Image Sharpening Explained, Simply

What is image sharpening? We’ve all heard about it and have undoubtedly heard about various image sharpening tools like unsharp mask or smart sharpen, but I’ve found that very few of us actually understand what image sharpening is.

So what is image sharpening?

No matter what image sharpening tool or algorithm you use, they all have the same end result, which is to increase the contrast of the lines and edges of objects in the image. That’s all image sharpening is. The primary differences between the various algorithms or methods of sharpening isn’t the end result, but rather how the lines and edges of objects in the image are detected. Likewise the various sliders or controls you get for each method of sharpening are to control the amount of sharpening and to fine tune the underlying line and edge detection for that sharpening method.

So there you have it. Image sharpening explained in 4 simple sentences. It’s not that difficult. The human visual system is extremely good at detecting lines and edges, so when we sharpen an image, all we’re doing is making what we’re visually sensitive to more pronounced. It’s a very effective visual perceptual trick that we’re playing on our brains when we sharpen an image.

The soapbox

I’ve noticed some image sharpening trends the last 5-6 years that really bother me and make me think that all these people on the internet that are dispensing photography information and advice and are supposed to be photography experts don’t really know what they’re talking about and doing. I can’t help myself. I have to say something about it.

The image sharpening aesthetic

This is a huge pet peeve of mine. All too often, people think that a sharp picture has lots of detail. As a result, they sharpen their images way, way, way too much. The Internet is riddled with posts on how and when to sharpen. You have input sharpening, creative sharpening, and output sharpening. You have tons of sharpening algorithms, plugins, and tools to increase the clarity of your images. There are companies out there whose entire business model is literally based on selling you something that will help you sharpen your images. On the camera hardware front, as of late, it seems that if a newly released camera doesn’t output a ridiculously over-sharpened image the Internet declares that camera as a piece of garbage. Ugh.

On top of that, the internet is flooded with images that are painfully over-sharpened (usually as a result of said company that sells image sharpening tools), all in the name of having a nice sharp image that is “crispy”. You know what else makes a nice sharp image? A nice moderately high resolution image that has a depth of field that is large enough so that the whole subject of the image is entirely in focus, assuming the person taking the picture actually nailed the focus.

Image focus and resolution to the rescue

I can’t believe how many people get a super fast prime lens, and then proceed to shoot with it wide open, then sharpen the ever living day lights out of the resulting images in an effort to try to get the subject sharp. It’s almost as if they don’t realize that when you’re shooting an 85mm+ lens on a full frame camera at f/1.4 or f/1.2, the depth of field is so small that the only thing in the image that’s going to be in nice sharp focus is one eye, or the tip of the subject’s nose, or one of the subject’s cheek bones, or their lips, or whatever the camera actually happened to lock focus on. Having a really small depth of field definitely has its uses, but if you want a nice sharp image, try stopping your aperture down to something like f/8. You’d be amazed at how much more resolution and fine detail is there and how much sharper your photos are as a result once they’re scaled to whatever the final image output resolution is. Again, assuming that you actually nailed the focus.

Have you ever seen a picture that actually had as much resolution and fine detail as what could be natively represented by the medium displaying the image? Probably not, but you’ll know it when you see it.

I’ll give an example: have you ever watched an HD movie on your iPhone in HD? You should try it some time. It looks incredible. The reason why isn’t because of image sharpening, but rather because you’re displaying the maximum amount of resolution and fine detail that your iPhone screen can natively represent.

Another example: ever seen Christopher Nolan’s “The Dark Knight” movie on Blu-Ray? He shot parts of the movie on very high resolution IMAX cameras and cut those scenes in with the rest of the film, which was shot on standard super-35 film. Even at Blu-Ray resolution (which is a whole whopping 2 megapixels image size), the difference in image resolution and fine detail between super-35 film and IMAX’s 65mm film is stunning. The IMAX sequences just look a lot sharper, not because of image sharpening, but because they contain as much resolution and fine detail as what can be packed into a 1920×1080 pixel image size, which results in a picture that looks very sharp with very little actual image sharpening applied or needed.

Huh. We just came full circle back to image sharpening. Imagine that.

OK. Sooo… When do you do image sharpening?

Ideally, you sharpen at the very end when your image is at it’s final output size. If you have a good image with good resolution and fine detail, you’ll discover that sharpening is often like what salt and pepper is to a great meal. A little bit goes a long way, and if used just right, it greatly enhances things, however, more is rarely better.

There are other places in your workflow where you can sharpen, like input sharpening, and creative sharpening, and those instances do have their uses, however, they tend to be really over-used and abused, so for the sake of simplicity, we’ll leave them off the discussion table for now and maybe visit them in separate posts.

You may have noticed that I brought up image resolution and fine detail a number of times in this post while talking about image sharpening. How much resolution you actually need is a subject for a different post, so we’ll get into that later, though suffice it to say, you don’t need nearly as much resolution as you think you do, the trick is actually acquiring that resolution in a way that makes for sharp images that will need very little image sharpening to look good.

Till next time.