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06-04-2019, 05:23 AM   #1
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Astophotography: K-200 F4 or K-200 F2.5

Hello dear Pentaxians,

Could you please help me to choose between the K-200 F4 and the K-200 F2.5 for trying to do astrophoto?

I have the opportunity to buy either one of those (not the two as the 200/2.5 is expensive )

Thank for your help.

Kind regards.

J.

06-04-2019, 05:30 AM   #2
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For astrophotography, faster is always better as it substantially reduces exposure time, and thus sensor noise is reduced as well.
Sharpness is an issue as well - yuo want tack sharp stars with an optimal FWHM.
If you have the option, I'd test them both on stars.
Remember, you can always stop the f/2,5 down a little bit.
06-04-2019, 06:33 AM   #3
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ProfessorBuzz pretty much said it all. The difference in light gain is the square of the ratio of f-stops so the f 2.5 will capture a little over 2.5 times as much light as the f4 lens (both being compared wide open). Also, it's often the case (but not always) that optically the f2.5 will be better than the f2.5 by the time it's closed down to f4 which would be wide open for the f4. That said, either will work for astrophotography with todays fast sensors and a mount which allows you to follow the stars and obtain a time exposure.
06-04-2019, 07:25 AM   #4
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What are you trying to shoot?

200mm seems to be a bit odd length for astro photo. If you want deep space objects, they are generally pretty small - the Andromeda galaxy is about as big as they get at around a few degrees across. Most are much smaller. I wish I had longer than my 300mm for these.

For Milky Way/Constellation shots, you want wider - tens of degrees field of view.

06-04-2019, 07:33 AM - 1 Like   #5
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Either one would do well, especially if you have an equatorial mount. I the 200mm f/4 is a good astro lens and would assume that the f/2.5 would be as good or better. For reference of what a good 200mm f/4 can do for astro in competent hands I refer people to this post from Pete_XL over in the astrophotography group. However the f/2.5 may have other issues like more CA, coma, or more field curvature that the f/4 doesn't have. However as Bob 256 points out in most cases stopping the faster lens down to the same level as the slower one the faster one will likely be optically better. Astrophotography is really punishing with lenses and will show any defects they have.

If I were in your position I would go with the f/2.5 and if I didn't like it, sell and get the f/4. You likely could get close to what you paid for the f/2.5 here provided it isn't overpriced to begin with. In that case it could considered a free extended trial. That was the approach I took with getting my Sigma 300mm f/4 APO macro. Found out that I could basically steal it as the store was selling it for $130 as I wanted a better 300 than the M42 S-M-C one I had and in that case if I didn't like it I could have sold it at a profit.
06-04-2019, 07:34 AM   #6
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QuoteOriginally posted by AstroDave Quote
What are you trying to shoot?

200mm seems to be a bit odd length for astro photo. If you want deep space objects, they are generally pretty small - the Andromeda galaxy is about as big as they get at around a few degrees across. Most are much smaller. I wish I had longer than my 300mm for these.

For Milky Way/Constellation shots, you want wider - tens of degrees field of view.
Probably objects like M42, IC 434, M8, M16...
For deep sky, I have a Celestron ultima 8...
06-04-2019, 07:39 AM   #7
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QuoteOriginally posted by MossyRocks Quote
Either one would do well, especially if you have an equatorial mount. I the 200mm f/4 is a good astro lens and would assume that the f/2.5 would be as good or better. For reference of what a good 200mm f/4 can do for astro in competent hands I refer people to this post from Pete_XL over in the astrophotography group. However the f/2.5 may have other issues like more CA, coma, or more field curvature that the f/4 doesn't have. However as Bob 256 points out in most cases stopping the faster lens down to the same level as the slower one the faster one will likely be optically better. Astrophotography is really punishing with lenses and will show any defects they have.

