Spot Metering and Neutral Density Graduated Filters

When shooting a landscape you will often be in a situation where the sky is a lot brighter than the foreground. The sky could be at least two stops brighter than your foreground mid-tone. The result is likely to be an overexposed image with little or no detail in the highlights.

The cameras histogram will be pushed over to the right hand showing that most of the pixels have values close to or at 255. Some cameras will show the image with a visual 'blinking' effect indicating the overexposed areas.

picture of an over exposed lake histogram showing over exposure

Over exposed highlights resulting from using a dark foreground mid tone.

You could try to avoid this situation by underexposing by a stop or more. Then open up the shadow areas with your imaging software. The disadvantage of this approach is that with an underexposed image a significant amount of data is lost by not capturing the bright tones on the right hand side of the histogram.

When you come to stretch out the histogram and spread out the tones you will be utilizing a far smaller set of information to generate your image and the result will be either noise or poor contrast.

histogram with blank lines

Levels stretched histogram resulting in data loss

As a rule it always better to obtain a correctly exposed image in the camera rather than to underexpose and then open up the shadows.

Neutral Density Graduated Filters

Graduated neutral density filter (Grads) can be used in order to expose for the mid-tones whilst keeping the sky area within one and a half stops. Grads come in different strengths. A 0.3 Grad equates to 1 stop, a 0.6 to 2 stops, etc.

So how do you know what strength grad to use?

You could use the cameras built in meter - particularly if your camera has a spot meter function - but this may not be very convenient if you are using a tripod.

A handheld spot meter is the ideal solution. You can set up and compose your shot and then meter the scene. If the light changes you don't have to disturb the camera.

Using the Sekonic Spotmeter to determine the Grad Strength Sekonic L-558 Spot meter

The following example is based on the Sekonic L- 558 in 'ambient' mode with shutter priority selected. i.e. the 'T' symbol highlighted as in the diagram below.

Sekonic meter reading - T Mode selected

Try the following:

  • Take a spot reading of your mid-tone and save it to memory. This would be the middle reading in the screen diagram above.
  • now obtain a second reading of the darkest area. This would be the dot on the left hand side of the scale. Again saving it to memory.
  • finally a reading of the sky which would be the far right dot on the scale above.

histogram - correctly exposedThe amount of Grad needed is determined by the number of stops the reading on the far right of the scale (the highlight) is from the mid-tone reading. In this example the highlight (say the sky) is two stops brighter than the mid tone so a 0.6 Grad would be used to bring the sky back to the mid tone exposure.

Providing you position the Grad correctly over the brightest part of the sky you should obtain an image which retains detail in the highlights and is correctly exposed for the mid-tones.lake picture - correctly exposed

Correct exposure using foreground and ND Grad

Related Links

Sekonic L-558 Review
Lee Filter ND Grad review
Lightroom Tips Library Index