who's good at math? at DVinfo.net

Go Back   DV Info Net > The Tools of DV and HD Production > Open DV Discussion

Open DV Discussion
For topics which don't fit into any of the other categories.


Reply
 
Thread Tools Search this Thread
Old October 16th, 2003, 03:26 PM   #1
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***
John Jay is offline   Reply With Quote
Old October 16th, 2003, 03:40 PM   #2
Warden
 
Join Date: Mar 2002
Location: Clearwater, FL
Posts: 8,267
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
Jeff Donald is offline   Reply With Quote
Old October 16th, 2003, 03:48 PM   #3
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***
John Jay is offline   Reply With Quote
Old October 16th, 2003, 03:56 PM   #4
Warden
 
Join Date: Mar 2002
Location: Clearwater, FL
Posts: 8,267
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
Jeff Donald is offline   Reply With Quote
Old October 16th, 2003, 04:09 PM   #5
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***
John Jay is offline   Reply With Quote
Old October 16th, 2003, 05:34 PM   #6
Outer Circle
 
Join Date: May 2002
Location: Hope, BC
Posts: 7,527
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. :)
Frank Granovski is offline   Reply With Quote
Old October 16th, 2003, 10:20 PM   #7
Warden
 
Join Date: Mar 2002
Location: Clearwater, FL
Posts: 8,267
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
Jeff Donald is offline   Reply With Quote
Old October 17th, 2003, 12:20 PM   #8
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***
John Jay is offline   Reply With Quote
Old October 17th, 2003, 01:24 PM   #9
Warden
 
Join Date: Mar 2002
Location: Clearwater, FL
Posts: 8,267
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
Jeff Donald is offline   Reply With Quote
Old October 19th, 2003, 04:39 PM   #10
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/
Dan Uneken is offline   Reply With Quote
Old October 19th, 2003, 06:05 PM   #11
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***
John Jay is offline   Reply
Reply

DV Info Net refers all where-to-buy and where-to-rent questions exclusively to these trusted full line dealers and rental houses...

Professional Video
(800) 833-4801
Portland, OR

B&H Photo Video
(866) 521-7381
New York, NY

Z.G.C.
(973) 335-4460
Mountain Lakes, NJ

Abel Cine Tech
(888) 700-4416
N.Y. NY & L.A. CA

Precision Camera
(800) 677-1023
Austin, TX

DV Info Net also encourages you to support local businesses and buy from an authorized dealer in your neighborhood.
  You are here: DV Info Net > The Tools of DV and HD Production > Open DV Discussion

Thread Tools Search this Thread
Search this Thread:

Advanced Search

 



Google
 

All times are GMT -6. The time now is 10:09 AM.


DV Info Net -- Real Names, Real People, Real Info!
1998-2017 The Digital Video Information Network