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Forum: Flashes, Lighting, and Studio 1 Day Ago  
Product Photography: What do I need?
Posted By AstroDave
Replies: 23
Views: 568
I wouldn't get fixated on needing flash/speedlights. Your products aren't going anywhere, so moderate length exposures are fine (even several seconds). Just use regular light bulbs (but beware of color effects), and as swanlefitte says - USE A GRAY CARD to set your color balance! Consider have a small gray card in the corner of every shot, and maybe even a scale/ruler. That is what I do for art works.
Forum: Troubleshooting and Beginner Help 6 Days Ago  
KS-2 astrophotography, long exposures
Posted By AstroDave
Replies: 7
Views: 462
Check the effect of custom setting 7 (in C1 menu). I'm not sure about the KS-2, but in (at least some) other Pentax cameras, you can set a bulb option so you can

1) hold the shutter button down for the entire length of an exposure

or

2) push the shutter button once to open the shutter, and it then stays open until you push the button again
Forum: Photographic Technique 6 Days Ago  
Studio How I photograph Scale Models and other objects
Posted By AstroDave
Replies: 12
Views: 813
Nice pix.

Your lighting will be more diffuse (if that's what you want), but perhaps a bit fainter, if you move your side speedlights farther away from the diffusing material. That results in a more uniform illumination of the diffusing screen.
Forum: General Photography 06-30-2020, 01:14 PM  
Just Chatting
Posted By AstroDave
Replies: 18
Views: 855
Yes! Give them the experience(s) you had with Dad and Grandad. I, too, owe my Dad for my photographic inclinations.
Forum: Troubleshooting and Beginner Help 06-28-2020, 04:15 PM  
*ist DS won't shoot in AF or MF modes
Posted By AstroDave
Replies: 13
Views: 556
What batteries are you using? I'm learning the hard way that the *ist series seems to be fussy about batteries, especially if it has sat for a while. Try repeatedly turning it off an on, and it might wake up.
Forum: Pentax SLR Lens Discussion 06-28-2020, 04:05 PM  
Ray Tracing and Other Optical Measurements from Patents
Posted By AstroDave
Replies: 14
Views: 609
Clearly, I did! Mea culpa.

I still think it would be nice to have that at the bottom/end of each article as well, since that's where you wind up as you are reading it, especially if you have scrolled down aways and don't have the top of the page on screen, so you don't have to scroll back up!
Forum: Pentax SLR Lens Discussion 06-28-2020, 12:43 PM  
Half-Step Aperture Value Incorrectly Labeled!
Posted By AstroDave
Replies: 31
Views: 838
As Not a Number points out, it has to do with computer binary.

I think IBM gets credit (blame?) from all the way back to their 360 mainframes from the 60s. By the way, what is half a byte? A nibble (4 bits)!

As to 1024, that's 2 to the tenth power (still more influence from binary computers, and as you discovered the hard way) almost a thousand.
Forum: Pentax SLR Lens Discussion 06-28-2020, 06:00 AM  
Ray Tracing and Other Optical Measurements from Patents
Posted By AstroDave
Replies: 14
Views: 609
Nice information.

It would be very convenient in your Optics Primer to be able to navigate between pages (or at least on to your intended next page) without having to go back to the first page.
Forum: Pentax SLR Lens Discussion 06-27-2020, 07:40 PM  
Half-Step Aperture Value Incorrectly Labeled!
Posted By AstroDave
Replies: 31
Views: 838
Very interesting!

I kinda have that "maximum aperture support" idea - make it look like that widest opening is a bit more than it really is.
Forum: Pentax SLR Lens Discussion 06-27-2020, 07:05 PM  
Half-Step Aperture Value Incorrectly Labeled!
Posted By AstroDave
Replies: 31
Views: 838
I'll agree with the scholar, but some might argue the gentleman part (although I usually do try!).

Interesting on the K100d (which was my first digital Pentax, by the way, but is long gone). So this seems to pretty much go all the way back to the earliest digitals.

Can somebody chime in with film camera values? I never had a fancy Pentax film SLR, so I am not sure what they display. The ME Super was as good as I got in the film line.
Forum: Pentax SLR Lens Discussion 06-27-2020, 03:47 PM  
Half-Step Aperture Value Incorrectly Labeled!
Posted By AstroDave
Replies: 31
Views: 838
Towards the end of this discussion (Optical Measurements of Pentax ME Super Shutter Speed - PentaxForums.com , forum member swanlefitte notes




This prompted me to check my K-1 and K-3. They, too, display the (incorrect!) value of 4.5 for the step selected between 4 and 5.6 when in half-step EV mode.

