How does the Speed Boaster reduce depth of field?

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akshaymadan

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How does the Speed Boaster reduce depth of field?

PostMon Feb 10, 2014 2:36 pm

Is it that the speed boaster reduces the depth of field using its own optics.
OR
Does it seem that depth of field has increased. Because the angle of view increases, so to get the same frame as a one without a speed boaster you now need a longer focal length lens. And the aperture of a longer focal length lens opens wider.

If the second statement is not the reason, can someone please explain the optics of the speed boaster, or share some link which'll help me understand. Thanks.
Akshay Madan
New Delhi, India
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Marc Tschudi

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Re: How does the Speed Boaster reduce depth of field?

PostMon Feb 10, 2014 3:33 pm

The Speedbooster does not affect depth of field.
http://www.metabones.com/article/of/faq
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Kyle Gordon

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Re: How does the Speed Boaster reduce depth of field?

PostMon Feb 10, 2014 3:55 pm

Marc is mathematically correct, but under that way of thinking, a 5D mkiii does not have any different DOF than a 7D, and we all know that in practice that is not true. You would not compare a head and shoulders shot without a speedbooster to the same physical setup with a speedbooster that now includes most of the torso, you'd move the camera closer to the subject to make the same framing, and in that case, there would be a definite difference in depth of field between the two images.

So yes and no are both the right answer in this case.
Kyle Gordon
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Mac Jaeger

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Re: How does the Speed Boaster reduce depth of field?

PostMon Feb 10, 2014 4:08 pm

Think of the speedbooster as a device that magically enlarges your sensor while everything else stays the same - then all conclusions are right.
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Denny Smith

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Re: How does the Speed Boaster reduce depth of field?

PostMon Feb 10, 2014 5:58 pm

The speed booster does not change the sensor size, it changes the image circle of the lens projected onto the sensor, giving a resulting change in FOV (field of view) too. By making the image circle smaller, it concentrates the light coming through the lens, resulting in a shift to brighter image being projected.
Denny Smith
SHA Productions
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DaleCampbellFilms

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Re: How does the Speed Boaster reduce depth of field?

PostMon Feb 10, 2014 7:17 pm

By focusing more of a lenses image circle onto the sensor it has the effect of giving a wider field of view.

So when framing a shot with/without the speed booster you would need to be closer to your subject to get it to fit in the frame in a similar way.
For example a head and shoulders shot, so without the speed booster you have it all nicely framed just head and shoulders in shot.
Add the speed booster and suddenly you have more in the frame, to get back to the head and shoulders you move closer to the subject or vice-versa.
Now your back to where you started, but you are closer to the subject so when you bring them into focus this is where the effective depth of field appears shallower.

Remember if you stay in the same place and don't re-frame the depth of field is identical, your just cropping into the centre of the glass.

Its what the speed booster encourages you to do that results in a shallower DoF.
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Mac Jaeger

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Re: How does the Speed Boaster reduce depth of field?

PostMon Feb 10, 2014 7:22 pm

Denny wrote:The speed booster does not change the sensor size, it changes the image circle of the lens projected onto the sensor, giving a resulting change in FOV (field of view) too. By making the image circle smaller, it concentrates the light coming through the lens, resulting in a shift to brighter image being projected.

Of course it does, that's a given. But people always get confused about the visual consequences because they tend to thik to complicated. But if you imagine the pocket cam + pocket speedbooster to be a aps-c camera, then you can use all your experience with this sensor-size and won't go much wrong...
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LoganStewartDP

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Re: How does the Speed Boaster reduce depth of field?

PostMon Feb 17, 2014 4:04 pm

did this test a couple months ago comparing a 50 on both an aps-c camera and the blackmagic with speedbooster.
Logan Stewart DP
Variables Of Light LLC
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Tim Callaghan

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Re: How does the Speed Boaster reduce depth of field?

PostMon Feb 17, 2014 4:17 pm

Depth of field is relative to subject distance (distance from focal plane to subject). Speedbooster moves you closer for a given subject size (size relative to the image frame).

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