OK, I've now re-read this a bit more closely and I still don't
understand what you're on about. It seems like you're intent on
disproving something he didn't claim.
The article starts with these words: "I don't know why this myth about
the depth of field being utterly independent of focal length (for
constant magnification) keeps on going...". Here he is literally
defining "independent of focal length" as "constant magnification". One
cannot maintain constant magnification with different focal lengths
without moving the camera. So what's the problem with moving the
camera? That's part of the discussion. It's perfectly clear to me that
magnifying the image after it's captured is not the same as changing the
position of the camera to maintain the same magnification. But that's
not his point. That's your point.
And I accept his first point implicitly. Focal length is clearly a part
of the formulas for depth of field both front and rear and it can't be
substituted out with magnification... because that is also dependent on
focal length and distance. You're just recasting the problem to get rid
of the camera and lens. The two of you are talking apples and oranges.
The DoF calculator I use is from Jonathan Sachs of Digital Light and
Color. The equations he uses are from Merklinger. If I feed the
calculater data for a 25mm lens at 1/2 the distance of a 50mm lens (to
maintain the same magnfication) the calculator says the 25mm lens has
greater total depth of field and that the distribution is
proportionately more behind the lens than in front with the shorter
focal length lens. I think that's all that Ctein has said and my
calculator agrees.
Chuck Norcutt
Jan Steinman wrote:
>> From: Chuck Norcutt <puhpxabephgg@xxxxxxxxxxxxxxxx>
>>
>> Very timely. Jan and Mike should find this interesting. Here's
>> more on
>> DOF and focal length (by Ctein) from The Online Photographer:
>> <http://theonlinephotographer.typepad.com/the_online_photographer/2009/06/depth-of-field-hellthe-sequel.html
>>
>
> I'm still willing to be convinced, but not by this article.
>
> Ctein writes: "I moved the camera to keep the on-film magnification
> [the same]..."
>
> He cheated. He clearly changed his position, and thus, the
> perspective. Sure, that changes DOF! But it would have changed it no
> matter the focal length.
>
> I will be convinced if he stands *at the same point* and shoots with
> the 25mm and 300mm lenses and a crop of the former to match target
> sizes shows DOF different than the latter. But I've actually done that
> to satisfy my own curiosity, and the DOF was identical.
>
> SUMMARY: if you move back and forth, it isn't the focal length that's
> changing DOF, it's the perspective.
>
> I'm still willing to be convinced, but you'll have to come up with
> something better than Ctein's article. :-)
>
> :::: There is no hope for any of us outside of a community. — Mike
> Ruppert ::::
> :::: Jan Steinman, EcoReality http://www.EcoReality.org ::::
>
>
>
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