Understanding Image Sharpness

Aside from having an interesting subject and composition, sharpness is one of the most important factors determining image quality.

The term sharpness is a description of the detail and clarity in an image. What is it that causes a photograph to be sharp? Is it technique, the gear you use, or maybe it’s the post-processing?

In this article, I hope to answer all your questions related to sharpness. First, lets dig deeper into what makes an image appear sharp.

understanding image sharpness

More About Sharpness


The basic sharpness of an image is determined by the camera and lens used. Post-processing can further improve the sharpness, but only if the image is somewhat sharp to begin with.

Sharpness is a combination of resolution and acutance. Images need both a high resolution and high acutance to appear sharp.

Resolution

Resolution is the part that you’re probably most familiar with. It has to do with the camera’s ability to distinguish detail. In digital cameras, the resolution is determined by the camera’s sensor.

The resolution of an image cannot be improved in post-processing.

resolution

Acutance

Acutance is how quickly detail transitions. An image with high acutance has sharp edges and very precise transitions from one brightness level to another.

acutance

The acutance is partly determined by the lens you use, but it can be improved with post-processing techniques. When you apply sharpening or an unsharp mask to an image, you’re improving the acutance which improves the perceived sharpness.

Magnification & Distance


Whenever you talk about sharpness, the terms magnification and distance have to be mentioned. Both play an essential role in the perception of sharpness.

Larger images always need more sharpening. An image that’s sharp when printed at 4×6 inches will be much less sharp when printed at 8×10 inches.

Viewing distance also has a large impact on perceived sharpness. A billboard or sign that is meant to be viewed from far away doesn’t need to be as sharp as something that will be viewed from much closer.

Before you start applying generous amounts of sharpening to an image, think about what you plan on using it for. The closer the image will be viewed from and the larger you plan to enlarge it, the more sharpening it needs.

Magnification is also why cameras with larger sensors can produce images that appear sharper. Even if the camera actually produces the same resolution, the larger sensor means the image doesn’t have to be magnified as much to produce a print of equal size. A 4×6 inch print taken with a medium or large format camera will always look sharper than an identical image taken with a 35mm camera.

Noise & Grain


Image noise is usually an effect of using a high ISO. Most of the time, we try to minimize the amount of noise in our images for the best quality. Small amounts of noise, however, can actually improve the appearance of sharpness. The image isn’t actually more sharp, but the added grain gives it texture and makes it appear sharper.

Technique


Even if you have the most up-to-date, expensive gear, and a rigorous post-processing workflow, your images aren’t going to be sharp if you don’t have a good shooting technique.

At slower shutter speeds and longer focal lengths, even a tiny amount of camera shake can reduce sharpness. Using a good lens is a start, but knowing when to use a tripod is even better.

Another common problem among beginning photographers is using a cheap, lightweight tripod. While the tripod may stabilize the camera enough to recover some sharpness, it may still shake enough to cause loss of brilliance. Using a stable, quality tripod is essential to ensuring you get the best quality your lens is capable of.

For more information on shooting technique, check out this article.

Conclusion


The sharpness of an image is determined by the resolution and acutance present. Using a camera that can produce high resolution and a lens that has high acutance is important, but it’s not everything. Knowing how to apply the right post-processing techniques to improve acutance and using a good technique when shooting the image all play a part.

If this article isn’t as technical as what you were looking for, here’s a link to an article I found helpful. It has a lot of good information on sharpness, and MTF charts with in-depth explanations and formulas.
Norman Koren article on Resolution & MTF Curves

If you have any more questions that I may be able to answer about sharpness, I would love to help. Leave me a comment or suggestion in the comments below.