Twice a month we revisit some of our reader favorite posts from throughout the history of Phototuts+. This tutorial was first published in October, 2009.
The most common complaints I hear from most photographers of any experience level are "My images aren't sharp" and "I can't get my focus to lock." Most want to blame their equipment and, while there are many instances that equipment is to blame, I have found a vast majority are just simple user error. This is often down to a lack of understanding of how an autofocus (AF) system works. This tutorial will give you a better understanding of focus and sharpness, and hopefully help you take photographs that you're very happy with!
1. Adjust your Diopter
My what? Your diopter - or your eyepiece. You will never know if your subject is in good focus if you can't see what is sharp yourself through the eyepiece. On the side of your eyepiece (as shown below) is a small wheel to adjust the focus of your eyepiece in accordance to your vision.
You can adjust the eyepiece to a fairly large degree, but if you need more eyesight correction, there are replacement diopters available for DSLR/SLR's from many of the major manufacturers in a range from -5 to +4 as needed. No, this won't help your auto focus work any better, but it will help you appreciate when it's failing and assist with manual focusing.
2. Understand your Viewfinder
What the heck are all those things in there anyway? Here is where you may have to take your manual out (remember that book of paper that came with your camera?) Most common DSLR's have 9 - 11 focus points. Top of the line, professional cameras can have 45 to 51 focus points (although really only 11 -19 are selectable, the rest are assist points).
There are two types of focus points, single plane and cross point. Single plane only work on lines of contrast perpendicular (at a 90 degree angle) to their orientation. So if you look through your viewfinder as we have above, most of the focus points are rectangles, some are oriented horizontal, some vertical. Single plane focus points will work only perpendicular to their orientation. So - say you were shooting a tree - a vertical orientation focus point would not be able to find the edge of the tree, a horizontal one would. You can use this to your advantage by choosing a focus point that would lock to the line you want and ignore lines you don't need the focus to seek.
Cross point focus points will work with lines of contrast that align either way. Most cameras consist of a single cross point in the center, surrounded by single planes around them. Newer high-end cameras now are featuring cross points at all focus points.
Each focus point also has a particular sensitivity. Most need a lens of at least f5.6 maximum aperture to even use auto focus. On most cameras, the surrounding autofocus points are of this sensitivity, and the center point offers increased sensitivity if you use a lens with a maximum aperture of at least f2.8.
So if you are working in a low light situation, you may obtain better AF using the centre focus point. If you are not using a lens of f2.8 or better there is no low light advantage to the center focus point but it still may be more accurate since it is a cross point.
Also, when looking at the focus point rectangles the actual sensor area is 2 - 3 times larger than what is shown. Bear this in mind when focusing. If you're focusing on the bridge of someone's nose as your point of focus, remember that the person's eye is also within that actual sensor area. The auto focus will lock onto the eye rather than the bridge of the nose because it has more edge contrast than the flat light of the nose. Often it may not matter, but if you are working with a very shallow DOF it can make a difference to which area of the image has the sharpest focus.
3. Give Your Lens a Hand
Most autofocus systems have a certain amount of error or slop and may actually overshoot the point of best focus because of the mechanics and inertia of the lens movement. You can help minimize that by manually focusing the lens close to focus and then let the AF system take up the rest of the movement. Or, if that sounds like too much effort, at least give the AF two attempts to get it right. Perform one half-press of the shutter to bring it close and then do another to fine tune it.
An advantage that higher end lenses have is full manual operation even when AF is locked. Cheaper lenses will not allow you to touch up focus manually once lock is obtained, though this is a good way to ensure focus is as perfect as it can be.
4. Find the Fine Line
Auto focus systems work on lines of contrast, so can struggle on subjects without this feature (e.g. a cheek or a forehead, a white dress or black tuxedo, sand, single color walls etc). On areas like this, the AF will hunt all day and may never lock. The approach is to find a "better line" - eyes, lines between the a contrasting shirt and suit, the line between land and sky, a doorway. Anything that has contrast will help the AF work better and faster.
A Poor Focus Area
A Better Focus Area
5. Don't use the "All Focus Point" Mode
Unless you are in an extremely fast situation that just requires incredibly fast focusing, it's best to avoid using the All Point Focus. This mode doesn't know what you want to focus on, and usually focuses on whatever is closest to the camera. There are some situations where this is perfect, but they are few and far between.
6. Focus and Recompose - But Do It Correctly
I used to focus and recompose by always using the center focus point. I'd lock my focus and then recompose. Then I read a few articles that said you shouldn't do that - that you should shoot with the focus point closest to the final subject composition. The theory behind this is that the amount of lens movement and the arc angle made the distance from subject to lens change as you recomposed. If you used the focus point over the subject and did not recompose, you would not have this change in camera to subject distance and therefore no focus error. So I set out to take some shots to show you that it was so - and it wasn't.
