To me, a "manual flash" means a simple non-TTL flash. When shopping for those, the five essential things to look at:

• Hot-shoe compatibility. Sony/Minolta systems use one layout, all the other manufacturers use what's considered a "standard" hot shoe with central triggering pin.

• Guide Number. This will tell how much light the flash can spit out in a pop. More is better. Different manufacturers may use different ways of measuring GN; while mostly it shows how far it would reach with an f/1.0 lens and ISO 100 imaging surface in meters, some manufacturers (e.g. LumoPro) show the same distance in feet so you'll have to divide those numbers by 3.28 to compare. Also, check what options you have for dialing down the power. More options will give you more flexibility in how you can use the flash.

• Ability to tilt the flash head up and down so you can bounce from ceiling for softer light.

• Ability to swivel (turn) the flash to both sides; how far? This will let you bounce the flash to a side to create directional light, and gives more freedom of placement when using built-in optical slave sensor (if the flash has one).

• Triggering voltage when buying an old second-hand manual flash - there are models that used voltage too high for modern cameras and might fry them. Research the flash model and ask the seller before buying one.

There are some other optional features that may or may not be important for you:

• Connectivity options, such as built-in optical slave (when triggered with a TTL flash, it should be able to ignore preflash) or whatever plug you need to connect to your radio triggers or an external optical slave.

• A zoom head and/or built-in diffuser to adjust beam width (as a bonus, a tighter beam will provide more reach).