Wide or Narrow Apertures -Depth of Field versus Diffraction

Adjustments to aperture, ISO and speed all have different advantages and disadvantages, so understanding what happens with each element of the triad is important in understanding which one you want to adjust when setting your exposure. In this post I am focusing on aperture.

Narrowing aperture settings to a high f-stop like f-22 runs the risk of introducing diffraction into your image and widening your aperture by setting a low f-stop like f-2.8 decrease your depth of field. So at either end you of your aperture setting you have to consider the advantages and disadvantages of these extremes.

Lower Apertures

When your aperture is at its widest at f-2.8 or f-1.8 your Depth of field narrows, the larger the sensor in your camera the narrow the depth of field. So at the widest aperture or lowest f-stop the advantage or disadvantage is a shallow depth of field.

This can be an advantage as narrowing the depth of field can highlight the subject by blurring the back ground as the following examples illustrate.

The following example illustrated aethsteic desicions you have to make while working with a low f-stop. In this image the the low f-stop means the photographer has to either set the focal point on the house in the background or the hedge in the foreground. Both can not be in focus unless the f-stop is increase and given the low light situation this woudl likely require a tripod to steady the camera well enough to use a higher f-stop.

Infinity Focus

The depth of field of course is not an issue if the things you are photographing are beyond the infinity point of your lens. Here technically what is happening is everything is far enough away that the incoming light rays are functionally parallel and reach the camera’s sensor as points. So everything is in focus throughout the image.

To discover where your lens is at infinity you need to look at the barrel of the lens for this symbol “∞;” Otherwise, you may need to check your manual for the lens online.

Nikon AF-S NIKKOR 24-70mm 

Nikon AF-S NIKKOR 24-70mm 

Higher Apertures

When you are using higher apertures typically above f/11 diffraction begins to make your images less accurate, this inaccuracy is seen primarily as a loss of detail or softening of the image. This does not mean you have a bad lens all lenses have diffraction, it is created by narrowing the opening of the aperture. If you click the following link you will see how as the opening narrows the light becomes more and more diffused making the light reading of the sensor less accurate. (https://upload.wikimedia.org/wikipedia/commons/8/82/DiffractionSingleSlit_Anim.gif)

Although apertures never get as narrow as the illustration suggests it does, however, get wider than the individual pixels on the sensor and at high f-stops begin spilling out of the pixel into nearby pixels. The three illustrations below show a grid representing the pixels on the sensor and the white circle light falling on the sensor. In the first diagram, the light is clean and sharp at f-1.4, at the lenses, sweet spot f-8.0 the light is still within the pixel, but at f-22 it is not spilling out into adjacent pixels. These higher f-stops make it difficult for the pixels to read the light accurately and the consequence of this is called diffraction.

The advantage of being able to use a high f-stop is if there is no other way to get the photograph you do get an image you can use but it may not be quite as good as the camera can get, but it is as good as you can get given the equipment you have on hand. There are ways of avoiding the need to have a high f-stop such as neutral density filters, which one can look into. These reduce the light coming into the lens which allows for a lower f-stop.

References

• Why You Should Stop Shooting with Small Apertures, by A Griffin.
• Lens Diffraction and Photography, from Cambridge in Colour
• Diffraction, Wikipedia
• Infinity Focus — What is it? How does it work?, from 1001 best photography tips
• Understanding Infinity Focus, John Phillips
• Depth of Field, overview by Cambridge Colour