There was absolutely no advantage to using the point over the subject without recomposing. In fact, focusing using the center point and recomposing actually was sharper in every shot except one case - macro. And I shot everything from 17mm to 200mm, and every distance from macro to 30 feet with the same results.
In every test, using the centre focus point and no recomposition resulted in the sharpest image. The next sharpest was to use the centre focus point and recompose, with the least sharpest being to use an outer focus point over the subject. To clarify - the theory is correct in that you do lose sharpness recomposing from the center point. What isn't correct is that using the point over the subject will bring back the sharpness - it does not.
Now one possibility that I think highly likely is that on my camera, the center focus point is three times more sensitive than any of the other eight points and is a more precise cross point. But that is the case with many cameras except for high-end models such as a Canon 1Ds Mark III or Nikon D3X. Another reason may be that most lenses are sharpest in the center and fall off to the side.
Here are three very typical examples of what I shot with two different cameras. The inset is a 100% crop of the lettering.
All I can say is that switching focus points is not, in my opinion, worth the time. But test for yourself as your results may vary.
A quick note on macro - this should always be done with a tripod and manual focus, because of the extremely shallow DOF and closeness of lens to the subject.
7. Use the Correct Focus Mode
Most DSLRs have at least two similar focus modes. The first is "One Shot" (Canon) or "Single Servo" (Nikon). In this mode it is assumed that the subject is stationary. Focus locks, you get a confirmation light in the lower display, and then the shutter fires. The shutter will not fire if a focus lock is not obtained.
The second type is "AI Servo" (Canon) and "Continuous Servo" (Nikon). In this mode it is assumed that your subject is in motion, i.e. sports, wildlife etc. The Camera will seek a subject with the focus point and focus will change continually to keep up with the subject but will never lock. The shutter will fire even if focus is not obtained.
There are other modes on some camera, such as "AI Focus" on Canon that are good for when you have a subject that is still, but has the possibility of movement like a small child. The AF will lock on the subject but if the subject moves it will go into AI Servo mode to keep track of the subject.
A third possibility - predictive focus - is for objects that are moving closer to or away from you. The camera will try to predict the movement and give you acceptable focus.
8. Don't Substitute Depth of Field for Good Focus
While using a deeper depth of field with a smaller aperture can improve "apparent" sharpness of the image, remember one thing: no matter what the depth of field, there is only one point of focus. So always practice good focusing techniques regardless of the aperture you may be using.
9. Use a Tripod or Take a Stand
When we stand taking a photo, we all have a tendency to sway forward and backwards - especially bent over a subject with a heavy camera/lens combo. It is natural; every one does it to some degree. And, if you are shooting with a very narrow DOF, that small distance you sway can influence how sharp and in-focus the area you want is. If you have a DOF of 4 inches, swaying two inches can make all the difference in the world. So, use a tripod.
Now I have to admit, as much as I use a tripod - I hate them. They interfere with how I work and most of the time with the way I shoot. So, if you'd prefer to avoid using on, at least take the time to use a good photographer's stance. One foot in front of the other with a slight crouch, arms locked to your sides, not up in the air (this is where battery grips with side controls come in handy), and body weight centered over your legs.
10. If All Else Fails - Use Manual Focus
I always hear a big gasp when I suggest this to photographers. They regularly tout "I only shoot manual, never auto" but suggest manual focusing to them, and they look at you like you just suggested they sell their children. Manually focusing in most cases (providing your dipoter is set correctly) can achieve the best and most precise focus. Especially in this digital age in which it is so easy to look at 100% or even 200% zooms on our monitors.
In fact, if you look at the unofficial specs for auto focus, you can see that they are not very precise. Here is the specification for "sharpness": The image is considered sharp if it appears sharp in a 6 x 9 print from 10 inches away. Yep that's it. No 100% zoom, no 20 x 30 print. Just that.
Now a number of new cameras come with "Live View". This can be a helpful tool in manual focus. Turn on your live view, zoom in to your subject/point of focus and check the sharpness on the LCD. This doesn't work well for me since I am almost always in bright conditions; deserts, beaches etc - but for some it works excellently.
