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12-16-2014, 05:08 AM   #1
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My Impossible Lens - Previously: "The Lens of Abomination"

NOTE: This thread was an attempt to start a discussion about a lens I would create however impossible that may be. I understand the title of this thread was misleading to many of you. I was trying to describe my dream lens and how awesome it would be if it could be created. It seems many readers assumed I was attempting to attack logic or put down the current lens line up. This is not the case. I see how this was misleading and apologize. I do appreciate some of the positive conversation in this thread! Also, I am happy to know this could draw the attention that it did. Thank you all for your comments and please consider my update to this post.


While I know the 20-40mm limited is already here, I would prefer if this lens were a bit wider and/or a bit more zoomed. Does anyone share my feelings on this?

If I could design and produce my own lens, I would build something with four main features in mind. This lens would be a wide zoom, bright at f2.8, compact, and weather sealed. As these features are a necessity, I would be willing to compromise on the zoom range and the constant aperture as long as it didnít go any darker than f4. Maybe it would be a 15-35 or 18-45.


Last edited by Blacknight659; 12-16-2014 at 01:17 PM. Reason: Clarification/Redirection
12-16-2014, 05:23 AM - 2 Likes   #2
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Wide F2.8 zoom and compact.

Rofl...
12-16-2014, 05:25 AM - 1 Like   #3
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Well, you have some existing choices. It seems like the DA* 16-50mm f/2.8 WR meets most of your technical requirements. But a lens with that kind of aperture and focal range isn't small. Sigma's 18-35mm f/1.8 is also very large and doesn't have WR. The old DA 16-45 f/4.0 can be found easily and isn't expensive. But it, too, lacks WR. The DA 20-40mm Limited has a shorter focal range for a reason: Smaller size is a priority with Limited lenses. On your list of must-haves there are longer focal range, fast aperture and compact size. Pick two.
12-16-2014, 05:40 AM   #4
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I have no issues with the 20-40 as it is. It's wide enough for me, and as I read it - it's usable on FF from 22-37mm, and I'll live with that based on the design. If they made it wide/longer, it may not have such a huge FF capable range at all.

12-16-2014, 05:45 AM   #5
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There is a question of how much wider? The wider you go the narrower the zoom range. Really anything wider than 16 you start to get in to ultra wide angle. Wide angle lenses traditionally have much shorter zoom ranges. The wider the zoom range, the more constraints you are dealing with - thus the more compromises that creep into the design.

Normal zooms usually have a zoom ratio of 4. Above 4 the designs get complex and expensive. WA zooms usually have zoom ratio of 2. Take a look at Tokina's 11-16, Nikon's 14-24, Pentax/Tokina's 12-24, to name a few. With 20mm you are really within the transition range between what has been normal and wide angle. Thus, the question of how much wider - since that is really going to determine a lot of the design tradeoffs. Also the aperture is going to really determine the physical size. If you are proposing something like 16mm, you already have an available lens the DA* 16-50/2.8 - which has a zoom ratio of a bit over 3, but its quite large and heavy, compared to the 20-40.

12-16-2014, 05:50 AM   #6
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But why f2.8 at the wide angle? You hardly ever need f2.8 at 20mm. The main problem with the DA 20-40mm limited is that at 40mm it is not f2.8, the zoom range where it would actually be useful to have a fast aperture. But the 20-40mm limited followed a different goal. Its goal was to make pleasing photos, with nice bokeh, not so much to be super bright, to have a long zoom range.
And there already is a lens that fits your wishes. DA* 16-50mm f2.8. Wider, longer, constant f2.8, WR. Compact? All Pentax lenses are compact compared to what the other brands are doing with similar focal length/aperture.

Btw, look at the Sigma 18-35mm f1.8 lens. It is not even WR, and it is huuge. Basically, a zoom faster than f4 will not be compact. Pentax did great making the 20-40mm as small as it is - try comparing it to other brand zoom lenses of similar focal length

And don't get me wrong, I understand that you enjoy thinking about awesome lenses, but there are many physical constraints. Optics is a complicated science and engineering

Last edited by Na Horuk; 12-16-2014 at 06:01 AM.
12-16-2014, 05:52 AM - 1 Like   #7
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Like others have said, what you want is a 16-50 f2.8 WR that has been magically made smaller. Only Gandalf and Dumbledore can do that.
12-16-2014, 06:27 AM   #8
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The 16-50 has been around a long time. There are probably engineers outside of Hogwarts who could update it to be more svelt. The Tamron 17-50/2.8 is between the 16-50 and the 20-40 in size.

I would never call it an abomination but I do scratch my head a bit at the two most recent Pentax zooms.

12-16-2014, 06:36 AM - 1 Like   #9
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Such a compact lens will not exist as it breaks some laws of physics.