What gives?! Is this just a mis-labeling or do these cameras actually shoot at the incorrect value? Let’s find out (I’ve got nothing much better to do during these days of stay-at-home orders).

So, off to take some photos of my Kodak gray card with my K-1 and the SMC Pentax-DA 50mm 1.8 lens across a range of f-stops. I chose the 1.8 lens so that I could measure over the widest range of stops.

I took frames at half-step and third-step values from f/1.8 to f/22. On the way up and down, the full-step values were shot 2 or 3 times. The repeatability is very good (a testament to my lights and power supply!).

I used RawDigger to extract the actual R,G,B pixel values averaged over a small rectangle in the center of the gray card, similar to what I did when I checked camera linearity a while back (How Linear Are Some Pentax Camera Sensors? - PentaxForums.com)

Just as with shutter speeds, it is important to note that the f-stop values reported for just about all cameras are not quite correct. The numbers have been rounded for convenience. This article has a comprehensive discussion: https://www.scantips.com/lights/fstop2.html

Here are the results, plotted with the (expected/hoped for) mathematically correct aperture values (a lot of fourth and sixth roots of 2 involved) - including the correct value (nearly f/4.8 rather than f/4.5) for the half EV step between f/4 and f/5.6




The data are pretty consistent, although not nearly as good as for the shutter speed results referenced above, and with some systematic deviations. The data clearly show some curvature - peaking in the middle, for all three colors. If pixel value versus aperture were exactly linear, the power law fits should have exponents of 2.00 , rather than the somewhat larger values here (typically 2.12 to 2.13). These values suggest that at the smallest aperture (largest f values), not as much light gets to the sensor as it “should.”

All in all, though, I am fairly impressed with the consistency and goodness of the data. The mechanical tolerances in lens aperture at the smallest openings have to be a small fraction of a millimeter. (At f/16, the aperture here is around 3mm (=50mm/16), and a 0.1mm error results in an aperture area (same as exposure) error of almost 7%.)

So, what about the original question: what is going on with our Pentax cameras when used in half-step mode between f/4 and f/5.6?

Here’s the same plot as above, except I have used the exact value of 4.5 for the half-step value. The points (circled) are clearly out of place.



Looks like Pentax has just mis-labeled this value, and it carries over to the EXIF data, too. Maybe in a future round of updates, this could be fixed.

This dichotomy appears here in the Forum article The Fundamental of Exposure (The Fundamentals of Exposure - Aperture - In-Depth Articles), where on page 5 the half-step values list F4.8, whereas the animated GIF on the same page has the lens stepping through F4.5 !

What value do you see for your camera(s) (including non-Pentax?) for this mystery stop? Does this error go all the way back to the first digital bodies, or even back to the film SLRs?
Forum: Pentax Film SLR Discussion 06-27-2020, 05:39 AM  
Optical Measurements of Pentax ME Super Shutter Speed
Posted By AstroDave
Replies: 15
Views: 639
Guess I'm lazy - I just bought my photodiodes (QSD2030)! They were only 60 cents a piece - the postage is as much. They have VERY fast response time - 5 nanoseconds! (Depends on load resistance - I use a few kilohms which slows them down a bit)

"Floppy drive"? or CD-Rom or DVD - much more likely to contain phototransistors!



Yeah - we are probably worrying way too much about this!
Forum: Pentax Film SLR Discussion 06-26-2020, 04:56 PM  
Optical Measurements of Pentax ME Super Shutter Speed
Posted By AstroDave
Replies: 15
Views: 639
I checked - the K-1 and K-3 indeed report f/4.5 between f/4.0 and f/5.6 when in half-step mode. That is just plain WRONG!! WOW - Pentax get your act together. I hope others can check other models. At some point, I'll check to see if the actual exposures that result are equally wrong.

The Q7 and G15 have only 1/3 step mode. And, the Canon Rebel XS was borrowed from a friend, so I can't check it conveniently.

