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10-24-2015, 12:26 PM   #31
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QuoteOriginally posted by UncleVanya Quote
And you base this on what? I think you are very confused about how cropping occurs and what the glassless adapter from Pentax does.

So. If we take a 50mm lens for 645 and put it on FF it will give the same field of view as the standard Takumar m42 lens mounted on the same FF camera. I think where the train is off the tracks is that you seem to think that the 645 lens will magically give the same field of view on both 645 and FF but on the 645 it will have a wider field of view.
If you bother to spend a little time you may wish to read what PENTAX themselves says about it reading the red circled text on the photo above from the 67 to K adapter leaflet!! It works the same for the 645! I have both adapters and I have a 35mm camera and several 67 and 645 lenses.

10-24-2015, 01:58 PM   #32
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QuoteOriginally posted by RuiC Quote
If you bother to spend a little time you may wish to read what PENTAX themselves says about it reading the red circled text on the photo above from the 67 to K adapter leaflet!! It works the same for the 645! I have both adapters and I have a 35mm camera and several 67 and 645 lenses.
Please read this:
The Crop Factor Unmasked - Articles and Tips | PentaxForums.com

What you seem to think is that the same lens on 645 and FF will show the same scene in the same proportions and field of view - which is not going to happen. There will be a reduction of the field of view. It is written right there in the circled box that you posted.

I concede that we may be in agreement but somehow talking in terms that make no sense to one another. Where you lose me is when you say that the APSC will be different than the FF - both will differ from 645 - but the APSC will differ more.
10-26-2015, 03:15 AM   #33
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QuoteOriginally posted by UncleVanya Quote
Please read this:
The Crop Factor Unmasked - Articles and Tips | PentaxForums.com

What you seem to think is that the same lens on 645 and FF will show the same scene in the same proportions and field of view - which is not going to happen. There will be a reduction of the field of view. It is written right there in the circled box that you posted.

I concede that we may be in agreement but somehow talking in terms that make no sense to one another. Where you lose me is when you say that the APSC will be different than the FF - both will differ from 645 - but the APSC will differ more.
I sustain what I have said ! Your problem is that you stopped reading the FULL sentence!! Read it until the end!!

In other words, I challenge you to mount a 645 lens on a FF body to check the real thing! Oh!! Yes you can't!! And why can't you?? Because you need an adapter!! Oh! Yes, and that adapter what does it do? It changes de FOV of view of that lens becoming equal to a 35mm lens! That's what they say on the paper!!

I went to the task of mounting both a 645 A-75mm with adapter on my LX and the FOV is EXACTLY the SAME as with my 75-150 Tokina K mount at the 75mm end! And yet another 67 /135mm macro with the 67 adapter on the same LX (got only one ) and it shows EXACTLY the SAME FOV as the M-135mm !!

For the record I will buy you a dinner if you can prove the contrary !!!
10-26-2015, 06:07 AM   #34
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QuoteOriginally posted by RuiC Quote

I sustain what I have said ! Your problem is that you stopped reading the FULL sentence!! Read it until the end!!

In other words, I challenge you to mount a 645 lens on a FF body to check the real thing! Oh!! Yes you can't!! And why can't you?? Because you need an adapter!! Oh! Yes, and that adapter what does it do? It changes de FOV of view of that lens becoming equal to a 35mm lens! That's what they say on the paper!!

I went to the task of mounting both a 645 A-75mm with adapter on my LX and the FOV is EXACTLY the SAME as with my 75-150 Tokina K mount at the 75mm end! And yet another 67 /135mm macro with the 67 adapter on the same LX (got only one ) and it shows EXACTLY the SAME FOV as the M-135mm !!

For the record I will buy you a dinner if you can prove the contrary !!!
Now we are getting somewhere. That is exactly what I said from the start. But - and this is important - your statement implies this is different than what happens on apsc. It isn't. A dedicated apsc 300mm lens and a full frame 300 lens and a 645 300 mm lens all still give the exact same field of view on the same camera body (at least when the image circle is large enough to cover the full height and width of the frame - otherwise you get black borders). Each lens gives a different field of view on the native platform - so the 300mm will have a narrower fov on the apsc than the FF and the ff will appear narrower than the 645. The focal length of the lens does not change at all on any camera - the field of view is set by the sensor or film. The 645 to K adapter is not special. It simply adapts the mounts and distances nothing more.


