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Forum: Photographic Technique 13 Hours Ago  
Studio Difficulty rendering true-to-life colors!
Posted By AstroDave
Replies: 20
Views: 536
Did you do a custom white balance????? You don't need a gray card - just a white sheet of paper works fine.

In my experience, this can make quite a difference in the colors that appear in your JPEGs. (I, too, shoot mostly jpegs, using RAW only occasionally for special occasions. Learning how to set custom white balance was a real "aha" moment for me, and I now do it ALL the time when shooting in the studio, where you have lots of time to get things right!)
Forum: Photographic Industry and Professionals 5 Days Ago  
Should Pentax make a digital TLR ?
Posted By AstroDave
Replies: 20
Views: 903
Yes - to reduce noise, but most importantly for astronomers using the dragonfly array, the (Canon 400 mm f/2.8 L IS II USM) telephoto lenses do not have any internal obstructions in the optical path. Virtually ALL other astronomical telescopes have stuff in the light path (typically struts that hold up parts of the telescope, like secondary mirrors and the detectors) that scatter light. The scattered light limits how faint you can see low surface brightness structure near brighter emission like the central parts of galaxies. Movement (of either the tracking mount or objects in the sky) is not a problem. discusses the dragonfly array(s). Quoting from the abstract: "The lenses are mounted on a common framework and are coaligned to simultaneously image the same position on the sky. .... The system has no obstructions in the light path, optimized baffling, and internal optical surfaces coated with a new generation of antireflection coatings based on subwave-length nanostructures. As a result, the arrayís point-spread function has a factor of ∼10 less scattered light at large radii than well-baffled reflecting telescopes."
Forum: Troubleshooting and Beginner Help 02-20-2018, 12:02 PM  
Deleting photos on K3
Posted By AstroDave
Replies: 26
Views: 703
No - have them absolutely stop for anywhere from 1 to 60 minutes!! I hate Windows updates!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!! especially when it decides to do them on its own schedule and locks up the computer.
Forum: Pentax DSLR Discussion 02-16-2018, 05:52 PM  
Format Question: can you switch to same model?
Posted By AstroDave
Replies: 18
Views: 798
You may screw up the file numbers! I've had this happen.
Forum: Contest Voting 02-08-2018, 09:20 PM  
Moon Dogs
Posted By AstroDave
Replies: 2
Views: 36
Moon Dogs from the Front Porch
Forum: Pentax SLR Lens Discussion 02-08-2018, 01:25 PM  
Need help with 150-450 focusing issue
Posted By AstroDave
Replies: 41
Views: 1,582
Anything other than heat shimmer as the cause is mostly a red herring for this image!

As I noted earlier, out of focus will NOT create those shimmery, wiggly lines.

Neither will camera shake (due to whatever: too slow shutter speed, wind shake, unstable tripod, mirror slap, ....) nor shake reduction. ALL these effects move a significant part (if not all) of the sensor in the same way - so all (at least) nearby pixels will show the same distortion - again not wiggly shimmer on a small scale.

All those effects may well have affected the rocket image, but the OPs chief problem is the shimmer, as others have noted as well. I'll bet a close comparison of his buddies' shots will show the same effect (at least for comparable focal lengths).

Faster shutter speed won't help either - it just freezes in what ever the shimmer is at the moment of exposure.
Forum: Pentax SLR Lens Discussion 02-08-2018, 08:12 AM  
Need help with 150-450 focusing issue
Posted By AstroDave
Replies: 41
Views: 1,582
Itís atmospheric distortion/heat shimmer as others have suggested - no question. You are shooting through 3 miles of warm, rising and therefore somewhat turbulent air - from an altitude of just 12 feet, judging from the GPS report in your EXIF data.

Look at the water tower legs and the right side of the slanted gantry on the right. Those should be straight as an arrow! And, they are not - there are lots of wiggles.

