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03-12-2018, 01:38 PM   #1
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the 25mm lens

my question, is the 25mm lens sized for the smaller 645d sensor or the larger 645 film area.

03-12-2018, 02:07 PM   #2
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QuoteOriginally posted by bull drinkwater Quote
my question, is the 25mm lens sized for the smaller 645d sensor or the larger 645 film area.
Technically it covers the film area, but the DA version that followed soon after the original D FA had a larger hood, which introduces dark edges on film.

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03-12-2018, 03:11 PM   #3
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I just got one today, came Fedex from Lens Authority (Lens Rentals' sales outlet). I no longer have my 645N, but this is what I understand from my research. I guess if there's ever a "FF" 645 from Pentax I'll have it adjusted (maybe. It's too bad Pentax didn't just make it an adjustable hood.).
03-12-2018, 05:44 PM   #4
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Just get the short hood version (DFA)to use with 645 film bodies. Good luck.

03-12-2018, 07:46 PM   #5
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you are missing my question. do the new lenses have the 1.26 crop factor with the 645d that the vintage lenses have?
03-12-2018, 08:23 PM - 2 Likes   #6
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Lenses do not have crop factors. Sensors do. The 645D and 645Z have the same size sensor so will behave the same.

The DA25 is capable of projecting a full frame image circle, but the hood is designed for the crop sensor so if you put the lens on a 645 film body you will get vignetting from the hood. If you hack the offending bits of hood away you will have a full frame lens.

As mikeSF said, the DFA version is optically identical but has a hood designed for full frame so there is no such problem.
03-12-2018, 11:03 PM   #7
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Can anyone confirm that this is definitely the case? I have heard it said many times, but also a few references to other, smaller, internal differences between the DA and DFA versions (such as baffles, etc. being larger in the DA version, taking advantage of the reduced need to deliver a large image circle). Is anyone in a position to say definitively, based on firm information, that the DA version would cover full frame if the hood were cut back?

Many thanks,

Ed

03-12-2018, 11:51 PM   #8
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QuoteOriginally posted by bull drinkwater Quote
you are missing my question. do the new lenses have the 1.26 crop factor with the 645d that the vintage lenses have?


Focal length is focal length, and image circle is image circle. These are related but separate attributes, but they are facts not affected by the size of the image the camera detects behind them.

A 55mm lens magnifies scenery elements the same size on the sensor as any other 55 lens. But a 55mm lens for a small-format camera might only have a 2-inch image circle, and leave the corners dark, if the sensor is bigger than that. The 55mm lenses for the 645 have 3-1/2” image circles, and the 55mm lenses for the Pentax 67 has a 4” image circle. But inside the circle, the scenery is magnified the same amount. That’s why lenses designed for larger formats work on smaller formats, but not necessarily the other way around.

So, a smaller sensor merely cuts off a part of the scene, rather that magnifying it differently. That’s why it’s called a crop factor. We can speak of “equivalent” focal lengths—those focal lengths that provide the same field of view for different formats. But for a larger format to have the same field of view, it needs greater magnification to fill the larger frame with the same scene. Greater magnification requires a longer focal length. So, 75mm on the 645 film frame is equivalent to 55mm on the 645 digital sensor, because it magnifies the same scene a sufficiently greater amount to fill the larger frame.

The 25mm focal length is less than half the diameter of the 645 digital frame, and a third the diameter of the film frame. That means it has to have an image circle at least three times its focal length to cover film. That is rather extreme, and it’s why that lens has such a bulbous front element. But it also means the light is approaching the sensor at a shallow angle, and any image spilling off the sensor has a tendency to light up the mirror box, which reflects around and lowers contrast. That’s why they limited the image circle mechanically in the later version.

And I bet Ed Hurst’s fears are well-founded—I bet there is other frame-limiting masking inside the lens in addition to the tighter lens hood.

At one-third of the film frame diameter, 25mm is about equivalent to 14mm in a small-format 24x36 camera. Those lenses have bulbous front elements, too. But the question is whether it will light the corners of the larger frame.