If I were in your position I would go with the f/2.5 and if I didn't like it, sell and get the f/4. You likely could get close to what you paid for the f/2.5 here provided it isn't overpriced to begin with. In that case it could considered a free extended trial. That was the approach I took with getting my Sigma 300mm f/4 APO macro. Found out that I could basically steal it as the store was selling it for $130 as I wanted a better 300 than the M42 S-M-C one I had and in that case if I didn't like it I could have sold it at a profit.
In fact, what should a K-200/2.5 to be priced?
06-04-2019, 08:14 AM - 1 Like   #8
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QuoteOriginally posted by Jome Quote
In fact, what should a K-200/2.5 to be priced?
eBay search results indicate a range of ~$250 - ~$800 USD. Review price average on this site is $349. This is not a common lens (expensive when new?) and the prices reflect the availability. Still though, one might be a good thing to get as a poor-man's A* 200/2.8.

You might want to look at one of the Tamron 300mm f/2.8 (Adaptall-2) lenses as a possible option in a similar price range.


Steve


Last edited by stevebrot; 06-04-2019 at 10:18 AM. Reason: accuracy
06-04-2019, 08:35 AM   #9
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Last year I used Pentax-K 200 F4 with K5 and Astrotracer O-GPS1 twice for astrophotography and the results weren't the best.

Dropbox - Astro - Simplify your life

Pleyades:
No of photos: 5 frames (2x ISO 800 60s, 2x ISO 3200 60s, 1x ISO 3200 30s) merged with DSS and edited in LR
Aperture: Different depends on exposure parameters
Place: Wrocław, Poland, some hill next to me, sky wasn't crispy clear
Date: 1st Nov 2018, 00:27-00:34 am

First photo is split in three parts. First one shows single frame, second - all 5 photos merged in DSS, third - final photo.
Second photo is a crop of final photo.

Orion Nebula M42 and Running Man
No of photos: 49 frames
23 Lights: F5.6, 30s, ISO 3200
13 Darks: F5.6: 30s, ISO 3200
13 Bias: F5.6, 1/8000, ISO 3200
0 Flats: Didn't have any white t-shirt to put on the lens

Pentax K-5, Pentax-K 200mm F4, Astrotracer O-GPS1 unit, remote YP-970 Rx II, tripod Bendro iT25

Place: Ngapali Beach, Myanmar. It wasn't really dark but I didn't want to go far away from my bungalow
Date: 18 Nov 2018, 04:22-04:51

I used Deep Sky Stacker to stack all 49 frames and edited it in LR later. I know it's not the best software for that work but it was my second attempt to astrophotography and I was so excited that I saw my first deep space object

PS. There is also Orion Belt + Orion Belt with Orion Nebula. Total number of frames for last two photos were a bit less because I was too tired to sit whole night on the beach
06-04-2019, 08:44 AM   #10
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the OP might want to look at this series of articles re: astrophotography

Astrophotography Series Announcement - Articles and Tips | PentaxForums.com
06-04-2019, 09:09 AM   #11
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QuoteOriginally posted by m4ciekk Quote
Last year I used Pentax-K 200 F4 with K5 and Astrotracer O-GPS1 twice for astrophotography and the results weren't the best.
For only attempting things twice and with such a small amount of total exposure time that isn't too bad. My first efforts started off much worse. The stars are bloated and you have a fair amount of clipping in all the images (I will shoot these objects at f/5.6 and 20 or 30 seconds at ISO 800 or ISO 400 with a 300mm lens). Things I would suggest doing would be to see about getting a Bahtinov mask made for your lens so you can nail the focus. There are online generators for them that you can then print out. Also shoot at a lower ISO as both of these objects are really bright.

After that another piece of equipment to consider would be a red intensifier filter. These are a poor man's light pollution filter and will do wonders for getting more out of your images.

I use Deep Sky Stacker (DSS) and the O-GPS1 for doing astrophotography and while a proper equatorial mount would be nice one can get some good results using astrotracer. Here are a couple of images I did last summer using my Sigma 300mm f/4, K-3, and O-GPS1 but I wish I had made a bahtinov mask then as the focus would have been better. I didn't take any darks, flats, or bias images, but really need to make some master flats for that lens. The stacking was done in DSS and each image used about an hour of images that were the best 25% of 4 or so hours of shots.

Lastly practice editing. I have gone back several times and reprocessed my images as I get better at it and learn new things and even with the same sub optimal input images I am getting better results.
06-04-2019, 12:12 PM   #12
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If you're willing to go older and screw-mount with adapter, the Takumar 200 3.5 is very highly regarded for astro.
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