Out of focus will blur straight lines but it doesnít make them wiggly!!!!!
Forum: Troubleshooting and Beginner Help 02-05-2018, 10:09 AM  
Can you explain what happened?
Posted By AstroDave
Replies: 26
Views: 1,551
That's why I never delete any shot until much later!
Forum: General Photography 01-29-2018, 04:03 PM  
True Shutter Speeds compared to Nominal Shutter Speeds - Actual Measurements
Posted By AstroDave
Replies: 14
Views: 683
A week or so ago, partially impelled by this discussion of how our cameras respond to changes in sensitivity (shutter speed or aperture), I used my K-1 to shoot my black/gray/white scale card at various shutter speeds covering the entire range of sensible exposures for all three brightness levels. When I plotted the results, there were some minor kinks in the response curves. After a bit of thought, a plausible explanation was that the claimed shutter speeds were not quite right.

Some web searching lead me to this article, which, while it concentrated on Nikons, suggested that indicated shutter speeds were indeed not necessarily correct. The essence of the argument is that whole exposure step speeds longer than 8 seconds or shorter than 1/8 second are just not right. Rather, our cameras really do change the exposure such that a so-called 15 second exposure is really 16 seconds - the true doubling of the 8 second speed. And, in this scheme, our "30 second" exposure is really 32 seconds.

After reading this, I adjusted the exposure speeds in my plot to the values suggested by a true mathematical sequence, and the results look much smoother. I will report these results elsewhere on the Forum.

This prompted me to try timing a so-called 30 second exposure, since a simple stop watch test should clearly be able to distinguish between 30 and 32 seconds. Lo and behold, I indeed got 32 seconds!

Thus, I started a thread in the Forum asking what folks here knew about this. I also set out to design an electronic way to measure true shutter speeds. In the olden (film SLR!) days, such measurements were ridiculously simple: you opened the camera back, put a photocell behind the shutter curtain, shined a light in the front of the camera, fired the shutter, and measured how long the light came through. You canít do that with DSLRs - you donít have access to the region behind the shutter curtains - the sensor lives there.

Hence, a different approach is needed. A reasonable scheme is to be to create a pattern of lights which flash at a rate such that many of them should be illuminated during an exposure. To make a reasonably accurate measurement (~1%), there should be at least 100 flashes during an exposure. Then you just count how many flashes you see in the image and compare that with how many flashes there "should be" during the exposure. For example, consider a 1 second exposure of lights which flash 100 times per second. You should see 100 flash spots in your image. If you see more than 100, the shutter was open longer than 1 second. Fewer than 100 flashes -> the shutter was not open as long as 1 second.

Itís trivial to flash an LED at appropriate rates - a few times a second to hundred of thousands of time per second. The trick is to have the flashes in separate locations - a picture of a single flashing light tells you nothing about how many times it flashed during an exposure!

So, I need an array of more than 100 LEDs that I can flash at a proper rate. Itís relatively simple to design such a circuit using integrated circuit counter chips and a bunch of LEDs. Unfortunately, wiring this up can be a nightmare - in the simplest circuit, you need at least 100 wires, one for each LED, not counting the rest of the circuitry. A simpler circuit results if you use a grid of LEDs. 8 by 8 arrays (64 LEDs in all) are readily available, and I had already made an exposure tester with one of these years ago. However, to get my 1% target accuracy, I need more than 64 LEDs.

Fortunately, there is another way to spread out the LEDs - you can move a modest LED array during the exposure. An easy way to move it is to hang up the LED array as a pendulum and swing it back and forth. This works nicely, as you will see below, for a range of exposures from around 1 second to about 1/20 second. So, I built a little 10 LED circuit with a decade counter (using a 4017 chip) and hung it up in my house. I have a signal generator that I can use to set the flash rate very accurately to any desired frequency. For some exposures, I set the frequency to produce as many as 400 flashes expected during an exposure.

Hereís a picture of my flasher. One after another, the LEDs are "on" for the duration of a cycle. The clamps hold a battery pack to the back side of the circuit board:

A web search such as "10 LED chaser circuit with 4017" will produce a number of hits if you would like to make something like this on your own. The parts cost with some astute Ebay searching should be well under $10. Here are a couple of examples:

LED Chaser using 4017 Counter and 555 Timer
LED chaser circuit / Sequential LED flasher using 4017 IC and 555 timer - Elonics

These examples include a 555 timer circuit to create the flash rate. This is OK for a simple flasher where you donít particularly care what the flash rate is. To measure shutter speeds, you do need a source of accurate frequency.

