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10-10-2012, 02:17 PM   #1
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Fast prime lens (35/40/50mm) for Pentax K-30

Hello.

I bought a Pentax K-30 kit with the 18-135mm WR a few days ago, and it has been great fun.

I also have a Pentax-A 50mm f/1.4 lens and I love it.

But I really miss the autofocus, specially in low light. I used the Pentax-A 50mm f/1.4 recently and missed lots of shots because I thought it was properly focused (throught the viewfinder or LCD screen) but it wasn't (when reviewing photos on the computer).

So, I'm looking for a fast prime lens that I can use. I saw several current Pentax alternatives (35mm f/2.4, 50mm f/1.8 and 40mm f/2.8), and I'm more inclined to the 35mm (price) or the 40mm (quality). However, I don't know if the aperture is open enough for low light situations, or if I'll still miss something more appropriate.

In the past I used Nikon's 35mm f/1.8G and Panasonic Lumix 20mm f/1.7 and I loved those two lens. Do you think I'll feel the same with the 35mm f/2.4, or should I wait a little more until I have enough money for the 40mm f/2.8? Or should I just get a 50mm f/1.8? I think 50mm is a little too close for me, I would prefer a 35 or 40mm, but I can bite the bullet if the aperture difference is significative.

Or do you have any alternatives you'd like to suggest (Pentax or third party)?

Regards.


Last edited by sena; 10-10-2012 at 02:34 PM.
10-10-2012, 02:26 PM - 1 Like   #2
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Reading between the lines, I think that you mean 35mm f/2.4 or the 40mm f/2.8, since there is no 40mm f/2.4 and the 35mm f/2.8 (macro) is not cheap.
10-10-2012, 02:27 PM - 1 Like   #3
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Are you held back by the prices? The 40mm F2.8 Ltd is around $400 MSRP and the DA 35 F2.8 Macro is about $600. Unless you meant the DA 35 F2.4...

Personally, if it's low light and you like your A50 F1.4, then you should really consider the FA 50 F1.7, FA 50 F1.4, and Sigma 50 F1.4 as well. Those will be in the range of the DA 40 F2.8 Ltd.

I will say though, the DA 35 F2.4 is pretty fantastic, and priced so well (usually can get it for $180). And if you want the DA 40 F2.8, you could get it's little brother - the 40mm F2.8 XS ($250). Get both, and you'd pay the price of a DA 40 F2.8 Ltd.
10-10-2012, 02:33 PM   #4
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Yes, yorik and JinDesu, I meant the 40mm f/2.8 and the 35mm f/2.4, sorry, I will edit the post accordingly...

I was thinking of the DA 35 f/2.4 because it's so cheap, I am kind of out of budget right now. But if it's worth it I can wait a bit longer. I didn't know about the 40mm XS. Is the image quality as good as the 40mm Limited?

I'll check out the FA 50 alternatives. I wish there was something like that with 35mm (f1.8 or 1.4)...

Do you think the difference from f/2.4 to f/1.8 will be noticeable in capturing pictures in low light?

Thanks.

10-10-2012, 02:41 PM   #5
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You have to consider a few things:

The DA 35 F2.4 is a 35mm lens. Assuming in body stabilization is on, you can effectively take pictures at 1/35s without worrying about camera shake. The 50 F1.8 can shoot at 1/50s. that's practically the same difference between F2.4 and F1.8... Of course, if you have sniper-steady arms, F2.4 is about 1 stop slower than F1.8, so it CAN make a difference, but it won't to most people. Both perform similarly wide open, and you prefer wider than 50mm anyways.

