[08amczb]

While I know the lens is very good, I don't understand why would someone buy it. If I would like to travel light/make family memories then the KP + DA 16-85/DA17-70 is much more compact, lighter and cheaper. I think the full frame camera is for the maximal image quality, so I prefer primes on the K-1, or the professional F2.8 zooms. Tele zooms are another story. At 105mm F5.6 I get a similar background blur as 70mm F4 on APS-C, so again, the full frame does not have advantage with the 28-105. The KP has a similar ISO performance, only one stop worse, but I can gain that one stop when I open the sigma 17-70 F2.8-4 to one more stop. (I also get the same DoF then.) Okey, I get 36MP insted of 24MP, but if I would like to make large prints, then the better lens would help a lot.
With the option of modern APS-C cameras I really not see the purpose of the consumer standard zooms on full frame.

[BigMackCam]

While I know the lens is very good, I don't understand why would someone buy it. If I would like to travel light/make family memories then the KP + DA 16-85/DA17-70 is much more compact, lighter and cheaper. I think the full frame camera is for the maximal image quality, so I prefer primes on the K-1, or the professional F2.8 zooms. Tele zooms are another story. At 105mm F5.6 I get a similar background blur as 70mm F4 on APS-C, so again, the full frame does not have advantage with the 28-105. The KP has a similar ISO performance, only one stop worse, but I can gain that one stop when I open the sigma 17-70 F2.8-4 to one more stop. (I also get the same DoF then.) Okey, I get 36MP insted of 24MP, but if I would like to make large prints, then the better lens would help a lot.
With the option of modern APS-C cameras I really not see the purpose of the consumer standard zooms on full frame.

I'm neither a K-1 nor D FA28-105 owner, but I can hazard a few guesses here.

(1) Whilst the K-1 may be bigger and heavier than an APS-C camera such as the KP, that doesn't mean it'll never be used as a walk-around camera. In such situations, you probably don't wish to carry several lenses in a bag. The D FA28-105 offers a versatile range of focal lengths / fields of view, and great IQ considering that range.

(2) The D FA28-105's image quality is widely regarded as excellent. I would generally agree that a very good prime lens is typically better than a very good zoom lens. But that doesn't mean that a very good zoom lens can't produce fantastic quality images. I have many lenses of different types, including some lovely primes, and by far the best lens I own - if we use sharpness / detail, colour and rendering as measurement criteria - is my Sony Zeiss 24-70 f/2.8 zoom in A-mount. Decent zoom lenses these days can be extremely good, and everything I've seen and heard suggests the D FA28-105 falls into that category. Furthermore, shooting primes is all well and good until you come across a subject or scene where you need a completely different focal length. While you're fumbling in your bag for an 85mm lens to replace the 28mm currently fitted to your camera, the D FA28-105 owner is busy taking the photograph

(3) I wouldn't say a full frame camera is for maximal image quality alone. That's just one possible reason for using it. Another is to get greater subject to background separation at the same field of view and aperture. Yet another is the ability to use old full-frame glass in the way it was intended. There are other reasons too. I shoot my Sony A7 MkII almost exclusively with vintage Soviet glass - some of which is remarkably capable, and some which isn't. But by using a full-frame body, I can enjoy the entire field of view those lenses were intended to provide. My point is, people have different primary reasons for shooting a full-frame camera like the K-1. For most of those people, a versatile lens that provides excellent IQ across a wide range of focal lengths is a very useful tool for the kit bag.

I could go on, but I think these points are a reasonable start

EDIT: To underline points 1 & 3 above, I bought a used Tamron 28-300 PZD which I occasionally use on my Sony-A99-based Hasselblad HV. It doesn't even come close to the IQ of my Sony Zeiss 24-70 f/2.8, but there are occasions where it's the most useful and appropriate lens I can take with me...

[Adam]

If you look at the number of photos posted on the forum, it seems the 28-105mm is (slightly) ahead of the 24-70mm with the K-1. I think there are lots of factors at play, including weight, cost, and reach compared to the 24-70mm. While a more optimal combo may exist (i.e. one of the crop alternatives you suggested), I'm sure many people want to stick to a single format, and thus may not have such alternative on hand.

