Long lens

With winter coming, and my lack of mobility, I decided to buy a new long lens to make it easier to photograph the birds on the feeders from the house. This is a 100-400mm on APS, and with the 1.4x converter, this means I have the full-frame equivalent of 840mm, and if I were to buy the 2x converter it would take me to a full-frame equivalent of 1120mm!