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01-06-2016, 05:10 PM   #1
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Focal length compensation using FA lenses on DSLR

So I have a K50, and I've been a Pentax user for over a decade now. Started off with a K1000 and switched to DSLR about 8 years ago.

Anyhoo.

So now I have a K50 and I usually use the DA lenses that it came with. I recently got an FA 50mm 1.7 (which I love) and am considering FA zoom lens that I could possibly get cheap. I am considering the FA 80-320mm.

I also recently acquired an O-GPS1 unit and plan to use it for astrophotography. When I used it with my DA 50-200mm lens, the O-GPS automatically detects the focal length and works as it is supposed to.

I am wondering though, if I get an FA lens, I am told that FA lenses are full frame, so using them with a DSLR will essentially make my 50mm a 75mm lens, and my prospective 80mm-320mm lens to a 120-480mm lens. Will the O-GPS have a problem compensating for this..? Will it be unusable? How will this affect the focal ratio? Does my f1.7 lens somehow become an f2.4?

So many numbers..!! Knew I should have paid more attention in math class.

01-06-2016, 06:21 PM   #2
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Fortunately, it's simple. DA focal lengths are actual focal lengths, not "35mm equivalent" focal lengths. As a rule it is only point-and-shoot cameras that use "equivalent" focal lengths in their advertising. No worries about the O-GPS1.
01-06-2016, 06:29 PM   #3
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What about the FA lenses? How does the focal ratio change if using the older lenses?
01-06-2016, 06:35 PM - 1 Like   #4
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QuoteOriginally posted by mylittlepwny Quote
What about the FA lenses? How does the focal ratio change if using the older lenses?
It doesn't. The best thing you can do is forget all that stuff about "35mm equivalent." Unless you're comparing FoV between sensor sizes, all it does is confuse.

01-06-2016, 06:39 PM - 1 Like   #5
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Just forget you ever heard about the crap factor.
01-06-2016, 06:41 PM - 1 Like   #6
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On your K50 an FA 50mm lens will show the same view as a DA 50 mm. Focal length is focal length, regardless of sensor size. What does change is the angle of view apparent to the sensor for crop sensor versus full frame. Also, f stop doesn't change. What does looks different for the two different sensor sizes is the depth of field that is in focus. Bottom line, you're letting all the talk about equivalence needlessly confuse you. In summary, an FA 80-320 mm lens would perform the same as a DA 80-320 if one existed. Hope this helps...
Daryl

Last edited by darylk; 01-06-2016 at 06:43 PM. Reason: Clarification of meaning.
01-06-2016, 06:44 PM   #7
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Ok. thank you everyone. I'll just forget all about it and just go take pictures. All this thinking has given me a headache.
01-06-2016, 06:52 PM - 1 Like   #8
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QuoteOriginally posted by mylittlepwny Quote
I'll just forget all about it and just go take pictures. All this thinking has given me a headache.
That's the spirit.

01-06-2016, 09:45 PM   #9
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QuoteOriginally posted by mylittlepwny Quote
Ok. thank you everyone. I'll just forget all about it and just go take pictures. All this thinking has given me a headache.
Hopefully this helps simplify things even more: a FA 50mm lens and a DA 50mm lens would give you exactly the same field of view and focal length on your K-50

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01-06-2016, 10:40 PM   #10
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QuoteOriginally posted by boriscleto Quote
Just forget you ever heard about the crap factor.
Absolutely correct.

It serves a purpose to scare you into upgrading to a camera you don't need, this whole DX/FX equivalence thing.
01-06-2016, 11:20 PM   #11
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As a side note. I found that my 80-320 needed a fair amount of AF fine adjustment to get the best quality ( -6 ).
01-07-2016, 07:18 AM   #12
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The beauty is with Pentax dslr's what your see in your view finder/lcd is what your going to capture 100%, then it is up to you to chose the lens with the correct magnification according to how you want to present your subject matter. That is one of the things that bugged me with my old Olympus gear with it's 95-97% then they slapped the EFL (effective focal length aka crop factor) on their lenses which only served to confuse and cause debate.
01-07-2016, 07:58 AM   #13
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QuoteOriginally posted by mylittlepwny Quote
Started off with a K1000 and switched to DSLR about 8 years ago.
Do you still have your k1000? A practical test is to shoot a roll of film with your FA50/1.7, and carry your k50 around with you. After each k1000 shot, put the FA50/1.7 on your k50 and take a photo from the same location with the same settings. The framing will be different and you get bonus points if you take another shot with the k50, but move backwards to try to get the same framing of your subject.

Compare prints. No math involved and you get to look at some pictures.

The 'forget about it' option also works.
01-07-2016, 08:12 AM   #14
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A particular focal length will have different uses on the two different sensor size formats. For example, a 50mm lens on an APS-C camera can be used to take a portrait showing the head and shoulders of the subject and will render the facial features of the subject in a way that meets the brain's perception of what a face should look like. However, when you put a 50mm lens on a body with a 24mm x 36mm sensor ("full frame"), you must walk closer to the subject to get the same field of view (head-and-shoulders only) as on the APS-C body. At that distance, the 50mm lens will make features closest to the lens (nose, brow ridge, lips, chin) appear distorted, i.e., disproportionately large. To achieve the same facial proportions as obtained with the APS-C, you have to put a 75mm lens on the 24x36 body and back away from the subject. On a 24x36 body, the 50mm lens achieves "normal" facial proportions when you back away from the person to include their entire body in the frame.
01-07-2016, 08:19 AM   #15
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Wen I buy a lens I look through the viewfinder...... that's everything I need to know.

I have never once employed the crop factor for anything, but arguing with the advocates for 36x24 sensor cameras.
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