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11-20-2009, 08:21 PM   #1
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Pentax k-x newbie question.


First wanted to say thanks to everyone who posts in these forums. I did quite a bit of reading on here before making a desc. to buy the K-X.

I'll be taking this to camera to a variety of tradeshows and conventions and would love to use its video functionality to record interviews ath vendor booths, or even in the hotel lobbies. I tested out both of the kit lenses (18-55, 55-300) in video mode, and outside I am pretty pleased with how it looks. Inside due to the lower lighting levels (i think) everything is just a bit too grainy for my liking. I improved things somewhat by adjusting and locking the apeture and shutter speed, but its still not where I want it to be.

From my reading, I think I need a faster lens, and was looking for some suggestions as to what might work best. In almost all cases the Camera will be fixed on a tripod with a stationary subject who will prob. be fairly close to the camera.

I see several of this lens, PENTAX-M 50mm 1:1.7 and was wondering if it would help out my low light video performance, or see if anyone had any other suggestions in the sub $200 range?



11-20-2009, 10:28 PM   #2
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Hi Tim,

I am also K-x owner now 18-55/55-300 kit. My opinion is.. K-x might not be well suited to record interviews etc. Couple of reasons.

1. I think the files are huge as they are recorded as motion jpeg avi.
2. No external microphone- even though internal mic phone is good, not sure how good it can capture a conversation.
3. If you are on budget and planning to use prime lenses, I think manual focusing may be a limitation if you need to switch between different subjects.. like people and products etc.

So I feel you are better off with a budget camcorder, as lengthy videos, good audio, quick focusing are your primary requirements.

Once you delegate that video part to a camcorder, you can use K-x for taking great photos.
11-20-2009, 10:54 PM   #3

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FA 50mm f1.4 is the king of value for money... Its twice your budget but it really needs to be on your camera where it will likely stay for a long time.

Id recommend selling the 18-55mm to help
11-20-2009, 10:57 PM   #4
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Well, FA50/1.4 is value king if you need AF, but if you can do without, the M50/1.7 is a *far* better value. does AF even work in video?

11-20-2009, 11:03 PM   #5
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Well the audio really isnt a concern I have a stereo zoom recorder I've been using for quite awhile now which works really well and I would continue to use that with this setup.

For the products I figure I would use the 55-300, then if i want to do a quick sitdown (10-15 mins top). I would setup the tripod and pop out the other lens.
11-20-2009, 11:11 PM   #6
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But wide open with your subject won't get focus. You can try it but is there some portable lighting that would be better...?
11-20-2009, 11:30 PM   #7
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Well as far as I can tell there is not AF with video anyway. Basically Im thinking I'll do some quick coverage of the products with the 55-300. I don't think anyone will be moving around too much. They will either be sitting or standing in front of the show floor. I won't be on camera. Just getting 15 min of them talking. In the end I'll be taking 2-3 min of that, plus product coverage with voiceover from myself for the finished product.
11-21-2009, 12:24 AM   #8
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Approximately you get 4min video per 1GB. So you will get about 1hr video per 16GB. It probably is ok.

11-21-2009, 11:31 AM   #9
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QuoteOriginally posted by Marc Sabatella Quote
Well, FA50/1.4 is value king if you need AF, but if you can do without, the M50/1.7 is a *far* better value. does AF even work in video?
No, AF does not work in video mode, and with the mirror locked up, you have to look at the LCD screen when filming. Keeping the lens (my Tamron 18-250) near wide angle will keep video sharp, but if you move to toward telephoto, you might have to adjust the distance manually, make sure you're in MF.
11-21-2009, 01:46 PM   #10

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For general use around the floor and product coverage, your existing 18-55 would probably work OK. It can do surprisingly well in not-great light, and by your description of the task, it sounds like most of your subjects won't be too far away. But if funds were not so limited, probably the ideal lens for you would be a constant aperture and bright zoom like a 18-50 f2.8 from either Pentax, Tamron or Sigma, or even something a tad longer like the Tamron or Sigma 28-75mm f2.8. You may be able to get a cheap one second hand. Rental may even be an option too, if you can find someone who rents K-mount lenses.

I'm not sure how the 55-300mm would work, given that it (f4-5.8) can be a bit dark. However there are some pictures in this forum of a guy who used the 55-300mm on a K-x with some success in very dim light indoors at a wedding, so on a convention or tradeshow floor, where the lighting is usually good, it could work better. I also wonder with the 55-300mm whether you will use any focal lengths past about 55mm, given the tasks you have outlined for yourself.

Another issue to perhaps think about is depth of field. If you are going to be interviewing someone and you (a) don't want to be manually focussing the lens at all during the shot and (b) you want the person's face to stay in focus (and not just their nose for example), you will want to plan your shot to achieve a generous depth of field around your subject's face, so that as they move around within the shot, they stay in focus. Achieving this may have an impact on the type of lens you use and the aperture you use, as well as your calculations about ideal subject to distance for interview purposes.

While 'fast 50's like the M 50/1.7 or FA 50/1.4 are great low light lenses (I have both myself), in general the higher the focal length you go or the smaller the aperture you use over a given distance with a lens, the narrower depth of field you get.

Eg using the FA 50/1.4 at f1.4 and focussing on a subject 2 metres away you will have a rather narrow depth of field of about 10cm, which may mean if the subject moves a bit during the shot parts of their face may move out of focus. However if you keep the same distance but stop down to about f4 with the FA 50 you'll get about 25cm DOF, which may be much more usable.

Alternatively, if you went with a shorter focal length lens (eg a 28mm) for the same distance at f1.4 you would have a very usable DOF of 28cm, at f2.8 a generous DOF of 53cm and at f4 a huge DOF of about 84cm.

However using a lens with a focal length of 200mm, even at f8 you'd only get a DOF of about 2cm for a subject 2m away, which would mean keeping the subject in focus would be really really hard.

So in general, to get the best DOF, when lighting conditions aren't ideal, it's probably best to work with shorter focal length lenses, rather than just longer lenses with faster apertures.

This is a great resource for making DOF calculations, btw:
Depth of Field Table

In making your assessment of which lens to use, as well as DOF, of course you will also want to consider the kind of framing you want to achieve. To do a head and shoulders interview of someone with a 28mm and fill the frame will require a different subject to camera distance than a 50mm. Etc. So it's a balancing act, between getting the framing you want and achieving the DOF you need.

If you can, I'd do a few simple 'rehearsals' of some interviews, experimenting with different distances, lens combinations and lighting conditions in order to arrive at a working arrangement for a standard interview setup in terms of lens, aperture and subject distance, as well as a standard 'product shot' setup.

I'd also be packing lots of fast Sandisk Ultra memory cards and several packs of Energizer Lithium batteries.

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