Author Topic: "Focus Point Revisited," Weekly Photography Ass. for Dec. 28 - Jan. 8, 2012  (Read 2279 times)

Michele

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I was walking outside and I was thinking about Spirit and all the things I learned since I joined.   Then I realized how much information I had forgotten.   When I first joined, there was an assignment Keith put together that really challenged everyone.  So I thought we should do it again, I know I could use a refresher!

Here is Keith's assignment from March 2010 revisited!

One common question photographers ask when photographing "grand scenic" or landscape images is "where should I focus?"  I've wanted to provide a simple answer to this question for some time, but have been struggling with how much background information I need to include in order to provide a sufficient answer.  Like many areas in photography, there is a "simple" answer, and a "more accurate" answer.  I always strive to reach a balance between "simple" and "accurate," which is sometimes a challenge.

The simple answer is "focus on your primary subject."  I will attempt to provide a more accurate answer in the discussion that follows.

We should start our discussion (and I hope you will ask questions) with a few basic concepts, starting with Depth of Field.

Depth of Field:  Depth of field is defined as a zone of acceptable sharpness within an image.  There is always a flat plane running parallel to the sensor/film plane of your camera at which everything is in sharp focus.  (Tilt lenses are an exception to this rule that we can talk about later.)  The focus scale on your lens determines the distance of this focal plane.  There is also a zone in front of and behind this plane where although the image is not perfectly focused it is still "acceptably sharp."  This zone of "acceptably sharp" is defined as the depth of field for that lens focal length, selected aperture and focus distance.  It is the combination of those three variables (focal length, lens aperture and focus distance) that determine the zone of acceptable sharpness (depth of field).  

We should also take a moment to talk about "acceptable sharpness."  It is important to understand that the guidelines for depth of field were determined in the 1950s when camera optics and film resolution were much more limiting with respect to the level of image acuity that was possible.  Although the criteria for "acceptable sharpness" varied slightly between manufacturers (and still does) the general guidelines were established so that a person with "average" vision would perceive the detail in an 8x10 print to be acceptably sharp if all of the objects reproduced in the print were within the defined depth of field of the lens.  I'm providing this background so that you understand that "acceptably sharp" as defined in the 1950s is generally considered to be less stringent than our perception of "acceptably sharp" today.  

You will generally see depth of field documented in three ways.  Although more and more rare these days, some prime (single focal length) lenses still have depth of field scales on the lens.  It's worth picking up one of these lenses on Ebay if only to play with the lens to see how depth of field was determined in the "old days."  You will also see depth of field conveyed in a depth of field chart that lists the near and far distances that define the range of acceptable sharpness for specific focal length, aperture and focus distance combinations.  Although useful, this method of conveying depth of field requires lots of numbers to convey all the possible combinations.  The third method of addressing depth of field is through the use of a hyperfocal distance chart, which is the method I find most useful for determining depth of field for landscape images.

Hyperfocal Distance Chart:  The hyperfocal distance chart is an attempt to define the optimum focus distance for a given situation (focal length and lens aperture) thus answering the question "where do I focus."  

The chart shown below is for a digital SLR with an APS size sensor and provides hyperfocal distances for lens focal lengths from 18mm to 60mm.  (You can find many online versions of this chart, just search for "hyperfocal distance chart" and then fill in the focal lengths you want to calculate hyperfocal distance for.)  For any given focal length and aperture, the hyperfocal distance is defined as the focus distance at which everything from half of the hyperfocal distance out to infinity will be acceptably sharp in an 8x10 inch print.  Focusing closer than the defined hyperfocal distance will result in objects at infinity being outside the range of acceptable sharpness, while focusing farther than the hyperfocal distance will result in closer objects being out of focus without any appreciable increase in sharpness of the objects at infinity.  (Again, the hyperfocal distance is an attempt to determine the "optimum" focus distance for a given focal length and aperture combination.)  For example, reading the chart below, if an 18mm lens set at an aperture of f11 is focused at the hyperfocal distance of 4.94 feet, everything from 2.47 feet (half of 4.94 feet) to infinity will be acceptably sharp.  Conveying the information in this manner allows us to build a much more concise chart for use in the field.

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Hyperfocal Distance (in feet) for APS-C sensor


We can see from the chart that depth of field is drastically reduced as the lens focal length gets longer.  For a 50mm lens set at the same f11 aperture and focused at the hyperfocal distance of 38.2 feet, depth of field is from 19.1 feet (half of 38.2 feet) to infinity, which is not sufficient to include everything in focus in a typical “scenic” picture.  It is because of this relationship between focal length and depth of field that most “grand scenics” are taken with wide angle lenses.

