Okay I know there are already some fantastic guides for this already, but I wrote out a guide on this for another site that apparently is gone now. So I figured I would contribute to TA by posting it here in case it might help someone, because more guides are always better than less. So on we go.
• Thuggish Splicer - The people that use melee weapons. (ie machete, flashlight, shovel)
• Leadhead Splicer - The people that use firearms. (ie pistols, machine gun)
• Nitro Splicer - The people who carry small grenade boxes and toss explosives.
• Spider Splicer - The people who use hooks to crawl on the ceiling and occasionally tosses them at you.
• Houdini Splicer - The people who can teleport from place to place and toss flaming or frozen projectiles towards you.
• Bouncer (Big Daddy) - The Big Daddy with a drill attached to it's arm and has a bit more of a bulky appearance. (Note: Both the normal and elite Bouncers count as the same research group.)
• Rosie (Big Daddy) - The Big Daddy that carries a rivet gun and wears a divers suit. (Note: Both the normal and elite Rosie count as the same research group.)
• Little Sister - The little girls who drain the Adam from corpses. (Note: Research can be done on both saved and not-saved Little Sisters.)
• Turrets - The stationary security drones that are equipped with a machine gun, rocket launcher, or flame thrower. (Note: All types of turrets fall under the same research group and cannot be researched while hacked.)
• Security Bot - The hovering security drones that are equipped with machine guns. [Note: Are only able to be researched while hostile (red), neutral (yellow), or deactivated while not hacked (no lights).]
• Camera - The security cameras that are attached to the walls and scan the area in front of them. (Note: cannot be researched while hacked.)
In order to max out research for these subjects you need the research camera, which can be obtained in the third area (including Welcome to Rapture) Neptune's Bounty. Maxing out research is done by taking pictures of your target, which adds to the completion of a series of progress bars that requires more and more pictures as you continue. These pictures get graded with scores that range from A-C. Of course an A is the best and is harder to get once you photograph an individual target, though not impossible. Think of the first picture as a starting point in where the score starts to drop with every consecutive picture. You cannot max out research with just one or two subjects, you'll need around ten or more for each group so take pictures frequently!
Here are some general tips that may help with getting better grades and filling the bars faster.
• Multiple Subjects - This is a score bonus for having multiple subjects, friendly or not, so long as one is a hostile and needs further research. The bonus stacks with every enemy, whether or not some are exempt from research such as hacked bots or other fully researched subjects.
•Action Shot - An Action Shot can be considered as many things, such as a melee move being executed, a firearm being shot, damage being taken, a Big Daddy in a defensive position, a Little Sister harvesting/drinking Adam, Spider Splicers crawling on the ceiling, a frozen enemy, and sometimes even a splicer noticing you.
• Also there are two tonics that help, Photographer's Eye and Photographer's Eye 2. These also stack giving a better result than just having one of them.
As well as bonuses there also some negative consequences, which I will list.
• Subject is dead - Surprisingly this is a score reduction for photographing dead targets. Don't let this stop you from taking pictures though as every bit helps.
• Subject Already Photographed - For taking a picture of a subject more than once. This shouldn't be worried about, just keep snapping pictures until you can't possibly take and more. Remember, if the score is too low you can lure them into an area with more enemies or a hacked camera. This will help with a Multiple Subject score boost which might help.
Here are some notes that may be useful.
• Little sisters, turrets, and cameras are at a set amount, so take as many pictures as you can of these when you have the chance. A good idea would not to hack any turrets or cameras/ harvest little sisters in the Neptune's Bounty level until you have the research camera and take some pictures of them first. Also sometimes when a Little Sister enters a vent a new one will take it's place, but not all the time. I recommend rescuing the Little Sisters, because this allows you to take some extra pictures if you forgot beforehand and allows you to take some of the ones that delivers Tenenbaum's gifts.
• Security bots can take awhile to fill up the research for, and I recommend taking advantage of moments when the alarm is on to take some pictures. Some moments for this includes accidental camera trips, enemy camera trips, when you exit the farmers market to administer the Lazarus Vector Andrew Ryan sets an alarm on you, after the last Andrew Ryan moment an alarm is set on you, and finally when you break glass in Fort Frolic.
• If you run out of ways to find security bots try staying around Arcadia and Point Prometheus as splicers will sometimes walk around with hacked bots.
• Nitro Splicers aren't as common as the rest, but some good areas to find them are the Medical Pavilion, Apollo Square, Olympus Heights, and sometimes in Hephaestus.
• Houdini Splicers might give you some troubles, the best area to find them is of course Arcadia, particularly in the beginning. If you're careful not to knock over the phonograph on the piano or otherwise be intrusive, you can take multiple pictures of the dancing Houdini splicers in Cohen's Olympus Heights suite. Speaking of which Cohen is also a Houdini splicer that seemed to give me a slight extra more than the others.
• If you killed all of the cameras and broke all of the glass in Fort Frolic, there are a bunch of deactivated security bots in a room in Point Prometheus. They give a tiny bit but they respawn so you can try there if you're out of options.
• Be sure to look out for some dead splicers and what not that you haven't killed, a few pictures can be salvaged from them.
• And finally I would obviously recommend this on easy, and make a final save somewhere before fighting the end boss, in case you hit a snare.