# I don't think I understand incident metering after all

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• 07-23-2011, 07:45 PM
2F/2F
Quote:

Originally Posted by ntenny
I don't understand this part. Isn't the idea that the dome looks to the meter like a gray card?

Thanks

-NT

I'm not capable of explaining it in technical detail myself from memory, but it is basically because most meters are not calibrated to reproduce 18 percent gray, which is what a gray card represents. Read the first post in this link for all the technical details: http://www.ohio.edu/people/schneidw/...d_musings.html.

It is a large part of the reason why Zone System speed testing usually results in lower working EIs than box speed, yet box speed usually works fine with incident meters.
• 07-23-2011, 07:46 PM
Chan Tran
I understand what 2F/2F was saying and it's a valid argument. But if you simply point the dome toward the camera from the subject position and point the reflective meter toward the scene from the camera position it's not often difference by 2 stops.
• 07-23-2011, 07:54 PM
David A. Goldfarb
If you think about it, the dome and the card are not seeing the same light, because the card is flat, and the dome is curved to read the light as it falls on a three dimensional object with a highlight and shadow side. If your meter has the option of a flat diffuser, it should match the gray card, and the flat diffuser is what you use, say, in copy work to make sure a flat object is evenly illuminated.

In a typical portrait or still life setting, where you can control the lighting ratio, I point the incident dome toward the camera.

Now the question is--how do you get the same reading with a gray card? You might presume the incident reading is correct and use it to fine tune your gray card technique. To photograph a three-dimensional object, you often have to angle the card toward the main light, maybe 30 degrees from the lens axis, to get a reliable average of the main and the fill. Experiment with the angle of the card to match it to the incident reading.

Take a look at the diagrams here for Kodak's instructions for gray card use in cinematography--

http://motion.kodak.com/US/en/motion...atographer.htm
• 07-23-2011, 07:55 PM
2F/2F
Quote:

Originally Posted by Chan Tran
But if you simply point the dome toward the camera from the subject position and point the reflective meter toward the scene from the camera position it's not often difference by 2 stops.

It certainly can be in bright light with bright objects in the frame...which is the light in which many (most?) people tend to shoot, and the light the OP described for his situation.

There aren't many situations that are what the OP describes as "sunny 22," so I am guessing that at least one stop, and maybe more, of the difference he is seeing between the two readings is due to the brightness of what the reflected meter was pointed at.

Incident readings simply should not match gray card readings, no matter what diffuser attachment is on the incident meter, for the reasons mentioned above. If reflected meters were designed to produce 18 percent gray, then they would match. But they are not designed to do this. Gray cards will cause underexposure unless you open up a bit from the reading they give you.
• 07-23-2011, 08:07 PM
Chan Tran
Yeah I never run into sunny 22 where I live. It's sunny 14.
Under even lighting and wih the flat diffuser my meter read exactly the same to within 1/10 stop. and yes my meter is K14 which is 18% reflectance.
• 07-23-2011, 08:14 PM
David A. Goldfarb
"Sunny 22" is bright mid-day on a white sand beach.
• 07-23-2011, 09:05 PM
ntenny
Quote:

Originally Posted by David A. Goldfarb
"Sunny 22" is bright mid-day on a white sand beach.

That's pretty close. I was in a light-colored section of fairly sandy desert near Yuma, AZ on a very bright (and very hot) midday---lots of reflection from the ground and the mountainsides in addition to the direct sunlight.

Thanks for all the information. I've never checked the calibration on my meter more precisely than "the slides look good", so that may account for some of the difference, and directional uncertainty for the rest. I was shooting negative film, so I don't expect anything to have been ruined; I'm just trying to understand why I wasn't seeing the behavior I expected.

-NT
• 07-23-2011, 09:06 PM
Leigh B
The dome has nothing to do with the gray card.

Your distant scene is illuminated from many different angles, including reflections from the area in front of it, like grass, dirt, sand, or water.

The dome integrates light from all different directions and includes that information in the meter reading. It should be pointed directly at the camera lens, not up or down or sideways, from the subject position, or from a position that mimics the illumination environment of the subject.

I normally just hold it above my head, pointed straight back. Works fine in most situations. I know when the subject and lighting are such that a correction may be needed.

Regarding the gray card... Very few people use these correctly.
It should be angled halfway between the camera line of sight and the illumination source. Then you take a reflected reading of the card from the camera position. As an example, if you're shooting a landscape at noon, with the sun directly overhead, the card should be at a 45° angle.

- Leigh
• 07-23-2011, 10:18 PM
Chan Tran
While the reflected meter will read the white sand beach, the incident meter is not influenced by the white sand beach. My meter in incident mode under sunny condition almost always read EV14.7@ISO100.
• 07-23-2011, 10:50 PM
David A. Goldfarb
An incident meter actually should be influenced by the white sand beach, because there's a lot more reflected light in that situation from the sand and the water than in most sunny scenes. If you look at, say, they old exposure guide they used to print on the instruction sheet they used to include in the box with a roll of film when film always came in a box, and oh, those yellow painted metal canisters...okay, I'm getting too nostalgic--anyway, the graphic would generally recommend stopping down to "sunny 22" for a sunny day at the beach, because there really is more light there. It is like the difference between photographing in a small studio with bright white walls vs. a large studio with high ceilings and black curtains along the walls to absorb stray light.

But in more typical lighting, yes, the advantage of an incident meter is that it isn't affected by the reflectivity of the objects in the scene. This makes it good for studio use, where you can control the contrast ratio of the light and the general level of the lighting. Incident metering in available light involves more awareness of the contrast of the light and the reflectivity of the objects in the scene, and whether it is possible to read the same light as the subject, which can be quite distant from the camera in the case of landscape photography. BTZS is an approach that adapts incident metering to the field.
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