[SpecialK]

The question has no answer.

There is no walk-around prime. Whatever you get will miss out on longer and wider. That is why there are different focal lengths.

[timo]

For some reason, I love 28mm Manual focus lenses. They seem perfect for my "Vision".

I also find good old 50mm and 135mm very useful.

Agreed. I have the DA35 but in some ways prefer using one of my 28/3.5s (K and M). The FOV just seems to 'work' for me. But of the options given by the OP I would definitely go for the 35, although one would choose a 50 in specific circumstances.

[Primes4ever]

I would say either 35 or 40 would be ok. I love my 40 Limited and it still gets a lot of walk around use, even since acquiring a 31Limited. The advantages of stealth and quality are important to me. I got the 40XS with my K01 but sold it because the Limited version uses 49mm filters, has a great hood, quick shift and has better build quality. So my vote is 40.

[HSV]

Since it's the 1st first prime, you must have at least one zoom. Looking at the last few hundred "keeper" images you've taken...which focal length is used on most of them? Also, get into the habit of choosing the focal length before putting the camera up to eye level, this will help you decide which prime to choose.

Keep in mind that your favorite focal length might not be your most used one. Also, consider the "type" of photography you're most interested in. There is really no right answer.

For me, my favorite focal length is about 70-90mm, and I'm almost always have my zooms set at the longest...but interestingly, my most used lens is the DA40.

If I were in your shoes, I will choose the 40XS. As a bonus, it has a metal mount!

[Kozlok]

Here's a fun idea, buy the 35/f2.4, and then get an old M50/f1.7 or f1.4 (avoid the f2.0). They are pretty cheap. If you are ready for screwmount, the Takumar 55/f1.8 or f2 (these are exactly the same lens) are super fun, and the slightly longer focal length makes them more different than the 35. Indoors, I use my Sigma 30/f1.4, and even that is a little long in some cases (museums where I can't back up any further for example). 50 is usually way to long indoors, but outside, with room to move, it's a great FL.

[MSL]

Others have given similar advice, but I'll weight in as I'm coming from the opposite angle. I have multiple 50s - F50/1.7, Tak 50/1.4, couple of 55/2 and couple of Helios (58mm). I also have the F35-70 and the kit 18-55. When I need speed the F50 is the one I turn to - something I can't get out of the two other autofocus lenses I have as they are both quite slow. But I often find 50 is too limiting, especially indoors. So I've been debating whether to get a 35/2.4 or 40/2.8 Limited. In the end I may opt to go for something like the Tamron 28-75/2.8 to give me a good range of focal lengths with speeds similar to the 35 or 40 primes. It is easy to get a cheap manual focus 50 so if you don't mind manual focusing (and possibly green button metering) you should be able to try that focal length for not much money.

You haven't said what camera you are using, or more importantly if you have some version of the 18-55 kit lens. If you do, why not start by looking at the photos you have taken and focal lengths they were shot at. There is a program that will do this for you if the images are in JPG format (I don't have the name at hand), but even a quick sampling in whatever program you use to look at the images should give you a quick answer. Or, go out shooting with the kit lens. Fix the focal length at 35, 40 or 50 and take a bunch of pictures at each focal length and decide from there what you need.

[dtmateojr]

To decide on a lens because of IQ or speed is quite short sighted. You should pick one that allows you to compose the shot according to YOUR style.

[MSL]

To decide on a lens because of IQ or speed is quite short sighted. You should pick one that allows you to compose the shot according to YOUR style.

It may be short sighted if that is all you base your decision on, but it is still important data. If you are a low light shooter, then it is important to know how sharp a lens might be wide open, for example. I think the OP is on the right track by looking at the various data that exist for these lenses and then asking the question that (s)he did.

[EarlVonTapia]

Pentax makes a lot of fantastic 50mm lenses. The F 50 1.7 in particular is such a superb lens. And the F series in general has this great Japanese 80's electronics aesthetic to it.

But 50mm on APS-C is sort of a no-mans-land focal length for me. It's too tight for indoors, but it's not really a telephoto either. It's great for portraits though, so if you are into that type of thing then a good 50mm is something to invest in.

I would say go for the 35. The DA 35 2.4 is a great value lens. It's not the fastest, but it's cheap, incredibly sharp even wide open, and it doesn't need a hood. It's really simple and a joy to use.

Buy it used for $140-$160 and try it out for a while. This will let you know if you want something that goes wider, or something with more reach. When you're done, sell it for the same amount and lose nothing. Or, maybe you will discover that 35mm is just right for you.

[dtmateojr]

It may be short sighted if that is all you base your decision on, but it is still important data. If you are a low light shooter, then it is important to know how sharp a lens might be wide open, for example. I think the OP is on the right track by looking at the various data that exist for these lenses and then asking the question that (s)he did.

My point is, 35 and 50 are not something to be confused about. They are very different focal lengths. Those who shoot 35 rarely use 50 and vice versa. Getting confused on something that is almost black or white is a clear indication that the OP should stick with whatever lens he has now, most likely a kit lens.

Save your money.

[NZUnicorn]

I think you are best not to choose a single prime in isolation. Think of the set of primes you would like to end up with, then select the most appropriate one from that set.

A 35mm may sound like the single best option now, but what if you later decide you want a 28mm and a 70mm, with something to fit between them. Perhaps then a 40mm might be better than 35mm.

[Clavius]

This is very personal. I can go for ages without the 35mm not leaving my camera. Same goes for the 15mm even. With anything longer, I quickly feel cornered.

[Juventino]

It is a personal choice. If I can just take one lens with my Kr, I go with the 40 or 50mm. On the ME Super, I started with the 50mm but I bought recently a 35mm and I like this focal length.

What you can do is playing with your 18-55 to see if you prefer 35, 40 or 50. You can also use Exposure Plot or similar programs so as to analyze your picture folder and to obtain a plot of your focal lengths. Anyway you can't go wrong with the lenses you mentioned.

[Tonto]

+1 for 35mm, I personally prefer 40mm as a walkaround but there is no subsitute for a wideish lens sometimes.

[macTak]

I agree it's a very personal decision of shooting style. If you've been shooting with a zoom, you can go back and review what focal lengths you have used--perhaps you will find where you most commonly used. Alternatively, try shooting for a day or a week with your zoom (if it has these focal lenghs) set to just one of these, then repeat with the next, and see which you are most comfortable with. For myself, unlike many of the other posters, I actually greatly prefer 50mm as my general lens, and a 50mm prime is usually what is on my camera. For some reason, that angle of view is just how I see things. Perhaps I just like the additional bit of compression and subject isolation that 50mm on crop gives compared to 35mm or even 40mm. Even when I was shooting film, I was always shooting around 80mm.