How to get super sharp auto focus
By Steve Kirsch
Shots not in sharp focus? Hereís are the things to check
the cameraís auto focus mechanism: do an autofocus setup using a tripod, a book cover with large type, and a ruler placed at 45 degrees next to the book (leaned against another book set back from the first). Set lens to a wide aperture that you use most often (e.g., 4.0 is reasonable choice), shoot at reasonable distance like you would if you were going to photograph someone. Focus on the flat book target, not the ruler. Make sure the shutter speed is 2*focal length or faster. Take 5 test shots where each time you rotate the focus to infinity before each shot. Use a tripod + 2 second delay. Image Stabilization wonít matter if you use a 2 sec delay. The shots should focus nearly identically when looking at the ruler images on every shot and the shots should all be the exact same image without shifting (without a 2 sec delay youíll be surprised how much the images shift). See next section for how to determine whether your camera or your lens could be at fault. Using manual exposure, and a great focus target, and not having the target or the camera move eliminates a lot of variables. Make sure your tripod is level and square to the target. Make sure the exposure on the ruler is proper. Over exposing it will make it less sharp. You can see how the ruler should look by taking a picture using Live mode. Focus in Live mode is extremely precise because the sensor itself is setting the focus. If the camera is at fault for autofocus, then there are ways to do a hard reset on the camera (remove both batteries) and that has fixed autofocus for some people.
the lens: one of your lenses could be bad (no repeatable) or in need of a micro-adjustment. So test each lens. Repeat the camera test above with a different lens. If the focus is consistent on one lens and not the other, the problem is your lens. If the focus is inconsistent on both lenses, it is the camera. If the focus is consistent on both lens, then micro-adjust each lens so that the midpoint of the ruler best focus is the book cover. Try repeating at different zoom settings if zoom lens and different f stops to make sure the micro-adjustment doesnít vary based on focal length, f stop, or camera to subject distance. To determine which way to move, look at the extremes on the ruler at equidistant points from the center and see which extreme is more in focus. That will tell you whether you are back or front focused.
your movement: when you press the shutter button, you shake the camera. If you are shooting at a slow shutter speed or have image stabilization off (it should be on unless you are shooting at very high speed), camera shake can lead to blurry pictures. General rule is to use 2X the focal length for the slowest shutter speed hand held, so a 50mm lens should be shot at 1/100 or faster. I was amazed that even at 1/200 on a 70mm lens that image stabilization made a huge difference even when taking pictures on a tripod! With IS, all the images were in the same spot. The only way to achieve that without IS is to use a tripod and a 2 second shutter delay! Also, in addition to camera shake, donít underestimate the impact of large body movements. I'm amazed at how often people shoot without steadying their camera first.
subjectís movement: 1/60 of a second might be fine if your subject does not move. But if you have any movement at all for your subject at 1/60, it will be blurry. Try a faster shutter speed if your subjects canít hold perfectly still or you are taking action shots.
camera settings: Check for the proper camera settings for your subject:
∑ Is the shutter speed fast enough for the lens and for the subject? In my case, this was the cause of my blurry images. Shooting a 50mm lens at 1/50 will always result in blurry images, even if your subject is stationary. Switching to 1/100 will make the image tack sharp!
∑ Are you using the sharpest f stop for the lens? For many lenses, you get the sharpest image 2 full stops from the widest opening, e.g., a 2.8 lens might be optimal at f/5.6
∑ Did you remember to leave image stabilization on?
∑ Is the image properly exposed? (low exposure images are not as sharp as correctly exposed images due to the noise, and if the section is slightly overexposed, it will not be as sharp since there is bleeding of the light
∑ time: exposure time isnít too long (a long exposure time will cause noise in your image so keep the exposure times short as possible,
∑ ISO: the lower the ISO the sharper your images. Try to keep it under 800 if possible. Different cameras have different noise levels so see what your own camera does to determine the max ISO to use. Canon 7D can have great images even at ISO 1600 if you are properly exposed.
∑ Resolution: are you set to the full raw camera pixel resolution? You want the lenses to limit you, not the sensor!
∑ Formats: Just to be sure you know what you are getting both color-wise and image quality wise, set your camera to record JPG and CR2 formats. Compare them after importing the CR2, e.g., use ThumbsPlus to read the raw and jpg and compare with Lightroom. When I did that I discovered for Canon is that using Camera Neutral didnít shift the colors on input (like the other settings did) so it more closely matched the actual image (which may or may not be what you want; Adobe Standard seems to work quite well in most cases for shooting people). I also have to add .4 EV to make Lightroom match the exposure on the camera (so blinking in the camera will blink in Lightroom). By comparing sharpness of your imported image vs. the JPG from the camera you can determine if Lightroom is doing any adding any processing during import you didnít expect that might be affecting image quality (e.g., changing the exposure, etc.).
∑ In Lightroom, the rendering in Develop mode is slightly better (and more true to the image) than the image in Library mode. Itís a subtle difference but it is there if you look for it; there are more artifacts in Library mode
∑ Using a lens wide open is not nearly as sharp in its sharpest point as if you stopped it down. That makes perfect sense optically. So when you use the widest aperture, you are giving up sharpness and gaining bokeh and ability to shoot at a faster shutter speed for a given ISO. You are, in general, not going to get any gain at all in sharpness. Too bad.
∑ My L series lens with IS beats my non-IS 50mm prime lens.
∑ Setting Luminance smoothing to about 20 gets rid of the noise in the image and is what ThumbsPlus seems to do on image import; in Lightroom the default is 0 smoothing so if you are importing and there are large underexposed sections of uniform color, you might want to smooth it.
∑ David Murray wrote: In your first note, regarding "wide open is not nearly as sharp", you are correct from an optical perspective. But aperture influences sharpness less than motion blur and high ISO noise reduction. So if you are shooting a moving subject in low light, you are much more likely to get a sharp looking image of that subject by shooting wide open on an f2.8 lens at 1/100 sec than using f4 at 1/50th or f5.6 at 1/25th on that same lens. The risk of motion blur at 1/50th far outweighs the benefit of stopping down. So rather than say that you are not going to gain sharpness "in general" by shooting wide open, you might instead say that you can gain in sharpness by stopping down, as long as you can still use a fast enough shutter speed and low enough ISO for the shooting context.