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11-12-2014, 09:33 AM   #1
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Full frame/Cropped Lens Confusion

Today I was reading the reviews about the Pentax DA 60-250mm. I was reading the reviews on the B&H website when I decided to ask a question. They have a question and answer link on the Review page. I asked the question: "Can this lens also be used on a Full Frame film camera ?" . I got my answer in the mail a few minutes later which was a resounding NO ! I should have known better since this one of the DA designated lenses. In any case if the lens cannot fit on a full frame camera then why designate it a s a 60-250mm ? The lens is actually a 90-375mm on a 1.5X cropped camera. Nothing more, nothing less. Why not just leave it at that ? If the lens could fit on a full frame camera, then you could say that it is a 60-250mm otherwise why confuse the heck out of people ?

11-12-2014, 09:39 AM - 1 Like   #2
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QuoteOriginally posted by hjoseph7 Quote
The lens is actually a 90-375mm on a 1.5X cropped camera.
No. The lens is actually a 60-250 no matter where its mounted. The 1.5x crop refers to the center of the frame being cropped out because an APS-C sensor is smaller than 35mm.

Focal length is an inherent property of the lens.

The reason it cannot be used on a full frame camera is because the image circle is designed to only cover APS-C. This is so the lens can be smaller and cheaper.
11-12-2014, 09:51 AM   #3
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If I'm correct, the focal length is always the same on either format. It's the field of view that changes. APS-C cuts off the edges of a FF image making a narrower field of view like longer lenses on FF. An APS-C image circle on FF would be just that, a circle or have severe vignetting but, the same focal length. I'm sure someone will correct me if I'm wrong.
11-12-2014, 09:53 AM   #4
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QuoteOriginally posted by hjoseph7 Quote
If the lens could fit on a full frame camera, then you could say that it is a 60-250mm otherwise why confuse the heck out of people ?
The lens will fit fine on a potential FF camera. However, the image might (will) vignette as it was designed for APS-C and does not cover the full image circle of a FF sensor.

The focal length of a lens is a property of the lens itself, not the sensor it is mated to. Changing the focal length designation of a lens every time it was mounted to a different size sensor would be very confusing. How about a FA 50mm? A FF lens that is 50mm but works just fine on APS-C. Do we rebadge it as a 75mm on APS-C? What if you mount it on a Canon APS-C sensor is it now 80mm? No, the lens is always 50mm but the angle of view does change depending on the sensor used.

11-12-2014, 09:55 AM - 1 Like   #5
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as others have noted, focal length is focal length. full frame or APS-C does not matter in the focal length. Lenses are labled for focal length which is a physical property, not for the 35mm equivalent because it is meaningless

As for using the lens on full frame, there is nothing that prevents this lens from being used on full frame, other than possible vignetting.

there is a thread running presently with APS-C lenses tested on full frame (using film body)

check if the lens has been tested. Just because it is a DA lens, does not mean specifically it cannot actually cover a full frame acceptably.

Note, while i do not own this lens, my sigma 10-20 covers 100% of my PZ1 viewfinder by 13mm.
11-12-2014, 10:03 AM   #6
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QuoteOriginally posted by hjoseph7 Quote
The lens is actually a 90-375mm on a 1.5X cropped camera.
People mean with that, that in comparison to a 60-250mm on a FF camera, it is 90-375mm. So on one hand you're quite right. The end result on a apsc is the same as a 90-375mm on a FF camera. But if it's put on a FF camera, it would be 60-250mm, because that is the distance the lens provides. the apsc just crops down on the sides, not increase the distance.

I hope I didn't add more confusion.
11-12-2014, 10:07 AM   #7
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At the 60mm end, your photos will look like they were taken through a port-hole.
I'm sure you could get some full frame coverage from the longer end.

If you won't buy a lens because it won't work on a camera that doesn't exist, that's an existential crisis.
11-12-2014, 10:10 AM   #8
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Not only might it vignette, but it also does not have an aperture ring. So unless you can control the aperture on the body, the lens will always be completely stopped down.

