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03-28-2016, 05:22 AM - 3 Likes   #1
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some star test results

This is mainly for astrophotography. For stars, the lens need to be sharp at wide apertures in both the center and corners, with as little coma and color aberration as possible. This is kind of different from the requirements of portrait or landscape photography. I am still new to the P645 system, and am trying to figure out what lenses may be used for astrophotography. The best way to find out the answers is to actually shoot stars. At this moment, I only have four lenses that can be attached to 645z. Here are what I got, and some comments. In the image, I show the center, and four extreme corners.

645 DFA 55/2.8
F2.8: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/645_55mm/f2.8_stack_mosaic.jpg
F3.5: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/645_55mm/f3.5_stack_mosaic.jpg
F4.0: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/645_55mm/f4.0_stack_mosaic.jpg
F4.5: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/645_55mm/f4.5_stack_mosaic.jpg

I feel this lens is borderline OK for astrophotography, and definitely not excellent. It only becomes barely acceptable after stopping down to F4.5. I may be forced to use it at F4.0 in some situations, but I won't have a smile face when I do so.

---------- Post added 03-28-16 at 05:26 AM ----------

This is for the FA 35/3.5.

F4.0: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/645_35mm/F4.0_mosaic.jpg
F5.0: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/645_35mm/F5.0_mosaic.jpg

This lens is incredibly sharp in the center. I have never seen such pinpoint stars from any other lenses on my D800, including the highly respected Sigma 50/1.4 Art and 35/1.4 Art.

On the other hand, it has strong coma even when stopped down to F5.0. I will never stop it down further for astrophotography, and F5.0 is acceptable to me give that this is a wide-angle lens.

I kind of wonder whether the new DFA 35/3.5 can perform better. If the coma can be reduced, it will be a killer.

---------- Post added 03-28-16 at 05:32 AM ----------

I have three Pentax 67 lenses. One of them is ED300/F4. On D800, the image shows misaligned optical elements. So I do not bother putting it on 645z. Below are the results from the 67 90/2.8.

F2.8: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/67_90mm/F2.8_stack_mosaic.jpg
F4.0: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/67_90mm/F4.0_stack_mosaic.jpg
F4.5: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/67_90mm/F4.5_stack_mosaic.jpg

There is also a sign of misaligned elements, as stars in the lower-right and upper-left corners are softer than the other corners. Fortunately, it's not too bad. It is acceptable at F4.0, and fairly good at F4.5. The center is a little bit softer than the 645 55/2.8, but the corners are much better. I probably will try to get another one from ebay and see if I have better luck on the corner sharpness.

---------- Post added 03-28-16 at 05:36 AM ----------

Finally, 67 165mm/F2.8

F2.8: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/67_165mm/F2.8_stack_mosaic.jpg
F4.0: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/67_165mm/F4.0_stack_mosaic.jpg
F4.5: https://dl.dropboxusercontent.com/u/15886389/645_lens_tests/67_165mm/F4.5_stack_mosaic.jpg

It's acceptable at F4, and quite decent at F4.5. I used to use this lens wide open on P67 with film. Obviously digital sensors demand much more. These are all the lenses I have that can be easily attached to 645z. I have many Mamiya 645 lenses. Unfortunately most of them cannot be put on 645z because of the flange-back distances. However, the M645 300/2.8 APO has a detachable rear end. I am working on modifying it so I can put it on 645z. I will let you know once I make some progress on this.

Cheers,
Wei-Hao

03-29-2016, 12:10 AM   #2
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Thanks for the tests!
03-29-2016, 09:01 AM   #3
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thanks. good test.
03-31-2016, 01:34 AM   #4
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I wonder how the new DFA 35mm would go?

05-12-2016, 09:40 AM   #5
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I added a few 67 lenses and create a separate Flickr account to just store the star test images. You can see them here:
https://www.flickr.com/photos/140916263@N04/sets/72157665841471914

Most of the test images are made from stacks of very short exposures (2 to 6 seconds). Later I realize that this cannot reveal color aberration, as shown by the P67 200mm/F4 case. So at some point all these tests need to be redone with longer exposures. This will be time consuming and also slightly more technically challenging. So I am not sure when I will have chances to do this, but I will try.