Hereís a picture of my flasher shown suspended on a lightweight rope so it can swing back and forth. The wire brings in the frequency signal from my signal generator.

OK - thatís nice, but how about some results!!

Using my K-1, I started at 1 second exposure and a flash rate of 100 Hz (Hz = cycles (flashes!) per second). Hereís one of my first shots, cropped to show just the lights. At this point, I was using only 5 of the LEDs in my array. You can readily count 100 flashes. The sequence starts at far left, with a very faint flash - the exposure started as this LED was going out (the LED array flashes in order from bottom to top - so the pendulum is swinging from left to right in this picture) and ends far right, where the last flash is at the top and a bit fainter than typical - it has not completely illuminated during the exposure. There are 100 flashes (counting the first and last as one, since each is only partly illuminated) as expected for a 1 second exposure.

Then I increased the flash rate to 200 Hz:

I got tired of counting all those cycles of 5, so I activated the rest of the LED array (10 LEDs in all) in the circuit - here are 19 cycles of 10 LEDs and 5 on each end, giving the expected 200 flashes for a correct 1second exposure:

Then itís on to 1/2 second - the lights are not as spread out. With a flash rate of 400 Hz, I get 19 full columns and 4 counts on the left end and 7on the right end - 201 counts in all - one too many for exactly half a second, so my camera shutter is staying open very slightly longer than it should (about half a per cent):

Hereís a shot of my system in action (with the kitchen in the background!). You can see the signal generator in the lower right. I ran a cable from this up to the second floor where the pendulum was swinging from and back down the rope to the circuit.

Hereís 1/4 second, where we again get an extra count:

I then upped the flash rate to give me an expected 400 counts for 1/4 second. And the count is 402. So my K-1 shutter is consistently staying open a bit longer than it should:

So far, everything is as expected. But all these measurements are for changes in shutter speed that are exactly steps of 2.

What happens at 1/15 second? We find that the speed is really 1/16 second, as hypothesized if sensitivity steps really are a factor of two!

First, I set the flash rate to 1500 Hz and took a shot. If the actual shutter speed is 1/15 second, I should see 100 flashes. I donít!! I get 94. Calculating the shutter speed from these counts, I get 94/1500 = 0.0627 second, quite close to what youíd expect for 1/16 second = 0.0625, especially if my shutter speeds are running a bit long:

Then I set the flash rate to 1600 Hz. We find 100+ counts (note the faint glow at the top of the right column), just as expected for a true shutter speed of 1/16 second:

I made a final measurement at a claimed shutter speed of 1/20 second. The result is way short of the expected 100 flashes - only 89 flashes. The resultant shutter speed (based on an average of 9 frames) is actually about 0.0446 second:

Bottom Line

I now firmly believe that the K-1 really is giving mathematically-correct exposure values, rather than the nominal values shown for many exposure lengths. Thus, I expect the 1/20 second exposure should really be (1/16)/sqrt(2), since I had the K-1 set to 1/2 EV steps, where a half EV step corresponds to a change in sensitivity of sqrt(2). The calculated speed for 1/20 second is therefore 0.0442 - very close to measured. (And assuredly different from the value one would get using 1/3 EV steps, where the correct "1/20 second" value is 0.0496 (again, not 0.05).)

To go to shorter shutter speeds, I will have to actually come up with a proper 100 LED flasher circuit. I canít swing my pendulum fast enough! To go to longer shutter speeds (greater than 1 second), I think I can use my existing 10 stage flasher and move it by hand across the camera frame during an exposure - here the pendulum period is too short: the period is less than 2 seconds, so longer exposures will tend to overlap.

