Cropping – A Closer Look

We talked a little bit about cropping in our earlier post “Megapixels and Pixel Peeping…” and have referred to it briefly many times both here on the blog and elsewhere in various birding and photography forums around the web. In this article we’ll discuss the topic in a little further depth.

I’d begin by repeating some usual and probably familiar disclaimers:
This article is written for the layman and intentionally sacrifices technical depth for the sake of simpler understanding. Further, there is no one right way to approach any aspect of photography… Various cameras, lenses, and photographers are all different each one to the next, and every photo opportunity presents its own unique set of variables and challenges as well. The concepts, examples and techniques presented here are based on our experience photographing primarily perching songbirds and other small wildlife at moderate distances, using small-sensor superzoom cameras. But we hope that our readers will find this useful, no matter which methods or equipment they use.

What is cropping?

In that above referenced Pixel Peeping post I wrote:
Even when using the longest telephoto lenses, wildlife is often still too far away to compose a shot where the subject bird or animal is large enough in the frame to present them as we’d like. In these situations cropping is frequently used to allow the target object to be shown larger in the finished image.
Wikipedia further explains that: “Cropping refers to the removal of the outer parts of an image to improve framing, accentuate subject matter or change aspect ratio…” And, of course removing any amount of the outer parts of an image results in the remaining portion being smaller (having fewer pixels) than the original. So some may wonder how cropping can result in distant subjects appearing larger in the image. This is neither magic nor magnification. It is because modern digital camera sensors capture much higher resolutions (more pixels) than can be displayed by the devices normally used to view them [note: printing is another matter]. So when we view a 12 megapixel camera image on our 2 megapixel computer screen it must be re-sampled and effectively reduced in size by discarding over 80% of the data originally captured. When we re-sample and downsize the smaller cropped version of the image to that same 2 megapixel display size however, less of the captured data is discarded and the cropped portion is therefore presented larger on our screen. I would emphasize that because less of the captured data is discarded during re-sampling, cropping can also preserve significant detail that would otherwise be lost to downsizing. But we must also be aware that the deeper we crop the “closer we get” to the pixels, where sensor level noise and artifacts will become more noticeable.

Bridging the distance

The amount and fineness of detail we see in our photos is dependent on many factors, including the angle of light and shadow, accuracy of focus, the sharpness and focal length of the lens, exposure levels, and of course the distance between the camera and subject. Two of the most important factors are: the number of “pixels on the bird” in the original capture; and the amount of detail lost when re-sampling to screen resolution.
When we shoot close enough that our subject is large in the frame, many of the sensors pixels are involved in resolving and recording the fine details of feathers and fur that we all strive for in our photos. But when our subject is distant and small in the frame, far fewer pixels are dedicated to resolving those details and far less detail is captured. Cropping in on distant subjects recorded with too few pixels tends to reveal this lack of captured detail and generally produces images of poorer quality. So ironically, the closer you get to your subject the more cropping you can do, and the less you need to.

Framing your shot

When shooting wildlife, and wild birds especially, perfect framing in-camera is both difficult and (for us at least), rarely achieved. They’re nearly always moving about, often unpredictably, and seldom pose long enough to allow composing a perfectly framed shot. Just keeping them in the viewfinder can be challenging, much less trying to capture them with ideal framing. We find it is usually much more productive to shoot with continuous drive for as long as our subject is in focus and in the frame, and hope to achieve well-framed images after the fact with cropping.
This juvenile Bluejay was in frame for about 2 seconds during which Temple managed to capture 3 usable images:


We chose the second frame and cropped it in a 16×9 aspect ratio:


We normally shoot all our cameras with the 4×3 aspect native to their sensors. But when cropping we frequently select various alternate aspect ratios based on the bird’s pose, perch, and/or other properties of the scene. Here are a few more examples:











There are many widely-used conventions and guidelines for framing and cropping, such as “the rule of thirds”; leaving more space in front of the bird or in the direction it is looking; and leaving twice the space above the subject as below it; etc. But I must admit that we follow most of these conventions loosely at best, and no two photographers are likely to agree on the “correct” framing for any given image. My advice is to just be conscious of the artistic impact of your cropping decisions. Avoid clipping tails and feet, or making your subjects appear too tight in the frame …except perhaps when intentionally zoomed in for a closeup portrait. Presenting your subject large in the frame helps to maximize and preserve visible detail, but too little space in the image will often be aesthetically unappealing.

You can also use cropping to remove various unlovely elements from the frame. I like to think of this technique as “finding a frame within the frame”, where a tighter window can isolate a usable or even lovely image from within an otherwise distracting or unattractive scene.


Digital zoom and “digital teleconverter” cropping.

And finally a few words on various special camera features involving in-cam cropping and digital zoom. Many cameras including most recent model superzooms offer various combinations of digital zooming and in-camera cropping, which extend the focal lengths (telephoto reach) of their optical lens systems. Each of these options involves tradeoffs which effect the quality of the images produced when using them.

Digital Zoom magnifies the captured image using in-camera software interpolation, which generates and inserts additional (virtual) pixels into the image by averaging the data values of the surrounding pixels actually captured by the sensor. To the manufacturers’ credit, this process can actually be pretty effective when used sparingly* for screen-resolution images. It certainly can be useful for species ID spotting and “for the record shots” at extreme distances. But we rarely use it ourselves and don’t recommend it’s use for any critical or print imaging because the processing necessarily results in softening of detail as well as exacerbation of any noise and artifacts in the captured image. [*Image quality degradation at 1.2x will often be un-noticeable or barely discernible @2mp, but higher magnifications or viewing at higher resolutions will reveal loss of image quality.]

Digital Teleconverter reduces the field of view (fov), and effectively increases focal length by recording to a cropped portion at the center of the sensor. The following diagram illustrates the concept:


Some implementations of this scheme subsequently perform digital zoom magnification back up to the full resolution of the sensor. Other implementations allow or enforce limitation of output resolutions to the size of the sensor crop which avoids degradation in image quality. This latter method can have benefits in specific applications which are beyond the intended scope of this article, but the process effectively reduces both the size and resolution of the sensor. We do not normally use any of these in-camera “telephoto enhancement” features in our bird photography, and don’t recommend them for any general use.

To crop or not to crop?

When, how, how much, and even if any given image should be cropped is certainly a matter of judgment. Some images may not require or benefit from cropping… and some may be either realized as photographic masterpieces, or be utterly destroyed by it. The great majority (nearly all) of our images are cropped to some degree… and many of them are cropped quite deeply. Getting close and putting sufficient pixels on the bird is very important of course (with any camera), but that’s only one of many vital factors in producing satisfying images. Cropping can also be a primary and critically useful part of the process.



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