[olenl]

I am a DSLR newbie, just bought k-x.

I am trying to choose the prime lens. This is going to be my only prime for a while. I want something not too wide but not too narrow, like 20-40 mm.

The main criteria is the "speed" of the lens, that is it should be fastest with the same aperture. At the same time, I do not want to use the widest aperture possible since this reduces depth or sharpness a lot, while I am interested in landscape shots.

So I want to use high aperture numbers. In fact I want to focus on infinity and it should still be sharp for close objects (like 10-20 meters).

So here are my choices so far:
SMC Pentax DA 35mm f/2.8 Macro Limited
SMC Pentax DA 21mm f/3.2 AL Limited
SMC Pentax DA 40mm f/2.8 Limited
Sigma AF 30mm f/1.4 EX DC HSM PENTAX

It would seem like 35mm/Macro is the best since it also gives macro, but doesn't macro capability decrease light passing through? That is, wouldn't other lenses be faster because of no macro?
Also, 40mm and 21mm are "slim" models, doesn't it mean they are the fastest (since there are less glass so less light is lost)?
I would also examine other affordable options.

Oh and another criteria is the lowest item variety possible... since I probably won't be able to test the lens before buying unfortunately.

[Adam]

You're way off on your light calculations. First off, just because a lens is a macro lens doesn't mean it's slower than a non-macro counterpart. The only truth to that is that is that macro lenses sometimes feature smaller apertures by design, to increase sharpness.

The aperture number (Fx.x) is what tells you how much light passes through the lens. An F2.8 lens always allows for 1/2.8'th of the ambient light to pass through the lens, an F4 lens 1/4th, and so on. That's what the number means- it's simply a ratio

I'd recommend the 35mm macro from that list. The 21mm is a bit too wide, and the 40mm is nice, but why not get the macro capabilities while you're in that ballpark already? The Sigma is obviously the fastest lens out of the bunch, but if you're venturing into that price class, then I'd recommend the SMC Pentax FA 31mm limited, which is quite a gem compared even to the DA limiteds.

[Ash]

I agree with Adam, but wouldn't hesitate to recommend any of your previous options, as well as that of the FA 31 ltd. Now being a prime, you'd have to be happy with the field of view you get out of the lens for the type of landscape photography you're after - so choose your focal length wisely. Bear in mind the effect the 1.5x crop factor has on your field of view. This is why I wouldn't think 21mm is too wide for most landscapes, and I'd think 40mm would be too narrow for many landscape scenes - it's all up to you with that one.

I solved that with the DA* 16-50, which I have used extensively at just about all the focal lengths it offers, and does a good job at all of them.

[RioRico]

No, macro capability doesn't reduce light, but macro usage does. That is, if you use a macro lens at infinity focus rather than very close focus, it admits just as much light as a non-macro lens. So my Vivitar 90/2.8M is just as fast as any other 90/2.8 lens, unless I extend it out towards 1:1 magnification.

Be aware that shorter macro lenses mean that, when doing macro work, you need to be very close to a subject. 28-35-50mm macro lenses are usually used for studio work, while 90-100-105mm lenses are more for field work -- you can put a bit more distance between camera and subject.

The lenses in your list are all fine glass, and are all new autofocus lenses, and not cheap. For macro, portrait, and landscape shooting, autofocus isn't needed. For macro and landscape, speed isn't needed. You could buy 5-10 fine used manual primes for the cost of one of those you list.

Stopped-down, any decent lens should be sharp. My Lentar (Tokina) 21/3.8 (US$22) stopped to f/8 and prefocused to 10 ft is sharp from 5 ft to infinity; my Sigma 24/2.8 (US$3) at f/8 and focused to 15 ft is sharp from 7 ft to infinity. Old Takumars like a 28/2.8 or 35/3.5 still sell for under US$25; a 50/2 might not even hit US$10.