One issue I found long time ago and confirmed by these tests is that P67 lenses become out of collimation very easily. A good fraction of the tested lenses are out of collimation. The symptoms are not-round stars in the center or the shapes of stars in two opposite corners not mirroring each other. In the Flickr pages, you can easily see this on the color fringes of the 67 200/F4 and the shape of stars of the 67 75/2.8 AL. This is also true for my 67 ED 300/F4 on D800. So you should be really careful when you invest on a used 67 lens. The lenses may be good initially, but not so after a few years of usage.

Based on the data so far, and assuming that color aberration should be less an issue on shorter lenses, my ranking of the lenses tested so far for astrophotography is (from high to low):

1. 645 FA35/3.5 (needs to stop down to F5 or F5.6)
2. 645 DFA55/2.8 (needs to stop down to F5 or F5.6)
3. 67 90/2.8 (needs to stop down to F4.5)
4. 67 165/2.8 (needs to stop down to F4.5)
5. 67 75/2.8 AL (needs to stop down to F5.6)
6. 67 105/2.8 (needs to stop down to F5.6)
7. 67 200/4 (strong color fringing that doesn't go away at F5.6)

If I have to draw a line, I would say #6 and #7 are not capable of doing good astrophotography on 645z. #5 is the minimum I can accept, if the copy does not have collimation issues.

I hope at some point I can put my hand on the 645 DFA35, but this won't be any time soon, unfortunately.
11-21-2016, 08:22 AM   #6
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A friend of mine got a like-new 25mm/F4 and let me use it for a short while. I took some star test images with it. You can view them here:
P645 DA25/4 @ F4.0 on 645z | Reasonable sharp at the frame c? | Flickr
Click the left arrow to see images taken under different apertures.

The results disappoint me. Strong coma exists even half way through the frame. Essentially only the very center of the frame is coma-free. Color fringes is also there in the corners. And all these do not improve much after stopping down to F5.6, which is the smallest practical aperture for astrophotography.

Last week I took my 645z and this lens to the Atacama desert to visit a few astronomical observatories. This is one of the pictures I took:
ALMA and the Magellanic Clouds | Stack of 8 20-second exposu? | Flickr
I stopped the 25mm down to F5.6. Here you can see that stars in the center appear to be fainter than stars in the outer parts of the frame. This is an artificial effect, caused by the coma of this lens. Actually, it is much worse. I had use several photoshop tricks to reduce this effect in the above Flickr image. The original one can be found here:
https://dl.dropboxusercontent.com/u/15886389/645z_25mm.jpg
You can see how bad it is in its original form. The apparent brightness of stars is not uniform, and this is very bad for astrophotography. For comparison, this is an image taken with Nikon D800 and Tamron 15-30/F2.8 at 20/F2.8:
https://dl.dropboxusercontent.com/u/15886389/D800_15-30mm.jpg
You can see that the apparent size of stars is much more uniform across the frame, and the stars are reasonably sharp. And this is achieved at F2.8, without any stopping down.

So far I had tried the 25mm, 35mm, and 55mm, and none of them impressed me. I don't have a high hope for the 70mm, given how these three perform. The 90mm might be still impressive, but its price is quite prohibitive. I will probably never have a chance to use the 90mm.

Looks like FF is the way to go for astrophotography. Then lenses for 645z just do not cut it.
12-03-2016, 05:55 AM   #7
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I have been looking into medium format cameras and the 645Z in particular. I would recommend not using camera lenses at all and using dedicated astrographs instead. Astrophotography pushes lenses in a way that landscape or portrait photography does not. For professional grade results take a look at the following options. These are astrographs designed for medium format:

Vixen VSD:
Ultra Fast Astrograph VSD100F3.8 | Vixen Co., Ltd.
"Vixen's goal was to develop a process to outperform the views from a premium photo lens...."
"The VSD100F3.8 has an oversized focuser that can be attached to the 645 medium format cameras without difficulty."

Celestron Rowe-Ackermann-Schmidt astrograph:
Celestron - Optical Tubes - Rowe-Ackermann Schmidt Astrograph
"11-inch f/2.2 optical design with rare-earth glass for images free of false color, coma, and field curvature
43.3mm optimized image circle maintains pinpoint stars to the far corners of even the largest astroimaging sensors, while the usable field extends even further to 52 mm for larger format sensors."

Takahashi FSQ-106ED:
Takahashi FSQ-106ED main features
"The modified Petzval design of the FSQ-106ED, with the field flattener integrated inside the tube, makes it possible to cover all the photographic formats, from 35mm SLR/DSLR to 6x7 cameras. Its photographic image circle of 88mm of diameter (field of 9,5°) does not have an equivalent on the market of the refractors of 100mm of aperture."

