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12-20-2010, 07:32 AM   #1
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Shallow DOF techniques??

Hi all I'm having a hard time getting shallow DOF with my pentax k-x. i'm speaking mostly about video.
Im using a 50mm 2,0 pentax M lens. even when i us the kit lens the dof is still not shallow.
Here is a opening sequence to a music video i'm working on.



<object width="640" height="385"><param name="movie" value="http://www.youtube.com/v/_bGaqps7FwQ?fs=1&amp;hl=en_US"></param><param name="allowFullScreen" value="true"></param><param name="allowscriptaccess" value="always"></param><embed src="http://www.youtube.com/v/_bGaqps7FwQ?fs=1&amp;hl=en_US" type="application/x-shockwave-flash" allowscriptaccess="always" allowfullscreen="true" width="640" height="385"></embed></object>

i was trying to moderately blur the background. i was about 5 feet from the characters and i believe the lens was set to f8. i'm not sure about the shutter speed.
if i set the lens to f5.6 or f4 everything is to bright.
is the live view supposed to show me the blur that will occur. what am i doing wrong?
thanks

12-20-2010, 08:53 AM   #2
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The opening line in your video says "This is a private video".

I don't do much video so this probably won't be much help but...

Shallow DOF is a function of aperture, distance from lens, and actual focal point. If you are far away from your focal point for instance, you might get a light blur outside of the range. If you are really Close to your focal point, then everything But the focal point will be blurred.

55mm f1.8 SMC Takumar - a set on Flickr

SMC Takumar 55mm f1.8, mostly wide open.

12-20-2010, 08:44 PM   #3
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here is a link to the video on vimeo

12-21-2010, 06:17 AM   #4
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QuoteOriginally posted by pwrlogic Quote
....
i was trying to moderately blur the background. i was about 5 feet from the characters and i believe the lens was set to f8. i'm not sure about the shutter speed.
if i set the lens to f5.6 or f4 everything is to bright.
....
For shallow DOF you need low f-stop. You say if you decrease the f-stop the image is too bright, therefore you must either shoot in a darker place or filter out some of the light. (I don't know if there are any camera adjustments you can make to extend the ISO range it uses....the manual isn't clear on that.)

You can filter out some light with a Neutral Density Filter (ND). You can buy a set of ND filters or make an adjustable one by stacking a linear polarizing filter atop a circular polarizing filter; as you rotate the filters with respect to each other the light transmitted varies over a wide range (1/2->1/500 or more).

Dave

PS Another trick is to use a longer lens to help isolate the subject; this doesn't really change the DOF but objects in the background look larger, hence look more out-of-focus - since there are fewer objects, they can be less distracting.

Here's an example - look at the photos on the left - same DOF but different lens length.


12-21-2010, 07:04 AM   #5
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hey thank you for commenting and your examples were great. I would have at least liked to get the blur of the 28mm at f4 but i did not even get that.
If it is possible to see the blur in live view that would help.
i do know i must get the nd filter and longer lenses as you say.
the video looks good but just not the look i was trying to get.
thanks again
12-21-2010, 07:43 AM   #6
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QuoteOriginally posted by pwrlogic Quote
hey thank you for commenting and your examples were great. I would have at least liked to get the blur of the 28mm at f4 but i did not even get that.
If it is possible to see the blur in live view that would help.
i do know i must get the nd filter and longer lenses as you say.
the video looks good but just not the look i was trying to get.
thanks again
It is difficult to see the blur in live view (it is difficult to see blur on any small display or print.)

I think you might prefer the polarizer approach as it is adjustable on the camera and might even be used for fades.

The example shown was for a 35mm camera; to get the same DOF would require an f:2.8 or so lens on your Pentax.

Dave

PS the practical trouble you'll have with shallow DOF is maintaining focus as the actors move.

Last edited by newarts; 12-21-2010 at 07:48 AM.
12-21-2010, 11:07 AM   #7
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QuoteOriginally posted by pwrlogic Quote
hey thank you for commenting and your examples were great. I would have at least liked to get the blur of the 28mm at f4 but i did not even get that.
If it is possible to see the blur in live view that would help.
i do know i must get the nd filter and longer lenses as you say.
the video looks good but just not the look i was trying to get.
thanks again
I don't know anything about k-x video, but you should be able to get some of the blur you want by forcing your aperture to f2 or f2.8 and letting the auto-ISO deal with the "too much light". Try it and see? With a 50mm lens it should be possible.
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