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DIY find fresnel focal length??

Started Apr 6, 2016 | Discussions
CardBoardBoxProcessor Regular Member • Posts: 328
DIY find fresnel focal length??

I used my better beamer and it was great.  So I want  to  make  my own for  my sigma 150-600mm S to make up for its poor appeature of 6.3 at 600mm.

Questions:

do I want  the focal length to be adjusted for the bulb itself or the front of the flash?

How do I figure out my fresnel's focal length t?

My theory is that  I open a window and perfect the image on a wall.  Where ever distant objects are most clear is infinity focus. Is that correct?. Refer to image.

ibiza123 Regular Member • Posts: 280
Re: DIY find fresnel focal length??

CardBoardBoxProcessor wrote:

I used my better beamer and it was great. So I want to make my own for my sigma 150-600mm S to make up for its poor appeature of 6.3 at 600mm.

Questions:

do I want the focal length to be adjusted for the bulb itself or the front of the flash?

How do I figure out my fresnel's focal length t?

My theory is that I open a window and perfect the image on a wall. Where ever distant objects are most clear is infinity focus. Is that correct?. Refer to image.

You could also focus the Sun light on the sheet of paper and find the distance between  the lens and the paper while the image of the Sun is smallest. But be careful not to burn the house!

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OP CardBoardBoxProcessor Regular Member • Posts: 328
Re: DIY find fresnel focal length??

Any other tips?

And bulb or front?

AustinMN Contributing Member • Posts: 674
Re: DIY find fresnel focal length??

To accurately determine FL for a fresnel, I would use the moon.  The sun is too hot and will start a fire.  Make sure the paper or other stock is parallel to the moon (a perpendicular line would point directly at the moon).  The fresnel needs to be parallel to the focus paper as well.

For your other question, I'm really not sure if you want to focus the front of the flash or the bulb.  If you use the front of the flash, then you probably don't want to focus it sharply.  If you do, thou will just end up projecting the pattern of the front of the flash with it's lines and patterns onto the subject.

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OP CardBoardBoxProcessor Regular Member • Posts: 328
Re: DIY find fresnel focal length??

Hmm. What flash power I want?  I've got  the  newer Yongnuo. Does it have  the  out put to bird?

OP CardBoardBoxProcessor Regular Member • Posts: 328
Re: DIY find fresnel focal length??

Made the thing  tips for getting it perfectly aimed at what  the  lens is aimed at

Aussibum Forum Member • Posts: 55
Re: DIY find fresnel focal length??

CardBoardBoxProcessor wrote:

I used my better beamer and it was great. So I want to make my own for my sigma 150-600mm S to make up for its poor appeature of 6.3 at 600mm.

Questions:

do I want the focal length to be adjusted for the bulb itself or the front of the flash?

How do I figure out my fresnel's focal length t?

My theory is that I open a window and perfect the image on a wall. Where ever distant objects are most clear is infinity focus. Is that correct?. Refer to image.

Your method is absolutely correct. I use the same method.

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