Also, if you ask me, if you're after maximizing image quality, primes are the way to go. On the other hand, a walkaround zoom will help ensure you have the right tools (i.e. focal length) to capture the moment wherever you are. I'm sure there are many others who, like me, use this lens as a complement to a large suite of primes.

HD PENTAX-D FA 28-105mm F3.5-5.6 ED DC WR and PENTAX K-1 | Sample Photos for Pentax Cameras & Lenses - PentaxForums.com
HD PENTAX-D FA 24-70mm F2.8 ED SDM WR and PENTAX K-1 | Sample Photos for Pentax Cameras & Lenses - PentaxForums.com

[interested_observer]

I like my K5IIs. I still have it. It was doing an ok job at astro - but I really wanted more. After a year of thinking about it, I acquired the K1 primarily for night astro and night ambient low light landscape images. From what I saw, the KP was not performing as well as the K1 - in particular on astro. It just comes down to sensor size and physics. I really don't think that the K3II replacement will match or exceed the K1, for my two specifics.

There are lots of locations around Arizona that I have not had an opportunity to visit that I want to see and shoot. Since I have the K1 - my plan is to use the two lenses I have, the DFA 15-30/f2.8 and the DA 60-250/f4 (modified and the parts are sitting right here), filling in with my FA31, A50/f1.7 and my two Contax Zeiss 28 Distagon and 85 Sonnar. However, for the most part these are manual lenses. There are times, I just want an AF zoom, that offers good image quality.

I would like a "walk around zoom lens", with good image quality to span the middle. To me, that's where the 28-135 comes in. Stop it down to f8 and use AF and from what I have seen - you get some excellent image quality, without the size and weight of the new f2.8 glass. For landscapes, I don't need f2.8 and I don't want to spend another thousand dollars plus on lens' speed. Is it a compromise? - sure it is, but it appears to be a reasonable compromise. I'm in the process of picking one up.

So, there are my thoughts....for what they are worth.

[stevebrot]

If I would like to travel light/make family memories then the KP + DA 16-85/DA17-70 is much more compact, lighter and cheaper.

Perhaps if the intent was not as part of a travel kit or if the intent of travel was photography other than snapshots. To be honest, if my intent is to make family memories only, I leave the camera at home. The phone cam does an excellent job for casual work.


Steve

[pentageek]

While I know the lens is very good, I don't understand why would someone buy it

Well, in my case it was pretty simple: I needed 'walk around zoom' and something wide, mainly for landscape. There are many combinations, but I decided to consider two:
'professional 2.8 zooms': 15-30 + 24-70, cost: ca. 2.6k USD, weight 1.8 kg
28-105 + Irix 15 mm, cost ca. 1150 USD, weight 1.1 kg, smaller and cheaper filters


I chose the latter and this combination works well for me. Sometimes I think about used 24-70, but difference in IQ is relatively small, so the upgrade can wait. Of course I also use primes, mainly A and F, but not as part of travel kit.

[kiwi_jono]

Well I was surprised to find the DFA28-105 was pretty competitive in many ways with most of my FA primes. Yes its relatively slow but in terms of sharpness and CA control, it does the K-1 36 MP sensor justice IMO. I use it a lot more than I thought I would.

In terms of FF versus APS-C, there are plenty of discussions to found on the forums, so I'm not going rehash those. I will say that I still have a K-01 and K-5 as well as the K-1. I use the K-01 (with DA40, DA15, and DA70) when travelling very light but I have not used the K-5 since I got the K-1 and I'm going to sell the K-5. Very happy with the K-1 (and the DFA28-105 for that matter).

[surfar]

Very happy with the K-1 (and the DFA28-105 for that matter).

My guess is that the owners of this lens is with you.I know i am.

Its wide enough and long enough for most situations.

[stevebrot]

Very happy with the K-1 (and the DFA28-105 for that matter).

My guess is that the owners of this lens is with you.I know i am.

