Choosing focal length for DOF & OOF blurring

[ajohnw]

John, field of view is dependent on the focal length. You cannot change that fact. A 100mm lens will provide a narrower field of view than a 28mm lens. What you may be alluding to is to use a larger magnification of the subject with 28mm by moving very close to it but keeping the magnification with 100mm lens smaller. You'd still not be changing the field of view.

Scratch head - that is what I said in the last post and in the part you have quoted. Surprisingly I am well aware of the fact too.

IE Same view in the frame irrespective of the lens in use.

John
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[New Daddy]

I just bought a 45mm F1.8 lens, more dof than 90 F1.8 on 35mm or so they say. I'm glad it has. This shows the difficulties in using it for full frames but it starts getting usable as the shots are reduced in size. Full frame it's 27 pixels blur 10cm away from the object. It also shows something along the lines of what I did with the flower. APS 85mm at F8. This can be useful at times.



John
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I think you put in the wrong crop factor. I don't know what system you're using but if it's micro four thirds - it looks like it's micro four thirds - the crop factor is 2x, not .25x. (Yes the sensor size is 25% of a full-frame). Regardless, the relative blurring differential among the four lenses would not differ, so I guess it served its purpose of showing you which lens at what aperture would result in most blur.

[ajohnw]

Correct along with thinking along the lines of visible angle to the eye - pass why I did that, I have me crop factors messed up. I thought it was a bit extreme at F1.8 on the 45mm. Must have been having a bad day.



John
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[xpatUSA]

Much of the confusion in photography is caused the use of linear measure when most of the optical variables are to do with solid angles or just plain angles. Thus it is that we get things like "equivalent focal length" instead of "angle of view", values of "circle of confusion" in mm instead of "cone of confusion" good for all calcs, etc, etc, blah-di-blah.

In a paper, the well-respected Dick Lyon proposes something a bit different which at least provides a better understanding of the subject although, dear Real Worlder, it may not be of great practical use. However, it does have the ability to solve the dilemma in the OP.

The usual DOF equations are formulated in terms of focal length, f-number, maximum acceptable circle of confusion (COC) diameter, and subject distance. Typically, the COC criterion is taken to be proportional to the camera format size, such as format diagonal divided by 1500. For example, it is typical to use a blur diameter criterion of 0.029 mm for a 35-mm full-frame format with 43 mm diagonal, and proportionally smaller for “cropped” DSLR formats.

The alternative developed here ignores, or normalizes away, the focal length, f-number, format size, and absolute COC, and substitutes the absolute lens aperture diameter, the FOV, and either the relative COC or the angular COC.

(my emphasis).

The rest is here for those interested:

http://www.dicklyon.com/tech/Photogr...Field-Lyon.pdf

(it's informative, but does take some reading!).