Lenses for BIG Detectors and High Magnification: Part 2

Here’s Part 2of the posting by optics specialist Spencer Luster of Light Works. If you missed Part 1, click here. And don’t forget to check out his website, www.lw4u.com .

In Part 1 we talked about using 35mm photographic type objectives and especially so-called enlarger lenses for big detectors. Prices can run from a few hundred dollars to a few thousand for longer focal lengths. Typical magnifications (defined as image size divided by object size) were 1/30 to 1/2, but certainly not usually more than 1/1. Now let's take a look at a cheap solution for much higher magnifications.

Reversing Lenses

Another way to provide high magnification and good imaging onto very large format detectors is to reverse your lens. For standard c-mount vision lenses this means pointing the c-mount end toward the object to be inspected. For example, a lens that's designed for 0.10X mag. in normal operation becomes a good 10X performer when reversed. (Lenses are designed for best performance over limited magnification ranges. Far outside the range, such as 10X instead of 1/10X, the results would normally be very bad -- unless you flip the lens around.) This reversing principal can be used with most lenses. Here's a quick example:

Say you have a 25mm fl. c-mount lens that normally might be operated at 0.10X mag. The lens to object distance would be 25mm/0.10 = 250mm, while the image distance would be 25mm*0.10 = 2.5mm. (See our Light Ideas #1 at http://lw4u.com/index.php/machine-vision-help/tech-tips/)

To be accurate, the object and image distances are measured from the front and back focus points of the lens respectively. As some of you will know, the back focus point of a c-mount lens is 17.53mm (0.690") from the mounting flange. Adding 2.5mm to this puts the image at 20.03mm from this flange.

When you reverse the lens, however, you have something like what is shown in Figure 2.

Figure 2

Now you can image your 6mm long object onto a 60mm detector without paying a fortune for a special lens!

Of course there are some caveats – there always are.

1. The short working distance on the c-mount side can sometimes present mechanical challenges.
2. You'll likely be rolling your own adapter to go from the front filter thread of the lens to your camera mount.
3. When using the focus ring, the rear barrel of the lens (now facing your object) will move in and out. For high magnification applications even this little bit of motion can have a large effect. It can make focusing and simultaneously achieving the desired magnification a bit iterative.

The same trick can be pulled with other types of lenses such as the enlarger and photographic objectives previously discussed. In fact, photographers already know this, and the manufacturers provide "reversing adapters" for 35mm type lenses. The nice thing about using these or enlargers is that the distance from the mounting flange to the back focus (now being used as the front focus) is much greater than for c-mount lenses. This gives you more space for lighting.

Enjoy!

Spencer Luster

LIGHT WORKS, LLC