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Forum: Troubleshooting and Beginner Help 07-27-2018, 10:50 AM  
K70 Astro lens advice
Posted By MossyRocks
Replies: 14
Views: 631
That makes more sense and is much more inline with what I have been advised. With the limited amount of light when shooting at night I just didn't want them to start off on the wrong foot. I usually try to stick to the rule of 200 when I shoot stars so with a 28mm I would be at 7 or 8 seconds depending on where I have the camera pointed. Then I just take as many frames as I can shooting what ever object I can until I have to leave, it falls below the trees, or I am too tired to continue.
Forum: Troubleshooting and Beginner Help 07-27-2018, 06:53 AM  
K70 Astro lens advice
Posted By MossyRocks
Replies: 14
Views: 631
For astro on APS-C my advice is as follows:
1. For the moon get as big of a telephoto as you can find. Even with a 300mm the moon doesn't fill much space. For example on my k-3 a moon photo taken with my 300mm is something like 600-800 pixels across (I should probably measure it). So if you want a moon photo with the moon the primary subject I suggest bigger.
2. For constellations I find that a 50mm works great and you can pretty much fit any of them in the frame. A fast old good 50 or 55 is cheap so just buy one.
3. For Deep Sky Objects (DSOs) you want something big, bright, and ideally with ED glass. DSOs are pretty faint in general so you want a pretty fast lens, add in that many are pretty small you need a big lens. A 135mm is the smallest I have used for DSOs and gotten reasonable results from but if you have a fast 200 or larger it would be better. You will need to stop the lens down 1 or 2 stops in almost all cases to help with chroma because DSO photography seems to be exceptionally punishing on lenses. That said if things are done right one can get some really impressive results even with old small lenses. Also for DSOs you will need to learn how to stack photos and do post processing because otherwise you will just get faint items just above the noise floor of the image.
4. For Milky Way and star trail pics go wide and stick something in the foreground. I have used by 28mm f/3.5 SMC Takumar and 17mm f/4 SMC fisheye takumar for these and have really liked the wide view from that 17mm fisheye.

Also with astro photography if you don't have a tracking mount you will need to remember the rule of 500. Basically to avoid star trails from being visable keep the shutter speed under 500/(focal length). This works best for constelations and milky way photos. For DSOs I will use the rule of 200 as this typically keep the object from moving even a couple of pixels so things stay sharp.
For the moon use the looney 11 rule, always use this as it gets you close enough that post processing can easily handle the rest.
If you are using astrotracer as your tracking get the astro object you want centered in the frame and keep the exposure under 1/4 of the max it recommends. This will help a lot with the errors that accumulate in tracking and also since astrotracer doesn't do rotation this will keep the trails at the edge of the image to a minimum.

My other advice for astrophotography is get a heavy stable tripod, cable release, use the 2 second mirror up delay, and finally get a focusing aid (a bahtinov mask is the preferred solution it sounds like) as auto focus really doesn't work all the well if at all when doing astrophotography.

---------- Post added 07-27-18 at 07:13 AM ----------



Have you tried extending the exposure, even with a 28mm lens on a tripod that is a very short exposure for star fields or any wide field astro pics. I've done DSOs with a 135mm and 1.5s exposure with good results using the rule of 200 and not gotten trails. I've done constellations with a 50mm and 4 seconds of exposure without trails so with a 28mm you should be able to shoot pretty close to 10 second exposures without getting egg shaped stars. If you use the rule of 500 you should be able to shoot pretty close to 20 seconds with that 28mm lens giving you about 8 extra stops to work with. So maybe put 1 or 2 stops into the f/stop to sharpen things up some more and the rest into a lower ISO or more exposure to bring out the details.
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