If you are on a budget then look at the Ritchey-Chretein designs. The Orion 8" f/8 is a bargain:
Amazon.com : Orion 08267 Ritchey-Chretien 8 inch F/8 Optical Tube Assembly (White) : Camera & Photo
"The Orion 8 inch Ritchey-Chretien Astrograph is ideal for use with small CCD-chip cameras such as the Orion Star Shoot line, as well as medium-format CCD and DSLR cameras. The telescope's wide, fully illuminated flat field produces pinpoint star clarity all the way to the edge of the camera's sensor."

The only caveat I have with the Orion 8" is that I would personally upgrade the focuser to something a bit heavier duty.

The least expensive option is an Imaging Newtonian coupled with a coma corrector. I've used these before and the results are great once you properly collimate them. For medium format this may not be perfectly flat all the way to the very edge but the imaging circle is still fairly large and should yield fantastic results especially for the price.
Vixen R200SS with Coma Corrector 3 Bundle | OPT Telescopes

I hope this is information is of good use to you.

obin
12-04-2016, 07:27 AM - 1 Like   #8
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Hi,

Thanks. I used the VSD a couple of times. I am not impressed by its sharpness, although the image quality is quite uniform (uniformly mediocre) across the FF.

Like what you said, I now believe it is not a good idea to use any lenses on 645z. I primarily use it on my TAK TOA150. I have its 67 flattener and 645 reducer, and both can cover 645z very well.

The cheap 8" Newtonians do not have coma correctors suitable for 645z, I am afraid. I have TAK Epsilon 180ED, and even that doesn't cover the 645z sensor. The very large flange-back distance of Pentax 645 also makes things extremely difficult. Not many correctors/flatteners in the market are design to have this much back focus.

Very unfortunately, I have to conclude that 645z is not a very good camera for astrophotography, despite the very good sensor in it. The new Fuji mirrorless MP has much shorter flange back distance, which will allow it to be put on many more telescopes. It is also possible that its lenses perform better than 645z's lenses. I am looking forward to this new system.

Cheers,
Wei-Hao

12-04-2016, 08:12 AM   #9
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QuoteOriginally posted by whwang Quote
Hi,

Thanks. I used the VSD a couple of times. I am not impressed by its sharpness, although the image quality is quite uniform (uniformly mediocre) across the FF.

Like what you said, I now believe it is not a good idea to use any lenses on 645z. I primarily use it on my TAK TOA150. I have its 67 flattener and 645 reducer, and both can cover 645z very well.

The cheap 8" Newtonians do not have coma correctors suitable for 645z, I am afraid. I have TAK Epsilon 180ED, and even that doesn't cover the 645z sensor. The very large flange-back distance of Pentax 645 also makes things extremely difficult. Not many correctors/flatteners in the market are design to have this much back focus.

Very unfortunately, I have to conclude that 645z is not a very good camera for astrophotography, despite the very good sensor in it. The new Fuji mirrorless MP has much shorter flange back distance, which will allow it to be put on many more telescopes. It is also possible that its lenses perform better than 645z's lenses. I am looking forward to this new system.

Cheers,
Wei-Hao
Thanks for that update. I am surprised to hear that the VSD didn't impress you as the reviews are quite positive. Of course I trust you assessment though. I also agree that lenses in general aren't going to do very well on the 645Z for astrophotography. Camera lenses in general are not adapted for astrophotography and only a few are really outstanding and can compare to dedicated astrographs with a large imaging circle. I built my own Newtonian astrograph with a larger secondary for imaging using an APS-C sensor. It works well but I admit it's tough to find a perfect coma corrector due to the inherent imaging flaws with the Newtonian visual train.

For astrophotography have you thought about the Moravian G4 series cameras? Large format G4-9000 and G4-16000 cooled CCD cameras

Edit: I forgot to ask if you've looked into the various Dall-Kirkhams and Harmer-Wynne designs from AG Optical? They are well known for their large imaging circles and flat fields.
http://www.agoptical.com/idk/default.html

obin

Last edited by Obin Robinson; 12-04-2016 at 08:43 AM.
12-04-2016, 11:27 PM - 1 Like   #10
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Hi Obin,

Yes, all the things (or the similar) you mentioned came across my mind at least once in the past.

