Photo IQ - Week 5 -- Focus and Focusing

Focus, people! We are getting close to the end of the series!
We are on part 5 of my 6-part series on what makes up Image Quality. Photo IQ.
Why, you ask, did it take this long to tackle something so basic as focus? I'm blaming global climate change.
Why bother talking about focus at all? It seems fairly obvious that when your subject is out of focus it will not be a quality image. True. But there is more to focus than just pushing the shutter button halfway down--although that is baffling enough for many.

How focus works:
The auto-focus system in today's cameras works on a simple principle. It evaluates the scene and finds areas of high contrast. By comparing the line between the light and dark portions it makes a decision on the point where the object has the highest contrast. Point and shoot cameras use the sensor to determine the focus of the entire image while DSLRs evaluate on a much smaller scale at many different points in the frame, that is why they are more accurate. If you want more detail on how these work, this is a great article.

This method typically works well until the light gets low and the camera can no longer discern between two different areas of value. At that point, the camera begins to hunt for focus and may eventually just stop.
In the dark when there is no contrast, your camera should have an option to fire a focusing flash. It may fire automatically or you can trigger it yourself. It fires a rapid succession of small flashes that allow the camera to find the contrast it needs.


Types of auto-focus:
Single Frame: The camera focuses when you push the shutter halfway down. It's the best option for subjects that probably won't be moving since when you reframe the shot the focus will already be fairly close and your chances of consecutive shots being focused correctly are higher.

Servo: The camera tracks the subject through the frame. It is one of the main reasons to upgrade to a more expensive camera since a truly effective servo focus is expensive and technologically complex. If you are going to shoot sports or anything that continually moves, you are a prime candidate for expensive cameras.

Hybrid: The different camera companies call it different things but it is typically single frame focus mode but the camera reserves the right to refocus if it detects that the subject moves. I rarely use the hybrid focus (AI Focus on my Canon) because I don't want the camera making the decision on what the subject is. If it's moving, I'll use Servo, if it's not, I don't want it switching focus on its own.

Auto focus options
Typically there are a couple options for autofocusing: Fully auto, and manually choosing a point.
It's typically more effective to choose your own focus point since the camera doesn't actually know what you want to focus on. It will typically focus on the front and center. If you're shooting past something you will almost always miss focus when you let the camera decide.
In a point and shoot you have much deeper depth of field because of the small sensor but the same concept applies. Most have a focus area of the screen that you can move around if you want to focus on something off-center. These days most p&s cameras have face-detection too, which is awesome. It searches for anything it thinks is a face and focuses there. It's quite handy and typically works very well. (My G11 even finds photos of faces that are minuscule.)

As far as Image Quality goes, focus is crucial.
Take the time to stop, brace yourself and squeeze the release. There is a lot of resuscitation you can do on missfire images in Photoshop but focus is not one of them. Trying to focus a missed image in software is mostly an exercise in futility. If your original file is big enough you can probably make it passable for web use but you simply cannot focus an unfocused shot after the moment is gone.
If you miss something, don't let it get you down. We all miss focus sometimes, especially when shooting with faster, riskier apertures. (At 6 feet away, f 1.8 and 50mm, the total depth of field is under 4 inches vs. about a foot at f4.)
If you are consistently frustrated with your focus, there are a few things you can do.
First, check your shutter speeds. A little bit of motion blur looks an awful lot like missed focus. Typically, if the focus missed, something in the frame will be in focus. If you can't find anything, perhaps you or the subject moved. I have seen a HUGE rise in the percentage of usable images since I started paying a little more attention to my shutter speeds. Often times we switch to aperture priority mode and just leave it while the light fades. Before long we are shooting at 1/30 or 1/20 of a second and the images end up blurred.
Second, same concept, different issue. The general rule is that you can hand-hold (not use a tripod) at shutter speeds equal to your focal length (lens zoom.) If you are shooting at 100 mm (3x zoom on a point and shoot) your shutter speed needs to be 1/100 sec or above. Don't forget to take into account the lens crop factor! If you are shooting with a standard aps-c sensor in a dslr (Canon 50d, 7d, Rebels, etc. Nikon d40, d60, d90, d300, etc.) you need to be at 1/150 for the same zoom. Image stabilization helps a lot so experiment at different shutter speeds to find out what your IS/VR will do.
Third, check your lenses for front or back focus. Sometimes a lens or a lens/camera combination isn't quite calibrated correctly. When your autofocus consistently misses there are ways you can test to see if it's a hardware issue. Some cameras allow you to actually adjust the focus plane to make your autofocus more accurate.
Here are a couple of methods to check out:
The complicated way
The less-complicated-yet-still-somewhat-complicated way
The second involves this graphic and the moire of your screen. Don't worry, it explains that too.

So what If you DO screw one up? Nothing can really fix bad focus but my method is pretty simple. Make it relative. I will often use a motion blur or lens blur in photoshop to put the REST of the image so out of focus that in comparison to it, the subject looks okay. It's not a perfect solution but it's about the only option I've found.

As always, I will delve more into these concepts as time goes on. Thanks for reading.
Feel free to let me know what subjects you'd like discussed in the future!

End of Line on week 5.