Thursday, February 1, 2007

Depth of Field Basics

Being able to effectively control what's IN focus and what's NOT is one of the basic techniques available to the user of a manually operable camera. For this fairly straightforward demonstration, I've got the camera on a tripod, and both images are taken of exactly the same scene. The only thing that alters how they look is a radical change in the aperture , or f/stop, at which they were shot.

Using a 28-75mm zoom lens at 40mm, I manually focused the lens on the intricate detail of the wooden decorative post in the foreground. The lens was also manually set to the widest aperture, which was 2.8, in this case. And, as you can see, there is very little that is in sharp focus....or what's termed "shallow depth of field" It's not right or wrong....just one way to shoot this scene. And because of the lack of overall focus, we have directed the viewer to look at the foreground detail. A nice way to create an almost 3-D effect, with the background so soft. The shutter speed for this "wide-open" exposure was 1/30th of a second. So, it could have possibly been handheld.

However, for the second image, my objective was to try and bring as much of the background into focus, and maintain crisp sharpness on the foreground, also. This is where the magic happens! And this technique holds true for all gain as much overall focus as possible, the aperture needs to be closed down to it's smallest this case f/32. The focal length remained constant, and the lens remained focused on the same point.....however, when the meter reading was taken thru the lens at this VERY small aperture, the shutter speed had increased tremendously. There's 7 stops difference between 2.8 and 32. So the shutter was open for four seconds! No way to even attempt to handhold this baby!

But, man, what a difference in overall focus. The clock in the background was so soft in the first example, it was unidentifiable.

There's even more to this than what was shown here.....but, for a BASIC lesson, I'll stop here.
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