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01-28-2017, 02:42 PM   #16
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QuoteOriginally posted by Digitalis Quote
The original Pentax SMC DFA 25mm f/4 Asph is a full 645 format lens.
I have one of these and it gives a wonderful 15mm equivalent field of view on my 645Nii.

01-28-2017, 11:18 PM   #17

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QuoteOriginally posted by Sarnian Quote
I have one of these and it gives a wonderful 15mm equivalent field of view on my 645Nii.
Excellent. Can you comment on what the quality is like in the outer parts of the image, ie outside the crop 44x33mm image frame?
01-29-2017, 04:34 AM   #18
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QuoteOriginally posted by 2351HD Quote
Can you comment on what the quality is like in the outer parts of the image, ie outside the crop 44x33mm image frame?
Like most 645 lenses the Pentax DFA 25mm f/4 lens needs stopping down to f/11~f/16 range to obtain consistent levels of resolution across the frame.
Slight Distortion and field curvature is evident, this lens is very front heavy on the 645NII and is difficult to use hand held.

Pentax 645NII - Pentax SMC 645 DFA 25mm f/4 Asph - 1/60th f/8 - Kodak T-max 100 developed in Pyrocat HD. Apart from some slight cropping, this is an untouched scan.

Last edited by Digitalis; 01-29-2017 at 04:42 AM.
02-19-2017, 12:32 PM   #19
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QuoteOriginally posted by 2351HD Quote
Also, a chap on the Fuji GFX Facebook site claims that because the flange distance on the Pentax is so long, that this directly affects the quality of the wide and ultra wide lenses, making them harder to manufacture and not capable of the same image quality as the short flange distance of the Fuji. I am calling BS on this. Can anybody confirm? I would have thought it was the other way around.
The closer the focal length of the lens is to the register distance the easier a lens is to build and the less it will cost. Making a lens that is shorter than the register distance (flange focal distance) is more expensive and more challenging. Look at the K-mount which has a register distance of 45.46mm. The cheapest (easiest) lenses for the system will be those closest to 45.46mm. Lenses that are shorter than that will be the most expensive and lenses longer than that will be relatively economical. Thus the 31mm is twice the price of the 43mm and larger because it take a lot more glass (correction) to design a 31mm lens for a camera with a 45mm register distance. The DA 40mm F/2.8 is 5 elements in 4 groups. The 43mm LTD is 7 elements in 6 groups. While the 31mm is 9 elements in 7 groups.

Take a look at the Pentax 645 and the register distance is 70.87mm. What is the cheapest lens for the 645 system? The 75mm. The 75mm is only 6 elements in 5 groups. Its a pretty simple lens.

A Fuji GFX has a register distance of only 26.7mm so, yes designing a 24mm lens for a camera with a 26.7mm register distance is going to be a lot simpler and cheaper than designing the same lens for a camera with a 75mm. The revers is also true. A Fuji GFX 75mm lens is going to be more expensive to build than a Pentax 645 75mm lens. The Fuji G-mount 63mm is a $1,500 lens.

If wide angle is your need then the mirrorless will have an advantage. If you shoot portraits then Pentax has the advantage in terms of lens cost. The Fuji 120mm is a $2,500 lens while the Pentax 120mm is $1,600.


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