dsjtecserv wrote:
Why? What exceptions to the physics embodied in the calculators do these lenses accomplish? And if different assumptions are made, why would they not apply equally to all lenses?
Consider the differences in background blur (bokeh) between lenses. With the same diameter blur disks, some lenses will produce very smooth, "creamy" backgrounds, while others can be quite harsh and "edgy," depending on details of the blur disk. The transition from in to out of focus is just "microscopic bokeh": the precise shape of the spot the lens produces as you go in and out of focus can make a big difference in whether the image quickly blurs out to smoothness, or instead still shows harsh, perceptually sharp, edges further from the focal plane. If you magnified the shape of the circle of confusion near the focus point for different lenses, you'd see that they vary far more than bigger bokeh circles: you get complex, colorful shapes nothing like the uniform circles that a simplistic depth-of-field calculation assumes.
OK. Sounds like a worthy subject to consider when evaluation the fine points between lenses, and I can see where it would affect a job where precise depth of field is critical. But I have trouble seeing how it would be an important consideration for an understanding of the principles of depth of field. Even so, thanks for the explanation.
I agree, the details aren't important to understanding the general principles of DOF; a simplified "uniform light cone" model is all you need for basic intuition and quick estimates. Beyond that is where "depth of field calculators" get pretty silly: if you're trying to predict things to the precision where you need a computer/calculator (instead of rule-of-thumb mental calculations based on general, simplified DOF principles), then "precise" DOF-calculator results (ignoring the real-world differences between lenses) are useless (experimentation using the real lens is necessary).
Huh? I always use dofmaster to tell me exactly, to the hundredth of an inch, where my depth of field begins and ends. I need to know whether every leaf "is" or "is not" in focus. How can I live knowing that this might not be valid for every lens?
dsjtecserv wrote:
Why? What exceptions to the physics embodied in the calculators do these lenses accomplish? And if different assumptions are made, why would they not apply equally to all lenses?
Dave
Dave,
Here is a lengthy article published by Dr. Nasse from Zeiss.
Start at around page 20. I can't say I have a firm understanding of the physics, nor a background in physics, but the article is well written and sheds a lot of light on the topic.
In practical terms, I was amazed at the differences in the images of the exact same subject, same aperture produced by the Zeiss 50mm lenses. And this is at web resolution, not peeking at crops. It convinced me that there is room in my bag for primes of the same focal length that I can swap out depending on subject matter and the look I'm going for, which extend beyond sharpness and color.
