Here's Why Some Lenses Don't look Good Wide Open

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Trevor G
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Here's Why Some Lenses Don't look Good Wide Open
2 months ago

I had a bad run with Pentax lenses a while back and got the run-around from Pentax in Australia and, on one occasion, just average results from CRIS in Chandler.  I'll explain about that later.

The purpose of this thread is to let you easily see why  a particular lens might be bad, and to give you a reliable method for testing your own.

I'm not suggesting anyone just goes out and does the tests, because the results can be quite frightening.  Rather I want to offer an easy way to check a problem lens, if you have one.

The Problem

Many lenses appear softer on one side wide open, and sometimes/often soft on both sides.

The Service Centre Test/Alignment Method

This normally uses just a central target and all alignment is based around this.

Why Doesn't This Always Work?

Faulty lenses frequently are skewed, or give skewed results. In other words, focus on one side will appear to be behind the centre test target, while the other side will appear to focus on target or even in front of it.

In addition, the DOF on one side can easily be 3 times greater than on the other side, in addition to being skewed.

The Test Setup

This isn't for taking scientific measurements.  However, as long as you don't have major horizontal tilting and the side verticals are tilted about the same, you will get an accurate enough result to know whether you have a problem or not.

Here's what it looks like:

By tilting the test target backwards at about a 30 to 45 degree angle you can easily see a "map" of the DOF at any aperture. I recommend using cloth tape measures (as used for sewing) because they are flat, and I now add a third one in the middle for accuracy.

The larger your target device and the further back you can be, the more accurate it is, and the less error there will be because the lens is not lined up perfectly.

It is a challenge to line up, especially with a pan and tilt head on a tripod, but I'll talk about that later if anyone really wants to know more. 

Samples to follow in reply.

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Cheers
Trevor G
Silkypix tutorials at: http://photo.computerwyse.com

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