[clackers]

. Sure, the Pentax autofocus is not 100% accurate all the time but if it were true that an autofocus is accurate only at f2.8 and above, I guess, I wouldn't use Pentax gear at all.

What camera would you turn to, Skyer? Canons, Nikons and Sonys suffer from the same f2.8 specification as far as I know.

[6x7II]

Is the thickness equal? If not different thickness shims must be used in order to keep viewfinder focussing OK. Is automatic exposure still OK?

The thickness is not that important as long as the screen is pressed upwards in the direction of the prism, the top of the screen is the place were it should be sharp, thicker or thinner is compensated by the springs in the screen frame. My K1 proved to be accurate, pff With my K3 I had to remove the metal sims, form the body, to get it right.
This was a body problem, not a screen problem. Most people rely on the AF and use the (standaard) screen only for composition and to judge the bokeh, with the standaard screen a small offset will not be noticed, but with a screen with focussing aids it is direct visible.

[stevebrot]

I know about "f2.8 sensitivity". However, in practice this rule is not correct. I regulary shoot with 31 Lim, DA*55, 77 Lim and FA*85 at maximum opened apertures.

This not my experience with focus confirm, though I am glad you are satisfied. For the type of shooting I do, critical focus is often required and focus confirm is inadequate with a flat subject and much less so with complex or curved surfaces. I don't shoot with a K-1, but the AF precision limitations are no better than on my K-3. I installed my split-image screen for a reason and also fall back on magnified live view for the same reason.

As for the lenses you are using...I am impressed. I only own one FA Limited, but do own dozens of fast 50s (f/2 and wider) as well two LZOS Jupiter-9 85/2 (M42 and Contax/Kiev mounts) that I shoot on both APS-C and 35mm FF. It was my experience with the J-9 using focus confirm that led to the purchase of the KatzEye screen. I thought the lens was a lemon until I started using a focus system that was not thrown off by limited DOF. Now the J-9 is no more difficult than any other lens in the bag.

Sure, the Pentax autofocus is not 100% accurate all the time but if it were true that an autofocus is accurate only at f2.8 and above, I guess, I wouldn't use Pentax gear at all.

Again, the issue is precision, not accuracy. I am sure there are equivalent concepts in Russian and it may be that in English the two mean the same to you, but that is not the case. In regards to Pentax vs. other brands, no PDAF camera offers greater than f/2.8 focus sensitivity regardless of brand. Again, I am glad you are happy. Installing and dialing in an aftermarket screen is a pain in the rear. Routine use of magnified live view with camera on tripod is also cumbersome. My preference would be a return to the excellent viewfinder focus systems typical of 1980s vintage film SLRs, but that is not an option at present.

Edit: In rereading your comments, I think I may have misunderstood you. Yes, it is very likely that focus confirm using PDAF will be more accurate, even if less precise, than a poorly calibrated split image screen. As such, it may be preferable. That is why it is important to confirm the screen accuracy using CDAF or magnified live view.


Steve

[stevebrot]

The thickness is not that important as long as the screen is pressed upwards in the direction of the prism, the top of the screen is the place were it should be sharp, thicker or thinner is compensated by the springs in the screen frame.

Unless something has changed with the K-1, thickness is a concern. On all SLRs, the intended focus plane is at the flat underside of the screen facing the mirror. On the K-3 and all other APS-C Pentax dSLRs, spring tension via the retention frame is applied from that side of the screen upwards. Shims may be inserted between the screen and the bottom face of the pentaprism* to allow for variation of mirror box tolerance and screen thickness. When properly shimmed, the flange focal distance to the face of the focus screen will be identical to that of the sensor.**

I am pretty sure I am not blowing smoke on this matter. On the shelf in the other room is a box having an assortment of original Pentax factory part shims appropriate for current model APS-C dSLR cameras. I offer them for sale to people needing shims. They come in about ten different thickness. I bought them from KatzEye after they went out of business. KatzEye used them internally for their custom shimming service and would also sell them directly to Pentax owners of KatzEye screens. Similarly, focusingscreen.com provides plastic shims as part of the kit that comes with all screens they sell. Their screens are various thicknesses and each brand camera has somewhat different requirements. Most (all?) current brand dSLR cameras have similar means to adjust with shims and focusingscreen.com provides helpful instructions on both placing shims and adjusting for both front and back focus of the focus screen.

So...to repeat the question, how is the focus calibration with your replacement screen? Do you have good agreement with magnified live view and/or CDAF or is there indication that adjustment is needed.


Steve

* The bottom face of the pentaprism is fixed into a static indent in the chassis.

** It is the total height of the stack; screen, shim retainer, plus shim, that determines the location of the intended focus plane. Note that registration of the focus screen to the shim retainer is fairly critical on the APS-C cameras.

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Last edited by stevebrot; 11-19-2016 at 04:41 PM.

[photoptimist]

Stevebrot is right about the need to carefully shim the screen to match the thickness exactly.

A 50 mm lens focused at 10 ft using PDAF or CDAF would find the manual split-ring focus point off by about 3.5 inches if the focus screen thickness was off by only 1/1000 of a inch (an error of 88 mm at a 3 m subject distance with just a 0.025 mm wrong thickness)

Note: the focusing screen, PDAF, and CDAF are three independent focusing systems. The thickness of the screen won't affect the other AF systems only the photographer's interpretation of correct manual focus as determined by alignment of the split image in center of the screen. If the focusing screen is wrong, manual focus will be wrong, too.

