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01-27-2014, 12:32 AM   #16
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QuoteOriginally posted by stevebrot Quote
My experience has been the exact opposite when shooting with anything even approaching fast glass.
The measuring basis depends on the prisms in the screen. They are normally designed to work with f5.6 or slower lenses. Ths is as bad as the normal f5.6 AF sensors. So, no improvement. The new K5II has an f2.8 AF sensor, which improves things.
There was available an old LX screen (SA-23) with an f2.8 basis.

01-27-2014, 12:56 AM   #17
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QuoteOriginally posted by lytrytyr Quote
The big advantage of live view
is getting a useful image at a taking aperture of f/8,
when using preset M42 lenses, you can stop down using the preset ring, so DOF can also be seen through the optical viewfinder, but I guess it will be significantly darker than EVF preview.
01-27-2014, 10:42 AM   #18
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QuoteOriginally posted by blende8 Quote
The measuring basis depends on the prisms in the screen. They are normally designed to work with f5.6 or slower lenses.
Can you provide a reference?

I am aware of the issue as it applies to AF systems*, but have generally been of the understanding that most split image focus screens are optimized to work with faster lenses with "tweaks" applied to avoid prism blackout. After all, that is their utility. I am also skeptical of the notion that the aperture-specific design goal for an AF system can be applied directly to the anticipated precision of a split image screen.

QuoteQuote:
There was available an old LX screen (SA-23) with an f2.8 basis.
The SA-23 was a specialty microprism (not split image) screen that was indeed optimized for f/1.2 to f/2.8 lenses, but was not be usable with slower lenses. I did a little look-see with my various camera bodies and found that the split image focus was consistent from f/2 to f/8** while the microprism on the same cameras was fairly imprecise at f/4 and narrower when compared to the split image, microprism, and ground glass fields wide open. It is also my impression that the microprisms on those cameras are less precise overall. Go figure.

For those wanting to know more than they ever cared to about the subject, here is a layman's description (no math) of how split image, microprisms, and phase comparison AF systems work.

http://dougkerr.net/Pumpkin/articles/Split_Prism.pdf


Steve

* Split image and phase comparison AF systems operate on similar principles, though they diverge on implementation details.

** All of my split-image screens (Ricoh XR-2s, Ricoh XR7, Pentax Super Program, Katz Eye Plus) are resistant to blackout to f/8

Last edited by stevebrot; 01-27-2014 at 11:47 AM.
01-27-2014, 11:18 AM   #19
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I don't know - when using MF lenses on my digital cameras (and analog cameras, too) - assuming I've properly calibrated my eye and I use a small enough aperture, if what I want in focus appears to be in focus in the optical viewfinder generally it apears to have been in focus when the image is printed. On careful examination (in my cases) when a printed image is other than sharp the cause is almost always motion blur caused by hand shake.

01-27-2014, 11:35 AM   #20
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QuoteOriginally posted by stevebrot Quote
Can you provide a reference?
You provided the reference!

QuoteQuote:
By changing the angle of the prisms, we can arrange for the red and blue rays to be picked up from portions of the exit pupil that are closer together—within the diameter of a smaller aperture. ...
This allows the “blackout” effect to be averted down to a smaller aperture.
However, doing so reduces the “sensitivity” of the split prism effect: the amount of visible misalignment of the image is less for any degree of focus error.
Split prisms normally work ok with f5.6 lenses.
Pentax increased accuracy by creating a special LX screen for large aperture lenses.
The phase comparison AF works with the same principle.
01-27-2014, 12:11 PM   #21
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QuoteOriginally posted by blende8 Quote
You provided the reference!
I meant for the f/5.6 design goal for split image. The article does not cover that topic.

The full quote from the linked article (p. 11) continues to describe modern screens such as the Katz Eye Plus with design features to allow for wide basis while avoiding blackout (emphasis supplied by me).

QuoteQuote:
"By changing the angle of the prisms, we can arrange for the red and blue rays to be
picked up from portions of the exit pupil that are closer together—within the diameter of a smaller aperture. (This is referred to as “reducing the baseline distance” of the system, a concept that is discussed more thoroughly later when we describe the application of the split prism concept to automatic focus systems.) This allows the “blackout” effect to be averted down to a smaller aperture. However, doing so reduces the “sensitivity” of the split prism effect: the amount of visible misalignment of the image is less for any degree of focus error.

There are special modifications to the prisms (not involving a change in angle) that can mitigate this “blackout” effect down to smaller apertures than usual while still maintaining a desirable large baseline distance (in the interest of retaining greater “sensitivity” when a large aperture lens is in place). These modifications are used in some split prism focusing screens available today. The principle involved is proprietary, and is beyond the scope of this article."
To be honest, I am at the ragged edge of my understanding of such things, but I do know that from personal experience with my faster lenses, I have found that the AF system on my K10D will signal that it has attained focus over a broader range than the split image on the same camera. The is true even when working with a flat target. As a result, I prefer to not use focus confirm when fine focus is the goal.

This may have changed on the new bodies, though doing so would require some mechanism for determining exit pupil size* (the lens maximum aperture). Which brings us back to M42 vintage glass where that information is not available to the camera. In my opinion and experience a split-image screen is superior to the AF system for manual focus.

End of thread hijack for my part


Steve

* Page 17 from http://dougkerr.net/Pumpkin/articles/Split_Prism.pdf

Last edited by stevebrot; 01-27-2014 at 12:21 PM.
01-27-2014, 12:38 PM   #22
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QuoteOriginally posted by stevebrot Quote
I meant for the f/5.6 design goal for split image.
You can check this yourself by mounting an f5.6 lens on your camera, e.g. a telezoom. If it blacks out, your prism is "faster" than f5.6.
01-27-2014, 07:21 PM   #23
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The EES or EEG precision focusing screen (made by Canon) is the best I've tried.
Its catered for f2.8 and faster lenses.
Very precise calibrated properly between viewfinder and sensor.
I think focusingscreen.com does one (mills down the screen to fit)

I find the K30 implemention of LV too slow in shutter response to my liking.

The focus indicator as mentioned by others isn't that accurate.
That said, sometimes for some cameras it may be more in focus coming in from near to far (or vise versa) than the other way round.
So no harm to try that out.

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