How does one

use an incident light meter?

Does the aperture/speed/iso combination indicate settings for ' grey ' or does one just plugs that into the camera?

Also could you please recommend me a small, cheap, accurate incident light meter.
I have read about Gossen and Sekonic, but no experience of them.

Thank you.

Hi, Fahim,

fahim mohammed said:

[How does one] use an incident light meter?

Click to expand...

Set the "ISO" setting on the meter to the ISO sensitivity you plan to use in the camera for the shot.

Place the meter so its "acceptor" is where the subject is/will be, or in a spot where the illumination is identical. If the important part of the subject is a plane surface (as when photographing a building front), orient the meter so the acceptor is roughly parallel to that surface. Otherwise, face the acceptor roughly toward the camera.

The meter will then tell you combinations of shutter speed and f-number that should give a "proper" exposure. Choose one that suits (you may prefer a certain f-number from a DoF standpoint, etc.) and set the camera accordingly.

Do not try and think about "gray". It is not properly involved in the story here.

The strategy of incident light exposure measurement is to try and "plant" parts of the subject having certain reflectance in a predictable place in the tonal scale of the image. For example, no matter what the distribution of reflectance is over the scene, or its average reflectance, a spot on the subject whose reflectance is, for example, 95% will have a consistent "exposure result" in the image (a little less than the result that would constitute saturation).

Best regards,

Doug

fahim mohammed said:

use an incident light meter?

Does the aperture/speed/iso combination indicate settings for ' grey ' or does one just plugs that into the camera?

Click to expand...

Hi Fahim,

Both reflected light metering of an average contrast scene (or an 18% grey/gray card), and incident light metering, tend to return an exposure setting that results in 'correct' exposure. Correct of course depends on scene content.

Incident light metering disregards the brightness of the scene itself, because it only measures the incident light. A black cat against a black background will therefore result in a low exposure on your camera's histogram. A reflected light meter however, would have overexposed that scene much more because it wants to render the average scene brightness as mid-gray, and you run the risk of burning out small areas of slightly higher reflectance.

So the incident lightmeter basically measures how much light falls on a scene, and figures out the correct exposure for a subject of average reflectance to render as average gray. This still allows to deviate from the advice that the meter gives!

For example, when your scene has a lot of contrast, from white to black subject matter, and shadows, you may still need to reduce your incident light measured exposure to avoid burning out the highlights, or increase the exposure to get more shadow detail. Low contrast lighting is much simpler, because that will fit easier in the dynamic range of the camera/sensor. in fact, you may want to increase the exposure level to expose more "to-the-right" on the histogram.

Also could you please recommend me a small, cheap, accurate incident light meter.
I have read about Gossen and Sekonic, but no experience of them.

Click to expand...

You can probably find some decently priced Sekonics on eBay. Just look for the ones with a light integrating dome, either integrated in the meter or as an accessory. Gossen and Sekonic are good brands, although even they may need calibration before relying on them. Low cost calibration can be as easy as using an ISO offset, to center the hump of a large lightsource in the histogram.

Cheers,
Bart

P.S. This might help:
http://www.sekonic.com/classroom/classroom_2.asp

and for slightly more complex scenes:
http://www.5min.com/Video/How-to-use-Your-Sekonic-Light-Meter-72416679

Doug, Bart...

Thank you for the explanation.

So, if I have you and me standing in open sunlit, take an incidence meter reading from either of our face,
plug it into the camera..I should do ok?

Or if you and me were standing in the same dark and did the same exercise, the exposure would be ok?

Have I got that right?

With spot I generally point to mid-grey and compensate right or left based on the zone system.

Do I still do that, I the incidence reading was taken of my face?

Thanks a lot.

fahim mohammed said:

Doug, Bart...

Thank you for the explanation.

So, if I have you and me standing in open sunlit, take an incidence meter reading from either of our face,
plug it into the camera..I should do ok?

Or if you and me were standing in the same dark and did the same exercise, the exposure would be ok?

Have I got that right?

Click to expand...

Yes, you meter at the subject, pointing the meter/dome towards the camera. That way the light falling on the actual subject is integrated into an average ambient light value. You don't need to meter at the subject (e.g. when shooting lions in the zoo), as long as the meter and the angle of the light and the brightness of the surroundings are in similar light as the subject.

With spot I generally point to mid-grey and compensate right or left based on the zone system.

Do I still do that, I the incidence reading was taken of my face?

Click to expand...

Kind of. If you want a medium zone to be recorded as medium, you just follow the meter. If you want it brighter or darker, then adjust accordingly. With very contrast scenes, you generally reduce exposure, with low contrast scenes you increase exposure a bit, for the best quality.

Cheers,
Bart

Bart,

Could you now link the measurement to the camera by addressing calibration of both. After all, each camera will transmit to the film/sensor a different proportion of light.

Asher

Hi, Fahim,

fahim mohammed said:

Doug, Bart...

