Infinity focus and focal length

[The Ever Optimist]

Hello to all....

I had an idea that when i want to decrease shutter lag, the best thing would be to keep my digicam focussed at infiniti manually and then keep clicking....this gave rise to 2 questions...
1. Will this degrade the sharpness provided the subject is at the correct distance?
2. What is the minimum distance which will be in focus? Is this a function of the focal length?? And if i consider a digital camera... do i consider the actual focal length or the 35 mm equivalent?

thanks in anticipation!!

Kunal

[another view]

You probably will decrease shutter lag by manually focusing on infinity because that's one less thing the camera has to do. My Coolpix is pretty slow in the AF department, too. However, you could also hold the shutter down halfway to lock focus and then shoot (if that's an option).

If you lock the focus on infinity, then you will probably have quite a few out of focus images. Digicams have such short focal lengths (we're talking actual focal lengths, no 35mm equivalents) so they will be better at this than anything else out there. Stopped down to f8, you'll probably get most everything in good focus but it's not optimum - you may have a slow shutter speed for example.

Time for the term of the day: "Hyperfocal Focusing" If you google that term you'll find some charts where you put in the lens focal length (again, actual number), the aperture and the distance you focus the lens at. The chart will tell you how far in front of and behind that point is considered to be in focus (YMMV). You'll get 1/3 in front of that mark and 2/3 behind it. If you have any prime lenses - especially manual focus ones - they have these markings right on the lens barrel so no math is involved.

So the short answer is... maybe! ;) The trick would be to play around with the chart using the focal length and aperture you want (might as well choose the minimum available on the camera, for starters). Keep moving the focus point forward from infinity (or 50' would do) until you get "infinity" on the far end of the scale and the closest distance you can at the close end of the scale.

[another view]

I found this great calculator on the web: http://www.johnhendry.com/gadget/dof.php

It doesn't list a format small enough for a compact digital (the sensor is probably about 1/4 x 3/8). The smallest listed is 35mm so I picked that. I used 21mm focal length because that's the longest that my Coolpix will zoom out to (about 80mm equivalent). If I focused it at 8', then everything from about 3.5' to infinity would be in focus at an aperture of f8. Shorter focal lengths will bring up the minimum distance and larger apertures will push it out. The numbers aren't entirely accurate because of the format size but it should be a good start. Hope this helps.