I have a Apogee F16M, which has the same CCD (KAF16803) as the G4-16000. I am not impressed by this camera (or cooled CCD in general), as its read noise is about 4x as large as modern CMOS. This severely limits its capability when used with RGB or H-alpha filters. The exposure has to be unreasonably long (under my F7 optics) to make the influence of read noise go away. I realized this after I wasted quite a few nights on this camera, and then I never look back. CCD is old technology, and CMOS is superior in many ways. Once there is a mono version of large format CMOS camera (cooled or not, doesn't matter to me, since I almost always image under cold temperature), I will seriously consider to abandon DSLRs.

AG's D-K has my attention for a long while. At this moment, I am more into wide-field mosaicking, so their focal length is kind of too long to me. However, I may want to shoot smaller things in the near future. Then I will give it a very serious thought. Indeed, it is the top one on my list if I do want to switch to longer focal length.

On the VSD, if one takes a somewhat shallow image, say 0.5 hr of total exposure, and compare it with TAK's FSQ, the two are basically on par with each other. They are equally sharp. The FSQ perhaps has a slight edge in the field center, while the VSD has a slight edge in the FF corners. However, if the integration becomes very long, the stars in the VSD image will start to blow up, while the FSQ stars remain tiny. This means that there is a tiny level of uncorrected spherical (or chromatic?) aberration on the VSD. It's also more expansive than the FSQ, which makes no sense.

Cheers,
Wei-Hao
12-05-2016, 11:35 AM - 1 Like   #11
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QuoteOriginally posted by Obin Robinson Quote
Camera lenses in general are not adapted for astrophotography and only a few are really outstanding
Without a doubt!
If camera lenses were designed for astro work, the emphasis would be on coma, lateral chromatic, longitudinal chromatic and distortion. But for terrestrial use, there is not the emphasis on these 4 aberrations but all 7. Camera lenses of a symmetrical design correct distortion, lateral chromatic and coma automatically so those are good candidates for astro work. The Plasmat design used on view cameras, especially the ones using low dispersion glass would be candidates for astro use.
12-05-2016, 03:14 PM   #12
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QuoteOriginally posted by whwang Quote
Hi Obin,

Yes, all the things (or the similar) you mentioned came across my mind at least once in the past.

Cheers,
Wei-Hao
It looks like you've covered a lot of the bases. Honestly at this point I would look into custom optics. Have you looked at Ceravolo Optical Systems?
Ceravolo Optical Systems

They have done many terrestrial and space-based optical systems. They are right at the top of my personal list if I were looking into a good imaging setup for a medium or large format camera. Once all is said and done you'd have an imaging setup perfected for your equipment. I'd also look at the Clement Bellerophon focuser as well.


QuoteOriginally posted by desertscape Quote
Without a doubt!
If camera lenses were designed for astro work, the emphasis would be on coma, lateral chromatic, longitudinal chromatic and distortion. But for terrestrial use, there is not the emphasis on these 4 aberrations but all 7. Camera lenses of a symmetrical design correct distortion, lateral chromatic and coma automatically so those are good candidates for astro work. The Plasmat design used on view cameras, especially the ones using low dispersion glass would be candidates for astro use.
The other major flaw in camera lenses is the iris diaphragm. It causes undesirable flares with bright stars. A better method for reducing aperture is using an aperture reducing filter ring. Very few camera lenses use high quality glass like fluorite or crown glass because it would make them prohibitively expensive. I personally like the Ritchey–Chrétien design and I plan on building one for astrophotography usage. I feel that it's the perfect compromise as it offers a large field of view with optics faster than a classic cassegrain.

obin
02-01-2017, 06:17 PM   #13
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Have you tried the 150mm f2.8 lens (645)? I'm not into astro as you are. Attached is a handheld picture I took two years of the moon with the 150mm and 1.4x converter, cropped to about 230%.

02-02-2017, 02:47 AM   #14
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Unfortunately I don't have access to this lens. I hope I can have a chance to test it.
02-02-2017, 04:06 AM   #15
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QuoteOriginally posted by whwang Quote
Unfortunately I don't have access to this lens. I hope I can have a chance to test it.
I also read good reviews about the DFA35mm, but from your experience with the FA35mm it sounds as if it might be an optical problem rather than a coating. I believe the optics are the same between the three models of the 35mm, just the DFA is supposed to have newer coatings, and of course a new price tag to go with it.
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