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06-08-2018, 02:58 PM   #1
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Aperture requirements using astrotracer (milkyway photos)

Hello all,

I have read a lot in the forums along with other articles trying to find this answer. But I haven't been able to find any sort of direct answer.

My question is; when using astrotracer how important really is the aperture size? You typically see people looking for the largest possible aperture for astro but that is normally based off not being able to use astrotracer with the Pentax cameras. You want the larger aperture so that you an bring the most amount of light in small amount of time available before you start to get star trails. But if you are using astrotracer then that kind of eliminates that requirement doesn't it?

Reason I am asking is I was looking to buy a wide angle lens for astrophotography and some minimal architecture photography. I have the 18-135mm Pentax lens. I was looking at the 14-15 range. But I am not sure that much of a difference warrants the price for the wider lenses and if I am using astrotracer on the K-3ii then wouldn't the normally terrible f/3.5 aperture on the 18-135 not really matter at that point?

Maybe my other question is; how much of a difference is there between 14-15mm and 18mm? I haven't used anything wider that 17 (used to have the sigma 17-50 f2.8) so I haven't experienced that first hand.

Thanks in advance for any responses. If this is a duplicate please let me know, but I have been searching the forums all day for this answer and couldn't find anything. 99% of astro articles on the web don't consider the astrotracer feature on the Pentax cameras. Or they do and its just a mention of "oh that's a nice feature if you have it"

Also I know the broad answer is no aperture doesn't matter you can always make due with what you have. But my question is would the larger aperture really give any better results while utilizing astrotracer? Is it still worth $300-$500 to get that wide large aperture lens if you have the astrotracer feature?

06-08-2018, 03:14 PM   #2
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I don't use the astrotracer, but will chime in for the (limited) experience I have with astrophotography.
First of all it's not only a matter of how wide the aperture is, but how good it is. Wide open on a kit zoom is pretty terrible, I tried.

Second, the astrotracer has a limited effect: its accuracy varies with the calibration, and the duration is limited, and you can still get small trails.

Depending on how serious you're going to be for astro, it's a rabbit hole. You'll find that everything short of a dedicated motorized astro mount and a nice lens with certain characteristics isn't going to produce optimal results.

There's still an alternative to the astrotracer, on the cheap, and that is stacking. It's a whole different ball game, but you can get sharp images of even small portions of the sky with a cheap manual lens. Downsides are the wear and tear on the shutter, the amount of pictures you're going to need, and the chore of processing them on a PC.
06-08-2018, 04:03 PM   #3
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That is a good point on reliability of the astrotracer feature I had read those. Honestly I didn't know how to calibrate it until maybe 3 hours ago ha. But I see people getting some really good results using it. I haven't used it yet so I have to do some trial and error. I am going to big bend in September and plan on getting milkyway photos clouds permitting. But I am also going to DC this month and thought it would be cool to get some night shots of the monuments with a wide angle. So I was just going too buy one now. But then I got to thinking about whether its really worth it if you are going to use astrotracer. I have gotten some good IQ using the 18-135, its pretty sharp.

Was looking at the 15mm f/4.0 Pentax prime / 14mm Rokinon f/2.8 / Irix 15mm f/2.4.

In my mind I don't think the 18 vs 14/15 would be a big difference but maybe I am wrong and it would be worth it for that reason. I see the comparisons online just google searching but its hard to really gauge it without being the person taking the photo and being there yourself. Also using the camera handheld would benefit from the low aperture like on the Irix at f2.4. For evening street/architecture photography.
06-08-2018, 04:25 PM   #4
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It is possible to achieve decent results with Astrotracer, however I've found what other posters here have noted. Calibration can be hit or miss. Here in Hawaii, some of the lava fields have high iron content, making it almost impossible to get a calibration as a result of poor compass function. I've even found moving 10-15 feet can make the difference between obtaining a Calibrated screen or Failure.

I've also found highly touted ultrawides, such as the Samyang 14mm 2.8 to have soft corners and coma tails at 2.8, requiring stopping down 3.5 - 4 to obtain adequate sharpness.

All in all, my IOptron Skytrackers functions much better than the Astrotracer function.

06-08-2018, 05:11 PM   #5
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QuoteOriginally posted by Justin.APR Quote
[snipped for for brevity]...I have the 18-135mm Pentax lens. I was looking at the 14-15 range. But I am not sure that much of a difference warrants the price for the wider lenses and if I am using astrotracer on the K-3ii then wouldn't the normally terrible f/3.5 aperture on the 18-135 not really matter at that point?

