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03-27-2009, 09:34 PM   #16
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QuoteOriginally posted by Sean Nelson Quote
Only if the two lenses have the same field of view (the 75mm on film vs. the 55mm on the 645D).
It should be.

03-27-2009, 11:04 PM   #17
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QuoteOriginally posted by Sean Nelson Quote
Only if the two lenses have the same field of view (the 75mm on film vs. the 55mm on the 645D).
QuoteOriginally posted by RiceHigh Quote
It should be.
It should be close, but it's almost certain not to be exact. There's no magic formula that the manufacturers adhere to which guarantees that the field of view for one format is going to be the same as the field of view for another.

As an example, take the "standard" lenses for Pentax's 645 film cameras (75mm) vs. it's 35mm film cameras (50mm). The larger format has a "standard" lens 1.5X longer, yet the imaging area has over a 1.6X longer diagonal - therefore the field of view with the film 645's "standard" lens is wider than for film.

So dividing the focal lengths will give you an approximate crop factor, but not an exact one. It's precisely the same reason why you can't use the ratio of focal lengths to derive the exact size of the sensor.

Until Pentax releases more specific information, we're just guessing at the actual sensor dimensions. The best guesses are probably those that match up the published 30MP specification with existing MF sensors. But even there we could be in for a surprise.
03-28-2009, 12:02 AM   #18
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True Standard Lenses

QuoteOriginally posted by Sean Nelson Quote
As an example, take the "standard" lenses for Pentax's 645 film cameras (75mm) vs. it's 35mm film cameras (50mm). The larger format has a "standard" lens 1.5X longer, yet the imaging area has over a 1.6X longer diagonal - therefore the field of view with the film 645's "standard" lens is wider than for film.
50mm is not the true standard lens as by the definition of Pentax for the 135 format. They later made a true standard lens of 43mm in late 90s which is the true standard, i.e., the diagonal length of the frame.

So, as for the Pentax film 645, 56 x 41.5mm is the frame size, the true standard lens should be 69.7 and thus a 70mm lens. The "standard" 75 is a bit longish, which is the diagonal size of a frame measured 6 x 4.5 cm, exactly.

As I have mentioned in my original blog article, the back focus register distance should match this standard focal so that the standard lens is the easiest to make with most simplified optical formula.
03-28-2009, 12:39 AM   #19
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Moved to the medium format forum.


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03-28-2009, 01:47 AM   #20
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QuoteOriginally posted by Adam Quote
Moved to the medium format forum.
But it is NOT film SLR anymore, but DSLR! 645D is a Digital SLR! :-)
03-28-2009, 10:51 AM   #21
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QuoteOriginally posted by RiceHigh Quote
50mm is not the true standard lens as by the definition of Pentax for the 135 format. They later made a true standard lens of 43mm in late 90s which is the true standard, i.e., the diagonal length of the frame.
Yet for many, many years the closest thing to a 'standard' lens for their 35mm cameras was the 50mm. Nothing has been published by Pentax yet that confirms one way or another that the 55mm on a D645 is any closer to the diagonal of it's sensor than a 50mm was to the diagonal of the film image.

Another example: Pentax's current APS-C cameras have a sensor diagonal of 28mm. Pentax no longer makes a 28mm lens - most people would consider that the closest thing to a "standard" lens is now the FA 31mm. Even the 30mm that was on the roadmap doesn't match the sensor diagonal.

My point is: "standard" lenses supplied for a camera vary - they're not always equal to the diagonal of the image. Some have slightly wider fields of view than "standard" and some have narrower fields of view. It was true in the past and it's still true now. Therefore any measurements of crop factor or sensor size based on this lens being a "standard" lens are just speculation.
03-28-2009, 12:13 PM   #22
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QuoteOriginally posted by RiceHigh Quote
But it is NOT film SLR anymore, but DSLR! 645D is a Digital SLR! :-)
Perhaps it is time for the MF forum to be moved up to a full fledged forum rather than a subsection of the film forum?
03-29-2009, 06:49 PM   #23
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Ok, I tried something a little unorthodox... I used the images here ->645d pictures
I know this is not the same camera that is going to be released, but what I did was: I took the picture from the base and used the threaded tripod mount as a reference - it's a known size. I then measured a common point in both the base picture and the pictures showing the sensor and came up with something completely different than what is being discussed. I measured the sensor as being 36mm x 27mm. Actually it makes sense when a 55mm is considered being the normal lens. This isn't very far off from the what was considered the full frame (36mm x 24mm) normal lens of 50mm. I'm not trying to piss on anyone's cornflakes, but that's the size of the sensor in the prototype in those pictures and makes sense with the upcoming 645d if it's going to use a 55mm lens as a normal.

-Brian


Last edited by Das Boot; 03-29-2009 at 08:11 PM.
03-29-2009, 07:29 PM   #24
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Ok, what I just wrote does fall exactly into the 1.36 crop. I'm really feeling friggin' gypped. All this hype over a full frame sensor? $10,000??? And those Nikon folks were mad about the $8,000 price tag on the D3x? It's nice, but not that nice.

Last edited by Das Boot; 03-29-2009 at 08:10 PM.
03-29-2009, 09:44 PM   #25
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Is there any concrete information on the sensor manufacturer yet? I know Kodak's name has been bandied about a few times, but is that definite?
03-29-2009, 11:42 PM   #26
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QuoteOriginally posted by Das Boot Quote
Ok, I tried something a little unorthodox... I used the images here ->645d picturesI measured the sensor as being 36mm x 27mm.
Forgive a skeptic, but I'm unconvinced that it's possible to get a very accurate measurement that way without knowing the focal length of the lens and the geometry of the shot in order to compensate for perspective. I certainly don't think it's conclusive enough to cry Chicken Little just yet...
03-30-2009, 03:09 AM   #27
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Yeah, but you have to admit that it's uncanny that the measurements ended up supporting the 1.36 crop brought up earlier. This and the normal lens being 55mm? Later on today, I'm going to try my picture measurement idea with something I have here at the house. I really don't think I'm that far off. Maybe, I'll post the points on the images I used. They aren't very far apart - maybe 1cm at most. Not much when considering perspective. There are also more images at different angles I could use. I'll try at lunch today.
03-30-2009, 04:54 AM   #28
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Das Boot, I don't know how you get 1.36 crop from 6x4.5 and 3.6x2.7... maybe if you print the picture onto a balloon and then give it some air, I think the sensor will come out a bit bigger. I trust that Pentax is able to determine what camera they making (MF vs FF wise).
03-30-2009, 05:25 AM   #29
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QuoteOriginally posted by Sean Nelson Quote
Forgive a skeptic, but I'm unconvinced that it's possible to get a very accurate measurement that way without knowing the focal length of the lens and the geometry of the shot in order to compensate for perspective. I certainly don't think it's conclusive enough to cry Chicken Little just yet...
I second the view of Sean. Its just almost impossible to measure the dimensions from photos, taking perspective and different focal lengths of taking the pictures into perspective.
03-30-2009, 09:47 AM   #30
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And, as it turns out, this is probably all moot anyway because according to this report at the Luminous Landscape Pentax hasn't yet made a final decision which sensor it will use. So there's really no value in trying to guess sensor sizes by measuring a photo of a 2-year-old prototype or assuming that the 55mm lens means a 55mm diagonal on the sensor.
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