Now that that image has been straightened as outlined in the article The First Stage of Crop: Straighten, we can turn to the second item whose actions might remove pixels from our image.
Optical Correction can fall into a couple of categories: Monochromatic and Chromatic.
Monochromatic aberrations are a function of a lens’ geometry and most often manifest as pincushion or barrel distortions. Correcting these can affect the remaining pixels available to your “rectangular-formatted” image. They must be considered in the context of the crop.
Chromatic aberrations are manifest in photography primarily in the colored-fringing we see in high-contrast areas toward the edges of our frames. As chromatic aberration correction does not normally result in reducing your pixel-count we will not concern ourselves with it from the perspective of cropping.
More detailed information on Optical Aberrations can be found at this Wiki article.
In addition to optical correction there is also the consideration of perspective correction. These effects are dependent upon lens field-of-view and camera placement-to-subject. They greatly effect the look of a photo and correcting perspective often removes large amounts of your image’s pixels. The reasons for both types of distortion, optical and perspective are not import to our story today, only that we correct for them before we finish our crop in the third stage: composition.
Lastly, there is the subject of correcting for lens vignetting. Vignetting can be caused by extreme open apertures as well as attachments exterior to the lens. In this case, vignetting was caused by the attachment of a polarizing filter on a 17mm zoom. If we can’t correct the vignetting with a lens correction tool in software, we will have to lop it off. All of these factors add up to fewer pixels once corrected. So let’s correct them before we move on to the third stage.
In this example photo we have lens distortion, perspective distortion and vignetting, each indicated by the red arrows.
Here, we have corrected the vertical perspective, removed the vignetting and corrected for lens distortion. At this point we haven’t lost any pixels-we have stretched the heck out of them but haven’t lost them. Remember that perspective and distortion are a to-taste adjustment. You can leave as much or as little distortion in your image as your artistic vision dictates. Here, I chose to leave a slight bow on the horizon for the “church on the hill” illusion.
With the crop overlay placed on the image to re-rectangle it again, you can see exactly where pixels will be lost. Most of the areas in this image that will be cropped off are not all that important. Had you cropped for composition first, you might have found important details in those amputated areas.
With the crop executed we now see exactly how much real-estate is left for safely adjusting our image composition. With these distortions corrected, we are ready to move onto the final stage of crop: Compose. Compose will be covered in the next article.
Most software packages for image editing offer some sort of correction tool(s). Adobe and Corel products for pixel-based editing offer these with a modest degree of control. There are also third party programs like PT Lens that allow you finer control of Lens Distortion. Newly-announced features of RAW and DAM software packages are starting to add lens corrections as well. With a wealth of products offering distortion correction it is easier than ever to ensure that your image is ready for that final stage of crop.
Of course, if your image has no optical or perspective issues it is permissible to jump directly to the compose section of the three stages of crop. If they are needed, however, you now know where best to perform the correction step.
Rikk Flohr © 2010