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Old June 10th, 2008, 09:22 PM   #1
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What is this circular background blur, home made DOF Adapter

Hi all

I have recently completed my first prototype DOF adapter. Utilising a vibrating wax GG.

I am just curious about a slight problem I am noticing which is beyond my knowledge of lenses.

It is attached and appears as a circular blurring of the out of focus background? It doesn't appear to affect the focused portions of the image and isnt following the vibrating pattern either which is almost vertical.

Only thing I can think of is the GG is too transparent?? not sure.
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What is this circular background blur, home made DOF Adapter-dof-circular-blur-example.jpg  
Craig Turner is offline   Reply With Quote
Old June 11th, 2008, 01:17 AM   #2
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It's called "vortex" or something to the sort.
I believe it can be resolved by using a different achromat.

The apefos had a similar problem. I'm almost sure it was the achromat or the diffusion screen that changed it.
It basically looks like massive barrel distortion.
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Old June 11th, 2008, 03:18 AM   #3
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I experienced the same problem with my DIY adapter. Like this: http://www.pettersen-prod.com/35mm%2...web/still4.jpg

I had great luck in changing to a different, larger achromat that I made out of two binocular front elements. The "Vortex" dissapeared completely. The GG was not changed.
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Old June 11th, 2008, 05:31 AM   #4
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ohhh ok,

I'm not actually using an achromat at all as the camera could focus on the focus screen without one.

is this a secondary reason for using a achromat? am i correct in it's main purpose, to reduce the minimum focal distance of the video camera?

might have to try and make one.

cheers
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Old June 11th, 2008, 07:29 AM   #5
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It really depends on what camera you use. I have heard that the HV20, for example, can be used without and achromat. However, It exhibits a lot more barrel distortion.

Here is the tutorial I followed when I made my own achro: http://www.jetsetmodels.info/pics/achromat_tutorial.pdf
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