Originally posted by braver For manual lenses like Voigtlander Nokton 58mm, what do folks do to improve MF accuracy?
We can start by complaining to Ricoh/Pentax regarding poor selection of focus screens. Here is the executive summary of the current state of manual focus on Pentax dSLRs:
- The stock focus screen has poor precision. In technical terms, its focus sensitivity is about f/4 meaning that the ability to attain fine focus with an f/2 lens is the same as with an f/4 lens. This is due to exaggerated DOF that is never narrower than that present at f/4.*
- The focus confirmation system also has a focus sensitivity of f/2.8 on your K-3II, but only for the center column of AF points. The other points are f/5.6.
- Catch-in-focus (CIF) has the same limitations as focus confirm.
- None of the above are adequate for fine focus with an f/2 or faster lens, particularly at longer focal lengths
- The conventional solutions are to either use magnified live view (gold standard) and/or replace the stock screen with something having higher focus sensitivity and/or use a viewfinder magnifier
- I have not used a viewfinder magnifier, but some users on this site report good results
- There are two screen variants that are popular by users on this site:
- Canon S-type (high performance matte field)
- Split-image/microprism with matte field
- The S-type screen works real well with faster lenses (focus sensitivity of about between f/1.2 and f/1.4) and not so well with slower glass (maximum aperture below about f/3.5). The screen gets very dark very quickly. There may also be problems with metering with some models.**
- The split-image types also work very well with focus sensitivity in excess of f/1.2, but depend on the presence of a vertical line at the intended plane of focus. The microprism ring on most versions may be used instead, but has fairly limited focus sensitivity (f/2.8 - f/3.5 depending on screen). The better split-image screens have a matte field that rivals the S-type.
- Options for the Canon S-type include:
- Cutting your own from a FF Canon screen. These may be purchased for reasonable money from B&H and other online vendors.
- Buying pre-cut from an Internet vendor. Users on this site have had good luck with screens from focusingscreen.com and that is my recommendation over the eBay vendors. When buying pre-cut, it is usually possible to order with various inscribed grids or other markings.
- Options for split image are similar to those for the S-type. The Nikon K3 screen is favored for DIY and also pre-cut from focusingscreen.com. Focusingscreen.com has example views for the screens that they sell. If Katz Eye were still in business, I would give their screen (split-image similar to the Nikon K3) a strong recommendation, but alas, they are gone
Well, there you go. My personal testimony is that I was getting ho-hum results from my manual focus attempts on my K10D back in 2007 until I got a Katz Eye. Making the switch made all the difference. Now that I have the K-3, having a good screen is all the more important and I would not be without it.
Having made a strong case for an aftermarket screen, here are the cautions:
- Neither the Canon S-type or split-image screens will be as bright as the Pentax stock screen with most lenses
- Adding a center focus aid (split image or microprism) will adversely affect spot metering. This is a historic problem dating back to the earliest spot-metered SLRs. When I move to a split screen, I kissed spot metering goodbye. As always, YYMV so it may be worth a try.
- Metering...Most screens do not have a huge impact on meter performance, though the potential is definitely there, particularly if there is an split-image blackout (see below)
- Prism blackout (half or all of the prism goes dark) is a characteristic with all split-image screens at narrower apertures. The point of black-out depends on the design of the screen and below that point the split is not usable. Resistance to blackout is one of the strong points of the Nikon K3 screen and also of the late and lamented Katz Eye. Both are generally usable down to between f/8 and f/11.
- Calibration...You notice I left this one until last. The good news is that a well-calibrated screen of decent quality will outperform the best PDAF systems currently available. The bad news is that most aftermarket screens will only be properly calibrated using the shims inserted when the camera was made if they are the same thickness as the stock screen. Unfortunately, most are not.
- Calibration is not hard, just tedious and made more so if you don't have access to decent shims
- Shims...yes, I said shims. They fit between the focus screen and the bottom of the pentaprism. Too thick and you have front focus. Too thin and you have back focus. (I think...)
- Some vendors include plastic shims (e.g. focusingscreen.com), but they tend to be flimsy and it is often not clear which ones to use
- Some users on this site have shimmed using thin strips of paper
- Ricoh/Pentax will not sell official shims to other than repair shops
- Katz Eye (RIP) had access to and would sell shims on their Web site, but alas, no more ***
Well there you go!
Steve
* The reason is complex, but has to do with screen optimization to offset the light lost to service the AF system (the main mirror is half-silvered).
** I had problems on the K-3 with an S-type screen that I bought from focusingscreen.com. I added brightline laser etching (AF zones) and the had severe problems with underexposure. As always, YYMV, but it should be noted that they do not accept returns for that sort of thing.
*** Forum rules stipulate that we are not supposed to sell outside the marketplace, but I know what happened to Katz Eye's stock of shims...