[pcarfan]

Yeah! the f number is the only thing you look at when considering how fast a lens is (amount of maximum light that can enter), however you should know an f1.4 lens shooting at f2.8 allows the same amount of light as an f2.8 lens at f2.8 (I am not too sure whether you are aware of this).

So, all you are going to do is shoot at apertures larger than f2.8, then a fast lens is not necessary.

Also, the larger the aperture, bigger the lenses usually get (The sigma 30/f1.4 is a large lens). Assuming you want a small lens to accompany your small K-x, yet want a fast lens to use wide apertures, and still want the best picture quality possible, I would recommend the Pentax FA43/f1.9 LIMITED. It cannot focus close objects and is slightly wider than your requirements, but will look great with your K-x, will take amazing pictures if you are up to it, and is fast.

[kytra]

Take the FA31mm if you can afford it.

Think about it: if you could afford a Ferrari, wouldn't you buy one? probably yes
If you could afford a Mercedes 600, wouldn't you buy one? probably yes
If you could afford a Vacheron Constantin watch, wouldn't you buy one? probably yes
If you could afford a Vertu platinum phone , wouldn't you buy one? probably yes

I could not afford any of the above but I could afford the FA31 and it gives me the feeling of having the best there is in the Pentax stable. Forecasting a FA 77mm in the future...

Regards,

K

[nulla]

I think in this case Adams advice is the best.

New user and a K-x.


Whichever way you go enjoy yourself


Neil

[olenl]

Assuming you want a small lens to accompany your small K-x, yet want a fast lens to use wide apertures, and still want the best picture quality possible,

Ok I probably stated it wrong

I have 2 fixed M42 lenses, and I noticed that when using them, at the same aperture number as my kit lens, I get much faster shutter speed (or I can lower ISO of course). I assume that this is because fixed length lenses pass more light, not because of aperture but because they are "brighter" by design.

This is what I meant by "fast" - it should allow for faster shutter at the same aperture than other lenses - because I want to shoot in darker conditions etc.

Sorry for my mistake, by "fast" I did not mean fast autofocus (which still isn't bad though)

[NaClH2O]

Hi olenl and welcome to the forums. What everyone has said above about fast lenses and macros is true. I'd like to add one small fact. When looking thru your viewfinder you will find that the faster the lens, the brighter the scene in the viewfinder. This is because during composition, the lens will be held at it's fastest aperture, to give you the brightest view possible. So looking at a scene thru a F1.4 lens will look brighter than the exact same scene thru a F2.8 lens.
As far a recommendations are concerned, I'll echo what several ppl have said about the FA 31mm ltd (but it is a very pricey lens) and the FA 43 ltd (my favorite prime as I don't own the 31) I'd also like to add the Sigma 24mm F1.8. Sigma calls it a "macro" lens but that's more marketing than reality, "close focusing" is a more accurate description. In any event it's a nice lens, it's only problem being it's a bit on the large side. Also if you can find it the FA 20mm F2.8 is an excellent lens.

NaCl(I own both the Sigma 24 and the FA 20 and can vouch for their quality)H2O

[axl]

Take the FA31mm if you can afford it.

Think about it: if you could afford a Ferrari, wouldn't you buy one? probably yes
If you could afford a Mercedes 600, wouldn't you buy one? probably yes
If you could afford a Vacheron Constantin watch, wouldn't you buy one? probably yes
If you could afford a Vertu platinum phone , wouldn't you buy one? probably yes

I could not afford any of the above but I could afford the FA31 and it gives me the feeling of having the best there is in the Pentax stable. Forecasting a FA 77mm in the future...