I just shook my head when I read the original post. The 28-105 is a very capable optic and is price-competitive to the APS-C lenses suggested at a weight that not unreasonable for a 3.75X zoom. I would expect there are many reasons other than family travel photos to own this lens*, though it may well be that now might be the time to begin discussions of Norwegian brown cheese and Inuit Igunaq and the destruction of Tokyo as in this PF classic thread:

why I won't buy a k3 (Warning: Satire Thread) - PentaxForums.com


Steve

* Use the somewhat heavier Sigma 17-70/2.8-4 (C) as walkaround on my K-3. The heft factor is close to that of the K-1 + D FA 28-105, but I don't mind.

[mikesbike]

Actually, the same factors apply to both the K-1 and the KP, which performs remarkably close to the K-1 as Pentax designers intended. There are numerous reasons for having a fine FF body, among them very fine WA capability and better control of perspective and background blur.

For both is the issue of why take a zoom lens of f/2.8 if you do not need the speed? It is otherwise the image quality the lens is capable of that counts. Why use a DA* 16-50mm f/2.8 on your K-3 if you would rarely need to shoot at f/2.8? Why not the DA 16-85mm instead? Or the DA 18-135mm which offer far more reach and can keep to f/3.5-4.5 out to 70mm- around 1 stop off f/2.8? The D-FA 28-105mm is smaller and lighter than the D-FA 24-70mm, with a far greater FL range, so it offers far more flexibility. The ensemble being no heavier than the K-3 with say the DA*16-50mm, but with greater reach. So for those whose work is better served with a FF format, this lens makes for a very good solution at a reasonable price. For work in which a faster aperture is desirable, some people who own the K-1 with this lens also have a prime or two for which these low light needs are addressed, such as the FA 35mm f/2, the FA 31mm f/1.8 Limited, etc. But when you need a zoom lens having a constant f/2.8 aperture, well nothing else will do.

[BigMackCam]

I just shook my head when I read the original post. It is one thing to troll. It is quite another thing to troll owners of a respected body and the most popular zoom lens option for that body.

Respectfully, Steve, I don't think the OP was trolling. But I do think it's sometimes rather easy to make assumptions that everyone has the same priorities and requirements from their gear, when the reality is wildly different. Hopefully, some of the answers in this thread, as well as the obvious popularity of the lens with K-1 & K-1II users, will demonstrate that

[Kiwipixter]

Not only its a good performer optically, good focal range, WR, low cost, but also there aren't many other options of modern glass for FF at this price point. This is my main lens, along with DA*55 prime for travelling.

[08amczb]

Okey, so for those who has a K-1, but does not has a K-70/KP buying only a DFA28-105 is cheaper, then an APS-C body + a standard zoom. This is a point.

Altough I still think, a recent APS-C body + a standard zoom is more reasonable: you get a second body and it's more compact and light with aprroximetly the same IQ.

[BigMackCam]

Okey, so for those who has a K-1, but does not has a K-70/KP buying only a DFA28-105 is cheaper, then an APS-C body + a standard zoom. This is a point.

Altough I still think, a recent APS-C body + a standard zoom is more reasonable: you get a second body and it's more compact and light with aprroximetly the same IQ.

That's a valid view-point, but - importantly - it's yours... and certainly not everyone's

Although there are occasions where I take more than one body with me, it's usually for backup in case one fails. So, if I'm shooting with my K-3, I might take my K-3II as backup. Or, if I'm shooting my Hasselblad HV, I might take the A7 MkII plus LA-EA4 adapter as backup. In both cases, the lenses I carry are compatible and have the same field of view on both the main and backup bodies.

I can see why you might carry a different format body with you, but that means different lenses (if you want to replicate the same fields of view), different batteries, different battery grip, etc. If cost is the main consideration, then it might work out cheaper, though I doubt it. Most people, I think, only want to carry one camera and one set of compatible lenses for the majority of amateur photography situations...

EDIT: I will say, there are times when an entirely different format and lenses make an excellent addition or alternative to one's main kit. On numerous occasions, I'll carry my Pentax Q7 with one, two or three lenses instead of an APS-C or full-frame DSLR. That setup gives me similar fields of view and decent IQ in a very small package. But these situations are quite different, for a number of reasons, than those where I would happily carry and APS-C or FF DSLR / mirrorless setup...

[Wheatfield]

If I was still shooting weddings, that lens would be a no brainer.