I thought it might have to do with what lens is attached (i.e. if the largest opening is close to 4.5), but I get the same results with the 100WR macro attached (minimum f-stop of 2.8). The 1/3 stops seem to be listed correctly.
Forum: Pentax Film SLR Discussion 06-26-2020, 02:31 PM  
Optical Measurements of Pentax ME Super Shutter Speed
Posted By AstroDave
Replies: 15
Views: 639
I never thought to check Wikipedia (or the Forum review of the Super)! I did look in Wikipedia about shutters, but not specifically the ME Super (duh)! I think that explains a lot - especially running mechanically at (more-or-less) 1/125. I wouldn't be surprised if my measurements of the fastest speeds are somehow biased by my technique - taking the half-power levels on the rise and fall does not necessarily give the effective shutter time. I'd need to integrate the curve, being careful about amplitude calibration.

I was impressed with how accurate the speeds were for what I was thinking was a mechanical shutter that was ~40 years old! Now I know better. Some years back, I used a similar set up to measure an old (surely mechanical!) Topcon that my father had, and the values there were a lot farther from nominal.

I've improved my LED flasher system to a 256 LED array. I need to report those results (for K-1, K-3, Q7, and a couple of Canons: G15 and Rebel XS). The surprise there is that there is a very systematic deviation from nominal when the big Pentaxes are used for 1/3 shutter speed intervals, alternating between 3% high and 3% low between the "standard" values.
Forum: Pentax Film SLR Discussion 06-26-2020, 11:51 AM  
Optical Measurements of Pentax ME Super Shutter Speed
Posted By AstroDave
Replies: 15
Views: 639
Could be, and I even knew about this - see my post on "true" shutter speeds: True Shutter Speeds compared to Nominal Shutter Speeds - Actual Measurements - PentaxForums.com and references therein.

I don't know whether cameras of the ME Super vintage used electronic shutter control or not. Maybe somebody here can enlighten us on this. If they do, your suggestion makes a lot of sense, and the results do hang together better except for the 3 fastest speeds. But there is still the mystery of the 1/125 speed.



Take a look here: PentaxForums.com - BB Code List

I used the table code to stick my results in. It took a try or two to get the syntax correct. If you'd like to see the actual text I used, let me know and I can send you a PM or email.



Thank you. The variations are indeed very small. At 1/125, there is a bit of systematic difference between the viewfinder value (values of 10.1, 10.0, and 10.1 milliseconds) and the X dial setting (9.38 and 9.40 milliseconds). If you take the overall scatter of these 5 values, the standard deviation is about 0.3 (only ~3%) and by far bigger than for any other of the speeds. For instance, at 1/8 second, I got 117, 120, 119, 119, 119, and 120 milliseconds. This is where I started and took a bunch of measurements to make sure I understood the system.





'Fraid not. Despite my technical prowess, I am a dinosaur when it comes to cell phones! I have a flip phone (which costs me about $20/year). I'm retired and we have no kids, so no need for instant/constant communications!!!!!
Forum: Pentax Film SLR Discussion 06-25-2020, 03:46 PM  
Optical Measurements of Pentax ME Super Shutter Speed
Posted By AstroDave
Replies: 15
Views: 639
Two month’s ago (!), inspired by this posting (Shutter speed tester - PentaxForums.com) about measuring shutter speeds for a film camera, I put together a simple system to make such measurements, using my Pentax ME Super A as the device under test (DUT in electronics jargon).

(Aside - the photoplug tester used by forum member Vendee is almost certainly using an integrating detector with a modest time constant - I could duplicate those swoopy light curves when I used my photodetector in an integrator circuit.)

Here’s a shot of the setup: I configured a cheap green laser pointer module from Ebay to shoot through the ME Super with its film back open and on to a high-speed photodiode light detector. The photodiode response (scope trace goes high when light shines on the diode) was displayed on a storage oscilloscope, from which I could make screen dump copies of the displayed light curves. The overall response time of the system is at the microsecond (millionth of second) level.




The scope also has a pair of cursors that can be placed over the time response trace. I set those at the half-power levels at the start and end of the light pulse that came through the shutter and read directly the time interval. This interval is how long the shutter is open. Typically, I made 2 or 3 measurements of the shutter time. The values given here are the average of those measurements.

For longer durations, more than a 1/30 second or so, the rise and fall of the light curve is very sharp at the time resolution of the scope when showing the entire open trace. For shorter shutter speeds, the rise/fall times are resolved, and at the very shortest times (1/1000 and 1/2000 seconds), the shutter is barely “open” for the given duration of the exposure.

