APS-C to Full Frame

For about the last decade I have been content with the Canon Rebel body that I purchased while a collage student.  I didn't have a lot of money to put into what was little more than a hobby, so I decided the money would be better saved for getting a couple good lenses.  It wasn't a bad plan, but what I didn't take into consideration was if they were APS-C or Full Frame lenses.

Now you might be asking yourself at this point, what is APS-C?  What is Full Frame?  Back in the days before digital photography the "frame" of a picture was based on the film used.  For example, 110 film had a 13x17mm frame while 35mm film had a 36x24mm frame.  Since 35mm film was by far the most popular, when digital photography took over film in popularity, sensor sizes that matched the frame of 35mm film were called full frame.  The more common, smaller sensors found in most digital cameras, are referred to APS-C.  You can use full frame lenses on APS-C cameras, but if you try to use an APS-C lens on a full frame camera, you get the following:

Unless you like this "port hole" effect on your pictures, using an APS-C is not the best option for a full frame camera.  In spite of not realizing this until recently, I hadn't done to bad on my lens purchases:

• Canon 100mm EF 100mm f/2.8 Macro USM
• Canon EF 100-400mm f/4.5-5.6L IS USM
• Sigma 18-250mm F/3.5-6.3 DC Macro

The first two on the list (and most expensive) are fine - they are full frame lenses.  The Sigma lens is an APS-C lens.  The only problem is, I am left with no capability of taking pictures at less than 100mm!