by Sean Setters
There are several variables that can have an adverse effect on image sharpness. Therefore, it's important to isolate each variable to try to determine the exact source of the problem in order to help formulate a solution that aids in achieving sharper images.
1. Subject and/or camera movement (Shutter speed is too slow)
Probably one of the most common sources of image softness is motion blur, either caused by subject movement or camera shake. Thankfully, diagnosing and counteracting the problem are fairly straightforward.
If you notice sharp areas of your frame, but moving subjects are blurred, you know that your shutter speed was not fast enough to freeze action. If you notice a fairly uniform blur across the entire frame, but the blur is directional (with sharper contrast lines running in a specific direction), or else your images' EXIF information indicates a relatively slow shutter speed for the focal length was being used, then your images likely suffer from camera shake induced by the photographer.
For more conclusive results, you can conduct a Control Test (found at the bottom of this article) to see what kind of sharpness you should expect when subject and/or camera movement has been eliminated from the equation.
Fortunately, the solution to the problem is also straightforward – use a faster shutter speed. How fast? That's a tricky one, but... "as fast as it takes" is the true (but seemingly unhelpful) answer. Fast action (i.e., sports) may require a shutter speed in the 1/500 - 1/2000 second neighborhood. For more static subjects, a shutter speed of 1/focal length [or with more dense sensor cameras, 1/(focal length * 2)] is a good place to start. Experience is often the best teacher when it comes to determining the optimal shutter speed for obtaining sharp images in any specific situation.
If your subject isn't moving, using a tripod (or some other form of solid stabilization) and 2-second timer (combined with your camera's mirror lock-up feature) can help eliminate the effects of camera shake.
One thing to note is that wider aperture lenses will allow you to use faster shutter speeds while keeping high-ISOs at bay. If you notice that you must use a very high ISO to freeze motion because the maximum aperture of your lens is f/5.6 at the focal length you require, it might be worth considering upgrading to a lens that features a wider maximum aperture at that same focal length (or focal length range).
2. Autofocus not calibrated properly
It only takes a small amount of front or back focus to make your subject(s) look unsharp. If your camera and lens are not calibrated properly to work together at achieve perfect focus, your subjects will be noticeably soft. Keep in mind, even a top-performing AF system may miss focus occasionally. Calibrating your AF will help if your lens is consistently focusing at a point in front of or behind your intended plane of focus.
The easiest way to tell if your lens is front or back focusing involves shooting several image of a distant, high-contrast object in the grass that's roughly the same height off the ground as your camera (shooting propped on a knee and pointed at a yard sign usually works for me). Reviewing the images on the LCD, the blades of grass and/or ground in focus should be on the same optical plane as the object you are trying to focus on. If the grass in focus is noticeably behind or in front of the original plane of focus, then your lens may not be properly calibrated for use with your camera body.
If your DSLR features Autofocus Microadjustment, then a little testing should help you determine the optimal setting in order for your camera and lens to focus properly. If your camera does not feature AFMA, then you'll need to send both your camera and lens to the manufacturer's service department for calibration.
For cameras with the AFMA feature, you can dial in an adjustment to correct for front and back focusing. However, you'll need to figure out what value works best. My suggestion is to read John Reilly's excellent article "AF Microadjustment Tips" and try the setup explained in the section titled "The better DIY approach."
For DSLRs without the ability to adjust focus in-camera, you have a few of options. The first option is to exchange the lens (if it is a recent purchase) and hope that the next lens is better suited for your camera. The second option is to modify the lens firmware yourself if that option is available to you. Both Sigma and Tamron offer optional devices such as the USB Dock (Sigma) or TAP-in Console (Tamron) which allow you to modify focus parameters of compatible lenses. The third option is to send your camera and lens to the lens manufacturer (either OEM or third-party) to have them specifically calibrate your lens to your camera body.
3. Surpassing your camera's DLA (Diffraction Limited Aperture)
If you're not familiar with the concept of DLA, the take a quick look at Bryan's full explanation here. However, a quick explanation of DLA is the approximate aperture at which diffraction begins to negatively impact image sharpness. The DLA value is derived by multiplying a sensor's pixel pitch (in microns) by 1.61. For example, the DLA formula for the EOS 7D Mark II with a pixel pitch of 4.1µm would be 4.1*1.61 = f/6.6.
That means that for the absolute sharpest results at the point of focus with the 7D Mark II, you should limit your aperture to f/6.3 (the next lowest aperture that the camera can be set to) or lower. If you'd like to see an example of the degradation that can occur when using apertures significantly narrower than the camera's DLA, check out these image quality comparisons. That's not to say that you should never use apertures smaller than the DLA; sometimes a small trade-off in overall sharpness is preferable to obtaining an increased depth-of-field.