When/if I make additional measurements, I will present them here. But for now, I think it is safe to say that actual shutter speeds have an exact mathematical relationship, where we can take the starting point to be a 1 second exposure, and for 1/2 EV steps, the change from one speed to the next is a square root of 2. For 1/3 EV steps, the results should be based on the cube root of 2 (~1.260). this reference (cited earlier) has a nice table of expected shutter speeds for both1/2 and 1/3 EV steps, about half-way down the page (the 1/2 and 1/3 EV values are interleaved - be careful!).

I will make some similar measurements for my K-3.

I will also try to find the old data taken with my 8x8 flasher - that was for my long-gone Canon Rebel XT (or XTi) and (I think) my also-long-gone K-10 (maybe K-20). Those results should be accurate enough to shed some light on actual versus nominal shutter speeds over a wider range of speeds.

I have embedded my images in with text where I want them above by reference to a web site. Is there any way to actually put images in the middle of text by attaching them? My experience on attaching images is that they always go at the end of a post. At some point in the future, the embedded images will disappear (i.e. when I die and my web sites disappear!).
Forum: General Photography 01-28-2018, 01:05 PM  
On Line Photography Courses
Posted By AstroDave
Replies: 28
Views: 1,208
Don't pass those college courses by too quickly. Many (community) colleges have "Intro to (Digital) Photography" courses (I taught a couple of such sections here at Northern Arizona University a few years ago). You can probably take just a course or two, you don't have to sign up for a degree program. Many universities and colleges (especially public ones) offer reduced- or even no-fee courses to seniors. Talk to the admissions offices.
Forum: General Photography 01-25-2018, 10:40 PM  
Actual shutter speeds versus "standard" values
Posted By AstroDave
Replies: 16
Views: 854
Imagine a row of, say, 110 LEDs that turn on one after the other at a rate of such that there should be 100 of them illuminated during the expected time that the shutter is open. When the count gets to the last LED, the sequence starts over again, so the row of lights cycles endlessly at this rate. You take a picture of the LED array. If your shutter speed is correct, you should see 100 of the LEDs lit up. It doesn't matter where they begin - just count the lights. If there is an extra light or two, then your shutter is open LONGER than you expected. A few less than 100, your speed is faster.

If the lights are indeed in a row (my current system has an 8x8 matrix of LEDs), across the frame, this works even for high shutter speeds where the aperture is a moving slit between the top and bottom curtains. Conversely, if you are far enough away from the matrix, you will still see all the relevant lights within the slit.

Conceptually simple, but getting all those LEDs to light up in sequence takes a rats nest of wires!! I've got the design ready, just need to wire it up.

Another somewhat easy way to do it is to hang a single flashing LED on a long enough rope and swing it across the field of view. Takes only one LED, but you need a longish rope. (Fortunately, I have a place inside my house where I can hang a 15' pendulum - maybe I'll try this way first. I did something similar along time ago and had forgotten about that technique.)

In this way, you really do measure how long the shutter exposes the CCD.

I'll post my results (and circuit(s)) when I'm done.
Forum: General Photography 01-25-2018, 06:53 PM  
an opportunity: Super Blue Blood Moon
Posted By AstroDave
Replies: 37
Views: 1,580
That's right. The moon is not going to burn out your sensor, unlike the sun without proper precautions.

In fact, our good old "sunny 16" is a good place to start for estimating a good exposure for the un-eclipsed moon: exposure for the illuminated part of the moon/or here on earth during a sunny day is shutter speed around 1/ISO for an aperture of f/16. The moon gets the same amount of sun that we get here on earth! However, though, the moon's reflectivity is somewhat lower than average here on earth, so adding a stop or two of exposure will give you a nice exposure.

The eclipsed moon will be several stops less bright, so a good place to start would be 4 or 5 stops below a sunny day: maybe shutter speed ~1/ISO and f/4 or f/2.8. Try that, and adjust as necessary.

The sidereal rate of earth rotation is 15 arcseconds per second of time. Unless you have a long telephoto lens (more than 300mm or so), your pixel scale will be several arcseconds per pixel or more, so exposures approaching 0.1 second are unlikely to blur the moon - as long as you've got a good solid tripod!

