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Understanding Back Focus tweaking.

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#1 scadvice

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Posted 28 March 2020 - 12:03 AM

Basically I'm trying understand more about back focus. When tweaking a calculated back focus...  can you use a Siemans Star? If you can, do you need to determined the number of spokes for your camera's pixels?

 

Yes... I have entirely too much time on my hands lately...



#2 Tapio

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Posted 28 March 2020 - 01:59 AM

What is a Siemans Star?

#3 sg6

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Posted 28 March 2020 - 03:09 AM

Back focus is simply a distance or separation.

The distance/separation between the rear face of the Reducer/Flattener and the sensor has to be X.

All the "tweeking" does is set that distance.



#4 ZL4PLM

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Posted 28 March 2020 - 03:36 AM

not really - as the stars might be fine in the centre image but out at the edges 

 

best to look at the whole field


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#5 scadvice

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Posted 28 March 2020 - 12:32 PM

Siemens Star charts are use to adjust BF on pro video cameras.

 

SG6 your point is well understood.

 

ZL4PLM I think your right, that makes sense.

 

I was just looking for better daylight method to tweak BF after setting. After putting together the adapers, spacers, and EFW...adding the filter space 1/3 you still will have a 'stacking error' usually making the BF slightly too long. So you end up taking sample pictures looking at the corners for flaring and shimming back and forth to get the final adjustment.

 

It seems to me there was a post here on making a device to check and adjust (tweak) BF but I can't find it now. 



#6 Tapio

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Posted 28 March 2020 - 01:00 PM

Basically the common method for BF tweaking (with flattener/reducer) is to look at edge stars.
If the stars look like comets pointing center then distance is too short. If the stars look like radially distorted then distance is too long.

Edited my answer.

Edited by Tapio, 29 March 2020 - 02:37 AM.


#7 raguramm

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Posted 28 March 2020 - 01:02 PM

Tapio, I think you said Short twice. Could you clarify?

#8 Der_Pit

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Posted 28 March 2020 - 05:39 PM

If the stars stretch radially out you have to increase the distance between corrector and sensor.  If they go around looking like concentric rings you have to decrease the distance.




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