11-12-2014, 10:17 AM   #9
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QuoteOriginally posted by cali92rs Quote
Not only might it vignette, but it also does not have an aperture ring. So unless you can control the aperture on the body, the lens will always be completely stopped down.
but every pentax camera since the A series could control the lens through the body
11-12-2014, 10:21 AM   #10
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QuoteOriginally posted by hjoseph7 Quote
otherwise why confuse the heck out of people ?
The confusion comes as a result of the continued misguided effort to express FOV equivalence to the 35mm film format in terms of focal length. If degrees of arc were used instead, there would be no confusion. This is incredibly silly because the number of people in the current dSLR market have never used a 35mm film SLR and no basis for the comparison. Instead, there is marketing value in making people think they are getting longer lenses than what they actually paid for.

As for FF compatibility, there are two considerations:
  • Mount. At present, all K-mount lenses are mount compatible with all K-mount cameras.*
  • Image circle. This is the diameter of the projected image at the sensor plane. Lenses designed for small sensors have smaller image circles than those designed for larger sensors.
The bottom line is that any K-mount lens covering the 24x36mm frame will also cover the APS-C frame.


Steve

* Supported features may vary. There is also the issue of the Ricoh P variant of the K-mount which may be difficult to remove from a Pentax AF body.
11-12-2014, 10:38 AM   #11
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you really need to reference this thread

https://www.pentaxforums.com/forums/10-pentax-slr-lens-discussion/31629-da-le...ts-thread.html
11-12-2014, 10:53 AM   #12
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QuoteOriginally posted by stevebrot Quote
The confusion comes as a result of the continued misguided effort to express FOV equivalence to the 35mm film format in terms of focal length.
Misguided? Try figuring out a 3.8mm - 5.9mm zoom on a Q and a Q7!

QuoteOriginally posted by stevebrot Quote
If degrees of arc were used instead, there would be no confusion.
Unfortunately, I don't think that would solve the problem, because of differing aspect ratios.

Like using an 80mm lens on 6x6 cm film to get an A4 print.
11-12-2014, 11:15 AM - 1 Like   #13
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Focal length is a mathematical formula based on the size and position of lenses and their distance from a convergence point (when focused to infinity).

Super easy version that I *think* is a pretty accurate, if simplified, explanation: a simple "sherlock holmes" style magnifying glass held above a piece of paper at 15 cm or 40 cm might put a big blob of sunlight on the paper, but at 25cm, it will focus to a fine point and eventually start the paper burning. Its focal length is 250mm (25cm). Adding additional lenses with different curves to correct for distortion, or to spread the light back out, or to concentrate the light even further, changes the focal length, because now instead of making a point of light at 25cm, it might make the point at 35cm, or 5cm - that would make its focal length 350 or 50 mm.

So, a 50mm lens is a 50mm lens is a 50mm lens. A 50mm lens from a 6x7 camera is a 50mm lens on the K mount, but if you want it to focus to infinity (or possibly at all) you need the lens to be as far from the K mount camera's sensor or film as it would have been from the 6x7, since that's the distance at which the light converges. (wouldn't it be nifty if someone made a LensTurbo/SpeedBooster style reducer for 67 and 645 lenses? going from a 3920mm^2 sensor to a 372mm^ sensor would be most interesting (and probably unusable lol))
11-12-2014, 11:17 AM   #14
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QuoteOriginally posted by lytrytyr Quote
Misguided? Try figuring out a 3.8mm - 5.9mm zoom on a Q and a Q7!
Exactly.

It is much more easy to use the real focal lenght even for APS-c then use the "converted" one.
I never had a full frame, so I know exactly how long is a 50mm on my camera. Easy.
If you tried a full frame, you know that the same lens will produce the same picture on it if you cut the picture in 2/3. If not, you will have a bigger field of view.
11-12-2014, 11:24 AM   #15
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QuoteOriginally posted by cali92rs Quote
Not only might it vignette, but it also does not have an aperture ring. ..
Huh? every AF lens made recently are like that, not only in Pentax land...
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