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landulf

True focal length??

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Im new to this and have a question. If I shot with a anamorphic 70 mm lens and a 85 mm taking lens, then whats the true focal length? If I shot with only a 50 mm then its 50 mm, but here its easy to get confused.

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It should be the roughly the focal length of the taking lens divided by the stretch factor of the anamorphic.

 

An anamorphic lens doesn't have a focal length unless it has a built in taking lens so I have no Idea how you can have a 85mm lens attached to a 70mm anamorphic.

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(Lens focal length x Sensor crop factor) / Anamorphic stretch factor

 

For example, I use a 2X anamorphic with a 50mm lens on the GH3 (sensor crop factor 2X)

 

(50mm x 2) / 2 = 50mm

 

So I actually get a 50mm image with the GH3. Correct?

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So I actually get a 50mm image with the GH3. Correct?

 

Yes, that would be the field of view (horizontally). However, this can actually change slightly if you have a lens that breathes. For instance, a close up shot might offer a true 2x stretch, but focusing to infinity with the same lens may give you a 1.7x stretch or a 1.8x stretch, and that can affect your final field of view. But this formula is good for a ballpark number.

 

So if I use my sankor 16D x2 with a nikkor 85 mm I get 85x2=170. 170/2=85. So is 85 correct?

 

If you are using a camera with a 2x crop sensor, then yes. GH3 and 2x lenses are always the easiest to calculate.

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forget sensors in the equation IMO.  

 

the focal length of your lens remains what ever the taking lens is.  So any anamorphic lens in front of a 85mm is still a 85mm.

 

fov on the vertical remains the same as when you use the 85mm lens without the anamorphot.  

fov on the horizontal depends on the squeeze ratio.  Yours being a 2x anamorphic will result in a horizontal fov of a 42mm lens when used on your 85mm taking lens.

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factoring in your full frame sensor...

 

your 85mm will give you the same fov as a 50mm lens on a super 35mm sensor.  

 

thus..  with the anamorphic lens and a 3.55:1 ratio you'll see the 50mm vertical fov most would expect from a s35mm sensor, and a horizontal fov of 25mm in the same terms.    if you crop to 2.40 the horizontal fov will be the same as what you would expect from a non anamorphic lens of roughly 35-40mm in focal length

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