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Forum: Pentax Medium Format 1 Day Ago  
Grand Lens Test 10: The 645 A* 300mm f/4
Posted By AstroDave
Replies: 9
Views: 403
I did go back and look now. For sure, as you stop down, things get less sharp (~ my "more blurry"), but I think (without doing some calculations) that this is happening faster than diffraction would be kicking in. This may be due to other effects such as commented upon by kaseki.
Forum: Pentax Medium Format 2 Days Ago  
Grand Lens Test 10: The 645 A* 300mm f/4
Posted By AstroDave
Replies: 9
Views: 403
Don't understand. For a given camera, ALL lenses (regardless of focal length/front lens diameter) will show the same diffraction effects as a function of f-stop.

If your 120 was looking blurry, don't blame it on diffraction (until f22 or so for medium frame).
Forum: General Photography 3 Days Ago  
What Are Your Favorite Pentax Forums "Marketplace" Pet Peeves?
Posted By AstroDave
Replies: 45
Views: 1,232
I wanted to just give some stuff away (broken lens, lens screws, camera pieces, old film, ... might be useful to somebody), but you have to have a price on it - "free" doesn't count. We need a 'freebie" section.
Forum: General Photography 4 Days Ago  
Those blunders...
Posted By AstroDave
Replies: 28
Views: 652
Way back in the film days: loading a roll of film, somehow tearing out the sprocket holes, thus film not advancing, despite frame counter advancing - so final result was 36 frames all on top of each other! Nice dense negative!
Forum: Post Your Photos! 5 Days Ago  
Nature Triple Iridium Flash
Posted By AstroDave
Replies: 11
Views: 240
Flagstaff proclaims itself the "World's First Dark Sky City" and I live about 10 miles outside of town. It really is a nice dark sky area. When I drive home at night, I can see the Milky Way as soon as I step out of the car.

Between Lowell Observatory, Northern Arizona University, the Naval Observatory Flagstaff Station ("highest anchor in the US Navy" at 7000 feet), and the USGS Planetary research center, there is probably more than $10 million of astronomical research/spending around here every year (and about 10x that for the state of Arizona as a whole). That is quite an incentive for the City godfathers to keep it dark! The City and (Coconino) County have fairly strict lighting ordinances.

This would have come about 2 hours earlier, but - we just experienced another joy (?) of country living: the power went out as I was about to hit send.
Forum: General Photography 6 Days Ago  
What if... with the new K-1
Posted By AstroDave
Replies: 20
Views: 676
My first shots with every camera are of my wife -- Gets me lots of brownie points!!
Forum: Photographic Industry and Professionals 6 Days Ago  
Looking for a Photo Service
Posted By AstroDave
Replies: 9
Views: 299
Check out an HP7470 plotter? Not sure how good the pens would be for this (and they may be hard to find these days). IIRC, you'll need a vector graphics file to start with, and ultimately an HPGL (HP graphics language) file (which isn't all that hard to create - I've made them from scratch with a BASIC program).
Forum: Photographic Industry and Professionals 6 Days Ago  
Looking for a Photo Service
Posted By AstroDave
Replies: 9
Views: 299
Wacko idea(?)(!): use a laser printer to print on a transparent plastic sheet. We made overhead projector viewgraphs this way in the "olden days" (10-15 years ago).

Don't know if the resolution or contrast would be adequate. I have an old LaserJet II which still works and prints at 1200 dpi, and some of those old plastic sheets if you want to experiment. If you need a negative, you can just invert the image before printing (may use up toner in a hurry!).

Wanna try it? Just PM me. I could print a file you send me, or send you some of my plastic.
Forum: Post Your Photos! 6 Days Ago  
Nature Triple Iridium Flash
Posted By AstroDave
Replies: 11
Views: 240
I don't know. I saw that, too, early on. Originally (and until I can find/think of something better!) I'm going to chalk it up to noise or temporary detector defect. I'll take a closer look at the original file, and another long exposure an hour earlier.

Many astronomical CCD exposures get somewhat similar defects, usually accredited to "cosmic rays" - some charge deposited in the detector not by light/photons but rather by interaction with a charged particle that tracks through the CCD. Those are usually streakier though (i.e. a line several pixels long) - at close look, however, so is this - it's a linear feature about 9 pixels long, but it's the ends which are brightest.
Forum: Post Your Photos! 06-12-2018, 11:22 AM  
Nature Triple Iridium Flash
Posted By AstroDave
Replies: 11
Views: 240
Every now and then, the universe actually conspires IN ones favor.

Last night, the Heavens Above website (Heavens-Above) predicted three relatively bright Iridium flashes within 10 minutes within a couple of degrees of each other, at fairly low elevation (10-12 degrees) and almost due west.

Here they are with my K-1 and FA 20-35mm at 35mm, using an 11-minute exposure at ISO 400 and f5.0 . There's a fair amount of noise in the image. Photoshop Elements despeckle did a fairly nice job at removing it, but at significant overall loss of detail in the star and satellite trails, so it was not used in the final image. Just a bit of level adjustment and conversion to B&W.

