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John Jay · October 16th, 2003, 02:26 PM

Putting aside for the moment the argument of whether slim DOF can be achieved with small chip cams, consider the following

1 DOF gets slimmer with increase in focal length
2 DOF gets slimmer with increase in aperture size

Most lo-end cams reduce maximum aperture with increase in focal length. eg 1.6 wide to 2.4 tele

Therefore there must be a sweet spot on the zoom scale or focal length where the combination of aperture size and zoom combine to give the optimal point for minimum DOF.

Anyone know how to calculate this?

Jeff Donald · October 16th, 2003, 02:40 PM

You would need to know the other factors in the DOF formula. But if subject size stays the same, the DOF will stay the same. You may want to read this article or post in this thread.

John Jay · October 16th, 2003, 02:48 PM

from your replies it would seem that the answer to my question is the last point on the zoom where the aperture is still maximum - is it not?

Jeff Donald · October 16th, 2003, 02:56 PM

Yes, but the aperture continuously changes as the focal length of the lens changes. This is a result of the zoom and the optics used, not the lens opening (diaphragm) actually getting larger or smaller.

John Jay · October 16th, 2003, 03:09 PM

yes Jeff that was my understanding too, until I noticed my cam actually stops down at full telephoto, hence my question

thanks

Frank Granovski · October 16th, 2003, 04:34 PM

DOF depends on these 3 things: the lens aperture, the lens focal length, and the distance in which the lens is focused. A wide angle lens creates a much larger depth of field. A tele-photo lens has a much smaller depth of field---hence, smaller DOF with zooming in. :)

Jeff Donald · October 16th, 2003, 09:20 PM

John, what do you mean it actually stops down? Your looking in the lens and can see the aperture actually get smaller?

John Jay · October 17th, 2003, 11:20 AM

yes Jeff,

aperture physically begins to shut down at about 60% zoom

I guess I have found the sweet spot at 60%

Jeff Donald · October 17th, 2003, 12:24 PM

The manufacture may do that to help with sharpness or optical defects.

Dan Uneken · October 19th, 2003, 03:39 PM

What camera is this? When zooming there are a lot of lens elements moving around. In my XL1 16x there is no trace of this effect.

John Jay · October 19th, 2003, 05:05 PM

this is the case with my sony cams

to see if your lens behaves the same do the following

switch to manual

set speed to 1/10000 to protect the CCD

shine a maglite down the lens when at full telephoto - full zoom

you may see the aperture closed down from its wide open position

October 16th, 2003, 02:26 PM	#1
John Jay Major Player Join Date: Sep 2002 Location: US & THEM Posts: 827	who's good at math? Putting aside for the moment the argument of whether slim DOF can be achieved with small chip cams, consider the following 1 DOF gets slimmer with increase in focal length 2 DOF gets slimmer with increase in aperture size Most lo-end cams reduce maximum aperture with increase in focal length. eg 1.6 wide to 2.4 tele Therefore there must be a sweet spot on the zoom scale or focal length where the combination of aperture size and zoom combine to give the optimal point for minimum DOF. Anyone know how to calculate this? __________________ John Jay Beware *PLUGGER-BYTES*

October 16th, 2003, 02:40 PM	#2
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	You would need to know the other factors in the DOF formula. But if subject size stays the same, the DOF will stay the same. You may want to read this article or post in this thread. __________________ Jeff Donald Carpe Diem Search DVinfo.net for quick answers \| Where to Buy? From the best in the business: DVinfo.net sponsors

October 16th, 2003, 02:48 PM	#3
John Jay Major Player Join Date: Sep 2002 Location: US & THEM Posts: 827	from your replies it would seem that the answer to my question is the last point on the zoom where the aperture is still maximum - is it not? __________________ John Jay Beware *PLUGGER-BYTES*

October 16th, 2003, 02:56 PM	#4
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	Yes, but the aperture continuously changes as the focal length of the lens changes. This is a result of the zoom and the optics used, not the lens opening (diaphragm) actually getting larger or smaller. __________________ Jeff Donald Carpe Diem Search DVinfo.net for quick answers \| Where to Buy? From the best in the business: DVinfo.net sponsors

October 16th, 2003, 03:09 PM	#5
John Jay Major Player Join Date: Sep 2002 Location: US & THEM Posts: 827	yes Jeff that was my understanding too, until I noticed my cam actually stops down at full telephoto, hence my question thanks __________________ John Jay Beware *PLUGGER-BYTES*

October 16th, 2003, 04:34 PM	#6
Frank Granovski Outer Circle Join Date: May 2002 Location: Hope, BC Posts: 7,524	DOF depends on these 3 things: the lens aperture, the lens focal length, and the distance in which the lens is focused. A wide angle lens creates a much larger depth of field. A tele-photo lens has a much smaller depth of field---hence, smaller DOF with zooming in. :)

October 16th, 2003, 09:20 PM	#7
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	John, what do you mean it actually stops down? Your looking in the lens and can see the aperture actually get smaller? __________________ Jeff Donald Carpe Diem Search DVinfo.net for quick answers \| Where to Buy? From the best in the business: DVinfo.net sponsors

October 17th, 2003, 11:20 AM	#8
John Jay Major Player Join Date: Sep 2002 Location: US & THEM Posts: 827	yes Jeff, aperture physically begins to shut down at about 60% zoom I guess I have found the sweet spot at 60% __________________ John Jay Beware *PLUGGER-BYTES*

October 17th, 2003, 12:24 PM	#9
Jeff Donald Warden Join Date: Mar 2002 Location: Clearwater, FL Posts: 8,287	The manufacture may do that to help with sharpness or optical defects. __________________ Jeff Donald Carpe Diem Search DVinfo.net for quick answers \| Where to Buy? From the best in the business: DVinfo.net sponsors

October 19th, 2003, 03:39 PM	#10
Dan Uneken Major Player Join Date: Sep 2001 Location: Andalucia, Spain Posts: 301	What camera is this? When zooming there are a lot of lens elements moving around. In my XL1 16x there is no trace of this effect. __________________ Film & TV locations & production Spain http://www.fotofilmvideo.com/

October 19th, 2003, 05:05 PM	#11
John Jay Major Player Join Date: Sep 2002 Location: US & THEM Posts: 827	this is the case with my sony cams to see if your lens behaves the same do the following switch to manual set speed to 1/10000 to protect the CCD shine a maglite down the lens when at full telephoto - full zoom you may see the aperture closed down from its wide open position __________________ John Jay Beware *PLUGGER-BYTES*