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03-18-2013, 02:36 PM   #1
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Normal lens for APS-C cameras

Hi all,

I'm new to dslr and to Pentax, just bought a k-30, an I'm Just wondering - sensor diagonal of the camera is 28.4mm. Then why the current Pentax lineup of lens has the compact 43mm and 40mm limiteds which are on the long side and the 21mm limited which is on the wide side, but nothing similar in between? Is there some technical reason why a 28mm or 30mm pancake is not possible? Would be such a great walk around lens! Basically I'd like to find a normal perspective (say, between 24mm and 35mm) prime lens that's smaller and faster than the kit zoom. Maybe any suggestions from older lens lines or other manufacturers?

03-18-2013, 02:46 PM   #2
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A lot of us on these forums are wondering the same thing, actually. Pentax used to make pretty small wide angle lenses, don't know why they can't modernize it a little and keep selling. But, you know, you have many alternatives.
You can buy an older 24mm or 28mm, or get the DA 16-45 zoom or even the DA* 16-50mm zoom. Pretty much all modern Pentax zooms are better than the kit (and the kit itself is better than kits from some other brands). And Sigma started making some nice primes, as well as other brands like Zeiss and Voigtlander. Tamron mostly makes zooms.
03-18-2013, 02:51 PM   #3
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The 40mm is only 5mm from what's considered 'normal' and the available 35mm lenses (DA35Ltd, DA35-2.4 & FA35-f2 or older A & M's) although not 'pancake' are not exactly 'huge'...

Of the lenses I own the FA35 is my most used...
03-18-2013, 02:55 PM   #4
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Technically, A 28MM would be the most "Normal" lens you could get for APS-C. Closest widely available 1:1 ratio for Focal Length:Sensor Size

That said, Sigma makes a nice 28MM, and there's loads of legacy glass with that length.

03-18-2013, 02:56 PM   #5
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The 40mm pancake design is made possible by the flange distance to my understanding. At 40mm, the K mount needs no retro focus group. Unfortunately it has nothing to do with the field of view or what is considered "normal" for a given crop factor. That's just my understanding, perhaps someone can correct me if I'm wrong.
03-18-2013, 03:00 PM   #6
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Maybe I'm missing something here, but on a 1.5 crop, would not 35 be a "normal" lens? 1.5 x 35 is 52.5, half way between a 50 and 55. How does it get better than that?
03-18-2013, 03:12 PM   #7
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QuoteOriginally posted by normhead Quote
Maybe I'm missing something here, but on a 1.5 crop, would not 35 be a "normal" lens? 1.5 x 35 is 52.5, half way between a 50 and 55. How does it get better than that?
Some prefer a lens with a focal length equal to the diagonal of the sensor. That's 43mm on 135, and 28mm on APS-C.

It's really just a matter of preference. Probably everything between 28mm-40mm could be considered normal on APS-C.
03-18-2013, 03:16 PM   #8
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QuoteOriginally posted by normhead Quote
Maybe I'm missing something here, but on a 1.5 crop, would not 35 be a "normal" lens? 1.5 x 35 is 52.5, half way between a 50 and 55. How does it get better than that?
I think the accepted 'Normal' lens for 35mm film is 43.3mm (diagonal of 24 x 36mm) and the 1.5 crop of that is 28.8mm so a 28mm lens is as close as it gets. I know 50mm was always consider 'Normal' but that's not the way the math works out.

Maybe someone else can explain it better or correct me if I'm wrong, that's just the way I learned it.

03-18-2013, 03:45 PM   #9
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I really like 28mm on APS-C Not too wide, not too tight. You're out of like if you want anything new or AF with Pentax's name on it though. Sigma does make a 28mm and a 30mm. If you don't mind used and turning a focus ring, there are a ton of 28mm's out there. Since you are lucky enough to have a K-30 which has focus peaking, enjoy all the cheap manual focus 28mm primes out there

And no, my K28mm f/2.0 is not for sale
03-18-2013, 04:17 PM - 2 Likes   #10
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To find the center of normalcy of any camera system, you take the square root of the sum of the sides of the frame squared. If that sounds like the Pythagorean theorem, it is. So for a 135 frame, the diagonal is 43.26mm, thus a 43mm lens will exhibit no perspective distortion. Anything longer than 43 will cause some degree of compression from front to back of the scene, and anything wider will cause the scene to expand from front to back. That being said, 50mm was always called "normal" in the days of film because the amount of compression is negligible, and "nifty fifty" sounds so much more hip than "nifty fourty-three", that doesn't rhyme at all. For Nikon's and Pentax's implementation of APS-C, 29mm causes zero perspective distortion, Canon is a bit smaller.

Pentax does something interesting though, they make APS-C equivalents of classic film lengths. The DA* 50-135 for instance is (very) roughly the equivalent of a 70-200, and the DA* 55 I firmly believe is meant to emulate the FA* 85.

