Many upright freezers are frost-free. Most chest freezers are manual defrost. Self-defrosting models circulate air through a box where a refrigeration evaporator is mounted. The frost builds up on the fins of the evaporator and this is melted off by heating the box with a small electric heating element, usually every 12 hours, while the refrigeration compressor is not running. Only the evaporator is heated, not the contents of the interior.
However, most chest freezers have coils of refrigeration tubing routed all over the interior surface of the food box. The only provision to defrost these is a drain plug in the bottom. You turn them off for a day and let the ice melt. If you are in a humid environment and you open one of these freezers often, you'll be removing the contents and manually defrosting every few months. What would be really sad is if you had an extended power outage while away and came back to find photo materials soaked to the bone then frozen in a solid block when the power came back on. That exact scenario happened to the father in law during Sandy recently and ruined a grand in paper - most not replaceable since Kodak discontinued those lines.