Maybe my other question is; how much of a difference is there between 14-15mm and 18mm? I haven't used anything wider that 17 (used to have the sigma 17-50 f2.8) so I haven't experienced that first hand...
Practice with what you have before spending more money on astrophotography specific equipment. Your 18-135 lens, while not ideal, can get you started started with wide angle Milky Way photography. Try 18mm, f3.5, 20 seconds, ISO 3200 to start. Use raw DNG and process to reduce noise, adjust color balance, and manipulate contrast.

14-15mm is considerably wider than 18mm. Samyang/Rokinon also has a 16mm/f2.0 lens that's a good option with APS-C size sensors.

One challenge of using the astrotracer or other tracking for wide images is that you'll get sharper stars at the expense of blurred ground features. Layering and extra processing can remedy that, but again it takes more software work.
06-08-2018, 07:03 PM   #6
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The wider the angle, the smaller will the stars look in your photo. Depends on what you are looking to shoot, ultra wide angle lenses may be good for astro landscapes in which you see the milky way and some of the brighter stars, but not so much to take pictures of constellations etc.

Stacking seems to be the way to go regardless of your equipment. Either to lower noise, or to capture different shades of faint deep space objects, many of the best photos I see taken with normal camera gear (not motorized trackers and telescopes) are stacked.

To answer your question: In my experience with single photos with astro tracer, f2.8 is perfectly fine, f4 means more noise but I still find it acceptable. Coma is a greater issue because that's not fixable via stacking.

Last edited by aaacb; 06-08-2018 at 07:12 PM.
06-08-2018, 09:06 PM - 1 Like   #7
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I have used astrotracer on both my K-5 and my K-1. I have had pretty good luck with calibration. One thing I found online was that at the end of calibration when it says Calibration Complete, you still need to hit OK or your calibration is not saved. This is not clear in the manual but is apparently important. Occasionally my calibration is off, so I check the first shot or two for star trails. If the stars look good, Iím usually good to go. If I see trails, I recalibrate.

What the larger aperture gets you is more light for shorter exposures. I shoot with a Samyang 14mm f2.8 and astrotracer does a good job shooting the Milkyway for up to 80-90 seconds at 800-1600 ISO. If you have a smaller aperture, it still works but you might risk going longer exposure and getting small star trails or you need to go to higher ISO and risk more noise. I have used the Pentax 10-17 f3.5-4.5 on my K-5 with pretty good luck. Sometimes I can get 2-3 minute exposures with Astrotracer on either lens but there are usually some short star trails.

I agree that you can go a little longer on focal length but lens quality for star shots is important. The Lenstip reviews measure coma which determines if your stars stay round or get distorted. For my Samyang at f3.5, I get nice shots with some distortion (streaks) in the corners on my full-frame K-1. I have not tried stacking.
06-09-2018, 02:37 PM - 1 Like   #8
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Absolute aperture (size) determines the brightness of point objects like individual stars. With wide angle lenses, this can be problematic because even with low f stops, the absolute aperture can be small. "Extended" objects like nebulae respond to the actual aperture (f2 versus f8), so they can be rendered brighter even with a wide angle lens while the stars might be considerably muted relative to the extended object. Hence, contrast between stars and nebulae will be controlled by the size of the aperture and the f stop, and that contrast will depend on the focal length of the lens (generally speaking). Large f stops don't necessarily mean bright stars but for nebulae and aurora they do what is expected of a "fast" lens and prov beneficial for shorter exposures.

06-09-2018, 06:59 PM - 1 Like   #9
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Aperture isn't as critical with the Astrotracer. A fast lens lets you lower the ISO (less noise). Learn post processing and how your gear works at night before you spend a lot on gear.


smc PENTAX-DA 18-55mm F3.5-5.6 AL WR. 120 seconds (with Astrotracer) 3200 ISO f5 on a K5ii.
06-09-2018, 09:47 PM   #10
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I guess it depends on your luck with astrotracer then. I have taken a few star photos with the kit lenses before and the Sigma 17-50 f/2.8 and got decent results but I never had astrotracer until I picked up my K-3ii recently. Sounds like I need to play around with it and get familiar with that mode. I hadn't planned on doing any DSO pictures since my biggest lens is 300mm. Mostly just milkyway/star photos with mountains as silhouettes cutting into the nightsky. Seems like the wide angle large aperture lens wouldn't be terrible to pick up in the case that astrotracer doesn't want to calibrate properly. I do have the 35mm f/2.4 and 50mm f/1.8 to use as well but with the crop sensor those are pretty narrow.

I have no experience with stacking or stitching the pictures together so I will have to try that out.