But getting back to the original question of the thread, it's the flange distance, not the sensor size I think that determines what focal length lends itself best to "pancakization". The K mount was originally designed for 35mm film, though it took it's flange distance from the M42 mount, which in turn came from the M37 mount. When the decision was made years ago to put an APS-C sensor in Pentax digital cameras, they could have, at that time chosen a different flange distance, as 45.46mm is more than is necessary, and then the "pancake distance" might have been 28mm. Instead they opted for backwards compatibility, and I think most of us thank them for that decision.

Last edited by maxfield_photo; 04-12-2013 at 03:21 PM.
03-18-2013, 04:19 PM - 1 Like   #11
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QuoteOriginally posted by jatrax Quote
I think the accepted 'Normal' lens for 35mm film is 43.3mm (diagonal of 24 x 36mm) and the 1.5 crop of that is 28.8mm so a 28mm lens is as close as it gets. I know 50mm was always consider 'Normal' but that's not the way the math works out.

Maybe someone else can explain it better or correct me if I'm wrong, that's just the way I learned it.
I think it is missleading to clasify wide / normal or tele lens by their focal lengths. Instead, it is way better to use their Angle of View (AoV) value.

The human vision has great visual ability in the central region (about 20 to the left and 20 to the right from the central axis (our nose)). Away from the center, our visual ability decreases dramatically, such that by just 20 off-center our eyes resolve only one-tenth as much detail. At the periphery, we only detect large-scale contrast and minimal color:

http://cdn.cambridgeincolour.com/images/tutorials/camera-eye_detail1c.jpg

So, a normal lens should have arround 35-45 AoV in order to equal our eyes "AoV". (a 50mm lens on 135 format (24x36mm) has an AoV of 40...
Finally, AoV is a function of format and focal length. I mean, AoV depends on the size of the negative (photosensitive material) and te focal length used.

In APS-C sensors (23.6mm x 15.6mm) you can see this AoV vs FL (Focal Length) chart made at: Field Of View Calculator

FL X AoV Y AoV
25 50 32 17" 34 39 29"
26 48 49 30" 33 24 4"
27 47 13 5" 32 13 44"
28 45 42 25" 31 8 6"
29 44 17 10" 30 6 40"
30 42 56 46" 29 9 4"
31 41 40 52" 28 14 53"
32 40 29 10" 27 23 56"
33 39 21 18" 26 35 56"
34 38 16 55" 25 50 35"
35 37 15 54" 25 7 44"

See this article for lot more info:

Cameras vs. The Human Eye

** when referring to AoV, always using the horizontal (X axis) value.
03-18-2013, 04:30 PM   #12
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QuoteOriginally posted by kristaps Quote
Hi all,

I'm new to dslr and to Pentax, just bought a k-30, an I'm Just wondering - sensor diagonal of the camera is 28.4mm. Then why the current Pentax lineup of lens has the compact 43mm and 40mm limiteds which are on the long side and the 21mm limited which is on the wide side, but nothing similar in between? Is there some technical reason why a 28mm or 30mm pancake is not possible? Would be such a great walk around lens! Basically I'd like to find a normal perspective (say, between 24mm and 35mm) prime lens that's smaller and faster than the kit zoom. Maybe any suggestions from older lens lines or other manufacturers?
Back in the 35mm days, before zooms became popular, I don't remember 42mm being a particularly popular (or even existing in most lens lines) focal length. So I'm not seeing this as an issue. If focal length is that critical, we now have zooms to provide the necessary exact lengths required. Back in those days, I owned a Vivitar 17mm, plus Takumars in 28, 35, 50, 135, and 200mm. Migrating to mostly zooms was an improvement - less lens swapping, and as best as I could tell, equivalent quality.

Paul
03-18-2013, 04:37 PM   #13
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QuoteOriginally posted by cali92rs Quote
I really like 28mm on APS-C Not too wide, not too tight. You're out of like if you want anything new or AF with Pentax's name on it though.
I too really like 28mm on APS-C. However, there is a Pentax AF lens that is 28mm wide. I'm an autofocus junky, so despite the plethora of 28mm manual focus lenses out there, I shelled out the big bucks to get me a Pentax F 28mm f2.8.

SMC Pentax-F 28mm F2.8 Reviews - F Prime Lenses - Pentax Lens Reviews & Lens Database

They pop up every now and then from anywhere between $200-$300. Might go up when the mythical full frame arrives.
03-18-2013, 05:14 PM   #14
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I'm also a fan of the Pentax-F 28, which is fairly small even with my screw on lens hood. I think they should make a weather sealed 28 and package it with the K-30/K-5.

Btw, that's the 28 on my K-5 in my avatar.

Last edited by drugal; 03-18-2013 at 05:16 PM. Reason: realized the 28 was in my avatar
03-18-2013, 05:19 PM   #15
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QuoteOriginally posted by EarlVonTapia Quote
I too really like 28mm on APS-C. However, there is a Pentax AF lens that is 28mm wide. I'm an autofocus junky, so despite the plethora of 28mm manual focus lenses out there, I shelled out the big bucks to get me a Pentax F 28mm f2.8.

SMC Pentax-F 28mm F2.8 Reviews - F Prime Lenses - Pentax Lens Reviews & Lens Database

They pop up every now and then from anywhere between $200-$300. Might go up when the mythical full frame arrives.
You are correct, I meant new Pentax lenses...
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