The spiderweb shot was a bit of a compromise, Peter. I had to shoot at a bit of an angle to the web in order to get the flower sharp. Also to prevent the background leaves becoming too dominant.
I originally tried a wider shot which had more flowers surrounding the web. But it didn't quite work. Neither did several other attempts. And I have had a few different crops of this scene before finally settling on this angle.
I'm not really sure what 'official figures' that 180 lens has. For me, it usually depends on the 'fly away' distance anyway. Which is frequently well off minimum focusing point; and I often add a 1.4x converter so that produces different results.
I have tried it with a 25 mm extension tube which works well for 'studio work' but would mean going closer than the fly away point for most insect species.
For flower work however, I often find the 24-105 actually achieves a better result.