Go Back   Steve's Digicams Forums > Digital SLR and Interchangeable Lens Cameras > Canon EOS dSLR

Thread Tools Search this Thread
Old Apr 22, 2004, 10:04 AM   #1
Senior Member
iblaineman's Avatar
Join Date: May 2003
Posts: 152
Default Infinity focus is how far?

Do lenese have an infinity focus? like maybe landscape are all objects say 20 feet away in focus? Or trying to take pictures of fireworks can you just turn the focus ring to a point or mark?
iblaineman is offline   Reply With Quote
Sponsored Links
Old Apr 22, 2004, 11:23 PM   #2
Senior Member
Join Date: Feb 2004
Posts: 175

I read somewhere that some cameras focus beyond infinity (not that there is such a place in this realm). That is to say the mechanics of the lens actually travels beyond the point where distant objects would be in focus.

The obvious result of this is that if you turn the ring to the stop, there is no focus point.

Unfortunately, I don't remember the rational for this design aspect. One explanation may have been to compensate for how temperature may affect the lens's focus point

David Y
Amateur is offline   Reply With Quote
Old Apr 23, 2004, 12:55 AM   #3
Frank Doorhof's Avatar
Join Date: Mar 2003
Posts: 2,320

Correct that's what I also read (temp).

Frank Doorhof is offline   Reply With Quote
Old Apr 23, 2004, 5:19 AM   #4
Senior Member
NHL's Avatar
Join Date: Jun 2002
Location: 39.18776, -77.311353333333
Posts: 11,331

IMO the beyond infinity focus is for design tolerance in the lens or temperature expansion/contraction like Frank explained, but the most logical explanation is in case the photographer add accessories like teleconverter lenses to the camera which might not bring parallel rays of light to the same focal plane... (a large DOF will take of this problem).

BTW the other need is for IR use which have a different focusing point on the lens!
NHL is offline   Reply With Quote
Old Apr 30, 2004, 3:50 PM   #5
Join Date: Jan 2004
Posts: 89

"Beyond infinity" is there - not sure if it's a design "feature" or a natural consequence of the way it works or what. But I was taking some shots of "distant" items and to speed things up I put teh camera on "MF" and just swung the ring full-trave... and found that it wasn't in fucus. Had to rotate it back a few mil. AF finds the "infinity" point fine, but you can still push it "too far" if you then turn the ring in the same direction.

This is with a 35-135 USM - a very nice lens so far. So I guess it's a feature and I'll just have to remember it and not "assume" that full-rotation is "infinity".
ccomley is offline   Reply With Quote
Old Apr 30, 2004, 7:41 PM   #6
Senior Member
iblaineman's Avatar
Join Date: May 2003
Posts: 152

If the object is 15, 20 or 50 feet away is the camera just going to focus to infinity? Maybe it depends on the lense?
iblaineman is offline   Reply With Quote
Old May 2, 2004, 5:05 PM   #7
Senior Member
Join Date: Oct 2002
Posts: 123

Yes, it does depend on the lens. Multi element lens have a variation in focus which is dependent on temperature for one thing.The change of focus due to temperature will only beapparent when a large temperature change is experienced. Also aperature size or "f" stop will allow for varying "depth of focus" depending on the focal length of the lens. It is not possible to state an exact distance that represents infinity but you can assume that it is probably around 100-200 feet or more for the usual range of lens and typical "f" stops thatthe typical photographer uses. In addition, attached filters can cause a slight shift of focus.

You can test a lens by manually focusing on a distant object with the aperature set at the smallest "f" number(largest aperature)and then without changing the focus photograph a picket fence line or similiar prespective and noting the nearest picket or object that is in focus.Try different lenses and different "f" stops and you will see the depth of focus for each combination. Then notice that you can change the focus to a closer objectwhile still have the most distant object in focus. The closest focus which will still have infinity in focus is commonly referred to the parafocal point for a given lens and aperature.

The focus range or "depth of focus" is the region within which acceptable focus is achieved and is a function of "f" stop andfocal length of lens. The focus control permits the photographer to select such things as an out of focus background by using a small "f" number(large aperature) and a longer focal length lens( or zoom in with a zoom lens).


PS If you are interested in the auto focus methodology which works by examining contrast of the image check out the following: http://www.navitar.com/download/how_af_works.pdf

twcoffey is offline   Reply With Quote

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 3:21 PM.