There is another question, too. And that is whether the lens performs well enough outside the digital frame. That may also be why they mechanically limited the image circle.

Rick “sensors don’t change focal length” Denney
03-13-2018, 07:37 AM   #9
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I remember (maybe I'm wrong) that I read somewhere that another potential difference between DA and DFA was about tolerances. With same optical design between DFA and DA, Pentax could afford to have wider manufacturing tolerances (on elements positioning, alignment, etc) on the DA version, because the effects being outside of the 44x33mm sensor, it wouldn't be noticed. In other words, a DA on 54x40mm sensor could have weaker corners than the DFA with its narrower tolerances.
Can somebody confirm this ?
03-13-2018, 07:53 AM   #10
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Well, I've read 10 pages deep into a google search, and everything I could find here, and haven't seen that. Sadly, there is not a boat load of info about this topic.
03-13-2018, 09:46 AM   #11
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QuoteOriginally posted by Ed Hurst Quote
Can anyone confirm that this is definitely the case?
It has been several years, but blackcloudbrew (Tim) mounted my DFA 25mm on his 645 film body and reported the full image circle was captured.
I can reach out to him for more details if you like.
03-13-2018, 09:51 AM   #12
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QuoteOriginally posted by SylvainB Quote
I remember (maybe I'm wrong) that I read somewhere that another potential difference between DA and DFA was about tolerances. With same optical design between DFA and DA, Pentax could afford to have wider manufacturing tolerances (on elements positioning, alignment, etc) on the DA version, because the effects being outside of the 44x33mm sensor, it wouldn't be noticed. In other words, a DA on 54x40mm sensor could have weaker corners than the DFA with its narrower tolerances.
Can somebody confirm this ?
What I was told back in 2016 was that it was to costly to keep the tolerances tight, hence the lens was killed off altogether. Compared to the D FA version, the DA optimized the image quality with respect to flare, but even at $5000 it wasn't profitable.

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03-13-2018, 10:33 AM   #13
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QuoteOriginally posted by Adam Quote
What I was told back in 2016
Told by Pentax? Pentax who? Because the N.A. guys don't always have the correct story, in my experience...
QuoteQuote:
was that it was to costly to keep the tolerances tight, hence the lens was killed off altogether. Compared to the D FA version, the DA optimized the image quality with respect to flare, but even at $5000 it wasn't profitable.
Wow! Tight tolerances? Don't just about all lenses have tight tolerances? $5K and it can't be profitable?....the lens isn't that good, is it? Something isn't adding up here....and what about what SylvainB said above?
03-13-2018, 11:00 AM   #14
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QuoteOriginally posted by texandrews Quote
Told by Pentax? Pentax who? Because the N.A. guys don't always have the correct story, in my experience... Wow! Tight tolerances? Don't just about all lenses have tight tolerances? $5K and it can't be profitable?....the lens isn't that good, is it? Something isn't adding up here....and what about what SylvainB said above?
It was one of the reps at the WPPI show, possibly Dan. He's one of the few folks who might know Pentax cameras better than we do!

They said they might be able to market that lens for $7-8k if people were willing to buy it at that price. Apparently quite a large percentage made it back to the factory, and that's what killed the profitability. If every unit sold had no issues, I'm sure the lens would still be around today.

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03-13-2018, 11:31 AM   #15
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QuoteOriginally posted by Adam Quote
It was one of the reps at the WPPI show, possibly Dan. He's one of the few folks who might know Pentax cameras better than we do!

They said they might be able to market that lens for $7-8k if people were willing to buy it at that price. Apparently quite a large percentage made it back to the factory, and that's what killed the profitability. If every unit sold had no issues, I'm sure the lens would still be around today.
Interesting, so, really a QC problem, less tight tolerances. I wonder who was manufacturing it. One of the most notable things about what Sigma has done in the past few years was the upgrade of their testing and QC system in house, from the ground up. They said at the time that the demands of the new cameras were such that it had to be done. Flew under most people's radar.
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