If you notice that your images are taken with apertures at or above the camera's DLA value, then your images will likely show varying degrees of diffraction (narrower aperture = visibly more diffraction).
Fortunately, this cause of image degradation is easy to correct – use an aperture wider than the camera's DLA (which can be found by referencing the site's Camera Specifications Comparison tool).
4. Heat waves
If you are using a fairly long focal length and focusing on subjects relatively far away, any heat source between you and that subject can cause heat waves which will negatively impact image quality. Common sources of heat waves include hot sand and asphalt, but even flowing water on a cold day can be a culprit.
Many times, heat waves are pretty easy to pick out. They cause your distant subjects to have a rippling look to them. The rippling effect will be especially noticeable when cycling between peview images that were captured in a burst sequence. You can also try photographing nearby subjects that do not have obvious sources of heat between you and that subject. If your nearby subjects are sharper, then heat waves may be contributing to the loss of sharpness visible in your distant subjects (though, this test does not conclusively isolate heat waves as the sole cause, as an incorrectly calibrated AF may lead to similarly unsharp distant objects).
As Bryan says in his Are Heat Waves Destroying Your Image Quality?:
What can you do about this problem? Heat waves are an image quality factor that you generally can't spend money to put behind you. For example, a sharper lens and a better camera are not going to be helpful. Selecting a different location, a different time of day and/or a different day completely or even a different season is often the best solution. A cloudy day with low temperature fluctuation may work for your image.
Many times, the photographer does not have control of the day and time of a shoot and will need to deal with the issue. Sports photographers typically fall into this group. For example, auto racing often takes place mid-day on asphalt tracks and photographers capturing these events will encounter this distortion.
If opting to shoot through the heat waves, move closer if possible (but not dangerously so – referring to the auto racing scenario). The less air that light passes through, the less likely that heat waves will cause strong distortion. Also, capture lots of images to allow selection of the least-influenced and to give your camera opportunity to lock in proper AF distances.
It's no secret that some lenses are simply better than others. If you're using the 18-55mm lens that came bundled with your camera, you probably won't be surprised to learn that a different lens may allow you to get sharper images. That doesn't necessarily mean that you need a more expensive lens (though that may generally be the case). For instance, our tests show that the EF 40mm f/2.8 STM is sharper at f/2.8 than the EF-S 18-55mm f/3.5-5.6 IS STM lens is at 55mm | f/5.6 (even though the kit zoom lens retails for $50.00 more than the prime).
Some may question my comparing a zoom lens to a prime, but I think it's a relative comparison from an image quality perspective. If you want to maximize sharpness, you may want to consider a set of prime lenses for a few reasons. For one, the low-to-mid range primes are quite affordable. For another, primes typically feature wider maximum apertures than zooms at their comparative focal length (which, as described above, can aid in obtaining sharp images by allowing for faster shutter speeds to be used). And finally, primes are typically sharper than zooms when compared at the zoom's maximum aperture at that specific focal length (because, inevitably, the prime is stopped down).
Perform a Control Test (see below) to see what kind of performance your lens is capable of. Analyzing the images, see how the fine details are resolved compared to our Image Quality Test Results at the same aperture setting (or closest setting if we didn't test that exact aperture). If your results are similar, you know that your lens is performing normally from an image quality perspective. If your results are noticeably less sharp, and you've eliminated the other softness-inducing causes mentioned above, then see cause #6.
If your lens is producing the best image quality that you can expect from it, but the sharpness level is below your satisfaction threshold, the solution is simple – upgrading your lens will be necessary to improve the sharpness of your images. The hardest part, of course, will be choosing which lens will represent the best upgrade for your needs. On that note, here are some helpful resources:
6. Lens malfunction
If you've ruled out all of the other causes of blurry images found above, then a lens malfunction is likely the culprit robbing you of sharp images.
Perform a Control Test. Compare your results to our own Image Quality Test results captured using a similar focal length/aperture/camera body for reference purposes. If your images appear noticeably soft by comparison, or else one side of the image appears significantly softer than the other, then there's a good chance your lens has a misaligned element (or some other design anomaly).
If you suspect your lens is exhibiting signs of malfunction, you'll need to contact the lens manufacturer to arrange for a repair. It may be beneficial to show the manufacturer control images to illustrate your concerns. If the item is under warranty, then the repair costs should be covered by the manufacturer (though shipping your lens securely to the repair facility – and insured – may result in a moderate amount of cost).
After the lens has been serviced and returned, it's a good idea to perform the same Control Test (and compare the new results to the old results) to ensure the repair was completed successfully.
So that's our top 6 reasons why your images may be blurry. Hopefully this list can help you "stay sharp" when capturing photographs on your next outing!
Here's what you do to find out how sharp your lens can be under ideal circumstances: