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Relative F stop?


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#1 peter orland

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Posted 18 October 2006 - 06:51 PM

What would the relative f stop have to be on a 3 X 1/3" CCD (HVX200) in relation to 35mm to achieve the same apparent depth of field? If the 1/3" is set at f1.6 what would that equate to in 35mm?

I believe for a 2/3" CCD it is around 2 1/2 stops difference.

Thanks
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#2 peter orland

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Posted 19 October 2006 - 08:47 PM

92 hits and no one knows?

I think it might be 7-1/2 stops. Anyone care to concur?
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#3 David Mullen ASC

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Posted 19 October 2006 - 10:13 PM

I'd guess 5 stops?
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#4 Matt Sandstrom

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Posted 20 October 2006 - 05:15 AM

i'm guessing it's two stops per half image size, since two stops is a doubling of the aperture diameter, which is really what's determining depth of field. so 4 1/2 then?

but does this really work? while you get the same depth of field it won't look the same, would it? i have a feeling the background gets more and more out of focus as the focal length increases, even if dof is maintained. what i'm saying is if you're trying to create or match a look it's not the dof itself that determines it but the amount of fg/bg blur.

/matt
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#5 peter orland

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Posted 20 October 2006 - 04:18 PM

My reasoning behind 7 1/2 stops is because I thought that the DOF is based (basicaly) on the aperture, distance from object, capturing frame size, and focal length of the lense. If the relative F stop changes 2 1/2 stops on a 2/3" ccd and this is attributed to the change in focal length of a 25mm prime becoming a 10mm, a 50mm becoming a 20mm etc... Then the 35mm equivalent of a 50mm lens on a 1/3" ccd is 6.5mm, around 7 1/2 (7.7) times, equating to 7 1/2 stops.
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#6 George White

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Posted 21 October 2006 - 01:41 PM

I did some calculations and used an on-line DoF calculator. I got 3 1/3 stops. Assumptions were:
coc for 35mm = .001 in (.025mm) , coc for 1/3 " CCD = .00025 in. (.006 mm) Then I took three data points (angle of view of about 50 degrees, focused at 5 feet away and 10 feet away, and angle of view of 15 degrees with object 25 feet away) for each point I calculated the 35mm focal length and the coresponding 1/3" CCD focal length and then used www.dofmaster.com/dofjs.html to find a pair of f stops that gave the same DoF. I used 1/1.4 for the 1/3" and at all three examples the closest 35mm f-stop was 4.5. I could not get the data to line up in a table but this should show my assumptions and method.

format 36mm : 1/3" CCD

coc: .025 mm | .006 mm
angle of view: 49 degrees | 49 degrees
focal length: 30 mm | 8.2 mm
f-stop: 4.5 | 1.4
subject distance: 10 feet | 10 feet
near focus point: 7.23 feet | 7.27 feet
far focus point: 16.2 feet | 16 feet

---george
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#7 George White

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Posted 21 October 2006 - 01:58 PM

(1/1.4 should be f/1.4)
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#8 David Mullen ASC

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Posted 21 October 2006 - 07:18 PM

(1/1.4 should be f/1.4)


What would be the results if you used the same CoC figure for both formats?
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#9 Matt Sandstrom

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Posted 21 October 2006 - 08:43 PM

i'm fairly sure my logic would work fine in that case, i.e two stops for each halving of the image size. i don't know the exact image size of either 35mm or 1/3" though, so all i could do was calculate the difference between 2/3" and 1/3" and extrapolate.

/matt
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#10 George White

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Posted 22 October 2006 - 12:09 AM

What would be the results if you used the same CoC figure for both formats?


I do not see a paticular rational for running that case, but there is an argument for increasing the 1/3" format by a factor of 2 (making it about 1/2 of the 35mm coc rather than 1/4). The rational would be that material shot in that format was likely intended to be viewed on a TV at home rather than the theatrical experience the 35mm was aimed at. The intended viewer's angle of view is a factor in coc. A bigger image (to the viewer) has to be better focused to be in-focus than a smaller image. If the total angle of view of typical home viewing was about 1/2 that of typical theatrical viewing then dthis oubling of the coc would be valid. Using .012 mm rather that .006 mm for the 1/3" format coc produced a 5 1/3 stop (f/1.4 --> f/9) difference. I would still say that the 3 1/3 calculation above (assuming no error on my part) would be the right answer if both images were intended for the same ultimate viewing angle.

(I thought I already posted this info, but do not see it so I'm trying again --sorry if it appears twice)

---george
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#11 Stephen Williams

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Posted 22 October 2006 - 04:54 AM

Hi,

Adding detail to the HVX 200 will increase the apparent DOF in a wider shot.
I would say that the theoretical Difference is in the 5-6 stop range, and the apparent difference can be rather more.

Stephen
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