(I got a couple of airplane tracks for free, as well.)
Forum: Pentax K-1 06-11-2018, 04:51 PM  
Shooting canvas paintings with the K1 Pixel Shift?
Posted By AstroDave
Replies: 11
Views: 502
Forum: Photographic Technique 06-08-2018, 03:08 PM  
Want to try Astrophotography
Posted By AstroDave
Replies: 58
Views: 2,475
I would start out with ISO more like 400-800 . For even 30 second to 1 minute exposures, the sky background (especially if there is any moon up), will start to be apparent. A few exposures without tracking at different ISOs and apertures will give you good ball park values as to what to use for your "real" exposures.
Forum: Photographic Industry and Professionals 06-06-2018, 12:55 PM  
Kodak shows off some Ektachrome photos
Posted By AstroDave
Replies: 12
Views: 459
If they were smart, they'd go back to those Kodak mailer envelopes as well! I always did my Kodachrome that way, with an occasional Ektachrome. The only time I tried somebody else for Kodachrome, the colors were nowhere near as good. When you bought a roll of film, you just automatically bought a mailer as well.
Forum: Pentax SLR Lens Discussion 06-05-2018, 11:49 AM  
Poll: Primes and Zooms... where do Pentaxians stand in 2018?
Posted By AstroDave
Replies: 233
Views: 5,283
I'm with Norm. (Depends on whether I get to keep more than one camera, too - I have both K-3 and K-1, and the usual lenses thereon are these two.)
Forum: Pentax News and Rumors 06-05-2018, 07:52 AM  
Amateur Photographers (UK) Predicts End for Pentax
Posted By AstroDave
Replies: 432
Views: 17,192
Probably won't happen after the last two, though!
Forum: Pentax SLR Lens Discussion 06-04-2018, 07:32 PM  
The lens that suprised you the most.
Posted By AstroDave
Replies: 75
Views: 2,653
My SMC Pentax-A 35-70mm F4. I don't even remember how/why/where I got it. Probably a throw-in with a flash or another lens on Ebay.

I use it all the time for my Ebay pictures, on both my K-3 and K-1. The focal range is just right for my compact "studio," and stopped down to f8 or so, it's nice and sharp.
Forum: Pentax DSLR Discussion 06-04-2018, 07:22 PM  
When the new K-3 camera line update?
Posted By AstroDave
Replies: 14
Views: 856
As the saying goes: those who know, don't tell, and those who tell, don't know!

Anybody with advance information is almost certainly under a NDA (non-disclosure agreement). If they spill any beans, Pentax will never speak to them again.
Forum: Pentax DSLR Discussion 05-26-2018, 09:09 AM  
Understanding ISO
Posted By AstroDave
Replies: 22
Views: 1,176
Here's a more numerical explanation of Bob256 and photoptimist's comments:

Clarkvision: Digital Camera Review and Sensor Performance Summary (scroll up and down for lots more info)(fairly heavy on canikon!)

The "full well" value is what really sets the low ISO behavior. Bob256 was a bit optimistic with his 64 million levels! It's more like 50000 - 100000 electrons. The quantum efficiency of a good photodetector these days is better than 50%, so almost every photon that shows up gets converted to an electron. The problem is storing all those electrons without their leaking into adjacent pixels.
Forum: Troubleshooting and Beginner Help 05-20-2018, 12:03 PM  
Image ghosting when doing water splash photography with K-1
Posted By AstroDave
Replies: 11
Views: 441
I like this explanation (different trigger times) - accounts for the different separations between various blobs and slightly different shapes. Would also explain why the (stationary!) glass is sharp.

Can you do it again? If so, try a shot with just one light!!

Do you have any way to make high speed light measurements? If so, you should be able to see two separate flashes. I have measured optical flash response time (i.e. one flash triggers the second one with its light flash) to be tens of microseconds.

How is the initial exposure triggered?
Forum: Pentax SLR Lens Discussion 05-18-2018, 09:04 AM  
Further Vivitar 120-600mm test : the Moon
Posted By AstroDave
Replies: 11
Views: 759
Actually, 1/25 sec should be fine for lenses up to 600mm or so. The moon moves less than an arcsecond, which is less than a pixel.

What exposure length you need is easy to calculate - at least in order to avoid blurring due to motion of the moon during an exposure. Of course, other factors like focus setting and tripod stability are important too. Be sure to check your lens for focus at infinity! Virtually every lens I've ever checked does not focus at infinity when the focus ring is set to the "infinity" stop limit. They go "beyond" infinity!! Use live view on the moon to set your focus.