We should probably take a moment to talk about how aperture affects the depth of field.  By selecting a smaller lens diaphragm opening (the larger "f" numbers like f11 and f16) you are in effect limiting the light to a smaller “cone angle” as it approaches the film/sensor plane. This will result in an image that appears to be in sharper focus than an image that is composed of light with a larger cone angle.  So for a lens of any given focal length, a smaller lens diaphragm opening (larger "f" number) will result in a larger zone of acceptable sharpness, while a larger lens diaphragm opening (smaller "f" number, i.e. f5.6) will result in LESS depth of field.  

There is one important caveat to this rule.  When the lens opening becomes too small, a greater proportion of the light traveling through the diaphragm opening will be diffracted (bent) as it passes by the sharp edges of the aperture blades.  This resulting diffraction will result in a less sharply defined image at the film plane, thereby making the image appear blurry when compared to an image with less diffraction.  A general rule of thumb for a camera using either an APS or "full frame" sensor is to stick to an aperture of f11 or larger (meaning a smaller "f" number), unless you absolutely need more depth of field for your image to be successful.  I very rarely use apertures smaller than f16, because I know that image quality will be severely degraded at these smaller apertures.  If a situation does arise where I need to use a smaller aperture in order to get everything in the scene acceptably sharp, I will use focus stacking instead.  (Please see the front page of the site for a link to a tutorial on focus stacking.)

As mentioned previously, three factors that affect depth of field are aperture, camera to subject distance, and focal length of the lens.  To reiterate, depth of field diminishes as apertures get larger ("f" numbers get smaller), focal lengths get longer, and focus distances decrease.  So if you wanted to blur out the background in an image, select a large aperture (small "f" number), a long focal length (50mm or longer) and get close to the subject.  If you want to retain detail in the background, select a small aperture (larger "f" number), wide angle lens, and don’t get too close to your subject.

Since our objective in this discussion is to determine "where you should focus" for landscape and scenic images, I will assume that you are primarily using wide angle lenses.  Since a "grand scenic" typically includes "near," "middle" and "far" elements and requires significant depth of field, using a lens with a focal length greater than about 35mm will result in insufficient depth of field and will require a different focus strategy.  (Actually a much simpler strategy is required for longer focal length lenses, which is essentially "focus on your primary subject."  )

So now that I have provided background information on depth of field and hyperfocal distance, how do we apply this in the "real world?"  First of all, I always try to keep in mind that my perception of "acceptable sharpness" is much more stringent than what is reflected in the published depth of field and hyperfocal distance charts.  However, these charts provide a very good point from which to start.
 
I can compensate for the differences in "acceptable sharpness" by using an aperture setting that is one stop smaller than the one on my chart.  So for example, if I'm using a 28mm lens and the closest object I want to be "acceptably sharp" is 6 feet away, the chart tells me I should use an aperture of f11.  Since I know that the resulting image won't render near and far objects quite sharp enough to meet my criteria of "acceptably sharp," I can compensate by focusing on the hyperfocal distance for f11, but setting an aperture of f16 which is one stop smaller and provides more "margin" for my depth of field.  An aperture of f16 extends the "near" limit of acceptable focus to 4.23ft (half of 8.46ft), putting my near object (which for this example is at 6ft) well within the range of "acceptable sharpness."  Make sense?

But the bottom line is that referencing the hyperfocal distance chart helps me to understand how limited the situations are where I can reasonably expect to have "near," "middle" and "far" objects rendered with acceptable sharpness in the final image.  I can only expect to achieve reasonably sharp "grand scenics" if I am using a lens with a focal length wider than 28mm, and my near object is at least six feet away from the film plane of the camera.  If I want to get closer to my near object in order to emphasize the "depth cues" provided by a "near" object in the foreground of my image, then I need a wider focal length lens.  Or, I need to make a conscious decision that I will focus closer and accept some degradation in the sharpness of the image at infinity.

So let's talk about the option of focusing closer for a minute, and how that might come to play in the "design" of our image.  For many years, photographers that specialized in "grand scenic" images attempted to produce images where every object, from near objects to objects at infinity, were sharply defined within the image.  As film and optics became better and better (and techniques such as focus stacking became viable) photographers were finally able to reach this goal.  However, it became obvious to some that sharply defining objects at infinity had the undesired effect of removing one of the visual cues that we subconsciously use to determine distance.  

