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Why Stop down to F8?


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wesmannmsu Silver Member Nikonian since 19th Mar 2011
Sat 19-Mar-11 03:36 AM

often when asking questions on the forum i get the Stop down to f8 as an answer, or that is issue is 4 inches behind the view finder, and while that maybe, i am just trying to figure this out.

I do understand the small F values increase the amount of light and consequently affect shutter speed. and of course the converse.

however, i do not understand why everyone suggest to go to F8. The only affect that can have is to increase the DOF (and decrease shutter speed).

but if i know the distance to my target and have plenty of light, why go to F8?

While taking a picture of my dog chasing a ball
If i am using a 50mm Lens at 20 Feet at F2.8 with an ISO of 100,
which gives me a Shutter speed of 1/3200..
Shooting AF-C (9 Points)

why would it be out of focus?

DOF calculations give a total "in focus" area of over 5 1/2 feet.
Subject distance 20 ft

Depth of field
Near limit 17.6 ft
Far limit 23.2 ft
Total 5.58 ft

In front of subject 2.41 ft (43%)
Behind subject 3.17 ft (57%)

Hyperfocal distance 145.2 ft
Circle of confusion 0.02 mm

Yes, i understand going to F8 and bumping the ISO would increase the DOF focus are to over 18 FT..

but why should i do that? I apologize the the issue is 4 inches behind the view finder.. but a little help here would serve to move that a little closer to the camera! thanks

Here is a picture of where the camera tried to focus (via ViewNX)..

clearly the focus point is on the dog.. but the Tree is in focus.

Click on image to view larger version

Attachment#1 (jpg file)

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