your meter may be difficult.

If there is a question, Gossen can check or recalibrate your meter if needed.

They don't measure the same way. Caucasian skin, for example, usually meters about one stop more with a spot meter reading than if you metered the light falling on the person. That is because the spot meter meters for grey, and most things aren't. You don't need to have a grey card to use a reflected meter, but you do need to have a good idea of what is brighter and darker than grey, and take that in to consideration. That's one of the most difficult things about a spot meter, and also one of the most useful benefits.

Jason,

They measure the same way. Only in incident metering the dome acts as ND filter transmitting only about 18%. In reflected light metering the object does this by reflecting only part of the light.

AgX said:

Jason,

They measure the same way. Only in incident metering the dome acts as ND filter transmitting only about 18%. In reflected light metering the object does this by reflecting only part of the light.

Click to expand...

Perhaps wording on my part. The dome averages everything to grey, and measures the light incident to where the meter is placed. It will read an average exposure that should render a decent scale of tones for most things within a scene, assuming there are no extremes. A spot meter measures reflected light using a narrow angle of acceptance. The area metered can in fact reflect part of the light, almost none of the light, or almost all of the light, depending on the reflectance of the area. It's readings will recommend an exposure that will render 18% grey, regardless of the luminance of the area metered, and so interpretation is needed concerning the actual exposure used. In that sense, which is what I meant, they do not measure same way at all.

AgX said:

Jason,

They measure the same way. Only in incident metering the dome acts as ND filter transmitting only about 18%. In reflected light metering the object does this by reflecting only part of the light.

Click to expand...

Perhaps poor wording on my part. The dome averages everything through diffusion, (it does not have any"ND" function, although the density of the sphere would factor in the design and calibration of the meter) and measures the light incident to where the meter is placed. A spot meter measures reflected light using a narrow angle of acceptance. The area metered can reflect part of the light, almost none of the light, or almost all of the light, depending on the reflectance of the area. It's readings will reflect an exposure that will render something middle grey, and so interpretation is needed concerning the actual exposure used. In that sense, which is what I meant, they do not measure same way at all. You can't just point a spot meter at something and set the exposure to the meter (unless it happened to be 18% grey, and you wanted to expose it that way).

Thank you very much to all of you, and yes, you are right. I knew about the 18% gray calibration, about grass and skin, but did not measure in the right way. The fact that I have been off by 1 f-stop consistantly will have been caused by missing grass: I have been taking pictures in a forest, with all the brown autumn leaves and no grass on the ground. I kept pointing the meter to the ground, thinking that not taking the (rather light sky) into consideration the metering should be fine. Only when I checked the results with incident metering did I notice the constant difference (incident 1 f-stop darker than reflective, pointed to the ground). With autumn leaves, i had not imagined that they could be so much lighter than grass or green foilage.

Testing with my gray card now got me identical results for both metering methods. So obviously my personal calibration has been off, I will now use the gray card for some time to recalibrate my eyesight.

Thanks a lot for the extremely fast and helpful feedback. Thanks again, and happy shooting, Holger

1 stop

Just a few thoughts.

In a landscape scene with grass, say, in direct specular light my reflective reading calls for an extra stop of exposure as compared to incident measure. The same scene in a diffused light, provided by sky light or overcast skies, the reflective and incident readings are the same.

Specular light creates all the shadows, and the reflective meter takes these into account.

For my work in b&w and with my exposure and processing system, the reflective meter gives me better results. If incident is faster, and often is, I adjust the camera setting according to the quality of the light.