Thanks everyone for their input. Seems like with astrotracer the aperture isn't completely vital but it helps to have, especially when the feature doesn't work properly.
06-09-2018, 11:59 PM - 1 Like   #11
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QuoteOriginally posted by Justin.APR Quote
My question is; when using astrotracer how important really is the aperture size? You typically see people looking for the largest possible aperture for astro but that is normally based off not being able to use astrotracer with the Pentax cameras. You want the larger aperture so that you an bring the most amount of light in small amount of time available before you start to get star trails. But if you are using astrotracer then that kind of eliminates that requirement doesn't it?
Well overall, aperture controls the amount of light that will actually get to the sensor. Exposure time is the amount of time that the star light will be collected. They are both extremely important in astro. You really can't ignore one and only concentrate on just one. You need to balance these two items.

Also, you just can't depend on just the astrotracking capability that Pentax provides. The problem is that even with today's excellent sensors, there is precious little star light falling on the earth. Also, when you couple the relatively small amount of light with the earth's rotation, you only have a small amount of exposure time to collect the light before you get star trailing. Using the astrotracer function, you can get up to - 5 minutes of sensor tracking. In reality, given wide angle lenses, some compass pointing error in the GPS system, the sensor moving in just an XY plane of the camera, etc. - you are only going to get an effective 60 to 90 seconds of tracking. However, this is OK, since it's 5 to 7 times longer than what you can expose for with out tracking. So, it's a winning combination.

So, the overall answer you are looking for is that - yes, lens aperture is important (arguably the most important component in all of this), as well as exposure time (which astrotracking/astrotracing extends). Additional, ISO is important - yes, higher ISO amplifies the light to a greater extent, however it also diminishes the dynamic range of the image - which will reduce the quality of the light (essentially the color of the light being captured). There is so little light being captured - you need to take every advantage you can in order to effectively collect it. The key is to shoot with the lowest ISO you can get away with, while still collecting enough star light to provide you with a good resulting image.

QuoteQuote:
Reason I am asking is I was looking to buy a wide angle lens for astrophotography and some minimal architecture photography. I have the 18-135mm Pentax lens. I was looking at the 14-15 range. But I am not sure that much of a difference warrants the price for the wider lenses and if I am using astrotracer on the K-3ii then wouldn't the normally terrible f/3.5 aperture on the 18-135 not really matter at that point?
Take a look at this recent thread on the topic. There is much more than just getting a reasonably wide aperture. Focal length does play an important part in this too.Yes, by all means use the equipment you have. Astro is a weapons race with yourself. What I mean is, that this is one of the few areas in photography where gear actually matters, and it does get expensive - if you go pursuing the Unicorn. It's almost an endless quest.

QuoteQuote:
Maybe my other question is; how much of a difference is there between 14-15mm and 18mm? I haven't used anything wider that 17 (used to have the sigma 17-50 f2.8) so I haven't experienced that first hand.
There is a relationship between aperture and focal length that relates to the physical aperture size of a lens. Longer focal length lenses have larger physical aperture sizes, and are thus more effective at collecting light. Having said that, you need to balance that against having a reasonably wide lens to collect the overall scene that you desire to shoot. Again, refer to the link above to the recent thread and discussion there.

QuoteQuote:
Also I know the broad answer is no aperture doesn't matter you can always make due with what you have. But my question is would the larger aperture really give any better results while utilizing astrotracer? Is it still worth $300-$500 to get that wide large aperture lens if you have the astrotracer feature?
Yes - a larger aperture does matter with the astrotracer - the combination of the two collects substantially more light and thereby provides a better resulting image. I would also suggest considering used lenses. There are now enough used lenses out there, that you can now find them - thus saving you quite a bit.

QuoteOriginally posted by Justin.APR Quote
That is a good point on reliability of the astrotracer feature I had read those. Honestly I didn't know how to calibrate it until maybe 3 hours ago ha. But I see people getting some really good results using it. I haven't used it yet so I have to do some trial and error. I am going to big bend in September and plan on getting milkyway photos clouds permitting. But I am also going to DC this month and thought it would be cool to get some night shots of the monuments with a wide angle. So I was just going too buy one now. But then I got to thinking about whether its really worth it if you are going to use astrotracer. I have gotten some good IQ using the 18-135, its pretty sharp.
Use what you have and then figuring out what you really want to capture in terms of the resulting image and image quality along with how you are post processing the images.

QuoteQuote:
Was looking at the 15mm f/4.0 Pentax prime / 14mm Rokinon f/2.8 / Irix 15mm f/2.4.
I would also toss the Rokinon 16mm/f2 in to the mix. It's an entire stop faster (collecting twice amount of light right off the bat) and you will get an even larger physical aperture size.

