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Forum: Pentax Medium Format 2 Days Ago  
P67-to P6 -adapters here: Rafcameras M68 (not real P6 mount)
Posted By kaseki
Replies: 4
Views: 243
Small gaps, particularly in combustion engine assembly, are typically measured with "feeler gauges." These gauges are manufactured so that one, two, or more gauges can be used together to get whatever thickness is needed. These should be widely available at very low cost.

Thickness is typically measured by calipers or dial calipers when the surfaces are flat and parallel. Types resembling "C" clamps are called "micrometers." These can have better interpolative precision than dial calipers. Some have ball ends for curved surfaces.

Look these up on USA Amazon for images.
Forum: Pentax Medium Format 09-25-2022, 09:15 AM  
Post your medium format photos!
Posted By kaseki
Replies: 18,847
Views: 2,975,918
Nice choo-choo train; looks like she was cleaned to the concours d'elegance standard. I like the sharpness over the entire scenes. Thanks.
Forum: Pentax Medium Format 09-25-2022, 09:00 AM  
Are my 135 and 200mm lenses really getting infinity? have big doubts.
Posted By kaseki
Replies: 11
Views: 727
One really needs such as a Nikon autocollimator on an optical bench to establish an atmospheric turbulence free 'infinity' projection for determining what the camera/lens is actually doing. As these are pricey, rental or lab testing may be more affordable. Otherwise, one might use Jupiter or the Moon as a subject on a still night where the local atmospheric temperature is nearly that of the ground. Even under these conditions, point sources such as stars will always blur and dance around in any near sea level atmosphere, less so at the top of Mauna Kea and similar altitudes.
Forum: Pentax Medium Format 09-16-2022, 08:29 AM  
Post your medium format photos!
Posted By kaseki
Replies: 18,847
Views: 2,975,918
Thanks!
Forum: Pentax Medium Format 09-15-2022, 07:32 AM  
Post your medium format photos!
Posted By kaseki
Replies: 18,847
Views: 2,975,918
Fascinating, albeit completely mysterious. Is that an undercut limestone hill?
Forum: Pentax Medium Format 09-09-2022, 12:01 PM  
Post your medium format photos!
Posted By kaseki
Replies: 18,847
Views: 2,975,918
Makes me think my retinas are bleached out due to forgetting my sun glasses.
Forum: Pentax Medium Format 08-24-2022, 07:17 AM  
Best apertures 67?
Posted By kaseki
Replies: 19
Views: 720
Thanks. I need to keep the focal plane sizes at hand for these recurring pixel discussions. 157 million 5-micron pixels, or 150 million 5.1-micron pixels.
Forum: Pentax Medium Format 08-23-2022, 06:28 PM  
Best apertures 67?
Posted By kaseki
Replies: 19
Views: 720
168 million 5-micron pixels in a 6 x 7 cm array. I wonder if that can be built without stitching using present or near-term technology.
Forum: Pentax Medium Format 08-23-2022, 09:23 AM  
Best apertures 67?
Posted By kaseki
Replies: 19
Views: 720
Someone here (I can forget a name in less than 10 minutes) was involved in detector array development, as I recall. He could elaborate on all the other real-life noises that need to be added (in quadrature) to the quantum intrinsic noise.

I have the latter DA zoom; I was fortunate to find a DFA 25mm. My intent was to also use it on film, but sadly the time I can allocate to photographic output has lately been too limited to deal with film also.
Forum: Pentax Medium Format 08-22-2022, 10:12 AM  
Best apertures 67?
Posted By kaseki
Replies: 19
Views: 720
Assume the wall is made of mini-bricks, each of which just fit one detector cell (pixel for the purpose of discussion). The noise in this cell cannot be less than the square root of the number of photoelectrons captured, rms, so the signal-to-noise ratio (S/N) is not less than the square root of the signal photoelectrons. Since a mini-brick maps to a pixel, the larger pixel will have a higher S/N if the f/no was conserved (larger lens objective) to go with the longer focal length (lens is scaled up by a factor of 10 in each dimension) and distances are the same. This implies that the larger pixel size camera will have better S/N and thus potentially greater dynamic range, ignoring other sources of cell noise that might scale with cell size or area. The larger pixels can store 10^2 more photoelectrons for the same well depth.

