Julian Loke wrote: > I thought you were going to bring the issue about aperture and DOF into > the discussion.
I'd never voluntarily open such a can of worms ... actually, I just was looking at focal lengths. > To produce the same print/projection size, you need larger magnification > if you start from a smaller frame. Consequently, the DOF requirements > are different, because the CoC (circle of confusion) has to be smaller. > The effect of aperture is also affected the sensor factor (1.6). The initial image magnification also is smaller. The net effect, within a certain range of object distances, is that DoF is inversely proportional to format size, so that for the same final image the EOS-20D gives 1.6x the DoF of a 24 x 36 format. More practically, perhaps, is that you can get the same DoF with 1/1.6 the lens f-number, e.g., f/5 on the 15 x 22.5 format will give the same DoF as f/8 on the 24 x 36 format. If you absolutely must know the details, I discuss this in http://www.largeformatphotography.info/articles/IntroToDoF.pdf. You get extra credit if you find the mistake in (at least) one of the formulae ... > Unfortunately, an EF 24 wide open at f/2.8 does not produce the same > effect on the EOS 20D as EF 50mm f/1.8. Thank goodness for the TS-E, > where I can use tilts to control the plane of focus. Excessive DoF unfortunately is one of the problems when lenses designed for 24 x 36 format are used on smaller formats. Aside from the difference in angle of view between the two lenses you mention, you'd need an f/1.1 lens to give the same DoF on 15 x 22.5 as an f/1.8 lens on 24 x 36. Changing the plane of focus can partially compensate for this in some situations, but the wedge-shaped DoF that you get with a tilted lens is quite a different animal than the DoF between parallel planes. I've seldom changed the plane of focus to reduce the apparent DoF, so I can't offer much in the way of practical comments. Jeff Conrad * **** ******* *********************************************************** * For list instructions, including unsubscribe, see: * http://www.a1.nl/phomepag/markerink/eos_list.htm ***********************************************************
