Thanks Soufiej. I think I would have to buy you a present or something in exchange of all your help. Do you like wine?
The same goes for everyone else here helping me (and the others that might have the same dobuts)!
It's very difficul just practice one aspect of the camera, when all the primary aspects (DOF, shutter speed, ISO) are so close linked toghether.
In these past days I was walking and trying to see pictures worth taken. Not buildings

. I haven't saw anything that catch my attention, so I guess that all the hipotetical photos that I would have taken will not made any impact in the viewer (however, they would help me to practice).
Just the other day it came to me the name of Pedro Luis Roata (
pedro luis raota - Buscar con Google He turns out to be a famous Argentinian photographer, and I started to look his work... And every single photo is memorable. I think (as some of you pointed out) that a good photo sticks in your mind. And that is why the composition is so important, and why I have to start learning and practicing a lot on that. IMO.
"It's very difficul just practice one aspect of the camera, when all the primary aspects (DOF, shutter speed, ISO) are so close linked toghether."
Difficult at first, yes. Not that difficult to grasp after a bit of practice though. I would say ISO is the most negotiable of the three, certainly with a modern digital camera it is. In the days of film we were more or less locked into a film and its "speed" (or light sensitivity) until we used up that roll. Changing to a faster film as light levels dropped was a PITA.
In a digital system it comes down to the flick of a switch or the automatic selection made by the camera. You should, by reading reviews of your camera, know about where noise becomes an issue as ISO value increases.
(Remember,noise also depends on your use of the image. If you never intend to blow up a shot to more than, say, 8X10, you have a lot of latitude in your ISO settings and can probably shoot at 800 or even higher should you want a slightly larger finished print.)
You should be able to set a maximum ISO for your camera (found in the menus) to ramp up to but not beyond. And you should also be able to have the camera do so quickly or slowly. Slowly is best for a student photographer. I'd say set your max. ISO at 800 and the camera will not exceed that value unless you alter the setting (or you switch to full "Auto" mode). Certainly, for daylight shooting, there shouldn't be many cases where you would require higher than 800 ISO and not even that typically. At this point I wouldn't be overly concerned with ISO since it is an automatic adjustment within the camera made to compensate for aperture and shutter speed changes. (The camera should warn you when ISO must be changed to accomplish a clean shot at your given aperture and/or shutter speed.) If you've set the camera to never exceed that 800 mark, set the working ISO at 100 or 200 and let the camera do the technical work on ISO while you concentrate on the composition and the other two values.
Most day to day photographers tend to shoot primarily in either Av or Tv mode and aren't too concerned about ISO nowdays. The three values are, though, the three legs of a stool and you'll eventually need to know how to see one as changing the other two. That comes mostly from experience and, on a cloudy and cold/rainy day when you can't get out, just sitting with a pencil and paper and working out the numbers until they begin sticking in your head. Many tutorials on the exposure triangle have automatic systems which allow you to practice the settings.
Have you watched or read any tutorials on your 5100? You can put in, say, "Nikon D5100 aperture tutorial" and get lots of useful information. Not only will the information discuss aperture as it relates to general photography, the tutorials should also show you how to specifically operate your camera to get the most from the aperture control.
If you're becoming confused about what one change, say, to aperture achieves, try working in an "Exposure Lock" situation. Now you can set the exposure compensation for a specific exposure level and lock the camera to always create the same exposure as you adjust either aperture or shutter speed (or ISO). As you change one value, the camera will do the calculations at this point and will adjust accordingly to achieve a consistent exposure. This might help you see the changes just one value creates. The camera still depends on the exposure triangle but you can more easily see the effects on DOF, let's say, when you are changing just the aperture setting.
Setting Autoexposure Lock on a Nikon DSLR - For Dummies
I see now what you were referring to earlier was the auto white balance and not just an overall auto mode for taking photographs. Eventually, you should know how to set a custom white balance but that may be beyond what you need to work on right now. Your camera has predetermined values (accessed through the menu) for various lighting types. In general, these adjustments work well and you might just want to familiarize yourself with their effect on the image at this time. Don't clutter up your lesson plan with too many variables. You are still at the "throw away most" stage of learning so don't be overly concerned about turning out perfect shots until you have more time under your belt.
More than likely, the coloration issue in the #3 image is a case of nothing in the image being the equivalent to the ideal 18% grey. Also, depending on how you had set the metering system, your camera may have tried to adjust to an impossible standard. The camera made an automatic selection and simply missed. That's all, just a lesson in light values at this point. This is the type of shot where you might want to bracket your shots. That is, take one shot with a midway setting and the next with "this"setting and the third with "that" setting. The "this" and "that" should be on either side (higher and lower) of your midway value. You can then decide which gave you the closest approximation of what you saw in real life.
Some cameras have automatic bracketing functions for several values and you might want to experiment with these as you are learning the camera's capabilities. Normally, you select a middle value and the camera will take two shots, one each on either side of your assigned value. It's a fairly decent learning tool and we would normally do the same when we were learning to shoot film - and we'd waste a lot of film doing so. But it was a good teaching tool. Bracketing can almost be considered mandatory when shooting sunsets because the values and qualities of the available light changes so fast in just a moment's time that bracketing is one of the only ways to assure a half way decent image of a wide bandwidth dynamic range image. (Which brings up whether your camera has HDR settings. If so, study them and learn when it is appropriate to use them and how to use them. Lots o'stuff, learn each in chunks of experimentation and don't be concerned if you miss a shot at this point. You simply can't expect to remember everything now.) Check the numerous functions for which your camera will automatically perform a bracketing situation. Bracketing is, though, best done with the camera mounted on a tripod. Not completely necessary in most cases (absolutely needed for in-camera HDR) but helpful.
Very nice work by your "found" photographer. Consider, though, the photos probably didn't look like that two months after he first picked up a camera. And it's a fairly safe bet that for each shot saved, there were a dozen or more that got thrown away. It helps to be talented but it really helps to be lucky.