bo_zhang

<p>I think I made a mistake above: 200mm Crop to 300mm, as long as the aperture does not change, will offer 300mm depth of field. It has something to do with the concept of "Circle of confusion"<br>

I know the DOF part is a bit brain twisting but here is a good read: <a href="http://www.film-and-video.com/dofmyth.htm">http://www.film-and-video.com/dofmyth.htm</a></p>

<p>No, perspective depends ONLY on point of view. It does not depend on the focal length of the lens. The reason why you switch to a 100mm to get the background bigger, is because a 100mm lens allows you to stand further, at the same time frame the foreground same as before. The effect of this action moved everything further from your point of view. Since object size on image plane is directly proportional to the distance, it has the effect of shrinking foreground object more than shrinking the background object.<br /> Take a numeric example, using 50mm: person = 1m, statue = 1000m, using 100mm: person = 2m, statue = 1001m. The person shrink to 1/2 = 0.5 times as big, the statue shrink to 1000/1001 ~ 1 times as big. Because 100mm lens has about 2 times magnification, the result is the status seems twice as big.<br /> So, perspective alone controls compression, focal length controls depth of field. Focal length and Crop factor together control FOV<br>

200mm crop to 300mm = 300mm FOV and compression + 200mm Depth of Field.</p>