Measuring Tonal Range with Histograms


How do you discern whether you got what you were after? Let's face it, when you shoot a digital image, you immediately check the LCD display to assess your results. Do you keep the first image, or do you need to reshoot? How do you decide, especially if you're away from your computer or out in the field as I was while shooting in Tuscany? One thing is for sure: Don't trust the paltry little LCD screen on the back of your camera. It might show you composition and framing, but it lies and deceives when it comes to exposureit cannot show you anywhere near the level of detail necessary for critical decisions. The solution is to use histograms.

A histogram is a graphic representation of how pixel information is distributed within the tonal range of an image. How much information is in the highlights? How about in the shadows? What's the darkest pixel? All these questions can be answered quickly using a histogram. When you're shooting in the field, you can access the histogram for the image you just shot to see whether you exposed properly for the highlights. If the highlights are clipped (too strong) or if there is no information in the highlight range, you know that the exposure is incorrect and you should probably reshoot. The next section explains how histograms are structured and how to read them so that you can make intelligent decisions about exposure and tonal range.

Anatomy of a Histogram

The histogram is a common tool in assessing the tonal range of an image. It's in most camera preview systems, as well as in Photoshop and other image-editing software. Because the histogram is a standard measurement tool, you must know how to read a histogram correctly and understand how it relates to the image as well as your creative objectives. In a histogram, pixel values are represented along the horizontal baseline, increasing from left to right. Rising up from the baseline are peaks and valleys that graph the proportionate number of image pixels corresponding to the baseline value. A quick glance tells you whether you have any peaks in the highlights or shadows, signaling that pixel data is present in that area (see Figure ). Check to see whether image data is present across the full tonal spectrum, especially in the areas that interest you the most.

A histogram and its corresponding tonal areas.

For a scene that's evenly lit, a good histogram shows some pixel data at both ends of the spectrum, with the highest peaks (the most pixels) in the middle. Such a graph signifies an image with moderate highlights and shadows and the majority of information in the midrange. But what about that snow scene or that twilight image? The histograms associated with those images will look very different from the ideal ones. If there are no bright highlights in the scene, don't expect them to be in the histogram. This lecture might sound basic, if not remedial, but it reinforces the point that each histogram is unique, and there is no target histogram shape, just as there is no universal formula for perfect exposure.

Histogram Shapes

There are as many histogram shapes as there are image types. Some emphasize the shadows, others the highlights. Don't fall into the trap of making all your image histograms look the same. What's more important is that you recognize the important areas in the image, and make sure that there is adequate pixel data in the corresponding area of the histogram. Figure shows a standard exposure, with bell-shaped histogram. Figure shows an image with the shadows dominant; notice that its histogram is skewed to the left. Figure shows an image with the highlights dominant; its histogram is skewed to the right. Figure shows an image dominated by a single flat color, with minimal highlights or shadows; its histogram is similarly flat (most of the pixels in the center of the histogram).

 Dried Flower II
Nikon D2H
1/320 sec, f/9
Focal length 75mm

 Yellow Flower
Nikon D2H
1/160 sec, f/10
ISO 640
Focal length 75mm

 Fresco Detail
Nikon D2H
1/60 sec, f/4.5
Focal length 80mm

 Table Cloth
Nikon D2H
1/250 sec, f/4
Focal length 20mm

EXposure and the Landscape

In a landscape, the sky is always going to illuminate the ground below and will almost always dominate in brightness. Remember to anticipate this and compensate by making the exposure lighter or darker to emphasize the sky or ground as your vision dictates. Here are some additional exposure tips to keep in mind when exposing for a landscape:

  • For normal landscapes, an incident reading from a handheld meter is the best approach to getting the exposure right.

  • If your camera allows (as professional cameras often do), add an extra 1/2 stop of exposure if there are large areas of dark foliage that are important to the scene.

  • Reduce exposure by 1/2 stop if you want to increase the saturation of a dominant color, such as the sky.

  • Take a spot meter reading in the darkest shadow areas and reduce exposure by 23 stops to maintain detail in the shadow areas.

  • Reduce the exposure for a snowy scene by 1 stop if you're taking an incident reading or up to 3 stops if you're taking a reflected reading.

  • Night shots look best when there is still some color in the sky. Reflected readings work best at night.

 San Gimignano
Nikon D2H
1/60 sec, f/19
ISO 640
Focal length 75mm

 Monteriggioni
Nikon D2H
1/200 sec, f/22
Focal length 24mm

 D'orcia Sunset
Nikon D2H
1/160 sec, f/4.5
Focal length 75mm


Exposure and the Object

When you're shooting an objectwhether it's a ball, a child's toy, or a portraityou're looking for ways to articulate and describe that object, bringing out nuance or emphasizing specific features such as the texture or rich color. The biggest challenge in shooting an object or a portrait is one of lighting. Is the subject evenly lit or should you use a fill flash or a reflector to balance the light? What about the background? Does it compete or is it too distracting? Ask yourself these questions as you shoot, and consider the following tips to improve the shot:

  • Keep the object far enough away from the background to avoid casting unwanted shadows from the light source.

  • Use a reflector to cast reflected light into the shadows.

  • For most portraits, position the subject against a background that's 2 stops darker than the subject's skin tone (2 zones).

  • Use an incident meter if possible, pointing it directly into the camera lens.

  • Dress your portrait subject in light clothes if you want dark skin tones. Conversely, dress in dark clothes to convey light skin tones.

 Grapes
Nikon D2H
ISO 640
1/125 sec, f/11
Focal length 75mm

 Florence Fountain
Nikon D2H
1/640 sec, f/4.5
Focal length 105mm

 Flowers
Nikon D2H
1/30 sec, f/5.3
Focal length 180mm

 Roses
Nikon D2H
1/60 sec, f/4.5
Focal length 60mm


Exposure and Close-Up Photography

The key in emphasizing texture and detail is to reduce glare and enhance sharpness. Although exposure can't help much in the sharpness department, it can reduce glare and bring out details. Slightly underexpose images with glare for a little added saturation and detail. Your TTL meter is probably going to do well with these shots because the entire viewfinder should be filled with the detail or texture area. Consider the following tips for enhancing texture and detail exposures:

  • Shoot textures outside on a cloudy day, which provides even, diffused lighting.

  • Position the lens at a 90-degree angle to the textured surface to maintain focus across all areas of the surface.

  • Take an incident reading and underexpose by 1/2 stop to reduce glare and deepen saturation.

  • If possible, set up directional side lighting to further emphasize surface texture and detail.

 Cross
Nikon D2H
ISO 640
1/90 sec, f/4.8
Focal length 28mm

 Paint
Nikon D2H
ISO 640
1/160 sec, f/6
Focal length 75mm

 Field
Nikon D2H
1/500 sec, f/5.6
Focal length 90mm

 Charred Door
Nikon D2H
1/30 sec, f/3.5
Focal length 28mm





The Art of Photoshop for Digital Photographers
The Art of Photoshop for Digital Photographers
ISBN: 0672327139
EAN: 2147483647
Year: 2006
Pages: 141

flylib.com © 2008-2017.
If you may any questions please contact us: flylib@qtcs.net