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Forum: Pentax SLR Lens Discussion
08-14-2017, 12:19 PM
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Yes! A corollary is that if distance is kept constant and magnification decreases (shorter focal length) a crop to maintain framing will have the same DOF as with the longer lens. What's more, your last point makes an extremely important qualification related to DOF preview, both with live view and the optical viewfinder. Live view will generally exaggerate DOF as will a low magnification optical viewfinder.*
It is somewhere around this point that the practical photographer will give up trying to fine tune where the limits of acceptable focus fall in their composition.
Steve
* This is the cause of user frustration with manual focus where "...it was sharp in the viewfinder/rear LCD, but OOF on my computer screen..."
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Forum: Pentax SLR Lens Discussion
08-14-2017, 11:56 AM
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Yes! The laws of perspective apply and the eye knows them very well. Even though the lens may allow a tight framing, the perspective is still one of distance.
Steve
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Forum: Pentax SLR Lens Discussion
08-14-2017, 11:38 AM
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Is your intent to maintain framing or a particular DOF? You can, of course, simply test it the difference in DOF yourself using a meter stick. This is not rocket science and does not require calculation. The time-honored approach to this problem is:- Choose the lens and focal length based on optical characteristics and desired working distance.
- Choose aperture narrow enough to ensure adequate DOF for your subject. DOF preview is handy for this task. The challenge with most focal lengths 50mm and longer is adequate DOF, not the opposite.
- Position subject relative to background elements appropriately for subject isolation. The deeper the background elements, the greater the isolation. One does not need razor-thin DOF to accomplish this.
FWIW, your larger concern moving from 70mm to 50mm should be the change of perspective to maintain framing. The purpose of a longer focal length for portrait is to move the camera (lens actually) further from the subject to equalize the relative size of features (e.g. nose to rest of face).
Steve
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