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02-11-2013, 09:23 AM   #1
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Hello pentaxians,
So now I have decided to go with the K30. The next decision is the lens......I would like to capture faces in low light, so after being completely confused by salesmen trying to sell me a 600 lens, even though he really wanted to sell me a nikon as that was his preference and it was end of line stock he needed to sell to get the next nikon of his choice on the shelf....helpful eh ? I need a straight forward answer please. One thing I got from him was the k30 standard lens would not give me the f-point to capture a good picture in low light, the lens is the standard 18-55 WR. He told me the nikon had a f1.8 standard tat would allow for very good low light pictures, now call me logical, but if nikon make a f1.8 to capture low light pictures, there must be a Pentax equivalent ? If so, which lens would be best to capture fast moving faces in low light scenarios ? Thank you in advance

02-11-2013, 09:45 AM   #2
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Yep, they do. There's the DA 50 f/1.8, or if you want something a bit faster you can go for the for the slightly older but still excellent FA 50 f/1.4.

You can find the FA 50 used for about the same price as a new copy of the DA if the extra 2/3 of a stop is important to you.
02-11-2013, 10:08 AM   #3
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Congrats on your purchase!
Here is a list of these lenses:
SMC Pentax-FA 50mm F1.4 Reviews - FA Prime Lenses - Pentax Lens Reviews & Lens Database
SMC Pentax-DA 50mm F1.8 Reviews - DA Prime Lenses - Pentax Lens Reviews & Lens Database
And the slightly more expensive, but really great for portraits:
SMC Pentax-DA* 55mm F1.4 SDM Reviews - DA Prime Lenses - Pentax Lens Reviews & Lens Database
SMC Pentax-FA 43mm F1.9 Limited Reviews - FA Prime Lenses - Pentax Lens Reviews & Lens Database

You can also buy a used 50mm film-era lens, like the K, M or A series, but it will be manual. If you can work with that, you can save some money and get a lens with IQ almost as good. (still much better than kit lens) Third party brands also make some fast primes for Pentax, but I think most users prefer to buy a Pentax equivalent, if available.
02-11-2013, 11:15 AM   #4
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The FA50 1.4 gets my vote, it has a few more features that I find useful (distance scale, depth of field scale, aperture ring) over the DA50 1. That being said the DA50 has slightly better contrast and flare suppression, but that's probably not a big concern if you're shooting in low light. You could even get an old screwmount Takumar. I have one from 1964 that I just love, but it is pretty soft when shot wide open. Super-Takumar 50mm F1.4 (Early 8-Element Variant) Reviews - M42 Screwmount Normal Primes - Pentax Lens Reviews & Lens Database The other advantage of a 1.4 over a 1.8 or something higher is it's easier to focus, both for you and for the camera's AF sensor. The wider aperture make it very apparent when things are out of focus.

But in very low light, even a 1.4 lens may be too slow, and you'll be stuck shooting wide open which has an effect on sharpness, so once you buy a lens and have had some time to get a feel for it, you might consider purchasing an external flash gun. The pop-up flash will probably give you all the power you would need at f/1.4, but the light is so ugly, in both direction and quality, not to mention that you subjects will probably have red eyes quite frequently. An external flash that you can bounce off the ceiling or a wall will give you much nicer light. If you know you're only going to be using it indoors in low light, something inexpensive and compact like the Pentax 280T might work, but you might consider spending a few extra pounds to get something that will give you the ability to work outdoors in backlit situations (i.e. more powerful). I don't own one but I hear good things about the Yongnuo YN560II, just be aware that most of the budget flash guns will limit you to manual flash, where you have to set the power level. It's very easy and consistent once you know how.

02-12-2013, 05:57 AM   #5
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QuoteOriginally posted by maxfield_photo Quote
The other advantage of a 1.4 over a 1.8 or something higher is it's easier to focus, both for you and for the camera's AF sensor. The wider aperture make it very apparent when things are out of focus.
Actually I don't think that's correct. As far as I can tell from the forums the AF sensors on the K-30 are all rated at f5.6, which means the AF system won't have any advantage in terms of focus accuracy from using a faster lens (for PDAF).

The optical viewfinder itself has an effective aperture which is something more than f1.8, which makes it tricky to accurately focus by eye with wide-aperture lenses. I've certainly found this with an old manual focus 50/1.7 on the K-30. Apparently this is because the focusing screen in modern SLRs is optimised for brightness at the expense of sharpness, so replacing the screen with an old-fashioned ground-glass screen designed for manual focusing helps, but then you get a darker image in the VF with slower lenses. I haven't tried this, but it seems to be the accepted wisdom.

