Following is my go relating to a number of the above recent posts. Comments are much appreciated and will assist in my own evaluation of deciding to go ahead with tech camera.
Graham, I was just pointing out Joe's analysis and that Alpa has no tighter tolerances or precision than any other quality camera maker (e.g., Sinar, Arca, Linhof).
I missed that part. Appreciate if you would please paste a quote, thanks.
That's what you pay the extra Alpa tax on the lenses for. They are supposed to be accurately calibrated to the mount and the precision of the lens mount/body/spacer/back plate is supposed to be set and consistent. The only part not set up by Alpa is the digital back so hence the shunning per application. (That said, I'm sure as an engineer you could argue that there's tolerance at every step in the chain ... Etc etc
oke
I am an engineer, dont hit me! :OT:. What Joseph Holmes mentioned of the various parts, camera body, lens etc contributing to the whole is correct for basically anything that is fabricated. Not only that, each camera body, lens assembly etc is assembled of various components that each are made to specified fabrication tolerances, in order for the summation of all tolerances to result in a total of the component (e.g. camera body) being itself within a specified tolerance. Certain expensive high tech machinery is capable to fabricate individual parts to within 0.01mm tolerance. My guess is that is what is used to fabricate an Alpa system, and perhaps also Arca and Cambo components? However, the individual tolerances indeed add up to a total whole that is likely adjusted in fabrication in various ways at different stages. In this regards my guess is that Alpa makes their system as whole to tighter quality control, thus tighter tolerances.
For a tech camera, it is also absolute important that the lenses be mounted and adjusted for the lens plane to be perfect parallel to the sensor plane (or back mount), and within tolerance to the acceptable tolerance to achieve focus. However, as Joseph Holmes mentioned, a loaner back was off by 175 micron, which is 0.175mm and which is huge in comparison (you will see that from what I type below). I would guess that back was a rare one…
Back to Alpa...
I would postulate that the precision and shimming of the alpa is only useful to ensure that if your lens is on the infinity stop, on the stop, not just the pointer on the mark, you are in focus.
at any other focus distance, the imprecision of the lens distance markings, coupled with however you measure distance to subject, completely override any high tolerance shimming. and if you are using the gg, shimming is not a factor.
Unlikely. The very nature of a lens being calibrated is that the infinity marking is calibrated since that is the reference mark for calibration. Thus, then consider that the turn of helical focus is large compared to the focus travel of the helical. This is why I believe what you state is not correct since you would be in control of focus point thanks to the calibrated tolerances of the system.
As example, per what I was told by Schneider the critical tolerance of camera systems with adjusted flange focal distance at infinity is 0.02mm. Thus the infinity index of the helical mount has to be adjusted individually to the effective focus length of each lens to get a sharp image on infinity, or if this is failed by 0.02mm one cannot get a sharp image (per Schneider). Thus, obvious the loaner back that Joseph Holmes mentioned that was off by 0.175mm had a big problem. I am not sure if backs are always made to within 0.02mm (perhaps they are not), but obvious the shimming of Alpas (or “fudge” factors applied to tables for Arcas) do bring to within that tolerance, provided of course... that the sensor is perfect parallel to that camera system (and if not Alpa is the only one that offer a system of corner shims to compensate…).
As we see, we are fighting for very precise tolerances…
Now, 0.02mm is indeed very, very small and the turn on helical focus will need to be larger, or simply it will be very difficult to adjust focus for an image at any other setting than the infinity lock (if… at all that has been perfect calibrated to within 0.02mm tolerance).
For the Schneider 24mm lens (which may be an exception with unusual small focus travel), the total focus travel from 1.5m to infinity is only 0.3mm. That is already very small. Thus it is perhaps not a wonder that Joseph Holmes found problem with this lens when focused at wide open. While the lens is recommended for use within f/5.6~11, what that means is that the lens is capable of being sharp within that aperture range. However that does not necessarily mean that the helical focus is capable to at all times reliable focus at those apertures, because a helical focus too has tolerances. Perhaps with use of a groundglass on an Alpa the chance of precise focus would improve, if assuming that the tolerances between groundglass and sensor are smaller than between sensor and helical and measurement to focus point??? Notably for the 24mm Alpa seem to offer their own helical focus... perhaps it is designed to be more precise??????
Alpa enable the user to shim a back using shim plates as small as 0.01mm in order to ensure that it be shimmed to their
system which is already within the critical 0.02mm tolerance. This is per my understanding (if I am correct) what really sets Alpa apart, including of course that all components also ensure of the total system (less back adapter plate) of being perfect parallel to lens plane. Thus all parts in Alpa system comes per my understanding already calibrated to very exact tolerances from the factory and which ensures to within 0.02mm critical tolerance. However please feel free to fill me in or correct me if I am wrong. I am keen to know and understand more.
Thus with any body or lens of the Alpa
system it should be possible to focus to high (acceptable) accuracy within the critical 0.02mm tolerance using any shimmed back. Other fabricators such as Arca and Cambo as example do not provide their systems with such readily precise tolerances from the factory but while perhaps fabricate
components to 0.01mm tolerance do not carry tolerances through to ensure the system being as exact as Alpa, or is my understanding incorrect??? Thus the lenses of those systems may need to be calibrated individually, or "fudge" factors may need to be applied such as for Arca, and which may or may not be different for different lenses (note). Correct? Indeed please feel free to shoot or correct me if I am wrong in what I write. This said, it is possible that the helical focus of Arca enables more exact adjustment of a focal point at wide open and with tape measure or laser, if a correctly calibrated “fudge” factor is applied to the scales, if that helical focus is fabricated to exact precise tolerances to match its turns…. For some applications perhaps that is desirable system and preferred means to work, albeit seem tad tedious. For landscapes I am uncertain if it is necessary with such system, Alpa's appear more simple to use to myself, yet we speak of perhaps preferences...
Back to tilts; with the critical 0.02mm tolerance it is my understanding that tilts cause complication, or??? 0.02mm is a very exact tolerance within which for each shot not requiring tilt that the lens will need to be zeroed to such tolerance. Perhaps the IQ backs with live view and focus confirmation a’la C1 “green mask” will assist in making that adjustment to point that it will not be much of an issue anymore, who knows??? However, there must also be risk that the tilt/zero adjustments get thrown out of tolerance between each shoot, or? Now, with this why would I speak of a Shen-Hao as a complement to an Alpa? Because the Alpa groundglass adhere to the within critical tolerance of the Alpa system and thus focus using the groundglass should result in acceptably sharp images using a shimmed back, if a custom mount with perfect flatness to fit the Alpa groundglass/back adapter is custom made for the Shen-Hao. The adjustments would be tedious and used for certain circumstances, but utilizing an aperture to yield a large DOF than wide open there will be some margin, perhaps even with a larger than required DOF the adjustment of tilt and shift will not be much issue???
Comments are very welcome and much appreciated. :thumbs:
Regards
Anders