“Sharpness” is a term that’s often thrown around in photography discussions. It’s used to describe images, as well as the cameras and lenses that produce them. “Is this lens sharp?” someone will ask. Or, “I just bought [name of camera] and my images don’t look very sharp. Should I have bought a different camera?”
Experienced photographers often request that an example image be posted. In other words, “Show me your problem, and I’ll see if I can help.”
What they’re saying (between the lines) is: “I don’t think it’s the camera. I think it’s your technique. If you post the image, I can prove it to you.”
This is where digital images have an advantage over film images in my opinion. Not only is it quick and easy to post a digital image online in such a discussion, but your camera records all the exposure information inside the image file. The experienced photographer will look at that information (“EXIF”) and usually spot the trouble within seconds. Often, it’s only necessary to look at the image itself.
The reason is that it’s usually not the camera, or the lens, that’s causing the problem, or the perceived problem.
Here are some common characteristics of images that are sometimes confused with “lack of sharpness”.
1. Narrow Depth of Field
The image above is very sharp — in the plane of focus. Because the object (part of my TV) is slanting away from me, the plane of focus intersects it at the “S”. (View it larger.) When viewed at great enough magnification, the image shows the grooves in the “S” and the speckled finish of the surrounding surface.
The rest of the image doesn’t lack sharpness; it’s just out of focus.
People using wide-aperture prime lenses for the first time are often startled to see such “softness” in their images. The 50mm f/1.8 is a great example; for both Nikon and Canon users, it’s an inexpensive first prime lens. But the depth of field is so narrow that sometimes a person’s face won’t be entirely in focus. The beginner thinks he bought a “soft” lens that lacks sharpness.
Solutions: (1) Use a narrower aperture to increase the depth of field. (2) back up a little — the depth of field is directly related to your distance from the subject.
2. Poor Focus
The image above is very sharp — again, at the plane of focus. But the subject is simply not in focus. This is not the fault of the lens, or the camera, but of the user. The user (me) set the camera to the fully automatic mode. So not only did the camera choose all the exposure settings (a bad idea), but it chose which part of the viewfinder to focus on.
In this case, it chose the blinds in the background, probably because of strong contrast.
Sometimes, the autofocus, even when used correctly, will misinterpret the information coming through the lens, and you’ll get a slightly “soft” image. This is the fault of the auto-focus system (none are perfect), and not the lens. You can help the auto-focus by selecting a single AF point in your camera, by making sure there’s enough light for proper focusing, and by aiming for high contrast areas at or near the subject.
Some cameras have large enough viewfinders to use manual focus; my camera’s viewfinder is small. Manual focus is nearly impossible with it. (But it’s still one way to be sure the image is focused properly).
3. Slow Exposure
The image above is (or would be) sharp, but you can’t tell because of (again) user error. I’ll blame my wife for this one (and I’ll suffer for it later). She’d accidentally bumped the ISO to 100. In this dark room, getting a proper exposure at ISO100 means using a very slow shutter speed. In this case, 1/4 sec.
Two issues crop up with slow shutter speeds: (1) motion blur, and (2) camera shake blur. This image has both problems.
The shutter was open long enough to capture the movement of the subject and the movement of the camera in shaky hands.
Solution: Use a faster shutter speed. It has nothing to do with sharpness of your lens or your camera’s abilities.
Sometimes you want motion blur — to show that something’s moving. But it should be something you do on purpose, not something that happens when you wanted a sharp and crisp image.
Here’s an image where I used motion blur on purpose, to show movement down the road:
Those are the three usual culprits: narrow depth of field, poor focus, and slow exposure. Explaining these three things has solved at least 99% of the “my images aren’t sharp!” problems I’ve seen.
I can think of at least one more culprit, but it’ll rarely happen for beginners:
4. Too Much Glass
If you introduce too much glass between your camera’s sensor and the subject, it’s going to eventually soften the image, no matter how excellent the quality of the glass, and no matter how clean it is. If you can help it, don’t try shooting through your car’s windshield, or the windows of your house or office. And if you’re going to add filters on your lenses, make sure you know why.
(Adding a UV filter for “protection” just because the salesman told you to isn’t going to help your images, and may actually harm your images. The lens’ front element is much stronger than the cheap glass in your filter.)
Adding teleconverters or closeup filters can also have a degrading effect on the sharpness of an image.
The image above was shot using one of the sharpest lenses that Canon makes, a lens that costs more than all of the cars I’ve owned. It’s the Canon EF 600mm f/4L IS USM, which currently retails for about $10,000. So why isn’t the image sharp?
Well, for one thing, I was about three blocks away from the subject.
Shutter speed plays a part here too — even though the camera was on a very heavy and expensive tripod, the slightest wobble will affect a lens this long.
And we made it worse by adding a bunch of glass to the lens. Not only did we place two 2x teleconverters behind the lens, but we also attached a 1.4x teleconverter. Even if it had been possible to shoot at a higher shutter speed (it wasn’t), adding that much glass will always make an image less sharp.
(For those of you doing the conversion for focal length, I’ll save you the trouble:
600mm lens x 2x TC = 1200mm
1200mm x 2x TC = 2400mm
2400mm x 1.4 TC = 3360mm
3360mm x 1.6 crop factor = 5,376mm equivalent)
For handholding (I’m not strong enough to handhold the lens I just mentioned), this would require a shutter speed of at least 1/5400 sec. Not only is my camera not capable of such a speed, but it would have been an all black image. Each teleconverter steals some light, so we used ISO1600 and still only managed a 1/400 shutter speed on a bright and sunny day.
But it was the extra glass (three TCs) that caused the lack of sharpness here. As I said, it’s not something that a beginner will run into often, but it was worth mentioning.
To see how far away I was standing, click here, and hover your cursor over the center of the image.
5. Viewing at 100%
It’s called “pixel peeping”. When you load your image into your editing software and then zoom in to 100% (or greater) to see individual pixels. There are only a couple of excuses for this. You’re using a clone stamp to remove dust spots? Okay. Zoom in and fix the spots, then zoom out again to view the image. You’re testing the effects of high-ISO noise or various unsharp mask settings? Okay, check those, and then zoom out
Viewing an image while zoomed into 100% is rarely helpful for determining what an image looks like. When you show the image to other people, will they be viewing it this way? No. They’ll be looking at a downsized version on the web (like my examples above). Or they’ll be seeing a glossy print at 4×6 or 8×10, or other standard sizes. Or they’ll see a framed picture in your living room, viewed from several feet away.
Think about the billboards you see along the highway. Don’t assume those pictures were shot with perfectly sharp 20-megapixel cameras. Because you’ll never view them from two feet away. You’ll only see them from 100 yards away; rarely closer.
(Edited on Dec. 17, 2011 to add point #5)