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DPChallenge Forums >> Individual Photograph Discussion >> Trouble with Focus
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01/13/2006 11:48:08 AM · #1
I have recently started to use a Nikon D50 and am having some trouble learning how to focus. The pictures below were shot with a 18-55mm AF-S DX f/3.5-5.6G ED Zoom lens. I want to get all the numbers in focus but the focus seems to fade away going from sharpest at 1 and softest at 11. This was shot from about 2ft away from the clock. What can I do to get all of the numbers in focus?

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01/13/2006 11:52:03 AM · #2
What f-stop was this shot at? Using a smaller aperture (higher number) would probably bring the 11 in focus more.
01/13/2006 12:10:36 PM · #3
At any given aperture (f/stop) and at any given focal point, there is a certain amount of "depth of field". DOF is the amount of the image, from front to back, that will appear to the human eye to be "sharp". DOF is subjective to a certain extent; for example, at a given viewing distance from an image, the smaller the image the greater will appear the DOF. For an example of what I mean, notice that the thumbnails here appear to have better sharpness throughout than do the 640-pixel images they link to.

Now, there are two issues here for you, basically; the first is "how do you maximize DOF" and the second is "Where should I actually focus so that the part of the image I wish to be sharp falls within the range of acceptable sharpness?"

The first is easy to answer; the more you stop down, the more DOF you have. There are limits to this as far as sharpness goes (something called the "circle of confusion" but let's ignore that for now); basically, to get more DOF you stop down further. It's an optical fact that the closer you are to your subject (the closer in you are focused) the less DOF you have at a given aperture, so you need to stop down further when shooting closeups to get an acceptable DOF. "Stopping down" means changing aperture from, say, f/5.6 to say, f/22.

The other issue is less intuitive but critically important. DOF, obviously, extends partly in front of and partly behind the point of absolute focus; that's the "depth" part. Roughly speaking, DOF is distributed 1/3 in front of and 2/3 behind the point of focus. So, let's say that you are focused at 20 inches and your DOF at the selected aperture with the chosen lens is 3 inches. This means that everything from 19 inches to 22 inches will be "in focus". If you remain focused at 20 inches but stop the lens down further, so your DOF is extended to 6 inches, then everything from 18 inches to 24 inches will be in focus.

On modern lenses without traditional barrel markings it's necessary to refer to printed tables to determine actual, working DOF at given aperture/focal point ranges, but don't let that deter you; when taking an image, determine for yourself what areas need to be sharp, determine what point within that area is 1/3 of the way into it, and use that as your focal point. Then shoot images at different f/stops and use the one that works best for you.

Over time, with practice, you'll be able to determine for far down you need to stop the lens to have the DOF you are seeking for a particular shot. There are benefits to not stopping down too far, as lenses typically begin to degrade somewhat in optical performance as their apertures get tiny. This is largely due to the aforementioned circle of confusion.

Summary: choose a smallish aperture (say, f/11 or f/16, f/22 or f/32 if you absolutely must) and focus at 1/3 of the distance into the zone you wish to be acceptably sharp. This means, on shots like yours, that you will need to use a tripod and, most likely, manual focus to optimize your results.

Robt.

Message edited by author 2006-01-13 12:12:13.
01/13/2006 12:19:58 PM · #4
To confuse matters a little, if choose quite a small aperture, then your shutter speed will drop quite a bit. If the clock is actually running, then the second hand will be blurred. It would probably be best to stop the clock, either by letting it run down or sometimes the movement stops when you adjust the time setting.
01/13/2006 12:26:07 PM · #5
Originally posted by AJAger:

To confuse matters a little, if choose quite a small aperture, then your shutter speed will drop quite a bit. If the clock is actually running, then the second hand will be blurred. It would probably be best to stop the clock, either by letting it run down or sometimes the movement stops when you adjust the time setting.


Or if it can't be stopped you can increase the light to obtain a higher shutter speed
01/13/2006 01:41:33 PM · #6
Thanks Everyone!
01/30/2006 05:03:12 PM · #7
Check out this cool calculator
//www.cambridgeincolour.com/tutorials/hyperfocal-distance.htm
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