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02-08-2015, 05:47 PM   #1
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help me understand...

OK so with the supposedly FF coming out soon ( I'll believe it when I actually see it announced and someone is holding it in their hand ) I want to get some idea of what this actually means for existing DA FA F M lenses.
I think the concensus is that if it is a K mount FF lens FA, F, A, M lens we can all assume that they will work FF at the designated focal legnth of the lens.
However it has been brought to my attention that some DA lenses might actually work on a FF. If so, would the designated legnth of said DA lens be the same or different if it indeed worked on a FF?
For the sake of arguement let's say that my DA15 covered the FF sensor of the new Pentax FF coming out. Would it not actually be wider that the 15mm designation on the lens?
Trying to figure out which of my DA lenses would work on the new FF....I"m hoping that my DA15, 21, 12-24, 10-17 might possibly work but not holding my breath...

I understand that the new camera will have a crop sensor setting for DA lenses but that's really not the same as shooting FF.

02-08-2015, 05:59 PM   #2
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QuoteOriginally posted by peterjcb Quote
OK so with the supposedly FF coming out soon ( I'll believe it when I actually see it announced and someone is holding it in their hand ) I want to get some idea of what this actually means for existing DA FA F M lenses.
I think the concensus is that if it is a K mount FF lens FA, F, A, M lens we can all assume that they will work FF at the designated focal legnth of the lens.
However it has been brought to my attention that some DA lenses might actually work on a FF. If so, would the designated legnth of said DA lens be the same or different if it indeed worked on a FF?
For the sake of arguement let's say that my DA15 covered the FF sensor of the new Pentax FF coming out. Would it not actually be wider that the 15mm designation on the lens?
Trying to figure out which of my DA lenses would work on the new FF....I"m hoping that my DA15, 21, 12-24, 10-17 might possibly work but not holding my breath...

I understand that the new camera will have a crop sensor setting for DA lenses but that's really not the same as shooting FF.
Why worry about it... it is still k-mount and you still can use it on the FF camera when it comes out. The only difference, as someone mentioned, like the Nikon D750, the result will be a cropped picture much like what you would have it mounted on the k-3 or whatever.
02-08-2015, 06:00 PM   #3
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Forum searching is your thread. There is a great post where these lens are test on FF formats. And no, the focal length doesn't change,
02-08-2015, 06:05 PM   #4
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The focal length of a lens does not change with the size of the sensor - it is a property of the lens. FF covers a wider field of view than APS-C since the sensor is bigger. All the rubbish about 'equivalence' and shallower depth of field stems from the fact that with a wider field of view you have to get closer to the subject to get a similar image.

Whether this extra that the FF sensor will see is useful or just black depends on the design of the lens. M, A, F and FA were designed for FF film so they will work without vignetting (although sharpness, CA etc in the now visible areas may disappoint). Some medium/longer DA lenses will work but the wide angle ones (such as your DA15, or the kit zooms at the wide end) will vignette.

02-08-2015, 06:05 PM   #5
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QuoteOriginally posted by tromboads Quote
Forum searching is your thread. There is a great post where these lens are test on FF formats. And no, the focal length doesn't change,
...maybe I'm not wording it correctly but what I mean is...if you mount a 50mm Pentax K lens on a K-3 you will in effect get a 75 FOV in stead of 50 FOV.
02-08-2015, 06:05 PM   #6
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Focal length just means magnification (how large the image is on the sensor). On a full-frame camera, you'll see more of that image than on an APS-C (the corners are the place where you'll actually see more), making the field of view effectively wider, but at same magnification.
An APS-C-only lens of 15mm focal length, like the DA15, has the magnification of any 15mm lens, but its image circle only covers the APS-C sensor. On a full-frame body, it would look like an incomplete 15mm image (you would see the edges of the image circle, and black past them).
On APS-C, you wouldn't notice any difference between a full-frame-capable lens and an APS-C capable one, same as on a FF sensor, you wouldn't notice the difference between a FF lens and a medium format one.
02-08-2015, 06:09 PM   #7
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QuoteOriginally posted by kh1234567890 Quote
The focal length of a lens does not change with the size of the sensor - it is a property of the lens. FF covers a wider field of view than APS-C since the sensor is bigger. All the rubbish about 'equivalence' and shallower depth of field stems from the fact that with a wider field of view you have to get closer to the subject to get a similar image.