The DA 40mm XS is optically as good as the 40mm Ltd. Maybe just a smidge weaker, but for all intents and purposes, it's the same. The only thing you are getting with the Ltd is better build quality (metal vs plastic), better looks, a proper hood, and better handling.
10-10-2012, 02:44 PM - 1 Like   #6
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Low light is where AF does not work so well. You will likely still have focusing problems, only it will take longer as the lens hunts.
10-10-2012, 02:50 PM - 1 Like   #7
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I am in the same line of thought as JinDesu here. I would also add that high ISO performance of the camera is more important for low light work. Since the depth of field is so shallow at such wide settings its not really much of an advantage. People mostly buy fast lenses for shallow DOF rather than low light work.
10-10-2012, 02:51 PM   #8
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Side note - you could also use focus peaking with your A50 F1.7. Can't imagine how that would be inaccurate.

QuoteOriginally posted by westmill Quote
I am in the same line of thought as JinDesu here. I would also add that high ISO performance of the camera is more important for low light work. Since the depth of field is so shallow at such wide settings its not really much of an advantage. People mostly buy fast lenses for shallow DOF rather than low light work.
Another benefit of the 35 F.24, as the wider lens allows for more DOF.

10-10-2012, 02:53 PM   #9
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I'm getting really inclined to the DA 35mm f/2.4, specially after reading the reviews here on the forums, it seems a very nice lens, even comparing to more expensive lenses.

And thinking about it, I used the Nikon 35mm f/1.8, which had no stabilization, on a Nikon D3100, where I avoided going over ISO 1600. And the Panasonic 20mm f/1.7 on an Olympus PEN E-PL3, where I really tried to never use ISO above 1250 (even though the Olympus had in body stabilization).

With the K-30, besides stabilization, I feel confortable going to ISO 3200, so I think all-in-all, I'll be very confortable with it in low light.

As for the auto-focusing, SpecialK, do you think that the DA 35mm f/2.4 will be worst than the 18-135mm WR? With the focusing aid light on the K-30 I can get satisfactory results. At least I know that when it manages to focus, It will be correct focus or very near. On the manual focus Pentax-A 50mm f/1.4, I would have to resort to the viewfinder, and the focusing aid light wouldn't work for focus confirmation.

Thanks for your input!
10-10-2012, 02:55 PM   #10
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Focus peaking is nice, but it doesn't work so well in low light, specially since the focus aid light doesn't work with my A50 1.4.
10-10-2012, 02:55 PM - 1 Like   #11
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ISO 6400, F2.4, something like 1/30s or something... the blur is mostly the jellyfish moving, so maybe it was even slower. And this is on a k-x, so the AF is much less reliable than your k-30.

I don't think you have to worry about the DA 35 F2.4's focusing. It's good. Dunno bout the DA 40 or the DA 50 1.8, but I can attest to the DA 35 F2.4.

But again - why not use focus peaking?
Edit - Ah, that is surprising. I haven't tried the k-30's focus peaking in super low light, so I don't know.

Last edited by JinDesu; 10-10-2012 at 03:15 PM.
10-10-2012, 03:05 PM   #12
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I would expect the 35mm to focus a lot faster than your zoom. Its faster so allows more light in and has a lot less glass to move. The other thing that makes a huge difference in auto focus speed is the gearing itself. Some lenses have a very short high geared shift and will shift from close focus to infinity with as little as a quarter turn of the focus ring. This is great for autofocus speed but it can be a delicate opperation when you decide to use manual and vice versa. Having a long throw will slow focus speed but makes manual focussing more precise.
10-10-2012, 03:09 PM   #13
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I think I'm sold on the 35mm f/2.4. Thanks
10-10-2012, 04:51 PM   #14
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There is a FA 35 f2.0 lens available used as well, would be FF proof.
https://www.pentaxforums.com/forums/photographic-equipment-sale/201546-sale-f...5-l-plate.html

Last edited by Franky2step; 10-10-2012 at 04:53 PM. Reason: added info
10-10-2012, 05:58 PM   #15
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If you need something fast, the Sigma 30mm f/1.4 is another option.
It can produce stellar results, and is really enjoyable to use.
I sometimes see them going cheap on the used market.
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