Regards,

K

This reasoning is kind of off IMO...
To buy lens (or any item - and no I wouldn't buy platinum phone, neither the watch, and I'd think twice about the Merc and Ferrari... ) just because you can afford it is downright silly....
To follow your example... why would you have superexpensive Ferrari in your garage if you don't like it's handling on the roads of your densely populated (mostly by other drivers) town.... you get me right?

to OP:
If you can, go to store, try various lenses, see which one suits your style, which FOV do you prefer... if you don't have Pentax stockist nearby, download Exposure plot freeware, and run it through your shots archive. It'll tell you which focal lengths have you used how many times among other things...
Then take results of the two above "researches" combine them with you budget and voila... just hope the lens you have chosen will be in stock

If I can chime in with my experiences:
31ltd - fantastic lens for indoor people pics, very versatile portrait/landscape lens. Stunning OOF, very good speed, slightly bulky
40ltd - one of the best lenses out there. Very versatile FL IMO... more so than 31 IMO. Fantastic IQ and great portability
43ltd - probably my best lens. Take DA40 with +1 stop, better MF and brighter VF.
When I'm taking one lens only, this is IT!!!

Generally 31/35 are regarded as the perfect standard lenses for APSC, I don't dispute this but for the only prime, this FL was always bit wide for me... I much prefer 40/43 as it's still wide enough for many vistas/landscapes/cityscapes/streets/indoors yet long enough for portraits....

[axl]

Ok I probably stated it wrong

I have 2 fixed M42 lenses, and I noticed that when using them, at the same aperture number as my kit lens, I get much faster shutter speed (or I can lower ISO of course). I assume that this is because fixed length lenses pass more light, not because of aperture but because they are "brighter" by design.

This is what I meant by "fast" - it should allow for faster shutter at the same aperture than other lenses - because I want to shoot in darker conditions etc.

Sorry for my mistake, by "fast" I did not mean fast autofocus (which still isn't bad though)

There is something fishy here...
IF you use your primes at the same f stops as your kit lens, you can't lower ISO neither increase shutter speed or you'll offset the exposure.
Good lens designs may allow more light through but with Pentax SMC the reflections should be minimal and there really shouldn't be any significant differences in exposure (ammount of light) coming through whether it's old prime or new zoom...
From this I assume you are shooting all the time wide open my friend....

There isn't any lens (I'd know of) that would magically let more light in at the same f stop as other lenses. It's actually the aperture that decides how much light comes through.... if you set it to f5.6 on zoom, prime, supertele, ultrawide whatever the shutter speed should* remain the same given you don't change ISO.

* I used should because different lenses with different coatings MAY result in SLIGHT differences in shutter speed but nothing like you say "much faster"

[Clinton]

If you're looking to experiment with fast glass, I'd suggest grabbing a Pentax FA 50/1.4 from Amazon before they're all gone.

[rover]

Let me throw out one more idea, the A 24/2.8? It is small, relatively wide on a DSLR, and less expensive than the new options if budget is an issue.

[olenl]

There is something fishy here...
IF you use your primes at the same f stops as your kit lens, you can't lower ISO neither increase shutter speed or you'll offset the exposure.
Good lens designs may allow more light through but with Pentax SMC the reflections should be minimal and there really shouldn't be any significant differences in exposure (ammount of light) coming through whether it's old prime or new zoom...
From this I assume you are shooting all the time wide open my friend....

There isn't any lens (I'd know of) that would magically let more light in at the same f stop as other lenses. It's actually the aperture that decides how much light comes through.... if you set it to f5.6 on zoom, prime, supertele, ultrawide whatever the shutter speed should* remain the same given you don't change ISO.

* I used should because different lenses with different coatings MAY result in SLIGHT differences in shutter speed but nothing like you say "much faster"

You are right.

My mistake was that I judged by my impressions with old 135 mm lens. Since I happened to have pointed it to the lighter areas, it received more light than wide kit lens, thus it was "faster". I have just checked with relatively the same light and those fixed lenses give similar shutter with the same aperture.

Another problem is that k-x's expometer seem to be way off with those lenses, I always have to either lower or raise the exposition (more often, raise) to get correct exposure. I also can't make flash work right with them but it's offtopic.

[mz-3]

vote for DA 40. You won't regret.