Sunday, August 29, 2004

Double check exposures

For the last few days, we've discussed the importance of accurate light metering. Most PJs use the reflective light meter integrated in the camera as their primary gauge. However, it's important to double check the accuracy of the meter for the most exact exposures.

Below are three alternate ways to check exposure to ensure accuracy. None are as good as an ambient light reading of the subject, but they're insurance to verify the camera meter is working properly.

Sunny 16 rule

The Sunny 16 rule is the quick standard for daylight shooting. It makes an excellent check for the camera settings. The PJ can look at the camera's suggested exposure, compare it to the Sunny 16 rule and see if the camera meter is operating properly under the conditions. If the PJ isn't sure, shoot at each setting.

The PJ sets the shutter speed at equal or slightly higher than the film iso and sets the aperture to f/16. Then, the PJ can convert it to the optimal combination through an EV calculation.

As an example, 400 iso = 1/500 at f/16. It could then be converted to f/8 at 1/2000 under clear, noontime sunny conditions.

Under cloudy conditions, additional compensation must be taken. For slightly overcast skies with soft shadow edges use f/11. For overcast days with barely visible shadows, use f/8. For cloudy days with even light and no shadows, use f/5.6. In most cities, smog drops it down one stop on clear-for-the-city days. Use typical cloud compensation otherwise.

Palm method

The palm of every human's hand reflects light at one stop above neutral gray (Zone VI). PJs can use this as a standard to check or to set accurate exposures.

Using the built-in reflective light meter of the camera, the PJ holds her/his palm out in front of the camera in the same light and light angle as the subject. The PJ doesn't need to focus on the palm because s/he is only checking the reflected light measurement. However, the palm must fill the frame in the same light and at the same light angle.

As a check
Mentally note the suggested palm exposure. It should be one stop above (+1) the general scene. If not, the scene is causing a meter variation beyond accurate. Use an ambient meter as a tie breaker, or go with the palm +1 reading.

As a primary method
Set the camera to manual control. Meter as noted above. Manually set the exposure one stop above (+1) the meter reading for the palm.

As a pneumonic to remember this process, hold a hand out in front of yourself where the palm is facing back at you and your fingers are together with your thumb pointing skyward. Note that your hand has OPENED UP. You can then look at your ONE thumb, which points UP. So, open up one stop to compensate for a palm reading.

In both cases, the PJ has technically opened up one stop to move Zone VI back to Zone V for a middle gray light reading.

Grass method
This is the fastest check for most sports shooters. Normal, green grass and turf is about middle gray to a reflected light meter. Occasionally point the lens at a patch of green turf and mentally note the exposure reading. If it's different than the general scene readings, set the exposure manually to the grass and use another method to triple check the light reading.

Spot-metered face method
This method works in all lights, but it's critical for events with a spotlighted subject (concerts, theater, guest speakers). This method requires a middle- to high-end camera with spot metering capabilities.

As always, it's best to arrive early and request a chance to get ambient light readings on the stage under the same light used for the performance. Often this isn't possible and must be done on the fly for the "first two, no flash."

When this happens, the PJ should switch to a spot meter rather than matrix or zone. Using the longest lens available, spot meter the subject's cheek.

If the person is a typical Caucasian or Asian, their skin is one stop more reflective than middle gray. Open up one stop. A typical central African (depends on skin reflectivity) is one to two stops below middle gray. Close down one or two stops. Most people who live around the Earth's equator (except in Africa) tend to have middle gray skin tone. Caucasians with deep summer tans are frequently at middle gray as well.

Obviously this method takes a lot of guess work. Some melanin-challenged folks require two stops of adjustment. Likewise, some makeup preferences (particularly in Asian theater) require two stops compensation. In general, if the cheek appears pure white without detail to the PJ, open up two stops from the reflected light reading. If the cheek appears pure black without detail to the photographer, close down two stops from the reflected light reading. If using a digital camera, start with these measurements and chimp it to the correct exposure.

Enough for now,


Unknown said...

Very nice web site, thanks so much.
Just one thing.
When talking about "palm" method, don't forget that humans have different skin colours.

Mark M. Hancock said...

Everyone's palm is within one stop of Zone Six regardless of individual dermal melanin. All humans have the same amount of melanin. It expresses differently depending on genetics.
However, there is less melanin in the palm, and it is located in a deeper layer of skin than elsewhere in the body.
Therefore, the production of melanin in the palm does not affect tonal reflectance in the palm of the hand.
This is why the palm method is universal. Extreme albinism might be the only exception.