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Author Topic: Depth of field vs. focal length  (Read 1977 times)

Mark Sawyer

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Depth of field vs. focal length
« on: November 29, 2006, 02:33:17 am »

re: http://www.luminous-landscape.com/tutorials/dof2.shtml

I know that Michael Reichmann's views are held as gospel here, but there is some bad info in the Luminous Landscape, this being some of it. Yes, focal length does affect depth of field. I think he played some slight-of-hand, saying that with the shorter focal length lens, you have to move the camera closer to the foreground subject to make it the same size as in the image with the longer focal length lens.

Well, if you're going to play that "make the foreground object the same size" game, then you have to play the "make the background object the same size" game too, by moving both the camera and the foreground subject closer to the background subject, and if you do that both will be more in focus with the shorter lens.

Or, leave the camera and all the subjects where they are, and again, both will be more in focus with the shorter lens.

Anyone who's dealt with longer focal length lenses in large format is very familiar with how longer focal lengths lose depth of field.  I've worked in 8x10 and 11x14 for decades, where the "normal lenses are 300mm and 450mm, respectively.

Sorry, I had to straighten this out in another forum tonight, and thought I'd leave a note here...
« Last Edit: November 29, 2006, 02:34:52 am by Mark Sawyer »
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Ray

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Depth of field vs. focal length
« Reply #1 on: November 29, 2006, 03:46:41 am »

I also thought there's a sleight-of-hand element to Michael's argument in this tutorial.

As you mention, if you keep the foreground objects the same size when using different focal lengths (by changing the distance to the foreground subject) then the background objects will always appear larger with the longer focal length lens and as a consequence of this greater magnification there will be less background.

We have essentially different compositions and I wonder how meaningful it is to compare the DoF of different images/compositions.

However, if we try to get away from appearances and consider DoF as being purely 'depth of definition' and if we only consider elements that are common to both images, then the background objects in the image from the longer lens will have the same definition as those same objects in the image from the shorter lens.

Now we all know that things appear sharper the smaller they get and fuzzier the larger they get.  An image in which background objects are large will appear to have a shallower DoF than a similar image in which the background objects are small.
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