Focusing issues – Hyperfocal distance

Focusing issues – Hyperfocal distance

Hyperfocal distance is the art in photography of achieving as much sharpness as possible throughout the image. In other words, it involves careful focusing adjustments to ensure that objects close to the camera and objects in the distance all have the same sharp focus.  More accurately, the hyperfocal distance is that point of focus where things are in focus from a point half way between you and the focal point all the way onward to infinity. Hyper-focal distance is more of a landscape photographer’s concept. When shooting landscapes, I never use auto-focus because this could interfere with some important focal point decisions.  You could use Auto-focus to set your focal point if an object is at the correct hyperfocal distance.

Circle of confusion

Although the Hyperfocal distance can be stated mathematically as an exact adjustment, lenses are rarely so perfect and are likely to vary n different temperatures. Also, we have the concept of ‘acceptable sharpness’ which related to the ‘Circle of confusion’.  The circle of confusion is the point at which a pixel in a digital image goes from being a dash (sharp) to a circle (un-sharp).  If your DOF doesn’t cover the whole scene, then the circle of confusion will set in at some point into your image.

Acceptable sharpness

Acceptable sharpness

The above image has interest points in the immediate foreground and also on the horizon, so hyperfocal distance was important for this shot. However, even at f/16, the distant ship and the foreground rocks are only just acceptably sharp. The hyperfocal distance for this image is out in the sea. In my opinion, it would have been better to take a shot focusing on infinity and then a shot focusing on the rocks and blended them together (see here). The sea is the part of the image with the best focus and yet it doesn’t really matter if this is sharp or not.

The perfect lens doesn’t exist although there are some very good ones. Typically lenses become soft as you deviate from the sweet spot (best aperture for sharpness) the sweet spot is often in the middle of the lens’s aperture range say around f/8, but some say that it is 2 stops down from fully open. So, as a consequence we might stop down to f/16 or f/22 to achieve a greater depth of field and thus sharpness throughout the image, but because we have deviated so far from the sweet spot, we are not really getting the best sharpness from the lens. All we can really say is that near objects are as sharp as distant objects. Lenses often degrade in quality as you deviate from the center. The outer parts of an image are often not as sharp as the central parts.

Acceptable sharpness comes into play when we consider how an image is going to be viewed. For example, reducing an image for the web will increase the apparent sharpness, whereas printing it large will have the opposite effect. Consider objects on the horizon. If you calculate the hyperfocal distance accurately, you should have perfectly sharp objects on the horizon. But, if those objects are just a few pixels across, then there is little point in achieving this absolute sharpness.

 

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Infinity Focus

Focus on infinity

Focus on infinity

The above picture was focused just short of infinity leaving the foreground boulders fairly out of focus. As it was night, I couldn’t use a small aperture. This was taken with f/5.6. For me the importance was the lights in the distance. There was no way I was going to get a balanced exposure with good detail of the boulders, so I chose to have them as un-sharp masses, they just add to the depth but their sharpness isn’t important.

Rule of thumb

In the days of film, lenses used to be pre-marked with hyperfocal information. For example if you were using f/11 aperture, there would be a mark on your lens where you align the infinity sign to f/11 and this would give you the hyperfocal distance. For some reaon this is missing on modern digital lenses although I do have hyperfocal marks on my Canon 24mm 1.4L.

If you don’t have the markings on your lens and you want a quick rule of thumb to give a rough hyperfocal distance, simply focus a third of the way into the scene.

Ok, so if you are still reading this you maybe have a near perfect lens and a huge mega-pixel camera, and so the achievement of absolute sharpness does matter. I suggest you browse the technical pages from the following links.

www.dofmaster.com
www.trenholm.org

 


Advantages of a prime lens for composition

Advantages of a prime lens for composition

Prime lenses are lenses with a fixed focal length. This article looks at the advantages of photographing with a fixed focal length. Choosing the correct focal length is important for effective composition.

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