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01-10-2010, 12:06 PM   #16
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QuoteOriginally posted by Wheatfield Quote
Do you know this for a fact or are you making this up?
I'd like to see some citations please.
With the 6x7 lens, you will be using something like the central 1/4 of the image circle, and are well away from the edges where barrel distortion is likely to rear it's ugly head.
here is one: https://www.pentaxforums.com/forums/822900-post4.html

01-10-2010, 12:25 PM   #17
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QuoteOriginally posted by Venturi Quote
I propose a ban on any "technical" term quipped by marketing teams.
It is usually used by ebay sellers to try to convince people that the 200mm lens they are selling is actually a 300mm lens

QuoteOriginally posted by rhodopsin Quote
The barel distortion is because the lens in question is a FISHEYE not rectilinear. A rectilinear lens for 645 will still be rectilinear for APSC and a fisheye lens for 645 will still be a fisheye on APSC.

The jist of this thread is FORGET crop factor and stick with focal length or field of view. Field of view can be calculated easily. The formula is:

fov = 2 x tan(l / 2 x f)
where l is the dimension of the sensor and f is the focal length of the lens.
For example the dimensions of the sensor on the K20D is 23.4 x 15.5mm (horizontally x vertically). With a 50mm lens the field of view will be 26.3 deg x 17.6 deg (horizontally x vertically).
01-10-2010, 12:59 PM   #18
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QuoteOriginally posted by rhodopsin Quote
I'm just seeing a picture taken with a crappy lens.
How about something a little more credible?
If you like, I could dig out my Pentax 6x7 45mm lens and mount it to a DSLR and show you how straight it is.
01-10-2010, 01:14 PM   #19
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QuoteOriginally posted by MattGunn Quote
It is usually used by ebay sellers to try to convince people that the 200mm lens they are selling is actually a 300mm lens



The barel distortion is because the lens in question is a FISHEYE not rectilinear. A rectilinear lens for 645 will still be rectilinear for APSC and a fisheye lens for 645 will still be a fisheye on APSC.

The jist of this thread is FORGET crop factor and stick with focal length or field of view. Field of view can be calculated easily. The formula is:

fov = 2 x tan(l / 2 x f)
where l is the dimension of the sensor and f is the focal length of the lens.
For example the dimensions of the sensor on the K20D is 23.4 x 15.5mm (horizontally x vertically). With a 50mm lens the field of view will be 26.3 deg x 17.6 deg (horizontally x vertically).
The other example I saw showed 67 45/4 compared to M42 50/1.4 on crop body; barrel distortion was significant on 45/4 photo examples. (still looking for link)

Thinking about crop factor is useful only when comparing formats, not for OP. I might be a seasoned 6x7 shooter and look through a 4/300mm lens on APS-C camera "oh wow that is L-O-N-G!" and then "why, I'd need a 900mm lens on my 6x7 to see that angle", "900/4 would be gigantic!", and others...

01-10-2010, 06:47 PM   #20
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QuoteOriginally posted by rhodopsin Quote
The other example I saw showed 67 45/4 compared to M42 50/1.4 on crop body; barrel distortion was significant on 45/4 photo examples. (still looking for link)

Thinking about crop factor is useful only when comparing formats, not for OP. I might be a seasoned 6x7 shooter and look through a 4/300mm lens on APS-C camera "oh wow that is L-O-N-G!" and then "why, I'd need a 900mm lens on my 6x7 to see that angle", "900/4 would be gigantic!", and others...
Yours was the most useful answer. I find there's too much semantics about this subject. Of course a lens is a lens is a lens. Putting it on a different sensor or film plane doesn't magically make its field of view wider or longer. But what proportion of the lens image circle is used by a given format (APS-C in this case) is what I needed to know. Now whether we all call it crop factor, FOV multiplier, yada yada makes little difference to me - but I bet lots of folks are curious what the ratio is, just like I was.

With your help, I know that APS-C would use only about the centre 1/3 of a 645 lense's image circle. So a theoretical 900mm 645D lens mounted to an APS-C camera would look roughly as "tele" as for example an A series 300mm lens on a 35mm film camera.

How about using 6x7 lenses on APS-C? What are the numbers involved, roughly?
01-10-2010, 06:52 PM   #21
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QuoteOriginally posted by Wheatfield Quote
I'm just seeing a picture taken with a crappy lens.
How about something a little more credible?
If you like, I could dig out my Pentax 6x7 45mm lens and mount it to a DSLR and show you how straight it is.
Text mentions barrel distortion of mf fisheye lens on crop camera.

After much searching I can't find the link ; I'm thinking you are right, I've probably got the lenses mixed up, that I saw another comparison with fisheye; they are grid-pattern photos, fwiw.

Not suggesting you go to such lengths, I would like to see on crop-cam 6x7 45/4 compared with 50/1.4 for crop cam, @ f/4, f/5.6, f/8. Examples to compare ("macro-")contrast, resolution, "3-D" "micro-contrast", "bokeh", "oof highlights", ...
01-10-2010, 07:39 PM   #22
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QuoteOriginally posted by Dubesor Quote
So a theoretical 900mm 645D lens mounted to an APS-C camera would look roughly as "tele" as for example an A series 300mm lens on a 35mm film camera.
Not exactly.

Any 900mm lens mounted on an APS-C camera would "look the same" as a 1440mm lens on 35mm camera, because 900mm * 1.6 = 1440mm.

Here's how the arithmetic works:

Any 600mm lens on 645 camera would "look the same" as a 300mm lens on 35mm camera would "look the same" as 188mm lens on APS-C camera because 600mm / 2 = 300mm and 300mm / 1.6 = 188mm.

Here is what the arithmetic means:

Three cameras, 645 with 600mm lens, 35mm with 300mm lens, and APS-C camera with 188mm lens, all "look the same" -- all have the same angle of view.

Three lenses, 300mm for 645, 300mm for 35mm, 300mm for APS-C, mounted on APS-C cameras, all "look the same" -- all have the same angle of view.
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