Apply Optimal Light Measurement


To determine the optimum exposure of a scene we trust on the built-in camera’s metering system. But did you know that these offer a wide range of different possibilities. To determine even more precise how the exposure should be? How can you make use of the different metering systems in the camera to optimum application.

light dark
A tricky measure situation with many tones, light parts on the right, and  dark subjects on the left

When the light and dark parts plays a role then the dynamic range of the camera sensor comes in. The dynamic range is the difference between the lightest and the shadows in a photo.

A digital camera can generally handle 7 F-stops contrast difference. In the ‘real’ world the difference between the lightest and darkest parts is almost always  10 F-stops. Therefore, it lacks some detail in your camera on the go.

This means that, in the determination of the correct exposure a choice must be made. Due to the limited dynamic range shadows are often show as black on the photo, and light parts in white.

Shade And Light

Completely white means that even lighter parts of the scene are no longer visible.

With the help of the luminance meter on the camera. You can set the exposure on the parts that you think are important that it shows sufficient detail. And that you also explicitly make a choice to certain details. You can not always 100% rely on the luminance meter. But you can improve the success ratio by using the correct method to choose for the circumstances.

18% Gray

Important to remember in this story is that what the camera sees as a normal exposure. Compared to what the photographer as correct exposure, does not always have to be the same.

The average brightness of a photographic subject is medium gray. Overall you get photos that show reasonable detail in dark areas and also reasonable detail. Shown in the lighter parts of the image (depending on your subject and the present contrast).

But this also introduces the risk that the beautiful colors (for example, in a sunset) are somewhat suppressed. Because the camera has a tendency to lighten a dark scene, the black areas tend to be more gray. Conversely, the camera also tends to darken a lighter scene to create gray snow in the picture.

light pole

Cameras have an internal database with a large amount of common lighting situations. But in extreme cases it is usually a choice for the photographer to choose for under- or over-exposure. You can do this by increasing or decreasing the aperture. Or the increase or decrease of the shutter speed with one to two stops.

Position On The Camera

What the camera measures to reach 18% gray is to be determined by the chosen position on the camera. The camera can look at the entire scene (more field/matrix measurement), to a part of the scene. With the preference for the middle (center measurement) and only to a very specific part (spot measurement). The various methods are suitable for different circumstances, depending on the exposure you want to reach.

In the fully automatic mode you don’t have the possibility to change the measurement. So you will have to do it manually or choose one of the semi-automatic positions.

Multi-Field Measurement

Nikon calls this position Matrix, in other camera brands Pattern or Evaluative.

In this position the camera is considering everything within the image and will search for the best possible average exposure. The image is built up in zones, the camera looks at the position of the object. Like the brightness, background, direction of the light, etc. Most of the DSLR cameras have many areas in which this is assessed.

Contemporary cameras do not simply measure the average exposure in the different areas of the image. But also use a set of comparing internal stored situations. The built-in computer tries to choose a as good as possible exposure.

subtle_light

For example, if a large part of the upper planes is much lighter in relation to the lower part. Then the camera will recognize that this is a landscape and already partially try to compensate for this. If you are trying to photograph a portrait against a dark background. Than the camera will recognize the situation and will not try to compensate for the dark background. Be aware that the camera does not always assess the situation at it’s best. Especially in back-lit or high contrast conditions.

Disadvantage of this method is that the camera does not always exactly knows what the eye sees and would like to capture, it is an interpretation of the scene, on the basis of a large set of examples, which can be right or wrong. It is for the photographer less easy to predict how the exposure will fall, it is not known how the camera makes it’s choice. In the majority of cases, however, this will be good, it is the default setting of most cameras.

Center Pointed Measurement

Virtually all camera brands call this position Center-weighted Average.

The Center-weighted measurement also looks to the entire image, but puts the emphasis on the middle, this counts for 60-80%. It sits therefore between the wide field and spot measurement. Also here the measurements are made in different areas, but the zones in the middle count in the overall calculation. The camera uses less automatic corrections, allowing you to calculate yourself if a scene should be under- or over-exposed to get the best results.

sunset building

This method is well suited for spontaneous photos, documentary, street photography, etc. With spot measurement you can accidentally have a dark or bright illuminating measurement so your end result is not good. To fully rely on to the computer as in matrix measurement is sometimes not a good decision, especially if you have a chance to capture a object and want to be able to rely on a predictable result.

I often choose for the wide field position, that in the vast majority of cases estimates the situation well in my camera, in cases that it is unsatisfactory and I have the time, then I will try a Center-weighted measurement.

Spot Measurement

Spot measurement at Nikon and Canon. Not every camera has this position, sometimes Partial Metering (9% coverage) is the best method.

This measurement method comes closest to the separate light meters that is used especially by studio photographers. Spot measurement, offers the photographer to determine the exposure very accurate, where the final result to other methods of measurement is sometimes less well to predict.