Whether this extra that the FF sensor will see is useful or just black depends on the design of the lens. M, A, F and FA were designed for FF film so they will work without vignetting (although sharpness, CA etc in the now visible areas may disappoint). Some medium/longer DA lenses will work but the wide angle ones (such as your DA15, or the kit zooms at the wide end) will vignette.
So then theoretically if for some reason the DA15 DID cover the FF sensor area completely without vignetting it would actually cover a wider FOV than the lens indicated.... correct?
02-08-2015, 06:12 PM   #8
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QuoteOriginally posted by peterjcb Quote
...maybe I'm not wording it correctly but what I mean is...if you mount a 50mm Pentax K lens on a K-3 you will in effect get a 75 FOV in stead of 50 FOV.
A better way might be to pretend that every Pentax DSLR has a 1.5x teleconverter permanently attached to it, but the full frame one will not.
(and putting a 1.5x teleconverter on a full frame body should make lenses (including those which make too small an image circle to fit the full frame sensor directly) behave exactly as they do on crop bodies, if you prefer that to using crop mode on the body)

---------- Post added 02-08-15 at 08:16 PM ----------

QuoteOriginally posted by peterjcb Quote
So then theoretically if for some reason the DA15 DID cover the FF sensor area completely without vignetting it would actually cover a wider FOV than the lens indicated.... correct?
It won't, but yeah, if it did -- on a crop sensor 15mm would be about 88 diagonal degrees, full frame it would be about 110.

And a 100mm lens is about 16 degrees on crop sensor, but would be about 24 degrees on full frame.

---------- Post added 02-08-15 at 08:25 PM ----------

https://www.pentaxforums.com/forums/122-lens-clubs/86234-15mm-limited-control...ml#post2817357 -- someone posted some shots with the 15mm ltd on a full frame (actually film) body a while back, in case you wondered what it actually looks like.

02-08-2015, 06:37 PM   #9
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QuoteOriginally posted by peterjcb Quote
So then theoretically if for some reason the DA15 DID cover the FF sensor area completely without vignetting it would actually cover a wider FOV than the lens indicated.... correct?
I'm not sure what you mean by a lens "indicating" a field of view,
and it doesn't help to think about the DA 15/4 covering the FF sensor,
since that isn't going to happen.

Let's imagine comparing what the DA 15/4 and the A 15/3.5 lens
will actually show on a full frame sensor.

The A 15/3.5 will produce an image across the whole sensor.

The DA 15/4 will produce a circular image stretching up to the top of the frame,
but not reaching out to the edges of the frame (in landscape format).

However, the circular image from the DA 15/4
will correspond exactly with what you see on the central part of the A 15/3.5 image.
02-08-2015, 07:23 PM   #10
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So instead of a 1.5 crop it would be just a 1, then the larger sensor would actually be smaller unless
of course the pixels would increase by .5 then everything would be the same.........UNTIL a 1.4 multiplier was
installed between the lens .
That would negate the previous multiplier ?????.......
I think I will keep using my Q ;( and save the money for therapy.
02-09-2015, 02:53 AM   #11
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QuoteOriginally posted by lytrytyr Quote
The DA 15/4 will produce a circular image stretching up to the top of the frame,
but not reaching out to the edges of the frame (in landscape format).
Like this.
02-09-2015, 06:51 AM   #12
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QuoteOriginally posted by peterjcb Quote

I understand that the new camera will have a crop sensor setting for DA lenses but that's really not the same as shooting FF.
If the new FF camera has a setting for an APS-H equivalent (1.2x or 1.3x), this would make me more interested in buying one. Cropping off the vignetting would be ideal in the camera. I photograph flying birds sometimes and have cropped wings off with the K-3. An APS-H would have given me just enough room to catch the entire bird.
02-09-2015, 07:06 AM   #13
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QuoteOriginally posted by kh1234567890 Quote
Thanks for the link.

QuoteOriginally posted by traderdrew Quote
I photograph flying birds sometimes and have cropped wings off with the K-3.
That sounds rather cruel!
02-09-2015, 08:25 AM   #14
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I have done a good deal of large format over the years. A 90 mm on 4x5 is a wide angle lens, equivalent to about a 24mm on FF and a 15mm on ASP-C. The equivalent on an 8X10 would be 180mm. It is a matter of angle of view. Depth of field would be absolutely the same on any format with the equivalent lens.
02-09-2015, 09:07 AM   #15
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QuoteOriginally posted by peterjcb Quote
So then theoretically if for some reason the DA15 DID cover the FF sensor area completely without vignetting it would actually cover a wider FOV than the lens indicated.... correct?
The lens does not indicate a FOV, it indicates a focal length.

Let's assume that your DA21Ltd works on FF without vignetting. 21mm on FF will give you (close to) the same FOV as the 15Ltd on an APSc camera. You can test this by putting the 21mm on your K1000 and the 15mm on e.g. your K3 and view the same scene through the viewfinder of both; you should basically see the same.
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