Realisations

Yesterday at work, we were discussing an exercise the students undertake to understand Depth Of Field.

Historically, we get them to take 1 shot at 3 different apertures in one location on a tripod then shift focus and repeat, finally focusing on infinty and repeating again, as well as a portrait at 3 different apertures, on a tripod.

A year or so ago I did this exercise using a Canon 350d, and the differences were NOT as apparent, as a traditional 35mm camera, I left it at that and haven't given it much thought since.

When I mentioned this in the meeting a passionate discussion then ensued as to why this was the case as we are contemplating getting the students to do this assignment using digital cameras. Beginning students need to see big differences quickly and the 350d wasn't giving the results I'd expected.

As a result, we found the answer. It has to do with surface area of the light capturing surface and size of circle of confusion. The smaller the film/sensor area the more D.O.F you have.

Let's let Bob Atkins explain it.

"So the bottom line - and all you really need to know - is that DOF is inversely proportional to format size. Note that format size is inversely proportional to the "digital multiplier". The higher the "digital multiplier", the smaller the format and thus the greater the depth of field. Note also that now you can see one of the reasons large format camera users need tilts and swings to get adequate depth of field. With an 8x10 camera you have about 8.5 times LESS depth of field than you do with 35mm for the same image. This also explains why consumer digicams, some of which have sensors 1/6 the size of 35mm film, have such a large depth of field and one of the reasons why it's almost impossible to get blurred backgrounds when using them."

The difficulty for us to teach the principles of D.O.F is that all cameras have different sized sensors and only cameras like the $4,000.00 Canon have 35 x 26 mm sensors. A camera few students can afford and the type that is not held in stock by us because of their delicateness.

Today I am going to shoot the exercise on the Pentax k100d, which has a larger CCD than the Canon 350d. I may post the results over at flickr, we'll see?

Some more links:-

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This page contains a single entry by s2art published on February 2, 2007 8:02 AM.

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