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03-27-2014, 07:25 AM   #1
Jordan P.

Manual Lens Assistance?

So first off, I want to apologize if this is very noobish, but I'm new to this whole manual lens thing. I've tried to figure this out on my own, but I've given up and am looking for some help.

So, I'm using a Pentax K-x with an Asahi 50mm f1.7 manual lens, and I'm trying to figure this thing out. I'm shooting in video mode, and it's frustrating me. If I set the f-stop to 1.7, the footage is clear, but the depth of field is overwhelming and ruins the shot, but if I turn it up to something that works for the shot, the footage gets way too noisy and grainy. How can I avoid this? I've been told that changing the ISO should fix that, but I'm not sure what I want to set it to?

Ideally, the shot should have a small amount of DOF, but not too much, probably around f14 or so. Can anyone possibly give me a solution?

Thanks in advance!

03-27-2014, 07:45 AM   #2
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Jordan, your issue has to do with lighting. You get too noisy and grainy because there is not enough light when you "stop down" your 1.7 lens.
You don't really get to set and change ISO settings in movie mode so really the big deal is coming up with more light to do the video. As far as the distance of focus - the 1.7 is pretty narrow, but f14 is way too deep (I suspect). F14 you will need daylight to do anything worthwhile. I am thinking you probably meant f/4. Do yourself a favor and go to take a picture with the lens and of the same object and change the Aperture everytime and see what you need. Also notice how the ISO changes and the noise changes during that time. This will give you a good idea of what you want in a picture, but the bottom line - graininess and noise is directly related to the absence of light. Stop it down to get more distance of focus and you lose light and add noise.

To use the Asahi 1.7 (I assume the M version) you will need to put your (using aperture ring) setting to 2.
03-27-2014, 09:05 AM   #3
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QuoteOriginally posted by First Poster Quote
Ideally, the shot should have a small amount of DOF, but not too much, probably around f14 or so.
DOF works the other way around. F/1.7 will be a very small depth of focus, f/14 will be a lot. f/1.7 will let in a lot of light, f/14 a very small amount. I'm not familiar with movie mode, never use video, so I cannot help you there but it sounds as if the camera is adjusting the ISO to maintain the same amount of light when you change the aperture. For stills I would turn off auto-ISO and set a fixed number. Not sure you can do that in movie mode or not.

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