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Forum: Pentax K-1 & K-1 II 12 Hours Ago  
Pixel Shift Question
Posted By AstroDave
Replies: 8
Views: 243
Take a look here: How Pentax Pixel Shifting Works - Articles and Tips |

And note especially this takeout: "The net result is that all 24 million pixels on the K-3 II's sensor will capture complete red, green, and blue (RBG) color data without the need for demosaicing." (hmm - note that "RBG" should be "RGB" to match the preceding text!)

So, you wind up with the same number of pixels (i.e. the image size is the same - as you note), but each of those pixels has, in some sense, more complete color information. I guess that makes the file size bigger.
Forum: Pentax SLR Lens Discussion 1 Day Ago  
Do I *need* to upgrade from the "kit" lens?
Posted By AstroDave
Replies: 47
Views: 1,180
This ^^^ And, you DO want to upgrade! I had 2 or 3 18-55s that came with various cameras, and they did not last here for very long!

I have the (original) Pentax 16-50 - it has not been on my camera(s) since I got a Sigma 17-50, and the 18-135 is now what lives on my K-3. It is versatile, covers a lot of zoom range, and for me is is quite acceptable optically.

I am unimpressed by my copy of the 50/1.8 - the focal plane is not flat. I can not find any f-stop at which it performs adequately. Lots of others rave about this lens, but not me. I think your 50 / 1.7 will outperform it.
Forum: Mini-Challenges, Games, and Photo Stories 2 Days Ago  
Thematic In 1990, Hubble Space Telescope had ___ megapixels.
Posted By AstroDave
Replies: 18
Views: 486
Virtually all astronomical imaging systems use monochromatic sensors. To make color images, or to isolate the light from a particular atom or molecule, color filters (sometimes quite narrowband - a few nm - for spectra lines) are used. A typical camera will have a filter wheel with anywhere from half a dozen or so up to perhaps 20 filters that can be rotated in front of the sensor, in a few seconds.

Not sure if you are kidding or not - but there have been several missions to repair/replace/upgrade Hubble cameras. (My wife is a Co-PI on one of these: STIS - Space Telescope Imaging Spectrograph, which went up in the 90s). The system is designed so that complete camera assemblies (about the size of a telephone booth - that's an anachronistic reference! Have you used a telephone booth lately!?) can be pulled out and replaced with another module. We lowly earthling DSLR users have interchangeable lens, whereas Hubble pretty much has a fixed lens (mirror!) and they change the camera as desired. Actually, HST has several "cameras" available at any one time which can be selected by a secondary mirror, as do all big earth-based telescopes, too. Typically, 3 or 4 instruments can be selected on a few minutes notice. Instruments can be swapped out during the daytime when it is time to put in a new camera (or whatever).

Once the Webb telescope is launched, there will be NO repair/servicing missions. Its orbit is such that it will be a million miles or so from earth, unlike Hubble in its few hundred mile high orbit.

Actually,it is the (much smaller!) secondary mirrors which will be flexed, at rates around hundreds of times per second - you can't wiggle a multi-meter piece of glass that fast!
Forum: Flashes, Lighting, and Studio 2 Days Ago  
multi flash-stroboscopic with cactus v6ii not having much
Posted By AstroDave
Replies: 4
Views: 276
Are you using one v6ii flash unit or five? If only 1, I doubt it will recycle/charge back up that fast. (HSS is different in that it fires the (a single) flash at very short intervals - a few milliseconds (at lower power per flash).) Sounds like your desired operation is somewhere in between?
Forum: General Photography 2 Days Ago  
Why are APS-C sensors pretty much around 24 MP
Posted By AstroDave
Replies: 38
Views: 1,275
Hmm - depends on your definition, perhaps.

Diffraction proper (i.e. the spread of a point source due to aperture size) is independent of the pixels. (For a particular camera) It depends only on the f-stop (and contrary to some pseudo-learned claims), not the lens focal length in terms of size in the image in terms of pixels (i.e. the Airy spot will be the same size in pixels - the nominally smaller spot for a longer lens* is spread out by the focal length - the effects exactly offset).

How you see the diffraction (i.e. how many pixels across the Airy spot is) does, of course, depend on pixel density. The pixel density just effects how the diffraction is imaged, not what the diffraction is.

* The angular size of the central diffraction spot (in radians) is ~lambda/D (lambda is the wavelength, D is the aperture diameter). For a longer focal length lens, D is bigger (more millimeters across) for a given f-stop value, so the diffraction angular size is smaller. But, when projected on to the sensor, the angular size is multiplied by the focal length, so the physical size of the diffraction spot in microns (or whatever unit you prefer) is the SAME!
Forum: Photographic Technique 3 Days Ago  
Glider ride
Posted By AstroDave
Replies: 22
Views: 641
Unless the air is very clear & dry on the day of your flight, you might consider a UV or haze filter for shots of the ground from the air.