Liveview with focus peaking works well, but even then there's a catch. I noticed when trying this on my K-30 with the 50/1.7 that when you use LV with the lens on auto it stops it down to about f4 by default when you select LV. If your lens has an aperture ring you can set to to wide open in LV, but then you have to use stop-down metering.

...all of which is probably rather technical and not much use to the OP, but I thought I'd chip in since I've become interested in this after acquiring the manual-focus 50/1.7 :-)
02-12-2013, 08:50 AM   #6
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QuoteOriginally posted by speagles2 Quote
Actually I don't think that's correct. As far as I can tell from the forums the AF sensors on the K-30 are all rated at f5.6, which means the AF system won't have any advantage in terms of focus accuracy from using a faster lens (for PDAF).
I believe, though I may be mistaken, that the f/5.6 rating refers to the maximum aperture of the lens or lens teleconverter combo at which the camera retains full use of it's cross type sensors. I'll use the Nikon D800 as an example, since they seem to tout this feature. With an f/5.6 or faster lens, 15 cross type sensors are available, with an f/8 lens that drops to 9 cross type sensors, and with anything slower than f/8 only the center point functions as a cross type. I believe the K30 is similar though perhaps not as well documented, but I can say with certainty that every AF camera body I've ever used works more effectively with larger aperture lenses, particularly in low light.

You're right about the focusing screen though, I tend to forget because I replaced mine with a ground glass screen. With a ground glass screen, particularly a snazzy one with a split prism and a micro-prism collar, the difference between 1.4 and 1.8 is noticeable, the subject just snaps into focus. So with the stock screen your rendition of the scene will be somewhere around f/5.6 regardless of the maximum aperture of the lens, but remember the AF sensors are located down in the well of the camera and are unaffected by the focusing screen, they will get a much truer rendition of f/1.4.

The liveview focus peeking thing seems like it might be some sort of Program line behavior, like maybe you have it set to normal or MTF (sharpness) priority, I can't speak to it as I don't own the K30. You might go into your custom menu and check your program line priority, and if it's not already, try setting it to Aperture priority.
02-13-2013, 07:06 AM   #7
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There's a lovely animation of PDAF here: Autofocus: phase detection

I think there are two things going on here. As I understand it (which is not very well), a simple (say, vertical line type) AF point comprises two sensors, one that just sees rays passing through the very top part of the lens, and one that only sees rays passing through the very bottom. For an object that's in focus, all its rays converge on the same point regardless of where they hit the lens, and the two sensors see the same thing. For an object that's out of focus, rays that pass through the very top of the lens end up arriving at a different place than the rays that pass through the bottom part of the lens, and the sensors see different things, which is how the camera determines that the object is out of focus. The physical distance between the sensors depends on the size of the lens (the distance between 'very top' and 'very bottom'), i.e. the aperture. The bigger the aperture the further the sensors can be apart, and the further apart they are the more they can detect small changes in focus. So for accurate (as in precise) focus we want them to be as far apart as possible. f/2.8 AF points have their sensors further apart than f/5.6 ones and so can detect smaller differences in focus. The downside is that when a slower lens is used (smaller aperture than f/2.8) there is no light coming in at the 'very top' and 'very bottom' that the f/2.8 sensors are looking at because the hole isn't that big. It's dark out there. Hence the mix of sensor types - some widely-spaced ones to enable more accurate focus when the lens aperture is wide enough to use them, closer-together ones that will work with slower lenses. As far as I can tell, the K-30 has only one type, which rumour has it are rated for f/5.6, but that may not be right - certainly my kit zoom focuses fine at max telephoto and that's only f/6.3.

So when I said the K-30 wouldn't have any advantage in focus accuracy from using a faster lens, that's what I was thinking of. However, as you pointed out, that's not right, because having a faster lens in low light is always a good thing. The sensors are only sensitive down to a particular light level, so the AF is going to work better in low light with a faster lens regardless of whether the sensors are f/5.6 or f/8.

Fnally, on the liveview thing, it's definitely nothing to do with exposure mode - on the f/1.7 it stops down the lens part-way as soon as LV is selected, regardless of aperture selected on the camera or the ambient light level. I don't know why it does this, because it does seem accurate enough to deal with the shallower DOF when the lens is set manually wide open - the depth of stuff highlighted as in focus by the focus peaking is definitely less when I do this, and so it's possible to focus more accurately.

Apologies to the original poster for hijacking this thread - if I knew how to drive pentaxforums I'd suggest starting a new one for further AF-at-wide-aperture discussions...

Last edited by speagles2; 02-13-2013 at 07:19 AM.
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