QuoteQuote:
In my mind I don't think the 18 vs 14/15 would be a big difference but maybe I am wrong and it would be worth it for that reason. I see the comparisons online just google searching but its hard to really gauge it without being the person taking the photo and being there yourself. Also using the camera handheld would benefit from the low aperture like on the Irix at f2.4. For evening street/architecture photography.
Yes, there is a large difference. It's arguable that the Sigma 18-35/f1.8 is one of the best astro lenses, due to its fast aperture and focal length. It also has no coma and is very well optically corrected.

QuoteOriginally posted by Justin.APR Quote
I guess it depends on your luck with astrotracer then. I have taken a few star photos with the kit lenses before and the Sigma 17-50 f/2.8 and got decent results but I never had astrotracer until I picked up my K-3ii recently. Sounds like I need to play around with it and get familiar with that mode. I hadn't planned on doing any DSO pictures since my biggest lens is 300mm. Mostly just milkyway/star photos with mountains as silhouettes cutting into the nightsky. Seems like the wide angle large aperture lens wouldn't be terrible to pick up in the case that astrotracer doesn't want to calibrate properly. I do have the 35mm f/2.4 and 50mm f/1.8 to use as well but with the crop sensor those are pretty narrow.
I would encourage you to use both the 35mm f/2.4 and 50mm f/1.8 and to stitch the images using Microsoft ICE (a free download). Both the focal length and the aperture size coupled with astrotracing will work in your favor.

QuoteQuote:
I have no experience with stacking or stitching the pictures together so I will have to try that out.
It's more post processing, but it works very well

06-12-2018, 08:57 AM   #12
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Thanks Interested_observer for all that info and digging into my questions. I did not know about the Rokinon 16 f/2.0. I really like what I see with that lens and found some awesome astro examples people have taken with that lens. Do you/anyone have experience with the samyang 24mm f/1.4. Seems like that would be good for astro as well especially if stitching is used. I am also in the market for a 21mm(ish) lens for street photography on the K-3ii crop sensor to better replicate the 35mm on FF. (Currently use 35mm on crop which can be to narrow at times). I know its manual focus but I mostly walk around using manual anyway. Might get two birds with one stone.
06-12-2018, 02:34 PM   #13
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QuoteOriginally posted by Justin.APR Quote
Thanks Interested_observer for all that info and digging into my questions.

I did not know about the Rokinon 16 f/2.0. I really like what I see with that lens and found some awesome astro examples people have taken with that lens.
There are some folks down in the Astrophotography group that have that lens, that have posted really nice shots of the Milky Way.
QuoteQuote:
Do you/anyone have experience with the samyang 24mm f/1.4. Seems like that would be good for astro as well especially if stitching is used.
Personally, I don't. If I remember correctly, it came out a while before the Sigma 35/f1.4 and was thus the largest physical aperture available that was good in astro, especially for the price. Then the 35/f1.4 arrived and folks somewhat moved over. I was "late" going to full frame, so I stayed with the 18-35, since it was both more general purpose, and I could shoot at different focal lengths - the longer of which opened up the aperture for more light. When I picked up the K1, I again took the same approach getting the 15-30, giving up a bit of aperture for a more generic set of uses.

I do have to say, when I was shooting with the K5, I always used the 18 end of the 18-35. Now with the K1, I am sliding to the longer end - opening up the aperture for more light, but have shot some at the 15 end, and have plenty of light. So, it all pretty much comes down to how you shoot or how you want to shoot, and what situations you find yourself in.

I was out shooting last night at not the darkest location available. The light pollution only get worse. On one hand it helps with the landscape segment, tending to brighten up that aspect (and reducing noise while enhancing the captured detail) - but plays havoc with the sky segment. Plus, the state park installed some super bright lights on the restrooms - which just blew everything out.

As I posted earlier - it all comes down to the balance you are willing to strike in terms of cost, acquiring light and overall convince. There really is no "right" or "perfect" answer - just a decision you are willing to live with (or spend on).

07-29-2018, 05:23 PM   #14
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I'm +1-ing IO on one of their early points too: It's much more important to get out there and get shooting and processing, rather than waiting till the One True Lens. Processing matters more in astro than in any other photographic discipline I have encountered, and the only way to get good at it is to do it. A lot of it. (Much more of it, apparently, in my case!) It's hardly unusual for me to spend several hours on a single image, throw that out, and start over. Bracken's Deep-Sky Imaging Primer is just excellent if you want to read up. The Brits swear by Making Every Photon Count, I haven't read that one.
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