Get some pre- and post-flight shots (if you don't get in the way) - airport ambience: you will be out on the field (depending on the size of the airport, things look much different out there than when walking down the jetway to board a passenger flight); hooking up the tow plane; when the tow plane releases your sailplane; coming in for the landing; ... .

No matter what, as others have said - Enjoy the ride!

Dave (an occasional motorized pilot, seen here doing the pre-flight for a trip to Albuquerque)
Forum: Weekly Photo Challenges 5 Days Ago  
Weekly Challenge WEEKLY CHALLENGE #541 Those who came before
Posted By AstroDave
Replies: 21
Views: 542
This may be the most powerful/moving image ever for me on PF. My wife is Jewish, and lost many relatives in the Holocaust. Brings tears to my eyes.
Forum: Troubleshooting and Beginner Help 5 Days Ago  
camera K70 has lost power since being connected to Laptop
Posted By AstroDave
Replies: 22
Views: 596
I guess we disagree - although it depends, I suppose, on what "completely discharged" is. This battery was reading ZERO volts on my voltmeter and had been that way for perhaps 10 minutes or so - I'd call that completely discharged. The battery was being run down with my test discharge circuit, NOT in the camera, so there was NO protection/detection of undervoltage and automatic shut-off. And, it did recover and was one of the ones that went through my several test discharge cycles with no apparent loss of capacity (after that, though, I did realize I ought to keep a better eye on the discharge cycle so nothing bottomed out again!)
Forum: Pentax DSLR Discussion 5 Days Ago  
Infrared white balance issue
Posted By AstroDave
Replies: 17
Views: 451
Not sure how "grey"! Read on:

Perhaps this begs the question: What, really, is “white” balance for an IR-converted camera?

As we all (should) know, our standard color digital cameras use a sensor chip (CCD or CMOS) with an array of R, G, and B filters deposited just on top of the actual sensor pixels.

In order to get a proper-looking image, the camera must adjust the relative proportions of the signals from the various filtered sensors. Selecting a WB setting in the camera sets up the expected ratio, depending on the spectrum of the light source (how much R, G, or B light there is coming from, e.g., the Sun). The actual application of the relative factors can be done internally by the camera for the creation of jpeg files, or be left to PP software in the case of RAW files.

One can do a “custom white balance” by taking a shot of a “gray scale” card and then having the camera adjust the amount of R,G,B from the sensor so that the card looks “gray” (or white - a sheet of white paper works just about as well). When we do this we are relying on the maker of the gray card to guarantee that over some range of light wavelengths, the card is truly gray: it reflects the same fractional amount of R, G, and B at all wavelengths. For normal color photography, the wavelength range will be from around 400 nm (very deep blue/violet) to somewhere between 650 and 700 nm (very red). And, I suspect, the average gray scale maker does not worry about the reflectivity of their product outside this range of wavelengths.

You can readily find the color response of cameras on the web. Look here:

RGB: All About Digital Camera Color

Bin space: Spectral response of Nikon DSLRs (D90 and D300s)

Nikon CFA Spectral Power Distribution | Strolls with my Dog

(Sorry about all the Nikons!)

Other cameras have quite similar responses, including our Pentaxes. Notice that sharp falloff at the red end of the spectrum - this is due to that IR-Cut filter.

What happens when that IR filter is removed? Colors from any standard processing program may well look bonkers!

You can find the intrinsic sensitivity of many camera sensors with a bit of web searching (i.e “Sony CCD sensors spectral response”). Here’s something you might find: This plot shows the non-IR-filtered behavior of a Sony IMX250 chip - note that the Blue and Green filters on the sensor allow a lot of leakage out in the IR and the overall sensitivity is quite high out to almost 1000 nm! The concept of blue or green or red color response is not so clear cut when that IR filter is removed.

Of course, the response of your modified camera will depend on the external filter you use, and the transmissivity of your lenses, along with the idiosyncracies of your sensor. Processing with RAW and choosing the behavior of various color weighting / temperature etc. may well give you some very strange looking “white”balance effects.

I think you will just have to play around to find what you like, and might even consider software that will allow you to pull out, say, just the Red part of the RAW data. This will correspond most closely to what you would image if your were using a true IR camera - i.e. something that responded only to wavelengths longer than ~700 nm.
Forum: Troubleshooting and Beginner Help 5 Days Ago  
camera K70 has lost power since being connected to Laptop
Posted By AstroDave
Replies: 22
Views: 596
It might recover if you leave it on the charger long enough. When I was doing battery testing, I (inadvertently!) let a battery completely discharge. I feared it might be kaput, but after 10-20 minutes on the charger, the green charging light came on and the battery seems to have fully recovered. Certainly worth a try to save a $50 battery!
Forum: Weekly Photo Challenges 6 Days Ago  
Weekly Challenge Weekly Challenge #540- Silhouette or Very High Contrast
Posted By AstroDave
Replies: 29
Views: 966
Thank you all for your entries - all of these are shots that should make somebody appreciative of your efforts.

