Sunday, July 15, 2012

Hyperfocal Distance for Landscape Photography


A Tutorial & Simple Calculations



What is Hyperfocal Distance?

In landscape photography, the question we repeatedly ask ourselves as we are composing an image is:
w h a t      w i l l      b e      i n      f o c u s ?

We frequently attempt to control the depth-of-field of focus of our image such that the image is in sharp focus at a far distance (e.g. infinity) and also sharp up to a desired point much closer to the position of the camera.
Note: this is just the opposite of what we want when creating portraits that typically benefit from a shallow range of sharp focus.

When your camera’s focus point is at what is known as the Hyperfocal Distance (HD) it is set for the greatest possible depth-of-field for a given set of lens settings and the camera system. Everything from about ½ of the HD to very, very far away will be in good focus. Consider a hypothetical example in the cartoon below that depicts this. Let's say you are hiking in the mountains of Tibet and come upon a few photogenic high mountain oxen in front of a lake and you want the oxen, lake and mountains to be in focus. If you focus at the HD point, the range of acceptably sharp focus will be from the first ox to the distant mountains.


Of course the challenge is to set the HD so everything you want sharp is in focus, determine where the HD point is located and then to put your camera’s focus point there. This sounds complicated but learning to do this quickly is the topic of this tutorial and will add strength to your images!


Why should you care?

Think of the mountain landscape described above with oxen in the foreground, a lake behind them and mountains in the distant background – and you want to have all of it in focus. If you put your camera focus point on the first ox, that ox will certainly be in sharp focus. However, it is likely the distant background will be out-of-focus as seen here in Image-1. This limited range of focus is sometimes desired, but for landscapes we generally want the background in sharp focus too.


Image-1: Focus Point is Closer than the HD Point


Now think of the same mountain landscape scene but with the camera's focal point far back in the water behind the oxen. This moves the range of focus further back, fixing the sharpness of the mountains, but taking the first ox out of sharp focus as seen below in Image-2. Again this is sometimes desired, but generally a good landscape image tries to keep all important objects in focus.




Image-2: Focus Point is Further than the HD Point


Using knowledge of the HD, we can have substantial control over what range of depth will be acceptably sharp to our eye in the final image. Focusing the camera at the HD point just behind the oxen at the edge of the water and adjusting the lens aperture so that the oxen are at least ½ of HD away from the camera, we can maximize the range of sharp focus. The result is the entire image being in focus, typical of a landscape image, as seen in Image-3 below.


Image-3: Focus Point is at the HD Point- everything is in sharp focus


Establishing the range of sharp focus is one of the most important elements you can control leading to a great landscape image. Next we will discuss how to adjust your camera to attain this result.


How does HD work?

As we know, the depth-of-field of an image is determined by the aperture of the lens and its focal length. It is also dependent upon the size of the image sensor in your camera. A larger lens f-number (smaller aperture) produces a greater depth-of-field. The depth-of-field is also a very strong function of the lens’ focal length - the smaller the focal length, the greater the depth-of-field and vise-versa. Finally, a smaller image sensor will produce a greater depth-of-field – which is why tiny cell phone cameras typically produce pictures with everything in focus, although they have other significant limitations.

You have reasonable adjustment of three of the four aspects of this camera system that determine the resulting depth-of-field in your image: the aperture, the lens focal length, and the distance from the camera to the focus point. You don’t have control over the fourth element, the image sensor size, unless you are willing to buy a new camera body!

The property of the image sensor that affects depth-of-field is called the “circle-of-confusion” (CoC). The CoC is the size of the largest blur-spot that the human eye will still consider to be a “sharp point” (rather than a blur) when viewing an 8x10 inch print at a typical viewing distance of 25 cm (or about 10 inches). As the CoC becomes larger, the smaller the depth-of-field that is possible from the image sensor.
Table-1 below describes the relationship between sensor type and CoC. Most consumer Digital SLR cameras on the market today use an APS-C sized image sensor, while most small digital cameras that will fit in your pocket use an image sensor closer to 6mm x 4mm in size. Not shown in the table are cell phone sensors that are approximately 4mm x 3mm in size.

