Monday, November 12, 2007

Metering Exposure.


Youth Soccer Team
Taken at 75mm. The on camera flash was fired in hopes it would trigger a slave flash that was about 45deg off the goal. But the sun was too powerful and the slave did not pick up the signals from the camera so it didn't fire. The on camera flash did not add much to the photo as I was quite far away from the goal at 75mm.

The saving grace in this photo was the K10Ds multi-segment metering mode that works well in back lit situations like this.




The first basic function of any camera is to figure out how the camera meters the light in a given scene.

The K10D like most modern cameras uses a built in light meter to calculate the amount of light and make internal adjustments to the exposure settings to obtain the best exposure.

Digital sensors have the ability to capture a wider range of light than negative and slide films, but they still do not come close to the range of the human eye. This is obvious when a photographer tries to shoot a scene that is light by bright noon sun and has many deep shadows. What the photographer sees and what the camera sees are completely different. The camera sees pure white in the brightest areas and pure black in the deepest shadows where the eye can see shading and details in both the bright and shadow areas.

It is the photographer's job to try to figure out where to set the midpoint so the camera can capture the optimum range of light. The camera offers a few tools to help communicate this data. Inside the K10D are 16 zones that can measure light levels. There are 3 metering methods available (pg 142).

MULTI-SEGMENTED METERING
The multi-segmented method relies heavily on the camera deciding what the light levels are and the photographer is left out of the metering loop. This can be a good thing or a bad thing depending on the skill of the photographer.

This metering method reads the light level in each of the K10D's 16 zones and adjusts the brightness in each zone. This is great for point and shoot where you want a very balanced even lighting with highlights and shadows both exposed to the best of the camera's ability. However, it is not the magical metering method it sounds to be. For one there might be some zones where there is both bright light and dark shadow. The camera then tries to find a happy medium and the shadows will be lowered if the brights are lowered and vice versa. However the zone next to it might be in complete shadow or complete bright and it will be adjusted differently and as a result the 2 adjacent zones will not be exposed the same. The result will be either a photo with a very compressed tonal range or a photo with a patchy tonal range. The former is what happens when we post process an image with a wide tonal range anyway, the latter would look more like a bad post process dodge/burn job. However, this mode is good for back light scenes if you want to see detail in the silhouette.

CENTER-WEIGHTED METERING
This mode also measures the entire screen but more emphasis is placed in the center of the screen. If you look at page 143 in the users manual you can see the graph that represents how the meter evaluates the scene. As the book says this is not to be used for back light scenes as there will be a substantial amount of silhouetting.

SPOT METERING
This mode is the one the photographer uses to decide what they want the exposure to be. You point the center of the viewfinder at the area in the scene that you want to meter and press the exposure lock button to lock in that reading.

METER OPERATING TIME
Meter operating time sets the length that the meter will hold the exposure value before reseting it. This is a handy feature as you will find certain scenes that take longer to compose and you want the metered exposure to last a little longer before it resets.

HISTOGRAM
One other thing that should be discussed in the metering section is the camera's histogram. The histogram is a graphical representative of the quantity of each tonal range. The left edge of the graph represents pure black and the right edge of the graph represents pure white. The area in between from left to right is a gradual increase from pure black to pure white. What the histogram will convey to the photographer is the amount of underexposed (pure black) and the amount of over exposed (pure white). There is no exact correct histogram shape and they will vary from scene to scene or could be the exact same shape for 2 very different scenes that have the same quantity of tonal values. For an average lit scene with shadows and bright spots there should be readings all the way across the histogram, but if the histogram shows all the data bunched up to one end or the other then it is indicating an improperly exposed photo. The experienced photographer looks at the histogram to judge exposure and the image to judge composition. This is called chimping. You may only have to chimp a few times to get the exposure you are after then just take photos until the scene changes and you chimp again.

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4 comments:

Max said...

I have a Samsung GX10, and I found that when take pictures in the spot metering mode, they are very, very dim. I have to raise the exposure by about 2 f-stops to get an acceptable picture. Too bad.

Photography Rulez said...

How long do you have your exposure timer set for? If it's 3 seconds then maybe it's reseting before you take the photo.

To properly use center spot more you should use a long exposure timer or the exposure lock button.

Point the spot at the area you want properly exposed and lock the exposure in. Then set the focus on the desired area and then compose the image in camera and capture the image.

If you don't lock the exposure, then recompose the scene in camera after setting the exposure (while half pressing the shutter still) and use the AF button to focus the scene. Then do a final compose in camera and capture the image. This is where the 3 second timer is not long enough.

Also make sure the center spot is on a large enough area to determine the lighting conditions.

If you're shooting skin then get the exposure reading from your hand (in the same light) and put the camera in manual mode. You may have to chimp at the histogram to get the correct exposure setting. Works every time. Then you can concentrate on focus and composition after that and not have to worry about exposure until the light changes.

I have heard that some Pentax (and clones) are a bit under exposed and have noticed my K10D at times can be 1 stop under especially with old manual lenses.

Shoot RAW and with compensation adjustment (like you are) and you'll be fine.

Hope this helps.

Tim

Photography Rulez said...

I found this site too if you really are having incorrect exposures.

http://www.esnips.com/web/DSCServiceManual

Max said...

Thank you very much Tim.
Do you know the software on the website is for, and how to connect it to the camera? It is called GX10 SR Gain Adjustment. When I trigger it, it asks to 'Stop the stage, and set the camera become center position'. Then I press Enter, and it asks whether the camera switch is on.