Spotted these lovely frozen cobwebs in the garden the other day. It was a perfect opportunity to grab the lens adapter again and try some more reverse lens macro photography... There is no best focal length for reverse macro: 28mm-50mm are the most commonly used because they tend to give the most pleasing results. The magnification is very simple: 50mm reversed 5 cm away from the sensor will give 1:1 (50/50), and 28mm 7.5 cm away from the sensor will give approximately 3:1 (74/28). To put it another way a long lens will give you less magnification and a wide angle will give you more if you wish to try it out.