Although the moon is moving with respect to the background stars at a rate of about 12 degrees per day, you can pretty much just use the sidereal motion rate (the amount by which the sky "moves" due to the rotation of the earth) of 15 arcseconds per second. (The lunar orbital motion means an error in this rate of 12/360 - or about 3%.) (Even more technical: the moon is always within 30 degrees of the celestial equator which is at declination zero degrees. As you move away from the equator, the actual angular rate decreases by the cosine of the declination. Since cosine(30degrees) = 0.87, the lunar rate will not really be much less.)

If you know the arcseconds per pixel scale for your camera (which will be about 1.4 arcseconds per pixel for your K-3 / 600mm set up. See Easy Determinations of Camera Scale Factors (arcseconds per pixel) - for how to do this. Or just check K-1 Scale Factors - Arcseconds per pixel for several lenses - where I mention my results for for a 300mm lens on a K-3.), you can estimate how much pixel blurring your exposure might incur.

At 600mm, the moon will move one pixel in about 1/10 second. Your 1/25 should be OK in this regard.
Forum: Pentax K-3 05-11-2018, 06:57 AM  
How accurate is K3ii GPS?
Posted By AstroDave
Replies: 13
Views: 734

Start here: GPS Accuracy | Errors & Precision | Radio-Electronics.Com

For a very technical discussion, check

Accuracy at 5-10-15 meters is pretty standard for unaided (i.e. consumer single unit, single frequency) GPS.

If you are standing on/within a metallic bridge, a phenomenon known as multi-path is quite likely, due to reflections of the GPS signals from the metal in the bridge. This can affect the accuracy. For instance, if the receiver picks up a signal that had an extra 20-30 meters of travel distance to the receiver, because it took an indirect (reflected) path from the satellite, it will include that error in its calculation of position, and could be off by that much.

Also, if you happened to be doing this on a day when the Earth's ionosphere was "disturbed," thanks to solar flare activity, errors will be larger. The GPS signal travel time from the satellite to you (which is the data that the receiver uses to calculate its position) will be longer than usual thanks to the ionospheric activity (changes in electron density affect the index of refraction, which changes the signal propagation speed). Fancier GPS receivers use two frequencies from the satellites to determine the propagation time - the effect has a frequency-squared signature, and can be corrected for by comparing the arrival time of signals at the two frequencies.

If you have two GPS units near each other (near can still be miles apart!), you can correct for some problems by comparing the results for the two units. This is called differential GPS (Differential GPS - Wikipedia). Surveyors use this all the time, using a network of reference stations at known positions. Some commercial units use broadcast corrections provided by the government to help (Differential GPS - Wikipedia).

So, your 30 foot errors are completely normal (and not unique to the K3II!).
Forum: Pentax Camera and Field Accessories 05-05-2018, 07:14 AM  
Ogps1 vs SkyTracker etc.
Posted By AstroDave
Replies: 16
Views: 859
I second Pentax Syntax!

For less than twice the price of the O-GPS1, you can have a nice motorized tracking mount, if you put these two together:

Meade 41-7100-00 LX70 German Equatorial Telescope Mount & Tripod Excellent Cond 709942997125 | eBay

Meade LX70 Dual-Axis Drive System - 670011

If you are not quite sure how this all works, I suggest you find a local astronomy club. It will be full of folks who will be glad to help you get going.

For somewhat more, you can get something similar, but ready to go (you will still need to do a polar alignment): Explore Scientific FirstLight Exos-2GT Equatorial GoTo Mount with Tripod, White 812257016501 | eBay

(I have nothing to do with any of the above vendors, although I have bought stuff from Ebay seller scopehed1 and was quite satisfied with it.)

In the long run, I think you will find a conventional tracking system to be more satisfying than using the O-GPS1. It will work with just about any camera and lens that you have now or in the future.
Forum: Pentax K-3 05-01-2018, 06:16 AM  
Double stacking rear multipliers?
Posted By AstroDave
Replies: 20
Views: 920
If you have plenty of light (the Sun!), you can stack at least 3: Magnification Factors for several Kenko Teleconverters on K5 -

The images were surprisingly good, but I was stopped down to f11/f13 or so.
Forum: General Photography 04-29-2018, 08:20 AM  
Fun for crossword/camera buffs
Posted By AstroDave
Replies: 0
Views: 365
Today's (April 29, 2018) Washington Post/LA Times crossword puzzle is titled "camera shy" and features a number of camera brands hidden in the answers, including our favorite as the first one! There are some other camera terms as puzzle answers as well.

You can find it here: Daily crossword puzzles free from The Washington Post - The Washington Post

For some help with answers, go here: L.A.Times Crossword Corner
Forum: Pentax SLR Lens Discussion 04-27-2018, 01:40 PM  
Manual K-Mount Lens Suggestion Time - Travel Kit
Posted By AstroDave
Replies: 33
Views: 1,347
What about the SMC Pentax-A 35-70mm F4? You lose a bit of light-gathering, but covers the same range. I have one, and like it a lot (I use it for many of my Ebay photos, on both a K-1 and a K-3). Check the lens review database.
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