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08-13-2016, 07:59 PM   #1
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Bulb mode help

I'm trying some star photography. I have a Rokinon 24mm f1.4 manual focus lens. I have to enter the focal length in when I turn on the camera. Settings are bulb mode, ISO 1600. Single frame. F1.4 (manually set on the lens, the camera doesn't read it.). RAW format. High ISO NR on. Slow shutter speed ISO NR off. Shake reduction off. I'm using a remote that has a receiver that plugs into the camera and slips on the hot shoe. Bulb mode set to push the button to start and stop exposure. I'm shooting for 14~16 second exposures. For some reason, when I push the remote, the camera takes a 1 second and a 0.9 second exposure, then a third where it stays open until I push the button to close and end the exposure. I'm not 100% certain, but it seems that it may be taking the 1 and 0.9 second exposures again when I push the button to close. I only want the single, manual exposure. Not sure why it's taking the others. Another strange thing is those shorter exposures are much brighter than the longer exposure. Not sure why that is either.

Another thing I'm not 100% clear on is the 'black frames'. I've seen them before when I tried night photography with my kit lens, but it doesn't seem to be taking them with the Rokinon. If someone can explain this process (why it takes the black frame and what the camera does with it), I'm interested to know.

Any advice is appreciated.

08-13-2016, 08:16 PM   #2
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Slow Shutter Noise Reduction = Dark Frame Subtraction
Any exposure greater than 30s will trigger a dark frame - no exposure is recorded for the dark frame, but the "noise" is subtracted from your previous frame. This feature is normally turned off for star trails.

QuoteOriginally posted by jtkzoe Quote
F1.4 (manually set on the lens, the camera doesn't read it.)
- the Rokinon 24mm f1.4 has an A position to my knowledge. If you are using the manual aperture ring, you must "enable" in the custom setting menu

Cant speak to your remote release, but something sounds hinky there.
08-14-2016, 12:40 AM   #3
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As mattt wrote, find the enable aperturer ring setting and then turn the aperture ring on the lens to A. After that the lens aperture will be controlled from the camera body as with any modern lens.

As for the weird extra exposures you get, maybe it is your remote that are doing something weird? Try the normal shutter button instead to see if the problem still remains.
08-14-2016, 01:22 AM - 1 Like   #4
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You have two options for how shutter is triggered in Bulb (C menu of the camera). Select the correct one.

08-14-2016, 03:29 PM   #5
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Three exposures? Check to see if you're actually set up for bracket shooting or HDR. I've never seen (or heard of) a remote like the one you describe. If your exposures are less than 30 seconds, use manual mode and use the 2 second self timer to trigger the shutter. If you're doing more than one exposure, you can use the intervalometer. Set the exposure, set the number of exposures you want, trigger the shutter and sit back and have a cup of something until the camera is done..
08-15-2016, 07:06 PM   #6
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About dark frames they simply are exposures of the same length and iso as your "light" frames but with the shutter closed. When long exposure noise reduction is "On" the camera with take a dark frame after each image you take and substact it thus eliminating thermal noise and hot pixels in the final image. you could make your own dark frames by shooting with your lens cap on once in a while than use software like DeepSkyStacker to substract your darks from your lights. You'll be able to take more lights that way.
08-18-2016, 04:04 PM   #7
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The issue of the second photo after closing the exposure is due to the High-ISO NR you stated you have turned on. Turn it off. It isn't necessary, and as you see it renders your camera useless for a period of time. It'll also use up your battery faster.

The dark frames for deep stacking are actually supposed to provide the same thing. I usually take them with the lens cap on. If I am out shooting with a lens for a period of time, I'll take a series of dark frames at the beginning of the shoot (up to 10) at the settings I'll use for the shoot (specifically ISO and Shutter speed). I'll then take the photos. At the end, I'll take another series of dark frames (up to 10 again). If I am out long enough I'll also take some in the middle or at any time I change lenses or composition.

Because I am going to stack photos, I usually just keep my photos at Shutter Speeds around 30-seconds to save having to keep track. I can just set all the items in M mode and go. I occasionally use the interval shooting to get my shots specifying the number of shots I want at what time spacing.

This sounds like what you are doing. Note that the time spacing is from the beginning of a shot. If you take a 15-second exposure, you need to make sure the interval is more than 15-seconds (this is my recollection, I could be wrong). So, if you instead said a 10-second interval, you will miss some shots because the camera is already taking an exposure when 10-seconds pass. I believe in that case it will then take another photo at 20-seconds after the first shot, which is 10-seconds after the time the second shot should have been taken. Only, the camera hasn't taken the photo.

I usually take my 30-second shots on 35-second intervals for sky photography. I recall finding problems if the interval is even exactly at 30-seconds. It's worth testing it out a bit indoors when you aren't actually trying to get a special shot. 1) I may be wrong and 2) these details could vary by camera model.

I have had successful results in the limited attempts I've tried. The only bad thing is light pollution. Photoshop can help that out at the very end. Noise is generally not an issue. For best results, you definitely want a lot of frames, especially if you are shooting shorter exposures. I try to have at least 20 images for my stacking + 20 dark frames.

I strongly suggest searching for other threads on the topic including outside the Pentax Forums. There are a lot of true experts of this type of photography and they use all sorts of cameras. There are even good specific forums for astrophotography.
08-18-2016, 05:10 PM   #8
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QuoteOriginally posted by biz-engineer Quote
You have two options for how shutter is triggered in Bulb (C menu of the camera). Select the correct one.
What he said ^ ^ ^

You can test the behavior before actually going out shooting. Remotes that plug into the wired remote jack emulate the shutter button.* As such, the behavior in B mode will be to remain open until the shutter is released. That failing, confirm that you are not using one of the intervalometer functions that are common with many remotes.


Steve

* The IR remote works the other way in B mode since its operation is pulse based.

08-18-2016, 05:23 PM   #9
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QuoteOriginally posted by jtkzoe Quote
Another thing I'm not 100% clear on is the 'black frames'. I've seen them before when I tried night photography with my kit lens, but it doesn't seem to be taking them with the Rokinon. If someone can explain this process (why it takes the black frame and what the camera does with it), I'm interested to know.
As noted above, High-ISO NR uses black frame subtraction to do its magic. The important thing to remember is that the second exposure happens with the shutter closed and is never written to the card. The most obvious indication it is in play is that it more than doubles the amount of time between shutter presses.

If you are seeing unexposed frames on your card, you should confirm that there is no good reason for them to be there and that they are not just Oops photos where the lens cap is on or where the camera settings were for much brighter conditions. If neither is the case, you may have a fault condition that should be addressed. (Are you still shooting with a K-50?)


Steve
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