I like the 645z's dynamic range. This leads me to not be as interested in revising the 645z in the direction of more pixels stuffed into the 44 x 33 focal plane, but rather more pixels to cover the original ~ 60 x 45 (or whatever) focal plane that all but one of our lenses (as I recall) is designed to cover. In other words, a 100 Mpixel focal plane with present 645z cell sizes seems more useful than providing existing field coverage with root 2 higher linear resolution. At least we could eke out more field angle with the 25 mm.

The real-world question, though, is whether a large print of a given field of view landscape taken in daylight with 100 Mp over 44 x 33 will look different than if taken with 100 Mp over 60 x 45 with appropriate change in focal length.
Forum: Pentax Medium Format 08-19-2022, 07:14 AM  
Best apertures 67?
Posted By kaseki
Replies: 19
Views: 720
At the risk of oversimplifying here, smaller apertures (larger f/no's) typically reduce lens spherical and other aberrations, but increase diffraction effects. So, for a given lens assembly, the size of the physical pixel can change where the best balance is between these two effects. Going from monochromatic to color, the photographic result can be further modified by the Bayer (or whichever pattern is used) color mosaic and its processing algorithms.
Forum: Pentax Medium Format 08-15-2022, 07:46 AM  
220 holder finishes after 16 exposures only
Posted By kaseki
Replies: 7
Views: 404
I recall that also, but don't have any 220 backs so I never looked into using one in detail other than the physical modifications to the backs that were being discussed. The switch should be addressed in the user guide for the 645N.
Forum: Pentax Medium Format 08-14-2022, 09:52 AM  
220 holder finishes after 16 exposures only
Posted By kaseki
Replies: 7
Views: 404
Somewhere on this forum is a long thread about converting 220 backs to 120 backs. Using that you may be able to check whether any of the suggested conversion steps were taken.
Forum: Pentax Medium Format 08-10-2022, 09:01 AM  
Post your medium format photos!
Posted By kaseki
Replies: 18,847
Views: 2,975,918
I'm pleased. Thanks.
Forum: Pentax Medium Format 07-20-2022, 11:16 AM  
Recommend 3rd Party lenses for 645Z
Posted By kaseki
Replies: 8
Views: 510
Let's not forget the ARSAT 645 30mm f/3.5 fisheye lens (although relative to film it is a bit less of a fisheye imaging on the smaller digital focal plane of the z). The ARSAT lenses are from the remaining stock of a Soviet optics factory in Kyiv, so we might expect in the not too distant future to have only eBay recycling as a supply source.
Forum: Pentax Medium Format 07-13-2022, 07:53 AM  
Post your medium format photos!
Posted By kaseki
Replies: 18,847
Views: 2,975,918
Of the three, as a photo-tourist, I like this one best. It establishes the natural environment and, perhaps, reduces man's work to its proper perspective.

And a question raised by the kitty dental exam: Can anyone here watch a cat yawn, or even view a photo of one, and not yawn in sympathy?
Forum: Pentax Medium Format 07-05-2022, 07:17 AM  
Post your medium format photos!
Posted By kaseki
Replies: 18,847
Views: 2,975,918
Perhaps to minimize reduced resolution to (I would think) not worse than 0.5X in each axis, the initial image could be up-sampled by 2X or more in each axis, these sub-pixels translated to their new coordinates and rotated to their new orientations, and then original-size or double-sized new pixels would be assembled from the sub-pixels that are fully or predominantly within a given new pixel boundary. Luminance adjustment for the potentially varying number of sub-pixels in each new pixel would seem to be needed.
Forum: Pentax Medium Format 07-03-2022, 06:53 AM  
Different colors in JPG and RAW (Custom Image option)
Posted By kaseki
Replies: 18
Views: 930
Thanks for the clarification and the link.
Forum: Pentax Medium Format 07-02-2022, 08:48 AM  
Different colors in JPG and RAW (Custom Image option)
Posted By kaseki
Replies: 18
Views: 930
It has been my understanding that when the 'z' creates a raw PEF file, it includes some jpegs for the purpose of providing thumbnail images. It has been my further understanding that when Darktable and Raw Therapee open such a PEF file, they use the color balance data that was used for the jpegs to render the file for initial display. I can't comment on other PP softwares, but if my understandings are correct, then they should be able to determine the same information as the low cost spreads.
Forum: Pentax Medium Format 06-30-2022, 09:14 AM  
What's the longest shutter speed you use with Pentax 645z freehand?
Posted By kaseki
Replies: 29
Views: 1,417
I think it depends on the fact that with distributed grain sizes, the film acts like a myriad of different sized tiny pixels being stacked upon one another and providing (as Kodak Portra data sheets show) a gradual roll off in modulation transfer function (MTF) with (spatial) frequency. I forget what transverse blur MTF looks like, but I suspect that when multiplied over the frequency range with a film MTF the result is more benign (in some perhaps non-intuitive way) than when similarly applied to a spatially sampled data system (pixels) MTF (sinc squared function, as I recall).

