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05-04-2014, 12:26 PM   #1
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Digital 645 questions from a digital 645 noob.

I am familiar with film MF, and have a 645N and a group of lenses, as well as a Fuji GSW690II. The difficuly I'm having is getting straight and simple answers to a several questions, in advance of my jump to a 645Z.

Question 1: I understand that the digital 645 in Pentax is cropped from real 645. I have seen various crop factors, 1.26, 1.27, 1.3---even one claim of 2x, because the former numbers were supposedly calculated using larger sensors than the Pentax ones. Oddly, I am having some difficulty finding a definitive answer after many searches. What is the crop factor, please for Pentax 645 D & Z?

Question 2: The crop factor conversion from 645 to FF is .79 (there seems to be general agreement here), but that assumes full 645 (Yes I know we should not be doing this, but instead working with AOV/FOV numbers, but for lots of us those 35mm FL's are etched into our brains from decades ago, so...). Therefore, for A and FA lenses, first we should multiply by the crop factor within the 645 formats---so, let's say 1.27 for now---and then multiply again by .79 to get a 35 mm equivalent. Is this correct? If so, interestingly the FL's of the A and FA lenses as designated are pretty much the same as 35mm FL's---so, the 35=35, 55=55, & etc.....

Question 3: In the case of the new D FA lenses, are these complete redesigns, and therefore without the 1.27 (first) crop factor? Example: The new 25, is it in fact a 19mm (35 equiv) ? If not, then besides the weatherproofing in some of them, what accounts for their high price compared to the later A and FA lenses---many of which can be had really clean for ridiculously low prices?

Question 4: Pursuant to 3 above, are some of the D FA lenses, like the 25, designed specifically for the D/Z sensors----but others not? If so, which ones are which?

Please pardon my noob ignorance here. I wouldn't ask these questions if it had not been so difficult to find the answers. Thanks in advance for your assistance.

05-04-2014, 01:02 PM   #2
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QuoteOriginally posted by texandrews Quote
I am familiar with film MF, and have a 645N and a group of lenses, as well as a Fuji GSW690II. The difficuly I'm having is getting straight and simple answers to a several questions, in advance of my jump to a 645Z.

Question 1: I understand that the digital 645 in Pentax is cropped from real 645. I have seen various crop factors, 1.26, 1.27, 1.3---even one claim of 2x, because the former numbers were supposedly calculated using larger sensors than the Pentax ones. Oddly, I am having some difficulty finding a definitive answer after many searches. What is the crop factor, please for Pentax 645 D & Z?

Question 2: The crop factor conversion from 645 to FF is .79 (there seems to be general agreement here), but that assumes full 645 (Yes I know we should not be doing this, but instead working with AOV/FOV numbers, but for lots of us those 35mm FL's are etched into our brains from decades ago, so...). Therefore, for A and FA lenses, first we should multiply by the crop factor within the 645 formats---so, let's say 1.27 for now---and then multiply again by .79 to get a 35 mm equivalent. Is this correct? If so, interestingly the FL's of the A and FA lenses as designated are pretty much the same as 35mm FL's---so, the 35=35, 55=55, & etc.....

Question 3: In the case of the new D FA lenses, are these complete redesigns, and therefore without the 1.27 (first) crop factor? Example: The new 25, is it in fact a 19mm (35 equiv) ? If not, then besides the weatherproofing in some of them, what accounts for their high price compared to the later A and FA lenses---many of which can be had really clean for ridiculously low prices?

Question 4: Pursuant to 3 above, are some of the D FA lenses, like the 25, designed specifically for the D/Z sensors----but others not? If so, which ones are which?

Please pardon my noob ignorance here. I wouldn't ask these questions if it had not been so difficult to find the answers. Thanks in advance for your assistance.

The film crop factor is 1.27x, but don't let it confuse you too much. Focal length is focal length and it doesn't change with the size of the sensor It only affects the field of view, so the only difference on digital is that your 645 film lenses won't seem quite as wide, which is why Pentax released the 25mm.

DA lenses (I.e.) the 25mm are designed exclusively for digital, whereas DFA lenses also cover the film format. In the 25mm's case, the only reason you get a bit of vignetting on film is due to the size of the built-in hood. Again, crop factor doesn't have any effect on the lenses' focal length.

Adam
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05-04-2014, 01:14 PM   #3
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As for FF vs 645 vs 645D, see this table, which shows you what focal lengths have equivalent fields of view.

https://www.pentaxforums.com/forums/54-pentax-lens-articles/93714-field-view-...d-645-6x7.html

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05-04-2014, 03:23 PM   #4
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See Adam's table. I believe the "crop factor" you want for 645D to 35mm is 0.79 or about 0.8; in other words a 500mm lens on the 645D has a FOV about the same as a 400mm on 35mm FF (0.8x500).
The factor for a 645N with a larger "sensor" and hence FOV, is closer to 0.65; so that same 500mm on the 645N has a FOV about the same as a 325mm lens on a 35mm.

05-05-2014, 07:08 AM   #5
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It's easy to find all the variances in FL - FOV factors confusing, and I can appreciate your desire to figure out the details ahead of time, but honestly, if you make the jump and start using the 645Z frequently I would guess that in a very short time you'll have no need to ever think about things in 35mm terms any more. Personally I shoot frequently in six different frame sizes (110, APS-C, 35mm, 645, 6x6 and 6x7) but all I have to do is look at my lenses and it's pretty intuitive to determine which ones are WA, standard FL and tele... although I confess to not having the FOV numbers memorized for any format!
05-07-2014, 03:44 AM   #6
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you guys keep confusing me. you mentioned as compared to 35mm. we are trying to compare the 645d to the 645 and the 67. since the 67 is bigger than the 645d what is the crop factor. for example fx to dx 500mm fx =750mm dx ,so with 67 to 645. 400mm 67=? 645d in effect.
05-07-2014, 05:08 AM   #7
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QuoteOriginally posted by bull drinkwater Quote
you guys keep confusing me. you mentioned as compared to 35mm. we are trying to compare the 645d to the 645 and the 67. since the 67 is bigger than the 645d what is the crop factor. for example fx to dx 500mm fx =750mm dx ,so with 67 to 645. 400mm 67=? 645d in effect.
Well, the 35mm is referenced because that was the question I started this thread out with.

Thanks for all the replies so far, and especially that handy table.
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