Circle of Confusion Calculations and A1 ccd at DVinfo.net

Cal Bickford · February 1st, 2008, 02:01 PM

I'm trying to calculate a reasonable CoC to use in calculating hyperfocal distance, DOFmax, etc for the A1.

First- Does anyone know the actual ccd dimensions of the A1? I know traditional 1/3" chips are 4.8mmx3.6mm but I'm assuming that the A1 native 1440x1080 results in alternate dimensions.

Second- Am I correct in assuming that a very large CoC can be used for a 1/3" video chip relative to a 1/3" piece of film - as the video's limited resolution would become apparent much more readily than would any circles of confusion as the image is blown up?

If this is the case, how on earth can we decide on a reasonable diameter for CoC?

Don Palomaki · February 1st, 2008, 02:39 PM

You should be able to calculate the active portion chip size it from the field of view and focal length. Video pixels typically are not square. Keep in mind that film grain is random, CCD pixels have a fixed size and fixed location.

Last I knew, camcorders rate CCD size using the old tube convention (not actual physical size) to retain consistent lens performance expectation; i.e., a given focal length lens would produce the same FOV with a 1/3" tube- or 1/3" CCD-based camera.

Cal Bickford · February 1st, 2008, 08:04 PM

Ok so basically I'm assuming that the A1's vertical dimension will be identical to a traditional 1/3" chip and I'm assuming that the width of one pixel (vertically - as this is the shortest dimension anyways) would be the logical choice for max CoC.

Therefore: 3.6mm/1080 = 0.00013 INCHES

This is considerably less than the traditional CoCmax in film of .001 in. and even the more contemporary (due to refined technology) CoCmax of .0005 in.

In fact, a safe CoCmax would probably need to be even smaller than this to take into account the fact that each pixel is probably not actually .00013 in. across vertically due to chip space around the individual light sensitive bays and around the borders of the chip (I am only assuming this as I certainly have little knowledge of how an actual chip is engineered.)

So maybe a CoCmax of .00005 or .000075 in. is more appropriate.

Can anyone see anything wrong with my methodology here?

Cal Bickford · February 1st, 2008, 08:22 PM

ok so i crunched some quick #s and with a CoC of .000075in. and the A1 at full wide (4.5mm) and f4 the Hyperfocal distance would be 8.7 ft.

So if you wanted max DOF at this focal length and and fstop you would throw your critical focus at 8.7 ft and have everything in focus from about 4.35 ft to infinity.

This sounds like a pretty deep field even for video. Does this sound reasonable to you all? I'd go out and test it out myself right now if I wasn't crippled with the flu : (

February 1st, 2008, 02:01 PM	#1
Cal Bickford Regular Crew Join Date: Jan 2007 Location: Austin, TX Posts: 157	Circle of Confusion Calculations and A1 ccd I'm trying to calculate a reasonable CoC to use in calculating hyperfocal distance, DOFmax, etc for the A1. First- Does anyone know the actual ccd dimensions of the A1? I know traditional 1/3" chips are 4.8mmx3.6mm but I'm assuming that the A1 native 1440x1080 results in alternate dimensions. Second- Am I correct in assuming that a very large CoC can be used for a 1/3" video chip relative to a 1/3" piece of film - as the video's limited resolution would become apparent much more readily than would any circles of confusion as the image is blown up? If this is the case, how on earth can we decide on a reasonable diameter for CoC?

February 1st, 2008, 02:39 PM	#2
Don Palomaki Wrangler Join Date: Sep 2001 Location: Northern VA Posts: 4,488	You should be able to calculate the active portion chip size it from the field of view and focal length. Video pixels typically are not square. Keep in mind that film grain is random, CCD pixels have a fixed size and fixed location. Last I knew, camcorders rate CCD size using the old tube convention (not actual physical size) to retain consistent lens performance expectation; i.e., a given focal length lens would produce the same FOV with a 1/3" tube- or 1/3" CCD-based camera. __________________ dpalomaki@dspalomaki.com

February 1st, 2008, 08:04 PM	#3
Cal Bickford Regular Crew Join Date: Jan 2007 Location: Austin, TX Posts: 157	Ok so basically I'm assuming that the A1's vertical dimension will be identical to a traditional 1/3" chip and I'm assuming that the width of one pixel (vertically - as this is the shortest dimension anyways) would be the logical choice for max CoC. Therefore: 3.6mm/1080 = 0.00013 INCHES This is considerably less than the traditional CoCmax in film of .001 in. and even the more contemporary (due to refined technology) CoCmax of .0005 in. In fact, a safe CoCmax would probably need to be even smaller than this to take into account the fact that each pixel is probably not actually .00013 in. across vertically due to chip space around the individual light sensitive bays and around the borders of the chip (I am only assuming this as I certainly have little knowledge of how an actual chip is engineered.) So maybe a CoCmax of .00005 or .000075 in. is more appropriate. Can anyone see anything wrong with my methodology here?

February 1st, 2008, 08:22 PM	#4
Cal Bickford Regular Crew Join Date: Jan 2007 Location: Austin, TX Posts: 157	ok so i crunched some quick #s and with a CoC of .000075in. and the A1 at full wide (4.5mm) and f4 the Hyperfocal distance would be 8.7 ft. So if you wanted max DOF at this focal length and and fstop you would throw your critical focus at 8.7 ft and have everything in focus from about 4.35 ft to infinity. This sounds like a pretty deep field even for video. Does this sound reasonable to you all? I'd go out and test it out myself right now if I wasn't crippled with the flu : (