Go Back   Steve's Digicams Forums > Digicam Help > General Discussion

Reply
 
Thread Tools Search this Thread
Old Jul 15, 2002, 7:09 PM   #1
Member
 
Join Date: Jun 2002
Posts: 70
Default Manual Focus and DOF

Basic question for all you experts out there. With my new Minolta 7i, I have greater control(confussion) over manual focus. If I know my subject is at 65-70 feet away. Do I set my focus on 20m, 30m or go to infinity. I can't really tell the difference in the EVF. I use manual to decrease the shutter lag( almost none in this mode). The subjest is always at the same distance so this makes it easy. Any comments?
Thanks
Ken E is offline   Reply With Quote
Sponsored Links
Old Jul 15, 2002, 8:32 PM   #2
Senior Member
 
Join Date: Jun 2002
Posts: 147
Default

Hi Ken,

Depth of field is a function of aperture and focal length. You can find DOF charts for the D7 here. The D7i will be the same:

http://www.arnason.no/modules.php?na...op=MostPopular

If you will use the 4X magnification function, you should be able to focus easily using the viewfinder. (Don't even think about focussing using the LCD.)

Note that your camera is capable of focussing beyond infinity. Most of us will dial up to the first click where infinity appears in the viewfinder and then back off one click. Or else make sure you stop when infinity first appears.

You've got a great camera. Have fun with it.

Jim
JimHunt is offline   Reply With Quote
Old Jul 15, 2002, 8:49 PM   #3
Member
 
Join Date: Jun 2002
Posts: 70
Default

Ok, how can you focus past infinity? When I say DOF, I guess I'm asking how much will be in focus. If I focus at 30m, will something at 10 or 20m still be in focus? Is "m" meters? I shoot at wakeboarders from a boat on a 65foot rope. I first would just spin the ring till I got way past infinity. I have had some blury photos and am trying to get control of it. With lots of zoom, it goes 20m, 30m, 50, infinity. I didn't know if it was wrong to go to infinity or not. You say back off one click?

"first click" I don't have any "clicks" whwn I dial. Should I?
Ken E is offline   Reply With Quote
Old Jul 15, 2002, 9:11 PM   #4
Senior Member
 
Join Date: Jun 2002
Posts: 385
Default

It's possible to focus "past infinity" if a lens is designed that way but I can't imagine any that are that way. Continuing past infinity means nothing will be in focus.

What you want is hyperfocal distance. That is the distance for a given focal length and aperture puts the far depth of field effectively at infinity. Traditional film lenses have depth of field marks for various f stops. Put the infinity mark on the far mark and you have hyperfocal distance.

Depth of field is very complex. It varies with focal length, aperture, magnification and bellows factor. Depth of field is also based on an arbitrary definition of in focus and out of focus. In perfect focus a point is reproduced as a point. As the image goes out of focus the point turns into a finite circle called the circle of confusion. For film this is commonly defined as 0.025mm

For some futher info and another link go here http://www.stevesforums.com/forum/vi...d.php?tid=1122
padeye is offline   Reply With Quote
Old Jul 15, 2002, 9:30 PM   #5
Senior Member
 
KCan's Avatar
 
Join Date: Jul 2002
Posts: 1,625
Default use autofocus to prefocus

Quote:
Originally posted by Ken E
I use manual to decrease the shutter lag( almost none in this mode). The subjest is always at the same distance so this makes it easy. Any comments?
Thanks
Ken,
regarding this , what I would do is use autofocus ( spot autofocus mode if necessary) to prefocus the subject, then immediately switch to manual focus and the subject focus will remain perfect , unless you touch the zoom ring .
KCan is offline   Reply With Quote
Old Jul 15, 2002, 9:39 PM   #6
Member
 
Join Date: Jun 2002
Posts: 70
Default

Don't have time to prefocus sometimes. Pluss it seems to have a hard time locking on with a moving subject. I need to read up on the spot focus, I must admit. Also I have taken a few pictures from getting bumped while holding the prefocus. I would rather set as much to manual and be able to snap quickly.
Ken E is offline   Reply With Quote
 
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off



All times are GMT -5. The time now is 8:58 PM.