Albano Garcia mused:
>
>
> Hi gang,
> I understand the APS sized sensors on DSLRs converts
> your 28mm on a 42mm cropping image area, but if I
> understand well, the depth of field remains the one of
> a 28mm and also the distortion of image proper of the
> original focal lenght remains, since the lens is still
> phisycally the same, but cropped, right?
> So, people who uses 50mm lenses as short teles for
> portraits, still gets the dof and face features
> rendition of a 50mmm but cropped, not the flatting
> effect of a 85mm...
> I'm right?
> Regards
Let's see if I can explain this simply.
The end goal is to end up with a resulting print (or image)
of given dimensions; this will require different amounts of
cropping, enlargement, etc., depending on what equipment was
used to make the original capture.
1) The perspective, or distortion (relative size of features,
relationship of near and far objects, etc.) depends on
where you stand to take the photograph, and on nothing else.
2) For a specified print size, the depth of field depends
on the actual physical size of the taking aperture
(not the f-stop; the diameter), and on nothing else.
Once you've decided where to stand to capture the image,
(and on what format camera to use, if that's an option)
you will have a maximum focal length you can use to get
the whole of your desired subject within the image area
on the sensor. But as long as you stay below that, you
can use any lens from your camera bag (although using a
longer rather than a shorter focal length is generally
better, to make the best use of the media capabilities).
Then all you have to do is pick the aperture diameter to
give you the depth of field you want, and take the shot.
The drawback is that an aperture diameter which gives
an f-stop of f2.8 with a 24mm lens will only give you
f22 with a 200mm lens, which will increase the exposure
time (possibly causing problems from camera shake or
subject motion).
