+1 - I've tried to clean the non-APO version of these lenses and whilst it wasn't hard to get apart, I found that the internal coatings seem to be very soft and the fungus had damaged them quite badly.

+1 (and I've also got it on my 350mm/5.6 mirror if that's any consolation!). I'm pretty sure it's not fungus, as others have said. Even in a really heavily fungus-infested Sigma zoom I saw, there were thread-like filaments at the edges.

I've had both (although not simultaneously) and they were pretty good. My main reason for getting rid of them (at various times) was a Sigma 30/1.4 and DA35 ltd.

As I understand it, the FA uses aspherical elements, which means that the bokeh isn't as smooth. The F28 has a bit of a cult following, which I found slightly bizarre, as I believe it has the same optical design as the A28/2.8, which gets mediocre reviews.

I preferred the rendering of the F. Both were pretty sharp although, on pixel-peeping, I'd say that the FA was marginally sharper, but it could easily have been copy variation.

I'd second the M50/1.7. It's an excellent lens and the extra speed at a slightly longer focal length over the M28/2.8 will make it easier to focus.

It was the first MF lens I bought when I was (re)learning to manually focus on my DSLR and in retrospect, one of the most accessible MF lenses I have used. You can do a lot with it that you can't do with the kit lens.

Either way, I hope she enjoys the K100D and you both have fun with whichever lens you choose - I hope that my kids show an interest when they're old enough to graduate from their P&S.

Weekend bump - open to reasonable offers.

The Tamron's pretty diminutive and light, but the Sigma's comparatively big and heavy (it takes 82mm filters!). That's its main drawback, to be honest. I don't really mind, as I've spent the past few years lugging around enough kit to maintain various babies/toddlers, so a slightly heavier camera bag is a drop in the ocean.

I haven't seen an 18-135 in the flesh, but I'd be surprised if it was as big as the Sigma 24-70. The Sigma certainly dwarfs my 18-250.

Without wishing to appear facetious, I find that, it depends on what I'm walking around.

In the range you're looking at, I think that a Tamron 17-50/2.8 is hard to beat, unless you need weather sealing.

Casting your net a bit wider, the Sigma 24-70/2.8 is just about wide enough indoors (equivalent to 35mm on FF) and 70mm at the long end is better in large spaces and outdoors and at f/2.8 it's good in low light and also useful for portraits. It's one of my most used lenses and offers excellent IQ.

As a jack-of-all trades, the Pentax 18-250 or its Tamron sibling covers most eventualities and really has very good image quality for such a zoom range.

PM replied - sold subject to payment. Thanks for looking.

I've bought M42 rear caps from eBay in the past ( item 150437640898 ) and have been very happy with them.

Well, from my experience, I'd recommend a few Takumars for a start:

3.5/35 (SMC or Super)
50/1.4 or 55/1.8 (again SMC or Super)
85/1.8 SMC or 85/1.9 Super
100/4 SMC macro Tak
135/2.5 SMC (later 6-element edition - see here for more info)
200/4 Super/SMC

One of the appealing things about M42 lenses for me is the diversity of lenses available. Among my other favourites are the following:

Jupiter-9 85mm f/2 (I've been through 3 copies to find a good one)
Carl Zeiss Jena Pancolar 50/1.8 (very different to the Takumars, but I like it)
Carl Zeiss Jena Pancolar 80/1.8 (has a cult following - in the same league as the SMC Takumar 85/1.
Meyer Optik Oreston 100/2.8

At the wider end, it's more of a challenge, There are some highly regarded CZJ Flektogon 20mm and 25mm variants, but they're expensive. If you can find one, a Komine made Vivitar 24mm f/2 in M42 mount, that's a cracking lens and comparatively good value.