Try autofocus, but you may want to use live view to focus.
Forum: General Photography 01-25-2018, 06:39 PM  
Actual shutter speeds versus "standard" values
Posted By AstroDave
Replies: 16
Views: 854
As this article explains, except for certain values (integer powers of two, and reciprocal integer powers - such as 1, 2, 4, 8, 1/2, 1/4, 1/8 seconds), the shutter speeds which we are familiar with are not quite correct.

The author refers specifically to Nikon, noting for instance, that a 30 second exposure is really 32 seconds.

With a really simple experiment - taking a "30" second exposure with my K-1 and timing it with my wrist watch - I get a similar result: a so-called 30 second exposure really is quite close to 32 seconds.

My simple search of the forum does not pop up any discussion of this, specifically with respect to Pentax.

Does anybody have good evidence to support these claims - or evidence to the contrary (i.e. real data from Pentax/Ricoh or elsewhere) for our Pentax cameras?

I am in the process of designing a digital circuit which will allow me to measure my K-1 and K-3 shutter speeds to about 1% accuracy (I already can do this to 2%) and will report results here when I have them. My system counts optical flashes from LEDs, recorded during the exposure, not the sometimes-hyped acoustic schemes using your computer audio input.

These differences are something to keep in mind if doing interval shooting with intervals of 15 or 30 seconds - not only do you have to allow for camera overhead such as writing to the SD card before starting the next exposure, but you should also keep in mind that the actual exposure is a second or two longer than you think!
Forum: Pentax DSLR Discussion 01-24-2018, 09:15 AM  
M42 to digital
Posted By AstroDave
Replies: 23
Views: 959
I essentially quit using film when DSLRs got to 6-8 megapixels, and haven't regretted it a bit. The ease of use is much better, both in getting the right exposure and in looking at your results. You do tend to get sloppy, though, since the next click is free (unlike slides, at maybe 50 cents a shot, back in the days). In 40 years of slide shooting, I took on the order of 7000-8000 shots. In just 10 years of digital, I'm approaching 100,000 shots!

The K-1 that photoptimist recommends (and with which I heartily agree - I have one) essentially provides the equivalent of around 5000 dpi scanning for a 35mm frame. I doubt your scanner is that good!
Forum: Photographic Technique 01-21-2018, 12:21 PM  
Studio Custom WB with the Fong Diffuser
Posted By AstroDave
Replies: 20
Views: 716
That's not quite what I said, but it's just arithmetic:

The usual convention is that "correct" exposure with an 18% gray card gives you R, G, B values at essentially 128 pixel value for the card - half way to full scale. Twice as much exposure, for a linear detector (one where twice as much light gives you twice the pixel count - what we use in astronomy) puts you at pixel value 255/256 - FULL SCALE.

Partially to save up from ourselves, and improve dynamic range, I guess, our cameras don't do that. The scale is not one-to-one. My experience shows that a 1-stop increase or decrease in exposure near mid-scale causes about a 40-50 pixel value change (for my K-1 and K-3), so the camera scale is not 1-to-1 for our DSLRs.

At any rate, you are right - you can blow out the highlights more easily with a white card, but a little bit of care easily gets around this.
Forum: Photographic Technique 01-21-2018, 07:06 AM  
Studio Custom WB with the Fong Diffuser
Posted By AstroDave
Replies: 20
Views: 716
Well, you first have to set the exposure level at something sensible when doing the custom WB - then, check the RGB histogram after you do the WB to check for clipping on your white or gray. I've never clipped it!

If you set your exposure so that the gray card is in the middle of the histogram, i.e. 50%, you are only a stop away from what you'd get with a completely linear sensor (this is not quite the case for our cameras) and a white card: twice as much exposure, and the gray card will be white (255 in R, G, and B). So, you can blow out the gray card rather easily, too, if you aren't a bit careful with exposure.
Forum: Photographic Technique 01-20-2018, 07:10 AM  
Studio Custom WB with the Fong Diffuser
Posted By AstroDave
Replies: 20
Views: 716
That should work, as will anything else that you deem to be white (or neutral gray). A relatively small piece of paper works, even if wrinkled.

