Adjusting Exposure in NEF files

Both the Adobe Camera Raw Plugin and Nikon Capture 3.5 provide the ability to adjust many image parameters in NEF (Nikon Electronic Format) files, perhaps most importantly, the exposure. Exposure is a measure of how much light is given to the sensor; to change the exposure, you can adjust the shutter speed, the lens aperture (f-stop) or change the sensitivity of the ccd (ISO film speed.) If you got it wrong during the actual shot, these editors allows you to change it after the fact.

Unlike changing the brightness, levels or curve, changing the exposure of a NEF file does not compress it's dynamic range. The difference between light and dark parts of the image stays the same as the image is lightened and darkened.

This gives digital photography 'latitude' - you need not hit the exact exposure value to get a usable picture. A digital sensor such as a ccd array behaves similar to slide film where the film's latitude is concerned: Under-exposure does not destroy the image, it only becomes dimmer. Over-exposure turns bright areas white. Once an area becomes white, all the white pixels are the same and no longer have any image data in them. (This is not to say that very successful images cannot have large areas of pure white.)

I wanted to look at the differences between the two NEF editors, so I deliberately made some (boring but repeatable) images with exposures different than those recommended by the camera's light meter. The first image has the 'correct' exposure, the second was one quarter of that exposure and the third was given twice as much light. To compare the editors, I then adjusted copies of the two mis-exposed images with each editor by adding two stops (+2.0 EV) to the second and subtracting one stop (-1.0 EV) from the third. Below are image pairs (at one third size) representing the output of the two programs for (top to bottom) the correct exposure, the under-exposure and the over-exposure:

Correct exposure:

Adobe Camera Raw - correct exposure, no adjustment

Nikon Capture - correct exposure, no adjustment

The correct exposures show that the Nikon Capture program yields an image with slightly more contrast and saturation. Since either image can be further adjusted with a curves tool, these differences are less significant than they appear.

Under exposure:

Adobe Camera Raw - underexposure, +2 EV adjustment

Nikon Capture - underexposure, +2 EV adjustment

These two images look very similar to the correct exposure, but the depth of field is greater, because a smaller aperture was used (f/4.5 vs f/3.2). The Nikon Capture rendition has darkened some shadow detail.

Over exposure:

Adobe Camera Raw - overexposure, -1 EV adjustment

Nikon Capture - overexposure, -1 EV adjustment

Here the difference is striking and reversed. The Adobe Camera Raw version has much more contrast than does the Nikon Capture edited one. In both images. the shadows are buried, and the highlights incinerated, but less so with Nikon Capture's image. The lesson from this is the expected one: avoid over-exposure, even by a small amount.

The image isn't completely safe when under-exposed however. Photo detectors, like the ccd in the camera, generate a small amount of noise. That is, each pixel will randomly activate at a very low level, giving the appearance of blotchy, very, very dim light. As the brightness of the pixels that make up the image become dimmer and dimmer the signal level from the sensor gets closer to the noise level. Thus the image is limited to under-exposures that do not get too near the noise level of the ccd, or the noise will show up in the image. (This is not to say that very successful images cannot have noticeable grain.)

In order to evaluate the noise level, the image must be looked at high resolution. Again each image was corrected to proper exposure with both tools and a dark area with detail was examined, magnified by two, for the presence of noise:

Adobe Camera Raw

correct exposure, no adjustment	underexposure, +2 EV adjustment	overexposure, -1 EV adjustment

Nikon Capture

correct exposure, no adjustment	underexposure, +2 EV adjustment	overexposure, -1 EV adjustment

Noise, which looks a lot like grain, shows up in the correctly exposed images in very slight amounts, and only in the darkest parts. In the middle, under-exposed images, noise can be seen throughout the dark parts, as well as the mid-tones. The over-exposed images are nearly grain free. The lesson: more light means less noise.

The two editors perform differently. Adobe Camera Raw has a greater effect than Nikon Capture has on the image. If 2.0 EV is added to the image with Nikon Capture, then adding 2.0 EV with Adobe Camera Raw will look like it had 2.1 EV added. Likewise when subtracting exposure, the image will appear slightly darker with Adobe Camera Raw. This increases the overall contrast of images adjusted by Adobe Camera Raw.

Because of this increase in contrast, the noise grain is more apparent with Adobe Camera Raw than with Nikon Capture. Looking at the middle images, the +2EV adjustment, shows a lot of grain in the Adobe Camera Raw processed image. By comparison, images with exposure added with Nikon Capture are noisy but the grain is more uniform in color and texture, so it is less noticeable.

The correct exposure shows almost no noise when processed by either program, and there is little noise in either over-exposed image. Again though, the increased contrast with Adobe Camera Raw caused problems by burying shadow detail in this image. Nikon Capture shows more shadow detail, with a slight amount of shadow noise.

Conclusion

To keep highlights safe from becoming white, and shadows with little grain, about two stops or so of latitude are available through correction with these NEF editors. Nikon Capture does a better job, both with controlling highlights, and at keeping shadow detail out of the noise, within that range of latitude. For images with the maximum amount of detail within both the bright and dark areas (that is, with the greatest dynamic range) Nikon Capture is the program to use.

Adobe Camera Raw however, can add up to four stops of exposure, and subtract two compared with Nikon Capture's two and one, respectively. This provides more opportunities to rescue incorrectly exposed photographs, and to engage in extreme photography where grain and blocked hilights are welcomed as artistic artifacts or at least tolerated in pursuit of the image.

Notes:

1) The Adobe Camera Raw plugin is intended for use with Photoshop 7. The same functionality will be included with Photoshop CS.

2) The globe of Mars in the test photo is in the same orientation as in this photograph.

3) Nikon Capture seemed to sharpen better too in this example.

4) Next: White Balance comparison - what is color temperature?