One quick note on the image above. I originally used it just to show the Manual Focus switch, but another switch raises an interesting point; the 1.2m to Infinity and 3m to Infinity. This switch relates to something I said earlier about not requiring your lens to hunt through the whole range of focus. If you know you will not be focusing on anything closer than 3 meters away, switch it to this mode and the lens will not have to travel as far to focus. This can lead to a more accurate auto focus first time around,
11. What Should I Focus On?
For portrait/headshots, it's fairly widely agreed upon: the eyes. For other portraits it is still the face, unless there is another body part that you want the "focus" of the image to be on. Have your sharpest focus on the area that you want to draw the viewer's eye to.
In landscapes it is not always as easy, but you still follow the same rule as above. Don't settle for "it's a wide angle landscape, focus on infinity". If you have a foreground subject, focus on that and let your DOF carry the image to the background. If the foreground subject does not have sharp focus, it adds confusion to the image, since we naturally see things up close sharper than far away objects.
Now I could get in to focusing at the "Hyper Focal Distance" but that may be beyond the scope of this Tut. If you are interested in it, and you should be, perform a quick Google search for it.
12. It's in Focus, but is it Sharp?
Focus and sharpness are two different things. Explaining sharpness could take another whole tutorial, so I'm just make a couple of useful points.
If an image is out of focus, you can't make it in focus by sharpening. You will just have a very sharp out of focus picture. Most RAW images need sharpening of some type. Whether you use Smart Sharpen, an un-sharp mask, or hi-pass filtering techniques, most RAW images benefit from some sharpening. That said, as I have gradually moved up in camera quality I have found less and less need for sharpening and now only use it in about 25% of my images.
Remember also that sharpening is final product dependant. You would not sharpen the same amount for a web-sized image as you would for a 16 x 20 print. And with that in mind, if you intend on selling an image to a stock photography agency, don't sharpen your image at all. Most ask that you do not, as you cannot predict the use and size of the image.
13. Considering Shutter Speed
Shutter speed is another issue that can lead to a lack of sharpness. Every person has a limit to how slow a shutter speed they can hand hold at any given focal length lens. Some people are steadier than others but if you don't have sufficient shutter speed to overcome the movement (shake) of your hands, your image will come out blurry. For standard and wide angle lenses, most people can hand hold down to about 1/30th to 1/60th of a second.
For longer telephoto lenses it usually requires much more. A general rule that people start with is "1 over the focal length of their lens". So if you have a 200mm lens, shoot at 1/200th of a second and go from there to determine your holding ability. Personally, I shake like California on a bad day so I usually shoot higher. It also depends on how far away you are from your subject, as movement is exaggerated the further away from the subject you are.
If your subject is moving, holding the camera still or a tripod won't help - you'll need to have sufficient shutter speed to stop the action. Most start at around 1/250 but it depends on how fast your subject is moving. Requirements also vary depending upon whether you are shooting static or panning along with the subject. If you pan, you can get away with a far slower shutter speed and also get some interesting effects. It lets you show movement in the background but stops the subject.
Image stabilization systems on lenses allow for hand holding at lower shutter speeds (up to 3 stops more) but will not stop action any better than a non-IS lens. You can only stop action with shutter speed (or a high speed flash)
14. Expose Yourself Correctly
Having the correct exposure and having great light (the essence of all photography) is essential to good focus and sharpness. Since sharpness is determined by a line of contrast, if you are underexposed or have dull lighting, an image will not appear sharp even if all your other focus criteria are met.
15. I did all that. I'm still out of focus!
There's a small chance that it may actually be a problem with your equipment. Since third party lenses have to be reverse engineered, they won't always work perfectly with brand name cameras. Some do great, others don't. But sometimes even the brand lenses are not perfectly produced.
Cameras such as the Canon 50D and 1D/Ds Mark III's have fine tuning for front and back focus for up to 20 different lenses, so if you know one lens in particular focuses in front of your subject every time, you can make an adjustment in the camera to fix that. If this isn't available, you'll need to determine whether it's just your lens - or your lens and camera - that need taking for repair.
Here is a test you can do at home to give you a good idea if it is you or the camera. Find a ruler or yard stick and place it on a table going away from the camera. Mount your camera on a tripod, and open the aperture up all the way. Shoot down on the ruler at a 45 degree angle, focusing on a certain mark - in this case, the 6" mark.
If that is what you see clearest when you open the picture, then your equipment is fine - go back and work on your technique! If the sharpest point is in front of or behind that point, then you know there is an equipment problem and it should be sent in for service.
16. Conclusion
I've covered a huge amount of ground in this tutorial - so well done for reading this far! I do feel that good focus and sharpness are two of the most important technical things you need to get right in an image. It can make the difference between what looks professional and what looks like the work of amateur (and we all want to look professional - whether we are or not).
Please feel free to leave your thoughts in the comments - do you ever have a problem with focus and sharpness?
