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Project:
Pentax 645 300 mm ED f/4 lens


Contents:

  1. Overview
  2. Internal filter holder
  3. Install the lens on the 300 mm rig and setup USB-focus for reverse
  4. Temperature compensating and tilted focus plane
  5. Autofocus with APT and N.I.N.A.
  6. Move the equipment to the observatory
  7. To be continued

Note:
I take no responsibility or liability for what are written here, you use the information on your own risk!


1: Overview

I have been very pleased with my earlier medium format lens, a Pentax 6x7 165mm f/2.8. No vignetting to talk about even at f/2.8 but the color handling was far from perfect. Another medium format lens I have read about many years is the Pentax 645/67 300mm ED f/4, it use more advanced glasses of ED type and has a reputation to be very sharp, even at full opening. I have bought one now, a used Pentax 645, a smaller medium format compare to the earlier 6x7 I had. Now I have to take some test photos to see how good the quality is compared to my more modern designed Canon EOS 300 mm L f/4. The Canon lens is very sharp but have heavy vignetting problems, about half the signal is lost at the corners at full frame.

Have a look at my vignetting test:Analyze of lenes vignetting.


Pentax 645 300 mm ED f/4 lens for astrophoto

This is how the Pentax 645 300 mm ED f/4 lens looks, almost exactly the same size as my Canon 300 mm L lens. It looks to be in good condition too. They are hard to find in Sweden, more common in Japan, due the huge import cost it's not interesting to import it. You find them at eBay among other places.

There are lot of space where to place the two tube rings and a pulley for the focus motor. At rear the diameter is 75.5 mm and at the front 80.5 mm.


Pentax 645 300 mm ED f/4 lens for astrophoto

The infinity focus point looks to be far away from the true focus point for infinity, very good because then it's easier to handle it with a focus motor. The focus barrel rotates very smooth.

Photos with my earlier Pentax 6x7 165 f/2.8 lens.


Pentax 645 300 mm ED f/4 lens for astrophoto

I got both end caps included. The lens have 8 lens elements and of them are two of ED glass.

Have a look at the internal construction:


Pentax 645 300 mm ED f/4 lens for astrophoto

The lens surfaces looks clean and they are in good condition, no scrathes. The filter diameter is 77 mm.


Pentax 645 300 mm ED f/4 lens for astrophoto

The rear end of the lens with its bayonet. The back focus distance is 70.87 mm compare to Canon EF's 44 mm. Some extra room for filter wheels, off-axis adapter etc.

Table of different lenses back focus:


Pentax 645 300 mm ED f/4 lens for astrophoto

To do some test with my Canon full frame camera I bought this adapter, Pentax 645 lens to Canon EOS camera.


Pentax 645 300 mm ED f/4 lens for astrophoto

If I find that this lens is good for astrophoto I will later make a special adapter. A thinner adapter that let a filter wheel or something else be installed between the lens and the camera.

I got a message from Greg in Australia, he sent me this link: Greg's photo 1 and this link: Greg's photo 2. Maybe enough to explain why I bought this old ens.

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