What the custom white balance really does is adjust the relative strength of the R, G, and B channels so that they have the same amplitude (pixel value) within the little square that the camera displays when you put it over your white/gray object. Doing WB in post processing does the same thing, except you get to pick the white/gray area ex post facto.
Forum: Photographic Technique 01-19-2018, 06:56 PM  
Studio Custom WB with the Fong Diffuser
Posted By AstroDave
Replies: 20
Views: 716
While a gray card is nice, a white piece of paper will work just as well. As noted above, just follow directions in the manual on how to do a custom white balance. Center the reference square on your white/gray card and hit OK. It is so simple, and REALLY gets you the right WB.

I do it all the time when I am doing something important.
Forum: Pentax K-1 01-17-2018, 01:54 PM  
K-1 shutter lag increase with MLU / Mirror Lock Up
Posted By AstroDave
Replies: 11
Views: 800
Is your flash your primary light source for the insect pictures?

If so, you want your trigger to fire the flash rather than the camera. There is essentially no delay here (well, maybe a few tens of microseconds - I've actually measured it).

PS I've also measured the shutter lag on the K-1 and get the same value as you do - 87 millisec or so. I used a common trigger to fire the camera and start a timer which I then took a picture of. I'll have to try my set up with the mirror lock up mode.
Forum: Pentax Film SLR Discussion 01-15-2018, 11:53 AM  
A question on DX reading capabilities of MZ-M
Posted By AstroDave
Replies: 7
Views: 535
Or, just cover up the bad code(s) (tape or paint over) - or make a new one that matches your film length, and paste it over the bad stuff. I never had a camera that used the codes, but I presume if you had a roll of uncoded film that the camera would work anyhow (you just had to set ASA/ISO and remember when to quit advancing the film).
Forum: General Photography 01-13-2018, 09:54 AM  
Inside one of America's last pencil factories
Posted By AstroDave
Replies: 15
Views: 845
If you like these pictures, for much, much more about the pencil, read

The Pencil: A History of Design and Circumstance, by Henry Petroski

Petroski is a wonderful author about the practice and esthetics of engineering!

I much prefer my (mechanical) pencils over ball- and roller-point pens for design and doodling.
Forum: Monthly Photo Contests 01-12-2018, 03:47 PM  
Was once a flower!
Posted By AstroDave
Replies: 19
Views: 388
I would like to nominate this photo
Forum: Pentax DSLR Discussion 01-08-2018, 08:59 AM  
Question about sorting through pics on a KP
Posted By AstroDave
Replies: 8
Views: 420
As a scientist, you learn to never throw any data away - what you tossed might turn out valuable. Maybe cross-out an entry in a notebook that you don't believe, but never obliterate it.

Seems to me in-camera pictures should be treated the same way, as noted by others above. I ALWAYS download every image. You can throw them away later if so inclined. Maybe a bit of a nuisance, but good insurance!!!!!
Forum: Pentax Film SLR Discussion 01-01-2018, 09:57 PM  
Parts for K1000 (prisms mainly)
Posted By AstroDave
Replies: 23
Views: 1,145
Are they really silvered, or aluminized? Maybe you could re-"silver" your existing mirrors: if aluminum would work (worth a try?), try to find a local amateur telescope maker (or telescope club) - they should have some experience in depositing thin reflective films on glass or at least know where to get it done.
Forum: Digital Processing, Software, and Printing 01-01-2018, 09:50 PM  
Prints on Fabric: What are my options?
Posted By AstroDave
Replies: 2
Views: 386
Walgreens does fleece blankets: Plush Fleece Photo Blankets - Create Custom Plush Fleece Blankets | Walgreens Photo

Looks like they can do around 4 by 5 feet or 5 by 7 feet - you could cut one in half.

I made one of these for my sister a few years back, with pictures of her dogs. As far as I can remember, it came out well, and she liked it (but she had to tell me that, and beside it was her dogs - her "kids"!!). We did one for my wife's grandniece, too.
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