SMC Pentax-A 28mm f/2.8 to complement DA 18-250 for travel kit?

GeneV · 01-20-2011, 07:13 AM

Originally posted by Spock

Given that APS-sized sensors are smaller than FF, and a 30mm lens in APS gives a similar field of view as a 50mm in FF, shouldn't it be possible to build a 30mm F1.4 or F1.2 that is no bigger than an old fast fifty? If not, why not?

I'd add to what Rico said that whether it is possible to build such a lens is a different question from whether it is possible to build, market and sell it at a profit.

RioRico · 01-20-2011, 10:12 AM

Originally posted by GeneV

I'd add to what Rico said that whether it is possible to build such a lens is a different question from whether it is possible to build, market and sell it at a profit.

And there we get to the question of just what a target audience (prospective buyers) would want or do with something WIDE and FAST. Mid-range is something else, and a 30/1.4 is mighty damn tempting from a low-light perspective, but it bumps against the DOF issue. Y'see, part of what makes ultra-Fast Fiftys (f/1.2, f/0.95, etc) exciting is ultra-thin DOF. The DOF of a 30/1.4 is only slightly thinner than a 55/2.8, and is rather thicker than a 58/2 like the cheap Helios-44. Note that the DOF of a 50/1.2 is slightly thinner a 55/1.4, and slightly thicker than an 85/2.

[You may ask: How d'ya knoiw that DOF stuff? And my answer is, the comparative DOF index: Divide the focal length by the maximum aperture; higher quotients mean thinner DOF. Y'all can build your own spreadsheet to compute this.]

Anyway, a shorter lens means thicker DOF, and that's just how optics works. If a large chunk of a target audience wants thin DOF as well as low-light performance, they WON'T be in the market for that wide fast lens. Would a 15/2 or 14/1.4 be tricky to use? A dSLR version would certainly be huge and costly. So, how many buyers will fork-out US$5K+ for such a monster? I'll guess that the bean-counters at the glass foundries have figured it's not enough, else we'd see some around.

grainbelt · 01-20-2011, 10:41 AM

To the OP: suggest you frame a number of shots at 28 and 35mm. If 35 is wide enough, consider the 35 F2.4 or 35 F2.8 Macro.

I'd particularly recommend the DA 35 F2.8 Macro. If you're out there fly fishing and whatnot, the macro capability will be fantastic. No, it's not alot faster, but it is much more capable.

I have the A 28 F2.8 as well, but use it more often on a film camera.

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01-20-2011, 07:13 AM	#16
GeneV Veteran Member Join Date: Oct 2008 Location: Albuquerque NM Photos: Albums Posts: 9,830	Originally posted by Spock Given that APS-sized sensors are smaller than FF, and a 30mm lens in APS gives a similar field of view as a 50mm in FF, shouldn't it be possible to build a 30mm F1.4 or F1.2 that is no bigger than an old fast fifty? If not, why not? I'd add to what Rico said that whether it is possible to build such a lens is a different question from whether it is possible to build, market and sell it at a profit.

01-20-2011, 10:12 AM	#17
RioRico Veteran Member Join Date: Jun 2008 Location: Limbo, California Posts: 11,263	Originally posted by GeneV I'd add to what Rico said that whether it is possible to build such a lens is a different question from whether it is possible to build, market and sell it at a profit. And there we get to the question of just what a target audience (prospective buyers) would want or do with something WIDE and FAST. Mid-range is something else, and a 30/1.4 is mighty damn tempting from a low-light perspective, but it bumps against the DOF issue. Y'see, part of what makes ultra-Fast Fiftys (f/1.2, f/0.95, etc) exciting is ultra-thin DOF. The DOF of a 30/1.4 is only slightly thinner than a 55/2.8, and is rather thicker than a 58/2 like the cheap Helios-44. Note that the DOF of a 50/1.2 is slightly thinner a 55/1.4, and slightly thicker than an 85/2. [You may ask: How d'ya knoiw that DOF stuff? And my answer is, the comparative DOF index: Divide the focal length by the maximum aperture; higher quotients mean thinner DOF. Y'all can build your own spreadsheet to compute this.] Anyway, a shorter lens means thicker DOF, and that's just how optics works. If a large chunk of a target audience wants thin DOF as well as low-light performance, they WON'T be in the market for that wide fast lens. Would a 15/2 or 14/1.4 be tricky to use? A dSLR version would certainly be huge and costly. So, how many buyers will fork-out US$5K+ for such a monster? I'll guess that the bean-counters at the glass foundries have figured it's not enough, else we'd see some around.

01-20-2011, 10:41 AM	#18
grainbelt Veteran Member Join Date: Nov 2008 Location: Minneapolis Photos: Albums Posts: 587	To the OP: suggest you frame a number of shots at 28 and 35mm. If 35 is wide enough, consider the 35 F2.4 or 35 F2.8 Macro. I'd particularly recommend the DA 35 F2.8 Macro. If you're out there fly fishing and whatnot, the macro capability will be fantastic. No, it's not alot faster, but it is much more capable. I have the A 28 F2.8 as well, but use it more often on a film camera.

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