Bernard said:
neuroanatomist said:
I can achieve critical focus where I want it when focusing a fast manual prime through the viewfinder with a stock -A screen. Maybe you don't have that skill. You're right...there is room for all sorts.
No you can not.
There's an optical reason why a focusing screen that is optimized for slower lenses can not be focused critically with faster lenses. The short version is that the screen itself only show f:2.8 (or 4.0) DoF at best.
Any number of basic optical texts explain this. I'm sorry if this comes across as snarky and pedantic, but it's true.
EDIT: I highly recommend reading "Camera Technology: The Dark Side of the Lens" by Norman Goldberg if you would like to understand this better.
In fact, I can.
Frankly, you're coming across as a jerk, starting with the implacation that putting the focal plane where I want it is irrelevant for 'my style of photography'. I'm really not sure who would consider the position of the plane of focus irrelevant to their photography.
But beyond that, you're coming across as someone who is very technically inclined (which applies to me, as well), but lacks substantial practical experience (and note that I'm not saying you lack such experience, merely that you sound that way).
Of course, I understand that the stock screen trades DoF for brightness with slower lenses. Perhaps you missed my previous reply in this very thread (not to mention the countless times I've explained that concept on these boards):
neuroanatomist said:
Those 'specialist' lenses are f/2.8 or slower, meaning the stock screen shows the true DoF. Even without a high-precision screen, a fast manual prime can still be focused with good accuracy. Granted, the matte screen makes it easier...but to say that the lack thereof means it can't be focused by eye is a pretty egregious bit of hyperbole.
Or perhaps you read it, and merely failed to grasp the implied converse of the above statement.
In fact, manual focusing is usually a dynamic process, not a static one. If you can assess the degree of defocus both in front and behind your desired plane of focus, and have an understanding of the distribution of the DoF around the focal plane as it relates to focal length and subject distance, then it's quite possible to reliably achieve critical manual focus with a fast prime and the stock -A screen, despite the fact that you can't see that true DoF in the viewfinder.
I could recommend ample literature from one of my fields of expertise – optical microscopy – stating that the resolution of a system is ultimately limited by diffraction...a 'fact' that was noted close to 150 years ago by Ernst Abbe, who approximated that limit as 1/2 the illuminating wavelength (e.g. ~250 nm for green light, and note that I'm significantly simplfying the relevant concepts here). That's technically true, except for some modern 'loopholes' that are collectively termed superresolution microscopy, and through various techniques make it possible for a light microscope to resolve structures separated by a distance of less than λ/2. Interestingly, and not coincidentally, many of those techniques have a temporal component, i.e. through dynamic imaging the system can overcome the physical limitation on performance.
If I give you a ruler with markings only in centimeters, technically you cannot measure an object to an accuracy of 5 mm. However, I suspect that in practice, you could do just that with a high degree of accuracy and precision. So please, before you so arrogantly declare that I cannot possibly achieve critical focus with a fast prime and a stock viewfinder screen because the true DoF cannot be directly seen in the VF, give some consideration to the fact that others may have abilities and/or skills that you lack. Fortunately, you can learn by practice, so if you do get a 6DII, I'd suggest you mount your fast, manual primes and learn to focus with them using the -A screen. You may be pleasantly surprised at the outcome.