Limited depth of field. In macro, depth of field is always the biggest challenge. Testing the sharpness of my lens - here are some images off the Tamron lens with f/stops between f/11 - f/22. A note on these images - the artifacts in the background are an effect of exporting the high-res files to the web (jpeg.) This happens with very smooth gradations - if you add some noise to the background, you can avoid this.