Here are the results in tabular form, with the nominal shutter speed (i.e. the speed shown in the camera display - the ME Super shows the shutter speed in the viewfinder display), the nominal speed given in milliseconds, the measured shutter speed in milliseconds, and the ratio between them, in the sense of measured divided by nominal.


Nominal Nominal Measured Ratio(sec)(millisec)(millisec) 1/20000.500.440.881/100010.870.871/50021.820.911/25043.820.961/12589.801.221/6016.715.40.921/3033.330.60.921/1566.7600.901/81251190.951/42502390.961/25004750.95110009610.962200019160.964400038200.96


And here are a few of the scope screen dumps.

For a quarter of a second (the vertical purple lines are the time measurement cursors) the edges are nice and sharp:




Originally, I wondered why there were those wiggles at the beginning of the trace (they were on most of the traces). Then, I realized that the camera was sitting on the board to which the rest of the system was mounted. The shutter slap was bouncing things around, in particular the detector diode with respect to the laser beam. When I picked the camera up off the board before I fired it, things were much better:




At short shutter speeds, the traces are not quite so distinct. Here’s what you see for 1/500 of a second. The open and close parts of the trace take place over a few hundred microseconds (the time divisions in this image are 200 microseconds):




And, at 1/2000 second, all you get is a bump, with about 400 microsecond rise and fall times:




Here’s a plot of the results. The speeds are consistently shorter than nominal, by about 5-10% (around a tenth of a stop) and even shorter at the two fastest speeds, except for the 1/125 speed. This is also the speed that is set for flash exposure. I measured the 1/125 setting for both the normal speed setting as well as the X (flash) position on the dial. In both cases, the measured speeds were comparable and longer than nominal. I wonder if Pentax gave themselves a bit of leeway here to make sure that for flash exposures, the shutter really would be open “long enough” for the flash.




Of course, these values apply only to my vintage ME Super (I'm the original owner), which has never been CLA’d and was last used seriously for a film class I took back in 2007 or so.


The rise and fall times tell us something about how fast the shutter curtains are moving. An a priori estimate of the curtain speed can be made by noting that the flash duration setting corresponds to a speed of 1/125 second. At shorter speeds, the second curtain starts its motion before the first curtain has had a chance to fully traverse the 24 mm vertical frame. So, we estimate that it takes around 1/125 second for a shutter curtain to move 24 mm. This corresponds to a curtain speed of

24mm/(1/125 sec) = 3000 mm/ sec (pretty fast!)

How should this affect my scope traces? Well, my green laser pointer produces a beam of light which is slightly bigger than 1 mm in diameter (with an unknown, but probably not too far from Gaussian, profile). If I naively assume a uniform dot 1 mm across, it should take a shutter curtain about 1 mm/3000 mm/sec = 333 microseconds to slice across the beam. This is remarkably close to what I see.

This image shows zoomed-in views of both the open and close light traces for a moderate shutter speed (1/60 second). (The close trace has been reversed and overlaid on the rise trace, with an eyeball best fit.) The whole process takes around 400 microseconds - very consistent with the estimate above for how long it should take a curtain to move across the laser beam. These timings are also consistent with the “bump” shown above for the 1/2000 second measurement.

Forum: Photographic Industry and Professionals 06-24-2020, 07:12 PM  
Olympus will sell their Imaging business to Japan Industrial Partners
Posted By AstroDave
Replies: 201
Views: 8,143
Well, I sure hope they stay in business. I am a devout WordPerfect user for the last 40 years!
Forum: Troubleshooting and Beginner Help 06-19-2020, 05:34 AM  
Questionable *ist D works just fine!
Posted By AstroDave
Replies: 8
Views: 914
Yep - I had that bookmarked already. I emailed him to ask whether he had ever tried to remove a Bayer filter as well, but no response so far.



Current plan (hope!) is to just remove everything from in front of the actual sensor. Maybe I should put the IR filter back in, to help keep the focus about where it should be.
Forum: Troubleshooting and Beginner Help 06-18-2020, 06:29 PM  
Questionable *ist D works just fine!
Posted By AstroDave
Replies: 8
Views: 914
I agree that the *ist D does some things that more modern cameras do not - I do wonder why this functionality has been lost!

