Bokeh and the 18-50 f2.8 lens

Earlier we discussed the formula for bokeh here
http://barrychengspictures.blogspot.com/2009/05/quantifying-bokeh.html
and the factors that contribute to bokeh, which are

The formula is

((Focal Length/minimum f-stop)/minimum focusing distance)/crop factor.

So the factors that will affect bokeh are
1) focal length- the longer the zoom, the more bokeh available
2) aperture- with a wider aperture (lower f number), more bokeh is available
3) the closer the subject is to the camera, the more bokeh is available. A subject 20 feet away will not provide bokeh the way a subject 2 feet away will
4) with a full frame camera, there is more bokeh available. When they discuss the relative merits of full frame vs digital size DX sensors, this is the one factor that is often overlooked. At any given zoom, there is more bokeh on the bigger sensor.

Here I have depicted that with a series of photos. This series of photos takes the same subjects using different F.stops to demonstrate the difference in Bokeh at each F-stop. This will also demonstrate that generally a wide angle provides little bokeh, and the more open F-stops really just assist with lower lighting situations.

F1.4


F1.7


F2.0


F2.8


F2.8 shot with the 18-50mm lens

In theory, these last two photos should be identical +/- the difference created from my holding the camera in a different place after I switched lenses

These last two photos are of the same subjects using the 18mm setting on the zoom lens. From the formula above, the focal length is decreased with the 18mm lens from the 50 mm lens, so the proportionate change in bokeh should result. As you can notice, there is little bokeh at all.



Here I recomposed the photo at 18mm to approximate the photo at the 50mm setting. While you can see more of the area to the sides of the subjects, the background does not have the same amount of bokeh as the 50mm pictures. Even though the distance to the subject was reduced, that is not enough to compensate for the change in focal length, and again there is little bokeh. A longer lens is needed to realistically create bokeh in your photos.