Last edited by UncleVanya; 10-26-2015 at 06:19 AM.
10-26-2015, 07:44 AM   #35
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10-27-2015, 06:58 AM   #36
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this is almost entertaining.
10-27-2015, 07:18 AM   #37
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QuoteOriginally posted by Arvid Quote
this is almost entertaining.
Enjoy !!!
10-27-2015, 10:43 AM   #38
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He did say 'almost', Rui. 😉

10-28-2015, 04:13 AM   #39
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QuoteOriginally posted by clackers Quote
He did say 'almost', Rui. 😉
Yes, I see ! But anyone can enjoy anyway!!
10-28-2015, 04:37 AM - 2 Likes   #40
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QuoteOriginally posted by RuiC Quote
645 lens with adapter to K will not crop on a FF camera. It will 1.5 on an APS-C camera !!
There is no cropping going on here, APS-C is just a different format like APS-H, 4/3,35mm,645,67,4X5,5X7,8X10.

Vanya is correct, a 300mm 8X10 lens is still a 300mm lens when used on 35mm or APS-C, the only parameter that changes when you use a lens from a larger format on a smaller one is the FOV is reduced: this is a property of the format, not the lens.

So to answer your original question, the Pentax 645 SMC 120mm f/4 Macro on a 35mm sensor will have a focal length of 120mm, however it will have a narrower FOV than it would ordinarily have on the 645 format that it is designed for - end of story.

Last edited by Digitalis; 10-28-2015 at 06:41 AM.
10-28-2015, 01:30 PM   #41
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QuoteOriginally posted by Digitalis Quote
There is no cropping going on here, APS-C is just a different format like APS-H, 4/3,35mm,645,67,4X5,5X7,8X10.

Vanya is correct, a 300mm 8X10 lens is still a 300mm lens when used on 35mm or APS-C, the only parameter that changes when you use a lens from a larger format on a smaller one is the FOV is reduced: this is a property of the format, not the lens.

So to answer your original question, the Pentax 645 SMC 120mm f/4 Macro on a 35mm sensor will have a focal length of 120mm, however it will have a narrower FOV than it would ordinarily have on the 645 format that it is designed for - end of story.
That's right !! Digitalis dixit !!

When you use a FA645-120mm with a Pentax adapter to K mount on a Pentax FF camera it will have the exactly same FOV as a M-120mm FF lens on the same FF body !!

Last edited by RuiC; 10-28-2015 at 04:59 PM. Reason: Silly error here, typed M-200 instead of M-120. I apologize.
10-28-2015, 04:23 PM - 1 Like   #42
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QuoteOriginally posted by RuiC Quote
FA645-120mm with a Pentax adapter to K mount on a Pentax FF camera it will have the exactly same FOV as a M-200mm FF



No, it won't. The adapter is an empty tube, if it is doing its job it will have no impact whatsoever on the optical properties of the lens. On a 36X24mm full frame camera 120mm lens will have a 20.4 degree diagonal FOV, A 200mm lens on the same format has a 12.5 degree diagonal FOV.


Angle of View can be calculated using plane trigonometry. The focal length of the lens and half the dimension in the image forms a right angled triangle so that half the Angle of View = arctangent of half the dimension divided by the focal length.
AoV = 2 x (tan-1 ( ( d / 2 ) / f )
where d is the dimension in the image (e.g. diagonal, width, height)
e.g. For a 45mm lens on a 35mm camera where the diagonal = 43.3mm, width = 36mm and height = 24mm, the respective Angles of View are 51.4, 43.6 and 29.9.

Some people use this variant: (AOV) = 2 x arctan (frame size (focal length x 2))

Last edited by Digitalis; 10-28-2015 at 05:12 PM.
10-28-2015, 05:00 PM   #43
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QuoteOriginally posted by Digitalis Quote
If the adapter is doing its job, no.





Angle of View can be calculated using plane trigonometry. The focal length of the lens and half the dimension in the image forms a right angled triangle so that half the Angle of View = arctangent of half the dimension divided by the focal length.
AoV = 2 x (tan-1 ( ( d / 2 ) / f )
where d is the dimension in the image (e.g. diagonal, width, height)
e.g. For a 45mm lens on a 35mm camera where the diagonal = 43.3mm, width = 36mm and height = 24mm, the respective Angles of View are 51.4, 43.6 and 29.9.

Some people use this variant: (AOV) = 2 x arctan (frame size (focal length x 2))
I apologize Digitalis. I meant to type M-120mm NOT M-200mm !
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