[Kerrowdown]

This thread has got the old Kerrowdown quite beside himself, does this mean K1 screens are changeable after all?

If so... what's the nearest equivalent to the old LX screen SA-23 (Microprism/Matte. Similar to the SA-21, but with sharper prism angles to aid focusing with high-speed (fl.2–f/2.8) lenses) which always seemed to work so well with my "Ladies".

[Bengan]

Unless something has changed with the K-1, thickness is a concern. On all SLRs, the intended focus plane is at the flat underside of the screen facing the mirror

Stevebrot is right about the need to carefully shim the screen to match the thickness exactly.

Yes, thickness is crucial. I've exchanged screens in booth my APS-C bodies and after inserting cut down Canon screens I had to use thinner shims in order to obtain accurate manual focus. I would be surprised if the K-1 screen and Canon screens are equally thick.

[skyer]

What camera would you turn to, Skyer? Canons, Nikons and Sonys suffer from the same f2.8 specification as far as I know.

Exactly, many cameras have a f2.8 limit in their specifications. However, usually it's an exaggeration. Before K-1, I was using Nikon D750. It's autofocus (with some lenses) is extraordinary, especially in regards to how accurate its autofocus is! When a mode with autofocusing on eyes is on, and AF.C is on, the AF system does wonders. One can point a cluster of AF points on a head and then be sure that the closest eye will be pin sharp even at f1.4!

The thickness is not that important as long as the screen is pressed upwards in the direction of the prism.

I also have doubts on this as many of us here. Please, compare manual focusing through the viewfinder to the manual focusing in LiveView.

[rawr]

Do all the viewfinder overlays (focus point squares, crop modes, level display etc) look OK when using a new focussing screen on the K-1?

[UncleVanya]

In theory thickness should only matter if the thickness caused the shim to partially compress reducing the distance to the screen. In practice, the calibration of the existing screen is so imprecise ( because that screen doesn't require high precision) that aftermarket screens often need adjustment to give the highly accurate focus desired. Again, in practice, the various screens behave different from theory and require minor adjustments to achieve critical focus even if the system was already setup for critical focus and all you did is swap one type of aftermarket screen for another.

[6x7II]

Ok here I am back,

I did a check with a FA 77mm 1.8 limited.

There is 1% front focus hence I need a thinner brass sim than the one that came with my copy of the K1. Removing the sim (0.45mm) result in a back focus of 2.15% so the sim needs to be (2.15/1+2.15)*0.45 = 0.307mm the sims normally comes in steps of 0.05 so the nearest sim would be 0.3mm. The question is if Pentax will provide those, or do i have to look for some other material and cut it my self from plastic.

The focus indication based on PDAF is most likely more selective than the focus screen. My only problem with it is that the actual position is not that clear, so the focus aid on the screen tells me if the central AF point sees the same as I do :-) with small objects the af point could look just to another object for instance you think you focus on an eye, but the central AF pint points at the eyebrow, this is directly visible on a screen with focus aids. So for me it is more an extra conformance that i focus on the right thing. It is a pity that there is no Canon screen available with both microprism and splitscreen.




The brightness of this screen is 1/3 stop less.

[skyer]

Ok here I am back,

There is 1% front focus hence I need a thinner brass sim than the one that came with my copy of the K1.

Q.E.D.
I wish I had a split-image focusing screen with very precise shims so that the accuracy would be 100%... Sometimes I think that old film cameras also didn't have 100% focusing accuracy. Film grain and small enlargements of film didn't need a very good accuracy. However, every time I look in the viewfinder of my Pentax Super Program, I wish I had the same viewing experience shooting with a digital camera.

[stevebrot]

The question is if Pentax will provide those, or do i have to look for some other material and cut it my self from plastic.

In North America, Ricoh/Pentax will only sell shims as replacement parts to authorized repair facilities. The practice may be different in Europe.


Steve

[stevebrot]

Sometimes I think that old film cameras also didn't have 100% focusing accuracy. Film grain and small enlargements of film didn't need a very good accuracy.

Film flatness is the challenge for accuracy with film photography. This is complicated by differences in film base thickness between films. If you are curious about the screen calibration on your Super Program, it is fairly easily checked with the film back off and the shutter open in Bulb. A quick and dirty way is to stretch a length of "magic" tape across the film rails to create a temporary ground glass focus surface. Check the focus with an 8x loupe. Even better is an actual piece of ground glass and best is a spare focus screen.* Repair techs have told me that factory screen calibration is traditionally very accurate for almost all manual focus film SLRs. Adjustment on your Super Program is via three grub screws on the focus screen retainer frame with access requiring a partial tear-down.


Steve

* I have an Olympus OM screen that I use for rangefinder calibration. Works great.

[6x7II]

In North America, Ricoh/Pentax will only sell shims as replacement parts to authorized repair facilities. The practice may be different in Europe.


Steve

I can always ask, but I have to send my camera in, then it is a different story with the original screen you don't notice the offset, with the modified Canon screen they do not have to support it... and second it might take a long time.