(Click to see on black :: Link to photo in gallery)
To get photos such as this one, which was taken wide open on a lens notorious for its focusing difficulties (the otherwise amazing Nikon 24mm f/1.4), autofocus fine-tune is essential. I have three lenses for my Nikon D7000, and each needs a slightly different value; +1 on my Nikon 24mm f/1.4, +8 on my Sigma 50mm f/1.4 Art, and -20 on my Nikon 105mm f/2.8 VR. Especially with the 105mm macro, fine-tune was a necessity.
Sometimes, although not as often as the internet would make it seem, there are manufacturing issues with a specific lens. For some of Nikon's highest-end lenses, such as the 24mm f/1.4 and the 35mm f/1.4, people online cried out that their focus was broken. Before you try to fine-tune your autofocus, it's important to make sure that it's not broken in the first place. Below I'll outline that process.
First, find a large (at least twice as wide and tall as the focusing squares visible in the viewfinder) and high-contrast subject, preferably outdoors. Don't use a tree or anything else with small patterns, because that can confuse the autofocus system. Put the camera on a tripod in single-servo autofocus to keep your shots consistent. Make sure that there are some objects slightly in front of and slightly behind the subject so you can judge exactly what the lens focuses on. Assuming your lens has autofocus fine-tune (which it probably does if you have a high-end body to complement this high-end lens), change the value to minus twenty. Now, autofocusing with the center point in the viewfinder onto the high-contrast subject, take several shots at f/1.4, making sure to defocus the lens after every shot (so that autofocus has to actually change instead of using its old position). Then, change the autofocus fine-tune to plus twenty and take another few shots of the same subject, resetting the focus ring after each shot. You should see a dramatic change in the photos.
Although both sets will probably be out of focus (even though one set may appear sharper than the other set, which is normal), the clear difference should be that the first set’s photos are consistently focused too close to the camera (between the camera and the subject), while the plus twenty shots will be focused consistently behind the subject. If this isn’t the case, try the test again because it really should work. That’s the whole point of Nikon’s autofocus fine-tune.
If your lens has bad autofocus, you will see no noticeable difference between the groups, and both will have most photos be way out of focus. It’s normal if photos from the same set aren't focused at the exact same point each time (although they should be close), but the real issue is if there is no general trend whatsoever with the minus-twenty set compared to the plus-twenty set.
The copies of these lenses that had issues DID NOT RESPOND in any way to autofocus fine-tune. If your lens responds to autofocus fine-tune, you’re fine.
If you’re not sure whether your lens has a focus problem, just set the fine-tune value back to zero and start taking some photos. If they are consistently out of focus, and they aren't consistent, send back the lens. Now, if all your shots are, say, focusing too far behind the subject, that’s actually not an issue; that's why fine-tuning exists!
(Note that due to the nature of phase-detect autofocus, photos from this lens will likely become less accurate as the subjects become farther away. If it really matters for a shot, you can actually jump into live view on your camera and focus using the LCD screen. Magnified manual focus works slightly better than autofocus. This works because the screen’s focusing system, contrast-detect focusing, is not related to the viewfinder focusing system, phase-detect autofocus. The contrast-detect focusing in live view will always be accurate, no matter the distance, especially if you use magnified manual focus. It’s worth noting that the autofocus fine-tune settings don’t affect the contrast-detect autofocus system at all, simply because it is already accurate enough in the first place.)
There is a tremendous array of focus-calibration products on the market. Some are free online, and some cost hundreds of dollars and automate the whole process.
Honestly, I had a hard time calibrating my autofocus until I learned about a new focusing method which is growing in popularity. As far as I know, the first person to invent this technique is a dpreview.com user named Horshack, who calls the method "Dot-Tune." Click here to see their post, or read my explanation below.
Dot-Tune is an easy method to fine-tune autofocus on cameras which have a viewfinder indicator of LEFT---DOT---RIGHT to say whether or not an object is in focus. Dot-Tune uses this indicator to make fine-tuning easy.
First, set up your camera at a distance which you typically may focus at. A landscape photographer may want to find the rough hyperfocal distance of their lens, and a portraitist may focus on something ten or fifteen feet away. Either way, the steps are as follows:
1) Put the camera on a tripod and place it in autofocus mode (manual focus mode increases the tolerances of the viewfinder focus indicators, which is bad for our purposes).
2) Pick a lens and a focal length on the lens, then open the aperture as wide as possible.
3) Point the center of the frame on the object you want to use for fine-tuning. Pick something relatively flat which has detail, but not extremely fine patterns. A painting in an area that receives a lot of natural light is ideal.
4) Now, enter live-view mode and manually focus on the object. Make sure that it is in the very center of the photo.
5) Go to autofocus fine-tune, and put it at a random value. You may as well put it at -20, since you'll cover all these settings eventually.
6) Flip to the viewfinder, then check the focus scale by holding down the shutter button half-way. Does the indicator say that the photo is in focus?
7) Make a chart with two columns. Label the first with all the fine-tune values that your camera offers, and then leave the second to write out which values give you a dot (in focus), an arrow (out of focus), or a blinking indicator which flips between a dot and an arrow (decent focus).
8) Ultimately, several values should give you an "in-focus" reading. That's okay! For my Sigma 50mm f/1.4 Art, the values from +5 to +11 all said they were in focus. How do you deal with that? Average the two extremes! Do not include the values which flip between a dot and an arrow. For my Nikon 24mm f/1.4, the values from -2 to +4 gave a good reading, so I averaged them to +1.
9) For some lenses, your range of values may give you an answer with a half value (values +3 to +8, for example, give you an average of +5.5). In this case, both values should give you good focus. If one side gave you a reading which fluctuated from a dot to an arrow, bump your fine-tune value up or down 0.5 towards that side. If both gave you such a reading, or if neither did, then both those values should be fine. As a landscape guy, I would probably recommend leaning towards the lower value because it is important that I have a foreground which is in focus.
10) If your camera says that the most extreme value is still in focus, like it did with my macro lens, just set your focus to that value and take some test shots. For me, I found that the macro lens was actually sharpest right at -20. Yours may be different, and this is one situation where you may need to do more testing between the extreme values simply by taking pictures at each one and finding whether they are sufficiently in focus.
11) If even the most extreme value is still not enough correction, then you have no choice but to repair the lens. It may or may not cost you anything; camera companies are finicky about stuff like that, even if you have a warranty.
12) Lastly, take test photos! This method should work, and it's how I fine-tuned my lenses, but any error in your testing method could throw the results way off. Also, because of the nature of autofocus, not every shot will be perfect. Hopefully, though, the results are more accurate than they were before the test!
Below is a photo that I don't believe would be sharp had I not fine-tuned my focus:
(Click to see on black :: Link to photo in gallery)