Camera Sensor TypeSensor Size (mm)CoC (mm)
Full Frame36.0 x 24.00.029
APS-C23.6 x 15.70.019
Four Thirds18 x 13.50.015
Point & Shoot6.2 x 4.10.005

Table-1: Sensor Specifications


The knowledge to take away here is that the larger the sensor (and correspondingly the more money you paid for the camera body), the larger the CoC and the smaller you can make the depth-of-field. This greater adjustment control over depth-of-field with a larger sensor allows you to place the range of focus more precisely and thus control where the viewers' eyes will be drawn.


Caution – here comes the Math!

In the “olden days” of film, lenses were mostly primes (i.e. a single focal length, not a zoom) and there was a scale on the focus ring that indicated the near and far ends of the range-of-sharp-focus. Now with zoom lenses with a fixed barrel (where there is only internal movement of lens elements for focus), we don’t have the depth-of-field scales anymore on the focus ring. To know the range of focus, you must compute the HD for your particular camera + lens settings. You can do this with a little math or refer to a chart (we’ll help with that a little later too). The HD calculation only requires three numbers: the focal length (FL) of the lens, the lens aperture f-number (Fn), and the circle-of-confusion (CoC) for the camera sensor (from the table above). And with these numbers, here is the equation to compute HD:

HD = (FL x FL) / (Fn x CoC) + FL
(Equation-1: Hyperfocal Distance)


Now if you focus your camera at a distance equal to HD, then the range of acceptably sharp focus is from approximately ½ x HD to Infinity. Let’s do an example using an APS-C camera, a zoom lens adjusted to a focal length of 100 mm, and the lens aperture set to f/8. With these numbers, then:

 HD = (100 x 100) / (8 x 0.019) + 100 and doing the algebra HD = 65,889 mm or HD = 216 feet

So if you now focus your camera at a point that is about 216 feet away, everything from about 108 feet away (which is ½ of HD) to Infinity will be in focus.
But what do you do if you want the object that is only about 50 feet away to be in focus? Well, looking at Equation-1, you have three options to get that object into focus:

   #1) Focus the camera at a point that is closer to the desired object as in Image-1. However, this will cause the
         image at a very, very far distance away to be a little out-of-focus

   #2) Change the lens f-number (or f-stop) to a larger value (which is a smaller aperture). If you doubled
         the f-number to f/16, then everything from about 54 feet to Infinity will be in acceptably sharp focus.

   #3) Change the focal length of the lens to a wider-angle view (or lower zoom). If you reduce the focal length to
         70 mm, this would result in everything from about 53 feet to Infinity being in acceptably sharp focus.


Simplifying the Math with a Chart

Wow – that’s a lot of math to be doing when you just want to make sure that your foreground and background are both in focus! Well we have a quick-reference chart in Chart-1 below for you to use that will allow you to estimate HD in a couple of seconds. It covers focal lengths of 10 mm to 600 mm with apertures of f/2.8 to f/32




Chart-1: Hyperfocal Distance vs. Focal Length for Aperture Series


Looking at the Log-Log chart above, you can see that intuitively it describes what you would expect to see: the bottom line that represents an aperture of f/32, has the greatest depth-of-field (or shortest HD) and similarly the top line of aperture f/2.8 has the smallest depth-of-field (or largest HD).

In Chart-2 below, we highlight the two options of #2 and #3 above to reduce the HD by a factor of two and thus extending the depth-of-focus. The red-arrow lines shows the original 100 mm focal length and f/8 aperture and how they lead to an HD of about 200 ft. The green-arrow lines shows the result of changing the f/stop to f/16, resulting in an HD of about 100 ft. Finally, the blue-arrow lines show the result of reducing the focal length to 70 mm and keeping the aperture at f/8, also producing an HD of about 100 ft. In both of these solutions (green & blue arrows), we have extended the focus range from about 100 ft to Infinity to about 50 ft to Infinity, producing a great landscape image!


Chart-2: Two Solutions to Extending the Depth-of-Focus


These HD charts were created in an Excel worksheet model that allows you to change the focal lengths, sensor type, and apertures for analysis. If you would like a copy of this worksheet, just request one at our email address:
 FocusInParadise@gmail.com


Conclusions

We hope this tutorial has provided some insight into how Hyperfocal Distance has a significant impact on your landscape images. With this knowledge you should be able to more precisely adjust your camera to define the desired range of focus that will be sharp to the viewers' eyes. Use this technique to keep objects important to the image in sharp focus and to put the other objects out of focus.

In a future blog post, we will discuss how to compute any depth-of-field, not just one that reaches to Infinity. This will be very useful for portraits and macro images.

Happy shooting,

Focus In Paradise

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