Wednesday, August 14, 2013

How to pick a lens that performs well

You probably know that MTF charts are the way to assess the performance of a lens. What you may not know is that those charts are not very helpful. Writing in the July/August 2013 edition of Vision Systems Design magazine Andy Wilson summed it up this way:

Although MTF charts may allow similar lenses from a single manufacturer to be compared, the different testing methods used by individual manufacturer makes it difficult to compare lenses from different vendors. Finally, since other factors such as filters, imaging sensors and camera readout electronics will also affect image quality, MTF charts should only be used as a starting point when comparing lenses.”

So if MTF charts are the start point, what comes next?

Actually, it’s really hard. I have two techniques, both of which rely on having the lenses in my hands. First, I set up a simple line pair test and measure the contrast. I think it’s important to do this out at the periphery of the field-of-view rather than at the center. Every lens manufacturer makes sure their products work well on the optical axis. And of course, make sure the field-of-view and resolution are the same for both lenses.

Second, I weigh the lenses. It’s a crude measure I know, but glass is heavy so I conclude that the heavier lens contains more of it. More glass implies, to me at least, more optical elements for optimizing the image quality, thus a heavier lens is better. Usually, a heavier lens is more expensive, and while price doesn’t always relate to quality, (ask any wine taster,) I do think there’s a closer correlation than say the price of a pinot noir to how much you’ll enjoy it.

1 comment:

Anonymous said...

detail explanation on how lens and extension tube is selected.