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Focus, Sharpness and Depth of Field

 

 

Cameras and lenses allow the creation of images that are different to those created by the eye – variable ‘depth of field’  is uniquely photographic. The plane of focus of the human eye is fixed at the distance we focus at and falls off rapidly on either end – in front and behind. On one hand, photographic images can be sharper ‘all the way through’ than the eye can see and on the other hand images can be more hazy and less specific than the (well-functioning) eye produces.

 

As Michael Freeman explains, the ‘norm’ is for the main subject of an image to be the point of (sharp) focus – for example, the eyes in a portrait or the main person or feature in a scene. So focus is used to guide the viewer to the centre of interest. Technically, it is therefore very important that where there is a centre of interest it is sharply focused and is thereby ‘sign-posted’ as the most important element in the image.

 

Being able to vary the depth of field (and thereby the figure/ground relationship) gives the photographer options. For example, a portrait may consciously place a person in his or her context by keeping both in focus to thereby equally emphasise (for example) the person and their job or the person can be picked out and the background blurred in order to isolate and emphasise the portrait. Selective focus therefore concentrates attention. The photographer often has choices – when photographing a group of people or objects, should they all be in focus or only a selection? This is both a conceptual and technical choice.

 

Philosophically, ‘Sharply focused photographs carry connotations of specificity and ……. truth and reality. Viewers more readily equate sharp focus with what was present in front of the camera, and the more descriptive it is the greater degree of trust in the accuracy and factualness of an image. Documentary, photojournalistic and evidentiary images rely on sharp focus for this very reason’  (Angela Faris Belt, ‘The Elements of Photography’, page 109).

 

On the other hand, some ‘schools’ of photography like the Pictorialists  http://en.wikipedia.org/wiki/Pictorialism have exploited the camera’s ability to produce ‘soft’ focus images which concentrate of producing ‘expressive’ rather than ‘realistic’ images. The term Pictorialism is used to describe photographs in which the actual scene depicted is of less importance than the artistic quality of the image. Pictorialists would be more concerned with the aesthetics and, sometimes, the emotional impact of the image, rather than what actually was in front of their camera http://www.rleggat.com/photohistory/history/pictoria.htm

 

 

‘Photographs ………. that use soft or distorted focus, remove [the] relationship between the contents of the image and the seen world. They connote a vague essence of things, and an atmosphere or more overtly filtered image of reality. In addition, the softly focused image guides the viewer’s attention more consciously to the act of seeing’  (Angela Faris Belt, ‘The Elements of Photography’, page 109).

 

There are also technical elements – see links below for full explanations.

 

In brief, when a lens focuses on a subject at a distance, all subjects at that distance are sharply focused. Subjects that are not at the same distance are ‘out of focus’ – in the sense that there is a gradual fall off in sharpness in front and behind the point of focus. However, since human eyes cannot distinguish very small degree of unsharpness, some subjects that are in front of and behind the sharply focused subjects can still appear sharp. The zone of acceptable sharpness is referred to as the depth of field and describes the ratio from foreground to background that the image is in focus from it’s ‘point of focus’ – the ratio of depth of field is always in the order of 1/3 in front of the point of focus and 2/3 behind.  (source: http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/depth-of-field.html ).

 

Depth of field is controlled by three things:

 

1. Lens aperture – one factor (the other two remaining the same) is the size of the aperture which lets in light to the film or digital sensor which is indicated by f numbers on the lens - with a wider aperture being indicated by a small f number such as f2 and a narrow aperture by a larger f number, for example f16. The wider the aperture, (the lower the f number) the smaller the depth of field.  Landscape photographers, for example, will commonly want all the ‘scene’ to be in sharp focus.

 

For full explanation of aperture see

http://www.great-landscape-photography.com/aperture.html  http://en.wikipedia.org/wiki/Aperture

http://www.photoxels.com/tutorial_aperture.html .

 

2. The Lens focal length – shorter or wide angle lens have a greater depth of field than long or telephoto lenses.

 

For a full explanation of focal length see:

http://www.paragon-press.com/lens/lenchart.htm

http://www.cambridgeincolour.com/tutorials/camera-lenses.htm

http://en.wikipedia.org/wiki/Focal_length

http://www.photoaxe.com/understanding-the-lens-focal-length-and-aperture/

 

3. Camera to subject distance – the further from the subject distance the camera is, the greater the depth of field.

 

 

Web links on Depth of Field

 

This section relies largely on Angela Faris Belt, ‘The Elements of Photography’ and Michael Freeman, ‘The Photographer’s Eye’. For more technical outlines of depth of field – and it is quite a complex subject - see web links below.

 

For technical discussions of depth of field – at various levels of complexity - see:

 

http://www.luminous-landscape.com/tutorials/understanding-series/dof.shtml

http://www.cambridgeincolour.com/tutorials/depth-of-field.htm

http://www.mir.com.my/rb/photography/fototech/htmls/depth.html

http://www.cs.mtu.edu/~shene/DigiCam/User-Guide/950/depth-of-field.html

http://www.dofmaster.com/dofjs.html

http://en.wikipedia.org/wiki/Depth_of_field

 

The images below use a variety of techniques - some compositional, some technical - to achieve a sense of depth.

In the first image - 'Bathtime in Bhaktapur' - a sense of depth is created by the focal point - the mother and baby - being in the middle 'layer' of the image  between the foreground wall angled towards the subject - a key visual tool as lead-in line -  and a background which is in focus because of the use of an aperture of f18. I made a deliberate choice not to use a wider aperture and so blur foreground and background because the context is a very important part of this composition.

Lead-in lines can also be seen in the portrait of the Cuban tobacco farmer but a wide aperture and narrow depth of field has been used to pick out the Fatah activist and bur both those infront and behind him. The street scene in Havana uses converging diagonals to suggest a recedding perspecting (vanishing point) and the travel magazine image of Nafplio harbour, Greece uses a conventional item of 'foreground interest' (the fishing nets) to give depth.

 

 

 

 


  

 

 

In the images of a Barber's Shop in Havana, a shallow depth of field has been used to concentrate the viewers attention on the key subject - the face of the man in the chair.

 

 

 

 

 

 

 

 

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