I don’t feel qualified to make quite as many comments on the artistry of all the entries, as noelcmn did last week. A bit sadly, I recognize that I am much more of a technician than an artist when it comes to my picture taking! :( (Please keep that in mind if you feel I missed the special element in your photo! ;) )

But, I have to pick a winner, and do have some comments, too, so:

Gump - the Winner! This one just grabbed me when I first saw it - it meets both the silhouette criterion and the high-contrast one. I especially like all the lit-up wave tips. (I’d suggest cropping off the bottom a bit for maximum impact.)

pacerr - a very close second place - love the trees, shadows (especially the diagonals at the lower front right), and the contrast of the snow (?) in a swamp(?) (Are those Cypress knees?). One gets pulled right in to the frame and wants to go for a walk (assuming I won’t sink!?).

noelcmn - third place - nice shot of the fisherman (who is way bolder than I would be out there in those waves!) and nice adherence to my perception/intention of the theme(s)

Honorable mentions (that and a few bucks will get you coffee and a donut!)

aaacb - nice silhouette, and the shadow detail (high contrast!) is fine for the theme - too bad you couldn’t get the entire shadow of the biker

SpecialK - I like the bird: great adherence to the theme - this is perhaps the most “silhouetty” of all the entires

Vaskebjorn - your well-isolated flower does indeed match what I had in mind

Perfessor5646 - as a dog lover, I think this may be the cutest of all the entries!

atupdate - nice silhouettes of interesting subjects

CedrusMacro - I like the stark, geometric pattern/contrast

Other entries

turbo_bird - The B&W conversion for this works, along with the black background. I like the dry ice-induced wisps. Playing around with this for a theme might make some really interesting shots.

charliezap - nice silhouette; another bold venturer amid the ocean waves. (Colors are a bit funky?)

jacamar - nicely contrasty, including the contrast between the action in the foreground and the stolid high rises in the background

The sunset silhouettes:

These are all nice shots, but they just have more visible other detail in them than I had in mind:

Bruce Clark - if my sister was judging, you might be first!

So, thanks again to all participants, and over to Gump to undertake next week’s contest.
Forum: Pentax DSLR Discussion 07-19-2021, 06:36 PM  
Is Pentax thinking about a new k1?
Posted By AstroDave
Replies: 71
Views: 1,900
The most wonderful "upgrade" would be an old split-image focus screen!!!!!!!!!! I can't imagine why that has disappeared, unless it messes up either (or both) modern light metering and electronic focusing.
Forum: Weekly Photo Challenges 07-19-2021, 03:59 PM  
Weekly Challenge Weekly Challenge #540- Silhouette or Very High Contrast
Posted By AstroDave
Replies: 29
Views: 966
Wow - you folks are gonna make this a tough one to judge/decide!

A few more hours to go - so keep 'em coming!
Forum: Troubleshooting and Beginner Help 07-19-2021, 03:15 PM  
K3 - images corrupt on Sandisk 128gb SDXC card
Posted By AstroDave
Replies: 11
Views: 385
You are putting a lot of eggs in one basket with a card that big. I tend to prefer smaller cards (16 GB is my sweet spot - but I don't do a LOT of shooting at one time) so if a card goes bad, both my lost investment and lost shots are minimized.

Forum: Pentax DSLR Discussion 07-19-2021, 03:11 PM  
"New" camera body suggestion
Posted By AstroDave
Replies: 71
Views: 1,882
My two cents (fraction of a krone): I went from a K-5 to the K-3 when the K-3 came out, and NEVER considered looking back. I'm not sure about shipping to Sweden, but there are K-3's on US Ebay for around $300.

I think you'd love one (and its heft :lol:)
Forum: General Photography 07-19-2021, 02:52 PM  
Why are APS-C sensors pretty much around 24 MP
Posted By AstroDave
Replies: 38
Views: 1,275
Well, you can sorta cheat.

Radio astronomers have been using the CLEAN technique ( for quite some time. The gist is to find the peak in an image, and subtract some fraction of what you know the imaging device point source response shape looks like (the diffraction spot, aka Airy Disk, for an optical camera) from the image, centered on that peak position. The typical radio astronomy interferometer beam map is quite messy, in terms of sidelobes, and this technique works AMAZINGLY well. It was an integral part in making that "first map of a black hole" for the galaxy M87 a while ago. (As an aside, I was the first person in the world to actually KNOW that M87 had a compact radio source in it, back around 1970 or 1971 when I was a graduate student manning the night shift at the computer center where we were processing our VLBI (Very-long-baseline interferometry - Wikipedia) data tapes.)