With spot measurement 2.5-3% of the light in the scene is being measured (some cameras have a larger surface area, this will actually come more closer to Center-weighted measurement), exactly in the center of the sensor (although you can set up some cameras that measure on the selected auto-focus point). This lets you determine exactly the correct exposure for a specific point in the scene.

However, this method is often difficult, because this requires more knowledge of the photographer about how the camera measures the light in the scene. It requires to be able to see light nuances in a scene.

Spot measurement

Spot measurement helps especially in difficult exposures, such as scenes with high contrast and strong variation in brightness. The idea is that if a major light tone in the photo is correctly exposed, that the other light tones are also correctly exposed. Not all cameras have this measurement method, the courtesy and beginners cameras will often have no spot measurement method. Check the manual of your camera.

For the best spot measurement you must look for an important area in the photo with an equal tone (for example, a forehead in portrait photography) and determine what the normal exposure is (what the camera thinks that a normal exposure is, but actually it looks for 18% gray).

The Correct Exposure

On the basis of this measurement you must interpret as the photographer if this is indeed the correct exposure or that there is over- or underexposure is necessary. Do the spot metering on a very light part, and the photo as a whole shall be darker, do the spot metering on a dark part and the photo is completely lighter.

You should try it with a white and a black slip, both will be exposed to gray with the same lighting conditions.

The spot measurement position requires reasonable knowledge of how to operate the luminance meter on the camera to get a correct exposure to fit your desire. Important is that you have the time to adjust the exposure, this method requires more work, and in rapidly changing lighting conditions this is not the ideal method.

Investing time in getting to know what is the spot measurement position, is worth the effort, spot measurement enables you to more specifically determine the exact exposure. Thankfully you can by using a digital camera see quickly what the result are using the histogram adjustments. In the majority of cases, the spot measurement position, is not the preferred measure method.

Back-lit Draw Trough

backlit

Back-lit fairly quickly creates a silhouette, the ambient light is much brighter than the light that is falling on the subject which the camera not draws through (detail) that the dark parts can register. By the spot measurement position you can ensure that the subject is correctly exposed. Point the camera directly at your subject, preferably on something that should be neutral exposed like someone’s forehead, and press the shutter button halfway down to measure the exposure of that part.

Then make sure that the stripe on the luminance meter is in the center. The camera has now determined that that part of the photo must be medium gray. Set the exposure if you’re not shooting in the manual (M) position. Then you make the composition and press the shutter.

If everything looks good your subject should be well exposed in the environment, and the detail disappears in white. If the image is still not completely to your liking (too dark subject), then you should probably still do some additional overexposing.

Silhouette

Nifty lady doing exercise at the sea during sunset

Nifty lady doing exercise at the sea during sunset

For the creation of a silhouette photo the spot meter the also the preferred method. Adjust the spot measurement position. Aim the camera at first on the (lighter) air (the center circle should fall on the part that you want to measure) and use this as a base for your exposure.

On most cameras, you can lock the exposure – if you are not shooting in the manual (M) position – and you can then make the final composition. If the contrast difference between light and dark areas are not large enough, then you can by using underexposure highlight the silhouette to ensure that it will be really pure black.

Partial Measurement

Often called Partial Metering

This partial measurement measures the middle part of the scene, approximately 9 %. This area is less small than a spot measurement, there is more of the ambient light included. In particular, a good choice if the ambient light is darker or lighter than the main topic, as long as your subject itself is not very light or very dark. If the camera has no spot meter, then this is often the position at which you can measure the closest to your subject.

Conclusion

A ‘best’ method does not exist. Everybody is working on his/her own way, and feels the most comfortable in one or more of the methods. If you choose a lot of the same position then it will make it easier to learn what the camera will propose and adjust the Ev compensation to that. Important is that you know what the different positions do, so if you end up in a difficult light situation, then you will be able to make the best choice.

A correct or incorrect exposure does not exist, this all depends on what the result is that you want to reach. In general, however, many people want to look for a picture that is not over-exposed in the light parts and that not all draw through loss in the dark parts, but why would you not go for a low-key effect which will show a very small part of your model and the rest disappears in black. Experiment with the different lighting methods and apply them.

Fully Automatic Mode

In all positions, except for the fully automatic mode, you can at any time underexpose or overexpose the camera. Also, remember to use your flash to add extra light in back-lit situations (fill light). If you’re shooting in RAW format you can afterwards use a photo editor to change the exposure, but try as much as possible to apply all the right exposure possibilities in the camera. The histogram helps in the assessment of the exposure.

With digital you can see immediately how you measurement has extracted so you can try different approaches on the spot – as long as your subject  permits it of course – until you have found a method to you satisfaction. Dare to experiment and discover the method for the subject and the camera shooting that works the best for you.

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