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Old May 26, 2005, 3:30 PM   #1
Randy G.'s Avatar
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I have read a few posts (here and there) about folks getting a new lens or camera and thinking that there might be something wrong with the focusing mechanism because of any number of faults (back focus, front focus, blurred images, focus hunting,etc.). I thought I would put together a short diatribe here on the subject.

There are a lot of very sophisticated cameras available at (relatively) affordable prices. This is putting a lot of control into the hands of folks who may not be accustomed to the relationships of aperture, focal length, and focal point, and how they affect depth of field. This can lead to thinking that there is something wrong with the lens. Let's take a look at a specific example:

My Canon 20D is now attached to a Sigma 24-70 f/2.8 EX. It is a very cool combination capable of excellent images when one knows what to do. I sort of do... :roll:

Taking close up shots of bugs and flowers can be a lot of fun, BUT with such a fast lens it can be a challenge if you don't pay attention. I have taken a lot of out-of focus shots.. was it the camer/lens or was it me?

To get a nice close-up of a small bug on a flower, you would zoom in (in this case, 70mm) and get as close as the lens allows (about 16 inches with this lens). Using the light gathering capability would seem like a good idea- opening the aperture all the way, but nothing could be further from the truth with this set-up. At f/2.8 the depth of field for this lens/camera combo is...? Show of hands? Anyone....?

Using Don Fleming's DOF calculator (http://www.dofmaster.com/dofjs.html)
The calculated DOF is just 0.12 in. That's a total of about 1/8 of an inch, or 1/16th in front and 1/16th behind the focus point.

That's not much, and only the smallest of insects will be totally in focus. What to do? Close down the aperture. That same set up, using f/22 gives a DOF of 0.95 in, or about eight times as deep.

What if you don't have enough light to shoot at f/22? What you should do is:
A-use the flash
B-use a slower shutter speed
C-use a higher ISO speed
D-use a tripod
E-All of the above

ANSWER- It depends... They would all be good choices, and depending on the situation you may need all of them. Starting with B and then C, and combinations of the two is a good place to start.

So why bother purchasing such a fast lens if the full-open aperture has such a narrow DOF? Because this anrrow DOF problem is mostly only a factor close up. If you are indoors and taking shots of a sports event or wedding with the subject fifty feet away, at 70mm and f/2.8, the DOF is 17 feet! The ability to gather light with a fast lens would allow a faster shutter speed, great for shots of fast action like hockey or the cake feeding-fight at the wedding reception.

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Old May 26, 2005, 8:20 PM   #2
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That's one lamest posts on DOF and DSLR I've read. Come on:?
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