However, I want a monochrome camera (that doesn't cost thousands of dollars). As I alluded to in my original posting, I've made a spectrograph and I want to see what the spectrum of various light sources (CFL and various LEDs) is. If I use my, say, K-1, the result is affected by the color response of the camera via the Bayer filter. As photoptimist suggested, I have made some efforts to calibrate the camera color response by assuming things like a black body spectrum from incandescent bulbs, but there are still questions in my mind. If I can go monochrome, most of that should be resolved. Yeah, I may wreck an otherwise functional camera, but .... . There seems to be many of these available (and for real photography, the 36 MP of my K1 is a heck of a lot better than the 6 MP of the *ist D).

If you want a 6 MP camera. let me know ASAP!!
Forum: Troubleshooting and Beginner Help 06-18-2020, 03:13 PM  
Questionable *ist D works just fine!
Posted By AstroDave
Replies: 8
Views: 914
A week+ ago, I asked the collective wisdom here if they thought a questionable *ist D offered on Ebay was worth taking a chance on: Is this ist D likely to work? - PentaxForums.com

Despite a few warnings and with some encouragement as well, I bought it, and it arrived quickly. After figuring out how to power it up (it just didn't like regular AA or NiMH batteries, so I went external), it works just fine. Other than the cracked view screen, it's A-OK as near as I can tell. Apparently, as some of us suspected, the seller had not tried it with a proper lens or had not activated the proper lens/shutter release mode.

And, miracles can happen: I thought I had some old CF cards, but I guess I sold them on Ebay eons ago, so I was wondering where to get a card to test it. But, the camera came with a 2 GB card in it!!

Now, I'm waiting for some JIS screwdrivers to arrive so I can get inside it and attack the Bayer filter.

The goal here is to get a monochrome camera for my spectrograph. Attached is a spectrum of a CFL bulb that I took with it - you can see why CFls might not be the best light source for good color imaging!
Forum: Pentax K-1 06-18-2020, 02:55 PM  
Astrotracer consistancy with a 360 mm lens
Posted By AstroDave
Replies: 16
Views: 677
What camera? How is telescope attached to camera - K-mount adapter, I presume.

How are you staying centered from frame to frame? What is your camera support? If you are moving the camera+lens between frames, there may be variable backlash and/or sag in your tripod (or whatever). Your rig is probably at least 3 or 4 kilograms (the lens alone is listed at 1.7 kg).

How close to the zenith are you really? If you are within a few degrees, the tracking might get a bit funky. At/through the zenith, alt/az tracking has a singularity (the azimuth changes by 180 degrees instantaneously). I think we've never actually figured out just what tracking motion Pentax does with the sensor. If it mimics hour angle/dec tracking, then going through the zenith should (?!?!) be OK.
Forum: Pentax DSLR Discussion 06-18-2020, 01:44 PM  
K50 body acting weird, time to upgrade camera body... Considering KP vs Sony A7 ii
Posted By AstroDave
Replies: 20
Views: 954
I'm a bit biased (I have one), but I agree with those above who suggest a used K-3

richfam beat me to it: There's one at KEH in your price range ($414): Pentax K-3 Digital SLR Camera Body, Black {24 M/P} at KEH Camera

(I have nothing to do with KEH.)
Forum: General Photography 06-14-2020, 07:50 PM  
What is your other camera ?
Posted By AstroDave
Replies: 49
Views: 1,670
If I want a small pretty-darn-good camera, I take my Canon G15. Otherwise, it's the K-3 or K-1.
Forum: General Photography 06-09-2020, 07:10 PM  
What is your everyday lens ?
Posted By AstroDave
Replies: 73
Views: 2,427
On my K-3, it's the 18-135

On my K-1, it's the 28-105

If I'm out for a day hike and on vacation, these are the must-haves.

Maybe not THE best lenses, but both offer a great range of focal lengths and WR (if that matters).
Forum: Pentax Camera and Field Accessories 06-09-2020, 12:20 PM  
Connect power bank
Posted By AstroDave
Replies: 5
Views: 535
I rather doubt it. The "power tank" almost certainly puts out 5 V - that is the standard voltage for USB charger systems. The K-3 battery is 7.2 volts.

There might be something useful in this thread: External power for K-3 - PentaxForums.com
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