A similar scheme was used to improve the early images from Hubble Space Telescope, back when it first went into operation with its mis-figured mirror and produced some rather ugly images.

How well this all works depends on the signal-to-noise ratio, and how well you really know the point-spread function (PSF). i.e. for our cameras, the apertures are not round (which is what an Airy disk is calculated for), so a more complicated PSF is needed to dig very deep. And, it probably varies across the image, as a function of look angle back out through the iris and lens.
Forum: Pentax Camera and Field Accessories 07-16-2021, 07:14 PM  
Turn old SLR into DSLR
Posted By AstroDave
Replies: 18
Views: 1,107
This ^^^^^

Although as a serious DIY-er, this looks interesting, if you are really after a small camera, Bob 256 has it right- there are any number of old point-n-shoots on Ebay for $20-30 or less!
Forum: Pentax Medium Format 07-16-2021, 07:06 PM  
6x7 slide projectors
Posted By AstroDave
Replies: 6
Views: 355
You might go check with the/a local university science department and see what they have in the way of old projectors. They would probably be glad to give them to you (depending on disposal rules for company/state-owned property!).

I suspect that most/all of their old scientific slide projectors have disappeared, but if not, they might serve your purpose. Back in the 60s/70/80s, every scientist had slides for presentation with their talks. These were somewhere on the order of 3"x4" in size (I have to go find some of my old ones to measure!) that were projected. These were generally just B&W (prepped by the local graphics folks; I think from Polaroid slide film).

As Bob256 hos noted though, the lights in these projectors (very hot incandescent bulbs!) might do a number on your slide film.
Forum: Pentax SLR Lens Discussion 07-16-2021, 10:17 AM  
To hood or not to hood?
Posted By AstroDave
Replies: 73
Views: 1,723
I seem to be running against the tide: I just about never use a hood. I think they are too big and cumbersome, and I doubt I have ever really had a shot messed up by flare. But, I don't shoot into the sun!

I always pop my lens cap back on as soon as I've taken a shot, so I don't worry much about lens damage.
Forum: General Photography 07-15-2021, 10:42 PM  
comments and opinions needed ASAP Phoenix trip Grand Canyon and Ball Games - thanks
Posted By AstroDave
Replies: 111
Views: 2,085
How much rain did you get at/going to/from GC? Upper Flagstaff nearly got washed away Tuesday and Wednesday both, thanks to several inches of rain each day on a forest fire burn scar.

We got REALLY rained on Wednesday here at home, and have had 2.35 inches total in the backyard rain gauge since Monday.

Hope you had a good trip (did you wave as you went by?!)
Forum: Pentax K-1 & K-1 II 07-15-2021, 06:16 AM  
K-1 Lens Advice
Posted By AstroDave
Replies: 36
Views: 1,192
What's "wrong" with this one? Unless you need wider, or a bigger aperture, you are not going to do much better in terms of flexibility. It's a pretty good "kit" lens, which you already seem to be comfortable with.
Forum: Pentax DSLR Discussion 07-13-2021, 07:38 PM  
Have brand new K-70 - worth taking a loss 7 trading for a KP?
Posted By AstroDave
Replies: 34
Views: 2,018
You need to upgrade your domestic companions?!

We are both (retired) professional astronomers here, so when I ask my wife to keep the lights on low, she knows what I am talking about! (Unless the dog HAS to go out!)
Forum: Do-It-Yourself 07-13-2021, 06:49 PM  
A New Macro Rail Set-Up
Posted By AstroDave
Replies: 6
Views: 343
Color me impressed! I do a lot of DIY stuff, and this is way beyond anything I've done/tried.

And what better as a subject than a resistor (at least that's what it looks like!)!
Forum: Pentax K-3 & K-3 II 07-13-2021, 07:26 AM  
Lightning Trigger
Posted By AstroDave
Replies: 12
Views: 470
And even then, you are still about a tenth of a second "behind" - that is the response time of the camera from an electronic (or finger press) "fire" command until something actually happens: Shutter Lag - K-3 and K-1 measurements -

I like your ideas about just leaving the shutter open until a flash occurs, as well as the thought that an AM radio static burst might be a slightly earlier precursor.

Not to mention the idea of modifying a remote flash trigger! Genius! I'd love more details on that (send me a PM if you will).
Forum: General Talk 07-12-2021, 08:38 PM  
What music are you currently listening to?
Posted By AstroDave
Replies: 8,637
Views: 631,453
Gabrielli, Monteverdi, Schutz - anything before about 1650 AD! Well, I might allow some Bach, Haydn, and Mozart, too.
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