DoF


I do hope someone is reading these.

Anyway carrying on from ASA/ISO, I mentioned at the end of the last blog that utilising higher ISO speeds on my DSLR allows me to use more usable apertures even when the light is low. In these days of full automatic cameras I think sometimes the bonus of having control over your aperture is forgotten about.

For the uninitiated the aperture governs the amount of light entering the camera and can be explained as┬áthe ratio of the lens’s focal length to the diameter of the entrance pupil. Because of the ratio the numbers are a little odd and the larger the number the smaller the aperture and vice versa. So a lens with a f stop of 1.8 will have a much larger “hole” to allow light in than say a lens with a f stop of 16.

This image shows it quite well:

640px-Aperture_diagram.svg

This has major influences on the outcome of the image one of which is known as the DoF or Depth of Field. As a kid of 10 I had this explained to me by Dad and my head hurt for days afterwards. Its actually quite easy to grasp once you understand the concept. The aperture effects the rays of light coming in to the camera and the angle at which they strike the film or sensor thus controlling the aperture can limit what part of the image is in and out of focus. A very small f stop e.g. 1.4 gives a very shallow depth of field, basically the area in front and behind the focus point will be almost completely blurred. At large f stop e.g 22 gives a huge depth of field which will result in an image been almost completely in focus.

See the image below courtesy of Big Sun Photography which shows the effect very well

dof

Depth of Field is often used in portrait work to isolate the subject (Small f numbers) or in Landscape work were you really want most of the image if not all in focus (Large f numbers).

Quite often you don’t really get a choice of what f stop you want to use. On a grey murky day at an airshow my f stop can be down to f4 or even lower. This proves very challenging due small depths of field. Ideally you would always like to shoot a lens at its “sweet spot”. This is the f stop at which any given lens performs at its best optically and produces the most sharpest images. This is quite often in and around the f4-f8 region and is due to diffraction and defocus. Stopping down (reducing the aperture) can lead to more diffraction of the light and thus impacting on the sharpness. Opening up (increasing the aperture) leads to defocus which also effects sharpness although some lenses use this to give rather special effects but thats a whole different subject area.

The other point to not about DoF is the size of the sensor or media you are using. Large Digital sensors or Full Frame as they are known have the ability to produce very small DoF. However the reverse is the case for small sensors which can basically leave you with infinite DoF and nothing out of focus. This is the major problem with small compact cameras and especially Smart phones where you have little or no control on the DoF.

Pro Sports photographers rely on high speed to capture the image they want and as such tend to use high speed lenses i.e f2.8. I have a range of these lenses from 35mm f1.8 up to 300mm f2.8. In between I carry 17-35 f2.8, 24-70 f2.8 and 70-200 f2.8 which give me high speed lenses for all eventualities

I also use 50mm f1.4 and 85mm f1.4 for portrait work as they produce what is known as “bokeh” when shot at their sweet spots. These prime lenses (none zoom) deliver fantastic images as they are designed around a single focal length and as such the glass elements inside are simpler and can be optimised. Bokeh originates from the Japanese word Boke or Blur and┬árelates to the quality of the blurred area around the in focus image. It sounds odd but when its right its produces the most fantastic images. This tends to present portrait work in the best possible way.

So I hope I have described f stops, DoF and Bokeh to a decent degree. My next subject will be Shutter Speeds. Sounds boring I know but its also critical to the perfect Airshow shot

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2 thoughts on “DoF

  1. Interesting read I’m trying to learn more, though my head hurts abit now, need to practice.

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