Thank you for the explanation.

So, if I have you and me standing in open sunlit, take an incidence meter reading from either of our face,
plug it into the camera..I should do ok?

Click to expand...

Well, one does not take an incident light reading "from" any subject. Recall that it measures the light arriving at a certain place - not the light reflected from some subject (which is what reflected light metering does)..

Perhaps you mean, "take an incident light reading at each of our faces".

In that case, if we assume your face has a lower reflectance than mine, yes, both faces would do fine. Mine would have a higher "relative luminance" in the image than yours, but that's probably what we want.

Or if you and me were standing in the same dark and did the same exercise, the exposure would be ok?

Click to expand...

Yes (subject to all the words above, and assuming you don't mean "complete dark"). Ideally, my face would be end up with the same relative luminance in the image as before, and the same for yours (a different one from from my face).

With spot I generally point to mid-grey and compensate right or left based on the zone system.

Click to expand...

First note that a spot reading taken on a target of a certain reflectance (suppose some known reflectance gray card) is equivalent to an incident light metering. Whether the result would be the same depends on the reflectance of the target. Most incident light meters are calibrated such that their "recommendation" for exposure is the same as we would get with a reflected light spot reading off a target with a reflectance of about 16%.

Now, as to making an adjustment with regard to the zone system, if you judge the gray things you metered on as worthy of being included in different zones, then that's fine.

Do I still do that, I the incidence reading was taken of my face?

Click to expand...

Again, note that an incident light reading is not taken "of a face". It is taken of the light arriving at a certain place (perhaps where your face is, of where it will be for the actual shot).

But an exposure taken based on an incident light reading in effect already "plays" the zone system" for you. That is, it fulfills the zone system objective of having the recorded luminance of each object depend (only) on its reflectance (we rarely describe the Zone System that way, but that's what is really going on).

So, assuming you really want your face "darker" in the delivered image than mine, then using incident light metering you would not need to make any different "adjustments" to shoot your face or mine.

Best regards,

Doug

Hi, Asher,

Asher Kelman said:

Bart,

Could you now link the measurement to the camera by addressing calibration of both. After all, each camera will transmit to the film/sensor a different proportion of light.

Click to expand...

Let's make sure what you mean "transmits a different proportion of light".

What actually matters in exposure control is the ratio of the effective illuminance on the film/sensor to the luminance of the scene. By effective illuminance, I mean taking into account the "corrected chimney effect". (By corrected, I refer to the matter of the camera bumping the sensor gain for larger apertures in an attempt to compensate, in exposure result, for the impact of chimney effect.)

That ratio involves three factors:

a. The f-number of the lens.
b. The transmission of the lens
c. The amount of uncorrected chimney effect.

(Note that of these, only "b" could reasonably be said to involve "transmitting a different proportion of the light". But the phrase is a dangerous one.)

Now, the "calculator" in an exposure meter takes "a" into account, but not "b" or "c".

In motion picture work, we have lenses for which we work with their T-stop (rather than f-number). This value of course takes the lens transmission into account.

Then, if we take the "recommendation" of the exposure meter regarding f-stop and set the lens' T-stop to that value, we now have accommodated both "a" and "b".

Best regards,

Doug

Doug, yes I meant , ' AT ' the face.

Thanks a lot, clears things up.

Now to find me an incidence light meter.

Just one more question please. Why would I want to take an incident reading as opposed to a reflective one?

Thank you for your patience..Doug and Bart.

Hi, Fahim,

fahim mohammed said:

Just one more question please. Why would I want to take an incident reading as opposed to a reflective one?

Click to expand...

Basing exposure on an incident light reading will give you appropriate exposure results for objects of different reflectance (the actual objective of the Zone System, although rarely said that way) without concern for the average reflectance of the scene (as would be the case with basic reflected light metering).

However, spot reflected light metering from a target of known reflectance is incident light metering.

So if that is handy for you to do, you do not need a purpose-built "incident light" meter.

Another kind of bona fide incident light meter is made if you put a genuine calibrated, cosine-response measurement diffuser (such as an Expo-Disk) on your lens. (The resulting instrument is not nearly so handy to place in the model's cleavage, however.)

Best regards,

Doug

Doug, my sincere thanks for putting up with me.

Regards to Carla and yourself.

Asher Kelman said:

Bart,

Could you now link the measurement to the camera by addressing calibration of both. After all, each camera will transmit to the film/sensor a different proportion of light.

Click to expand...

Hi Asher,

Handheld meters can be calibrated differently compared to the in-camera meter. AFAIK Sekonics are calibrated to 12.5% medium gray, cameras can be calibrated to different values.

So in order to match them, one can take a camera reading of a uniformly lit surface, and then match the handheld incident meter to that same response (histogram peak) by varying the ISO on the meter. This will avoid the need to spend money on a factory calibration.

All one needs to make a note of is the difference between ISO settings.

Cheers,
Bart