Straightening Images -- Part II

Article and Photography by Ron Bigelow

www.ronbigelow.com

Photoshop CS or Photoshop CS2 Used in this Tutorial

The previous two methods work with images that are in a format such as JPEG, TIFF, or PSD. However, with raw images, there is another option; the image can be straightened in the raw converter at the time of conversion.

Straightening the Image: Raw

Figure 1 shows a raw file that has been opened in a raw converter (Camera Raw). It is easy to see that the shoreline on the far side of the lake is tilted. This is easy to fix using the Straighten tool in the converter.

Figure 1: Image in the Raw Converter

The zoom tool can be used to enlarge the image, if so desired. However, for this image, enlarging the image is not necessary. The Straighten Tool is chosen and is used to draw a line along the bank (see Figure 2).

Figure 2: Applying the Straighten Tool

As soon as the Straighten tool is released, the image is rotated (see Figure 3). Obviously, the entire image is now angled. This is not a problem as the converter will straighten the image during the conversion process. The converter will also automatically crop the image. At this point, if it is desired to undo the rotation of the image, simply select the Crop tool, right click the mouse, and select Clear Crop. The image will be returned to its original state.

Figure 3: Image after Application of the Straighten Tool

Clicking OK closes the dialog box and reveals the corrected image (see Figure 4).

Figure 4: Corrected Image

Which Method is Best

Inevitably, the question will come up, "Which method is best?" The answer to such questions is almost always, "It depends". It depends on ones priorities. For me, this is a rather straightforward issue; image quality is almost always my first priority. With that priority in mind, straightening images in the raw converter is my first choice. In general, manipulation of images in raw converters produces less image degradation than manipulation of images in image editing programs (the technical reasons for this are out of the scope of this article).

In those cases where it is not possible to straighten an image in a raw converter, and the file has layers, my next choice is to use the Measure tool. This is simply because this method can be used with layered images without any unusual problems (I have had some problems with the Lens Correction filter when used with layered images). If an image does not have any layers, then, the Lens Correction filter will work equally well.

The Lens Correction filter is the best choice if additional corrections need to be made to the image (i.e., barrel distortion, pincushion distortion, vignetting, perspective, or chromatic aberration)

Reminders and General Considerations

In order to get the best quality when straightening images, a few points should be kept in mind.

Prevention is far easier and less time consuming than a cure. As mentioned at the very beginning of Part I of this article (see Prevention is the Best Cure), straightening images causes image degradation. The image quality is degraded when the image is rotated and again when the image is cropped. Therefore, the best way to have a straight image is to make sure that it is straight before the shutter is pressed. This is extremely easy with a bubble level.
Make sure that the interpolation algorithm is set to Bicubic or Bicubic Smoother. This will produce rotations of high quality. The other algorithms may work faster, but the image quality will not be as good.
If an image is being rotated in an image editing program, the resulting image degradation will be less if the image is a sixteen bit file. Sixteen bit files contain far more colors than eight bit files. Thus, image degradation is less noticeable. This is not an issue if the rotation is done in a raw converter as all raw files are high bit.
Perform only one rotation. Multiple rotations mean that the image will be degraded multiple times. If a single rotation does not get the desired result, undo the rotation and start over.
The longer the line that is drawn with the Measure or Straighten tool, the more accurate the straightening.