Perspective is controlled by the distance between camera and subject.
If 100mm gave you your desired perspective on 35mm film then 50mm will
give you the same perspective on a 4/3's frame.
If you use the 100mm on the 4/3's camera you will have to stand twice as
far away in order to frame the subject the same way as with a 35mm
camera. That change of distance is what will change the perspective.
If you use a 50mm lens you can still frame the subject the same way at
the same distance you used with the 100mm lens on the 35mm camera.
Preserving the distance to the subject preserves the perspective.
To make this a little more visual, imagine a post sticking out of the
ground at the very edge of the frame half way between you and the
subject. If you have to back up to maintain the same angle of view of
the subject, the position of that post will move inward on the frame as
you back up. That's perspective change.
The bottom line is; to preserve the same perspective abide by the "crop
factor". The same rule pertains to the difference in film sizes between
35mm and medium format and between medium format and large format. A
"normal" lens for any given format is approximated by the diagonal of
the film frame. Adhering to that rule will define focal lengths for
each format having the same perspective since it allows shooting the
same angle of view from the same distance. The rule is only approximate
since using the diagonal of the film alters relationships between image
sizes because of the varying aspect ratios between film/sensor sizes.
Chuck Norcutt
Don Shedrick wrote:
> I have not yet made the digital SLR switch, although I am getting
> closer. With 35mm film cameras, a good focal length for portraits is
> generally thought to be in the 75 - 150mm range. (I really like my
> Zuiko 135 f2.8) My question is, with the 4/3 system with the 2X crop
> magnification, is the same 75 - 150mm still the best for portrait
> work, or would 1/2 of that focal length be best to use, with the
> resulting image with the 2X factor being the same size image. My
> thought is that the angle of view and perspective with the 75-150mm
> range is the same with the 4/3 even though the image size is greater
> due to the 2X factor, so the ideal portrait lense would still be the
> 75 - 150mm. This means one would have to stand about twice as far
> from the subject to get the same size image unsing a 4/3 camera.
> Any thoughts or experience from others?
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