Now, supposing you'll forego the "compactness", but insist on a wide long zoom and at f/2.8 at that, chances are it will break our wallets (and probably marriage), not to mention our existing tripods.
12-16-2014, 07:05 AM   #10
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The reasons that the lenses are spec'd and designed as they are is that they simply have to be designed like that to make the best compromise. The next possible revolution could be negative refraction optic materials that would make extremely compact designs possible. But that isn't even close to reality at the moment and may never be.

---------- Post added 12-16-14 at 03:08 PM ----------

QuoteOriginally posted by Na Horuk Quote
But why f2.8 at the wide angle? You hardly ever need f2.8 at 20mm. The main problem with the DA 20-40mm limited is that at 40mm it is not f2.8, the zoom range where it would actually be useful to have a fast aperture.
Well, it's easy to design lenses that are brighter at the wide end and it won't increase the physical size much in many cases. You simply just get it as an extra plus when designing the lens.
12-16-2014, 07:21 AM   #11
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I happen to own both the DA 20-40 and the DA 16-50. Unfortunately there are always compromises in our photographic gear. I bought the 20-40 specifically to use as a walk around lens that replaced my DA 21. 20mm is wide enough for building shots in the city and having the little bit of zoom to 40mm is icing on the cake. The lens is half the weight of the 16-50 and very nice to carry around in a small bag. I would not want it to be any larger. If it were, then it definitely would not make sense to buy it over the 16-50.
I have taken several pics at night with if so far and have not felt limited by its 2.8-4 aperture range. I will still use the 16-50 when I need to have the extra range but when possible, it is nice to leave the extra 10 oz at home.
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12-16-2014, 07:28 AM   #12
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QuoteOriginally posted by dcshooter Quote
If you find that funny, there is another thread here where certain PF users are insisting that a 10mm f/2.8 lens that is K mount, compact, fully rectilinear, AF, and that offers Limited-level image quality is not only possible, but that such a lens could be made affordably if only Ricoh engineers were open-minded enough to make it happen. Oh and also that the well-understood laws of optical physics that make such a project futile aren't really "laws," since they are only "models" that might change in the future.
Those members can't be serious. Which thread is that? I couldn't find it using the search function.

Anyway, if Sigma can make a reasonably compact 8-16, why wouldn't Pentax be able to a 10mm? Speed would have to be compromised on though.
12-16-2014, 07:40 AM - 1 Like   #13
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QuoteOriginally posted by drypenn Quote
Such a compact lens will not exist as it breaks some laws of physics.

Now, supposing you'll forego the "compactness", but insist on a wide long zoom and at f/2.8 at that, chances are it will break our wallets (and probably marriage), not to mention our existing tripods.
At some point, it is time to look at the place of the DSLR in the future of photographic tools.

If you want to make a DSLR compact, put the kit lens on and shoot at smaller apertures or use a DA Ltd prime. With the ISO capabilities of modern sensors, that is more easily done. When I travel, I don't usually take the 16-50, the 17-70, or any other larger "normal" zoom which aspires to replace primes. I take a few compact primes, the kit lens and the 55-300. Put the kit lens or one of those in the wide normal range on the body, and you can pack it in a large waist pack with another lens. I see the higher quality zooms as wedding or close-in event lenses.

However, since starting to use the Sony E-mount APS-C cameras, I am more and more convinced that the days of the DSLR are numbered (though the number may extend plenty far for my purposes), and its main advantage is with high quality, less than compact glass. For portable with compromises, nothing beats the mirrorless cameras with larger sensors. An A6000 with the oft-maligned 16-50 kit lens will fit in the pocket of a pair of cargo shorts, and, online reviews aside, you won't really see the difference in most shots over the Pentax kit.It is nice to have a lighter bag, but really compact is not where the DSLR's strength is. My future travels will be with the mirrorless for light walks and the DSLR with truly excellent glass for the special shots.
12-16-2014, 08:13 AM - 1 Like   #14
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What a poor choice of words for a thread title. I mean, seriously?

This thread shows a lack of understanding of physics and optics in the original post.

You might as well ask why Mercedes doesn't make a luxury car that is very spacious inside and fits 8 people, and still is the size of a subcompact on the outside. What is foolish is the question, not the fact that those things don't exist.

Last edited by ChristianRock; 12-16-2014 at 08:23 AM.
12-16-2014, 08:17 AM   #15
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This lens with the same focal length is nothing new for Pentax. I think there was an SMC all manual 20-40 model which was manufactured and sold starting in the 1980's. Also a FA 20-35 f4 starting in 1998.

Last edited by hjoseph7; 12-16-2014 at 08:22 AM.
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