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Forum: Pentax SLR Lens Discussion
09-21-2010, 10:25 AM
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Without getting deeper into techical stuff, the depth of field is depends on some parameters.
One is distance to subject. Get closer, you get more blur on the background. The limit is your lens minimum focusing distance.
One is focal length. Wide angle = more depth of field (less blur) -> narrow angle (70-100 mm+ lenses) less depth of field (more blur).
The third is aperture. Wider aperture = more blur.
The size of the sensor also counts, but not in the way most of the people think. The same lens projects the same image into the viewing plane (take for instance your 28-75), it don't cares about that you're using a 35mm film or an APS-C sensor. What does count is the size of the photosites in your sensor - smaller they are, the more blur you recognize at really small sizes. But it does not affect your image in big way, like the 3 thing I mentioned above.
Also when you compose, you have to think about your subject, and its distance from the background. With a close background (like a wall behind your model) you will get less separation in DOF. You can get separation by colors and texture, but that is another view of the whole thing.
So if you place your 2 people correctly, you can get nice results with the 28-75 on 2.8-4.0 and with 50-75mm.
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