Question about focus accuracy at DVinfo.net

Jason McCormy · November 5th, 2008, 06:09 PM

If the back focus is set correctly, and you go outside and focus it on a fence (not zoomed in) at about 1500 yards away, and then zoom into it, will it stay in focus?

I'm not sure If I'm asking this correctly, but I think that is enough info.

Thank you again for your help.

Shaun Roemich · November 5th, 2008, 07:26 PM

Do it the OTHER way around: zoom into the fence, set focus and pull out and it will STILL be in focus, assuming back focus is set properly. At the wide end of the lens, the depth of field is greater (depending on iris, possible infinite). However, at the telephoto end, depth of field is MUCH narrower.

We call this presetting the focus.

Jason McCormy · November 6th, 2008, 09:03 AM

It does seem to stay in focus if I do that. Hmm. So perhaps it isn't really in focus when I'm not zoomed in and then zoom forward. I just notice it then.

Hmm.

Shaun Roemich · November 6th, 2008, 11:23 AM

Quote:

Originally Posted by Jason McCormy

It does seem to stay in focus if I do that. Hmm. So perhaps it isn't really in focus when I'm not zoomed in and then zoom forward. I just notice it then.

Hmm.

Remember, you are setting the distance from the object to be in focus to the lens when you "focus" the lens. Depth of field is your friend in terms of keeping stuff visibly in focus when focal distance isn't set accurately, but as your depth of field becomes more shallow your ACTUAL point of focus becomes more pronounced.

For example, in full wide an object at 100' distance appears to be in focus, even though the lens distance indicates you are focused at 10'. Zoom in to the object 100' away and it's out of focus. Why? Because you are STILL focused at 10' Making sense?

Claude Mangold · November 6th, 2008, 11:55 AM

I am never really sure if backfocus is totally ok on my standard lens.
i don't seem to find the point where BOTH extremes in focal length are TOTALLY in focus, using either SD monitor or HD monitor or beamer (and a tape measure).

Shaun Roemich · November 6th, 2008, 12:08 PM

Quote:

Originally Posted by Claude Mangold

I am never really sure if backfocus is totally ok on my standard lens.
i don't seem to find the point where BOTH extremes in focal length are TOTALLY in focus, using either SD monitor or HD monitor or beamer (and a tape measure).

I find the same thing Claude. Compared to the 2/3" Fuji lenses I've used on Betacams, I find the stock 16x soft in wide angle, no matter HOW much fussing I've done with backfocus (and yes, I do own a TRUE high def monitor for field adjustments).

Claude Mangold · November 7th, 2008, 01:40 AM

Which keeps me dreaming of a nice fully manual high quality zoom. the question has been much discussed here, but still no real anwer other than the vy expensive optional fuji lenses.
most of my film work is done with 35mm adapters and cineprimes, but i like to use this cam myself for spontaneous clips and for scouting and for the odd documentary situation. the standard lens is ok for that but something nice and not too expensive would be great.
i believe canon has an all-manual zoom around $3k for their hdv cameras.

November 5th, 2008, 06:09 PM	#1
Jason McCormy Major Player Join Date: Apr 2008 Location: Agoura California Posts: 268	Question about focus accuracy If the back focus is set correctly, and you go outside and focus it on a fence (not zoomed in) at about 1500 yards away, and then zoom into it, will it stay in focus? I'm not sure If I'm asking this correctly, but I think that is enough info. Thank you again for your help.

November 5th, 2008, 07:26 PM	#2
Shaun Roemich Inner Circle Join Date: Sep 2002 Location: Vancouver, British Columbia (formerly Winnipeg, Manitoba) Canada Posts: 4,088	Do it the OTHER way around: zoom into the fence, set focus and pull out and it will STILL be in focus, assuming back focus is set properly. At the wide end of the lens, the depth of field is greater (depending on iris, possible infinite). However, at the telephoto end, depth of field is MUCH narrower. We call this presetting the focus. __________________ Shaun C. Roemich Road Dog Media - Vancouver, BC - Videographer - Webcaster www.roaddogmedia.ca Blog: http://roaddogmedia.wordpress.com/

November 6th, 2008, 11:55 AM	#5
Claude Mangold Major Player Join Date: Dec 2006 Location: Berlin and Geneva Posts: 259	Backfocus I am never really sure if backfocus is totally ok on my standard lens. i don't seem to find the point where BOTH extremes in focal length are TOTALLY in focus, using either SD monitor or HD monitor or beamer (and a tape measure).

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November 6th, 2008, 09:03 AM	#3
Jason McCormy Major Player Join Date: Apr 2008 Location: Agoura California Posts: 268	It does seem to stay in focus if I do that. Hmm. So perhaps it isn't really in focus when I'm not zoomed in and then zoom forward. I just notice it then. Hmm.

November 7th, 2008, 01:40 AM	#7
Claude Mangold Major Player Join Date: Dec 2006 Location: Berlin and Geneva Posts: 259	Which keeps me dreaming of a nice fully manual high quality zoom. the question has been much discussed here, but still no real anwer other than the vy expensive optional fuji lenses. most of my film work is done with 35mm adapters and cineprimes, but i like to use this cam myself for spontaneous clips and for scouting and for the odd documentary situation. the standard lens is ok for that but something nice and not too expensive would be great. i believe canon has an all-manual zoom around $3k for their hdv cameras.