(An excellent question for a late friend and coworker of mine whose understanding of this topic was unsurpassed, but who unfortunately is not with us to answer. His purchase of a 645 in the 1970s at a Boston area camera show we attended eventually led to my purchases much more recently.)
Forum: Pentax Medium Format 06-29-2022, 08:18 PM  
What's the longest shutter speed you use with Pentax 645z freehand?
Posted By kaseki
Replies: 29
Views: 1,417
First, it must be emphasized that we are addressing angular motion, and not translation motion, which would have an effect in the near field. (We have to note that it is pixel size vs. focal length that determines image translation due to angular motion.) When you use the word "more sensitive" I interpret it to mean blur extending farther over across some number of pixels. Fundamentally, we are addressing "smear" and any motion will cause smear; the only question is how noticeable it is. In principle, one could take the image from a 50 Mp camera and derez it to a 25 Mp equivalent (to the extent that a linear root 2 derez was achievable) and the blur would be less noticeable even as the pixel resolution would be more noticeable (in a large enough print).

Second, we must keep in mind that pixel size affects the noise level that a given ISO value will yield in that camera, so in reality, smear sensitivity and noise sensitivity may be interactive in a given result. Other ways of looking at pixel size effects include:

  • From the point of view of modulation transfer function effects, the product of all the MTFs to yield the MTF of the actual read out image is being compared to the same final MTF times the MTF of a smear motion. The effect will be a reduction of the higher frequencies in the image, but if the pixels are large enough in angle space the effect will be initially hidden for small smears.

  • As another attempt at description, assume a diffraction free optical system, and infinitely fine grain film. The image will have infinite resolution. Then read it out by sampling the density pattern over a series of contiguous areas representing pixels. Compare to reading out a smeared version of this image. I can't claim that larger pixels provide an improvement over smaller pixels when the end result is observed with some level of degraded image resolution (eye, monitor, etc.). No matter what the pixel size is, smear always spills the image across more than one pixel, but not necessarily over exactly two pixels.


tl;dr: I think you got it right, given a lot of assumptions (that if ignored may lead to misleading understanding). I think I prefer more resolution because there is more information to work with such that post processing may be able to be more effective (for someone skilled in the art).
Forum: Pentax Medium Format 06-25-2022, 06:24 AM  
What's the longest shutter speed you use with Pentax 645z freehand?
Posted By kaseki
Replies: 29
Views: 1,417
An angular rotation is an angular rotation, and thus the image of a point at infinity's focal position movement on the focal plane is proportional to the focal length of the lens times the rotation angle. The field of view of a pixel is the pixel size divided by the focal length (in radians). Hence, a focal plane with double the reference focal plane pixel size using a lens of twice the focal length of that used with the reference focal plane will be equally sensitive to rotational motion.

So, a 100 Mpixel focal plane of the 645z size will have more sensitivity to angular motion -- pixel-peeping-wise -- than the present 51 Mpixel focal plane, whereas a 100 Mpixel focal plane covering the original film plane of 45 x 60 mm will have the same sensitivity. (Roughly)
Forum: Pentax Medium Format 06-14-2022, 07:15 AM  
Post your medium format photos!
Posted By kaseki
Replies: 18,847
Views: 2,975,918
Thanks!
Forum: Pentax Medium Format 06-10-2022, 09:41 AM  
Post your medium format photos!
Posted By kaseki
Replies: 18,847
Views: 2,975,918
As Minor White once remarked: "One should not only photograph things for what they are but for what else they are." Hence, even if only you see the the background, that is sufficient. :D
Forum: Pentax DSLR Discussion 05-29-2022, 07:41 AM  
Sticky: Pentax Proud Owner Signature Banners : K-3 III, K-1, K-5 II, K-70, and many more!
Posted By kaseki
Replies: 340
Views: 300,491
Take the images you want and combine them in software (e.g., Gimp) into a single photo. My image below is a combo of two.
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