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12-15-2011, 02:17 PM   #1
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different between Pentax 6x7 300 F4 ED(IF) and 645 300 F4 ED(IF)

Hello group,

My name is Pornchai POP who is very new to this forum. I'm looking for an information about Pentax medium format lens. I'm looking for a 300 mm. medium format lens which I found a Pentax 645 300 F4 ED(IF) and also found Pentax 6x7 300 F4 ED(IF). The shape of the lens is very look similar as well as F4 ED(IF) too.

Is it different in the optical formula and sharpness, CA, other aberration? I saw many successful shot with 6x7 300 F4 ED(IF) but never see in a 645 version. The price of 645 version is much cheaper than in the 6x7 version even both are F4 ED(IF) designate.

Thank you very much.

Pornchai POP

12-15-2011, 02:46 PM   #2
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Have you looked at the 67 lens database?
Pentax 67 Telephoto Prime Lens Reviews and Specifications - Pentax Lens Reviews & Lens Database

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12-15-2011, 03:52 PM   #3
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QuoteOriginally posted by Adam Quote
Thank you very much Adam. I already read it. It is very useful based-on real use. I would like to hear information in comparing each-others

POP.
12-15-2011, 07:24 PM   #4
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What camera are you going to used this on?

12-16-2011, 01:35 AM   #5
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I don't have any medium format camera. I only have a Nikon D700. But I don't these medium format with my D700 camera. Actually, I will use for a Astronomical CCD camera to shoot star in the sky. I would like to take a shot in wide field. Wide field in astronomy should mean 300 mm. focal length or below.

The reason that I have to use a medium format lens because of the registered flange distance or back focus distance of these medium format lens are larger than in the 135 format lens like Canon or Nikon lens. Penatx 645 lens has a back focus distance around 70mm+/-, Pentax 67 lens has 84.95 mm. of back focus distance. These larger number allow me to an astronomical equipment such as camera filter wheel, focuser between CCD camera and the lens itself.

Astronomical CCD camera mostly are in black and white (monochrome). If I want in color shot, I have to shoot 4 set of color, red, green, blue and no filter (luminance). And then combine them together, after that, the photo will be in color.

I need high quality medium format lens because in the bright point light of subject like star, it usually introduce every aberration such as chromatic, spherical, coma and astigmatism. And I want to shoot wide open or just a little less of a f-stop due to the supper dim light of star. But if I shoot wide-open, it will introduce every aberration in the picture.

Not much medium format that can surpass this quality. I knew that Pentax 67 300 F4 ED(IF) is very good as well as Mamiya 645 200 F2.8 APO. If I use these two lens for a reference, can anyone recommend which lens should be good enough for wide-field astrophotography? I'm looking for a lens which have range of focal lenght around 100 - 300 mm.

1. First pic is a setup for what I described above.
2. 2nd pic is a sample of shot that taken using Mamiya 645 200 F2.8 APO
3. 3rd pic is the enlarge from very corner (red box) from the 2nd pic. Star shape at the corner of the frame should be
- sharp, rounded star shape
- no chromatic aberration. only show pure color of its star.
- no curvature of field. (center frame and corner frame should be same sharpness.
- no coma. no star trail radius from center frame to every corner of the frame.
- no astigmatism. (star must be rounded shape, not triangular, regular shape)

All photos is not mine, there are from Mr.Rolf Geissinger at his website: http://www.stern-fan.de/Seiten/frameset.htm

Thank you very much.
Best Regards,
Pornchai POP
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12-16-2011, 04:55 AM   #6
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QuoteOriginally posted by popkrab Quote
....., can anyone recommend which lens should be good enough for wide-field astrophotography? I'm looking for a lens which have range of focal lenght around 100 - 300 mm.


Thank you very much.
Best Regards,
Pornchai POP
The 645 120mm macro and 67 100mm macro are both superb. I'm not sure how using lenses designed for close focusing would affect their application for astro photography however.
12-16-2011, 06:46 AM   #7
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I don't think anyone can answer your question beyond both of these lenses are fine. I would say the difference between the two lenses would be small with the edge gong to the 645 version as it is a smaller format lens and the spot size would need to be smaller.

I have used the 120mm 645 Macro at f/5.6 and it has pin point stars to the edge of a 645D frame--I never shot stars wide open so I don't know how well it would do. I would imagine the 645 300mm is better. Pentax did make telescopes and I am sure their long lenses benefited from that.
12-16-2011, 01:26 PM   #8
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If money is unlimited, then you can try to find one of the Carl Zeiss 300mm f/2.8 FE Tele-Superachromat for 6x6:
Zeiss 300mm f/2.8 FE Tele-Superachromat Lens for 200 1041695 B&H
Carl Zeiss 300mm f/2.8 FE Tele-Superachromat with APO-Mutar 1.7x E Overview at DentonImages.com

12-16-2011, 01:51 PM   #9
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QuoteOriginally posted by feverbeaver Quote
If money is unlimited, then you can try to find one of the Carl Zeiss 300mm f/2.8 FE Tele-Superachromat for 6x6:
Zeiss 300mm f/2.8 FE Tele-Superachromat Lens for 200 1041695 B&H
Carl Zeiss 300mm f/2.8 FE Tele-Superachromat with APO-Mutar 1.7x E Overview at DentonImages.com
At 33,000 euros, it is a steal...
12-17-2011, 05:46 PM   #10
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I have the cross sections of both lenses and they are similar. They both have two low dispersion elements. The 67 version has 9 elements total and the 645 version has 8. Since they use ED glass as opposed to conventional glass, there is bound to be better correction in all lens zones for Longitudinal Chromatic aberration (low spherochromatism). Since 300 mm is fairly short for color aberrations to appear, the use of ED glass makes color fringing undetectable, even wide open.

I would say that both lenses are similar in performance and probably superachromatic.
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