Achieving Sharp focus in photographs

Firstly, let me share this quote; “Sharpness is a bourgeois concept” – Henri Cartier-Bresson.  While sharp focus is important we often overemphasize the concept today.  Our lenses today, even those consumer level versions, are quite capable of achieving sharper focus than their predecessors and often beyond what we really intend, think portraits where sharp focus shows every blemish and wrinkle in the skin in great detail.

Understanding focus modes

Before we jump into the “how to” I believe it is important for us as photographers to have a basic understanding of the focus modes available to us.  Typically, these modes fall into two categories, manual and auto.  Where things become more complex is when we use autofocus.  In autofocus, we have a number of options.  On my Nikon D750, in “Live View” we haveAF-S (single-servo) AF-F (Full-time-servo), then we add “Face-priority”, Wide-area AF, Normal Area, Single Tracking.  While looking through the viewfinder, non-Live View” we have AF-A (auto), AF-S (single), AF-C (continuous).  We then break those down into a Single point, Dynamic-area with subsets of 9-point dynamic, 21-point dynamic, 51-point dynamic, which also adds 3D tracking group area and auto area.   Whew, who knew autofocus was so complex?

Next comes understanding how our camera focuses.  Here is a somewhat simple explanation from the website “How stuff works“;

Passive autofocus, commonly found on single-lens reflex (SLR) autofocus cameras, determines the distance to the subject by computer analysis of the image itself. The camera actually looks at the scene and drives the lens back and forth searching for the best focus.

A typical autofocus sensor is a charge-coupled device (CCD) that provides input to algorithms that compute the contrast of the actual picture elements. The CCD is typically a single strip of 100 or 200 pixels. Light from the scene hits this strip and the microprocessor looks at the values from each pixel.

Is it important to really know how each of these different modes of autofocus works?  Yes and no.  This is a time when reading this part of your owner’s manual works.  In most manuals I have read, the manufacturer gives examples when to use each mode.  I will also say, I don’t use each and every mode of autofocus.  I do make use of AF-S and AF-C.  If my subjects are stationary it is AF-S, if moving AF-C at which time I often utilize the 3D tracking.  Is it possible that these choices will work for you?  Yes and no.  It all depends on you, your technique and your style.  Again, this is where reading this portion of your owner’s manual will work best.

As you can see from the photograph below the manuals of my most used cameras are well used.

The technique

Ah, the meat of this article you may think.  The previous section is just as important.  We often blame the camera or our lens but often, in reality, it is our fault we didn’t get sharp focus.  One of the complaints I hear most often is blaming out of focus or blurry images on a so-called “kit lens”.  In a previous article, I showed you how a kit lens is quite capable of achieving sharply focused photographs.

Before I get too far into the “how to” let me define, for clarity, a couple of terms.

  • Out of focus –  When I talk about out of focus I am referring to an image or portion of the subject that is NOT IN FOCUS as a result of a missed focal plane, think a person without their eyeglasses.

 

  • Blurry – When I talk about a blurry photograph I am referring to an image or portion of the subject caused by camera or subject movement.

Use the appropriate shutter speed

One of the most common issues I see is often caused by camera shake or subject movement, these two things are what typically cause a photograph to become blurry.  While we may think 1/60th of a second is fast it may not be fast enough to prevent camera shake or subject movement.  Think about the expression, in the “blink of an eye”.

An easy way to ensure you are using a shutter speed fast enough to minimize camera shake is to use a shutter speed equal to or faster than the inverse of the focal length of your lens.  For example, if you are using a 50mm lens your shutter speed should be, at least, 1/50th of a second, if you are using a 200mm lens your shutter speed should be, at least, 1/200th of a second.

Also, consider the speed of your subject.  Take the graphic above as an example.  If you are photographing a person running very fast you may need a faster shutter speed.

So, how do I know how fast the shutter speed needs to be to freeze the motion of my subject, you ask.  Consider the graphic above,  If your subject is running at full speed you may need 1/1000th of a second to freeze the action.  If they are merely walking at a normal pace you may be able to freeze the motion at 1/125th of a second.  This is where practice ahead of time pays off, you gain an understanding of how you can do freeze motion.  Don’t wait until the day of or the day before an important event to practice for the first time.

Use an appropriate aperture.

So many people believe that when they buy a fast lens, f/2.8 or wider, they MUST shoot “wide open” (in other words the widest aperture) to achieve that elusive look of “bokeh”.  This isn’t true.  You DON”T have to shoot wide open, in fact, I very rarely shoot at the widest aperture of my lenses.  Shooting wide open often leads to a depth of field too shallow, resulting in your subject to become too much out of focus.  The photograph above was taken at f/4 with a 70-200mm f/2.8 lens from approximately 15 feet away from the subject.  Even at f/4, sharp focus starts to fade towards the back of the subject.  Had I used a wider aperture I would have begun to lose focus on more of my subject.

The depth of field is affected more by the focus distance to your subject than by aperture.

The photograph of my dog above was taken with the same camera, same lens, and also at the same aperture (f/4).  What changed was the distance to my subject.  The photograph of my dog was taken at about half the distance as the first portrait.  Now the dog’s nose is out of focus as well as just behind his eyes.  The depth of field is much more shallow.

The photograph above of the flower was shot at f/11 from a much closer distance, approximately 12 inches.  Notice the edges of the flower itself are out of focus and the elements behind the flower are completely out of focus.

The key?  Learn to control your depth of field.  Controlling depth of field is understanding the relationship between aperture focus distance.  There are many wonderful tools to calculate the depth of field.  Additionally, practice the technique by using the same aperture and moving closer and further from your subject.  Adjust your aperture to a smaller opening (larger number) repeat the process.

Controlling your focus plane.

We often call it a focus point but in reality it is a focus plane.  The only portion of an image that is in sharp focus is the plane where the focus point lands.  As an example, if you focus on an object at 10′ 5″ everything at 10′ 5″ is in focus.  At 10′ 6″ or greater objects are falling “out of focus” and at 10′ 4″ or closer everything is falling out of focus.  Depth of field refers to everything within the “depth of field” as acceptably sharp.

Don’t allow your camera to chose the area of focus.  The camera, more times than not, gets it wrong.  You need to chose the area of focus.  This doesn’t mean you have to use manual focus.  You can use auto-focus AND choose your area focus.  You can learn how by studying your camera’s owner’s manual and using a single point focus.  Your manual will describe preciously how you can do so with your specific camera.

Conclusion

The key to obtaining sharp focus is understanding some basic concepts such as depth of field, reducing camera shake, and stopping motion as discussed above along with understanding how to control your camera.  When reading articles, such as this one, it is vitally important to have your own camera owner’s manual and your camera.  This way you can read the concepts and determine how you can make your camera work the way you want rather than the way it wants.

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