Intuitively, we understand that a distant object in a scene will be "softer" due to atmospheric distortion.  We use this "softness" to help us gauge the distance of far away objects.  So, attempting to overcompensate and reproduce this portion of the scene as "sharply defined," can sometimes be counterproductive if one of our goals is to establish the overall setting of our image.  Reproducing objects at infinity with an unnatural sharpness removes one of the visual cues that we use to convey a two dimensional image as a scene with "depth."  My point is that some softness at infinity is not all bad.  

The level of "acceptable" softness will be determined by the importance of those distant objects in the overall scene, and the degree of magnification necessary to reproduce your image at the desired size.  (A larger reproduction, viewed from the same distance, will require sharper objects at infinity.)  In general I have found that my "one smaller aperture" adjustment on the hyperfocal distance chart provides enough acuity to keep distant objects effectively sharp for images up to 20x30 inches.  This of course assumes that I am using a high quality lens that has the ability to render fine detail for distant objects.  If the image will be reproduced smaller (such as on the web) I can get by with the listed aperture and no adjustment.

Given the choice of having closer objects in the scene out of focus, or having distant objects out of focus, I will always opt for the distant objects being out of focus and attempt to keep the near objects acceptably sharp.  This is much more consistent with our natural way of "seeing" the world.

So with all of these variables, what is my strategy for determining where I should focus?  Well, there's one other factor in human visual perception that I use to determine my focus distance and construct my image.  I know that if just one dominant object in the scene is sharply defined at the plane of focus, then the viewers' overall perception of the image will be that it is well focused.  Keeping in mind the depth of field available for my chosen focal length and the adjustment to the chart I will make based on my more stringent requirements for "acceptable sharpness" (by setting the lens to the next smaller lens diaphragm opening on the chart), I will determine the hyperfocal distance and then adjust my camera position in order to place a primary object within the scene at the hyperfocal distance, which will be the actual focus setting on my lens.  In this way I ensure that the viewer perceives the primary subject as being very well defined, while keeping the remainder of the scene "acceptably sharp."  Sometimes this technique may required a bit of "iteration" as I refine the focal length and distance from my "near" element in order to compose a pleasing image.  

The image below is one example where I placed my "near" elements at the hyperfocal distance for my selected focal length and aperture.  In the full size image the sharply defined detail in the ropes imparts a perception that the image is "tack sharp," while the depth of field available at 17mm and an aperture of f11 ensures the rest of the image is rendered "acceptably sharp."


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To summarize, my recommended strategy for focusing a landscape or grand scenic image is to use a hyperfocal distance chart to determine the optimum focus distance for a given lens focal length and aperture, adjust your aperture to the next smaller lens diaphragm opening if your idea of "acceptably sharp" is different than what was defined in the 1950s, and then adjust your camera placement (and focal length if needed) to place a primary object in your scene at the hyperfocal distance.  Obviously using a tripod will help tremendously, both because you will be using smaller apertures, and thus longer shutter speeds, and because it will help you to focus on the subject at your hyperfocal distance and then recompose your scene without changing the focus distance.  After a time, judging the acceptable limits of depth of field for specific focal lengths will become more intuitive, and you will need to consult the hyperfocal distance chart less often.  
 
For those of you that have survived this far, congratulations, and thank you for sticking with me.  Your assignment for this week is to compose a scenic or landscape image while consciously determining where to focus in your scene in order to attain the desired level of acceptable sharpness throughout the image.  Using a hyperfocal distance chart will help in this endeavor.

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So there you are!  I am sure it will challenge you as it still does me.  May I suggest that you save the little chart for future purposes.  Please upload your photos into the "Focus Point Revisited" folder.  

Have a great New Year's!

Michèle

« Last Edit: December 28, 2011, 11:59:14 AM by Michele »

keithsnell

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Thank you Michele.

Michele

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You're welcome, Keith.  (Did we not talk about that?)  I'm a little late because I've caught some sort of throat thing.  I'm feeling a bit better, so I will be more on the ball again.  I re-used your assignment because, at the time, it was one of the hardest assignments to me.  It was one of the first ones I printed out in my Spirit of Photography binder.  I still have my little card in my photo bag.  It's the kind of assignment that needs to be reviewed from time to time.

Have a great New Year's, Keith!  
« Last Edit: December 28, 2011, 12:00:34 PM by Michele »

Cindy Miller Hopkins

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Always a good reminder of important information! Thanks, Cindy