On my Nikon D7100, the f/stop will not go down from 14 when I hold down the +/- button and turn dial. All I get is a little number below but the photos do not change exposure when I change this number. Thoughts?
I'm guessing you moved to the Nikon D7100 from a lower tier model Nikon camera that only has one control wheel. With such a camera you must hold down the +/- button while moving the only control wheel in order to change the aperture in Manual exposure mode. Moving the control wheel alone changes the shutter speed.
The aperture setting is controlled a little differently with the D7100. Since there are two control wheels, when you are in Manual exposure mode one wheel (on the back of the camera) controls shutter speed and the second wheel (just in front of the shutter button) controls aperture without the need to press any additional buttons. Using the +/- button with the main control dial in Manual exposure mode on the D7100 will not change the aperture. Rather, it will change the readout of the light meter in the viewfinder by the entered amount of compensation.
Use the dial on the front (under your forefinger) to change aperture in M mode.
The cool bit about having a dual-wheel control body, like the D7100, is that in manual mode, one wheel controls aperture, while the other one controls shutter speed, and you don't have to mess with the freaking "compensation" mode button, like you have to on lower-end single-wheel control bodies. It's a much more direct way to get to your settings.
The compensation only works in the auto modes (P,S,A) to adjust shutter speed or aperture. Page 87 of the manual states:
In mode M, exposure compensation affects only the exposure indicator; shutter speed and aperture do not change.
You are trying to use exposure compensation while in manual mode, and the two are incompatible.
Manual (M) mode is a mode in which you tell the camera you want to take care of aperture, shutter speed and ISO without the camera's help.
The +/- button you are using is for exposure compensation. That tells the camera to adjust its exposure calculation when it is in control of some aspect of the exposure (so in S, A or P modes). In those modes, if you want more or less exposure than the camera is giving you, you can set exposure compensation. Otherwise, if you change the aperture, the camera would adjust the shutter speed to yield the same exposure and you would have no control over situations where the camera was over or underexposing the shot.
In Manual mode the camera isn't meant to make any adjustments, so you don't have that problem. If you change the aperture, the other settings don't change. So exposure compensation in Manual mode is not needed, and in fact it has no effect on the exposure.
So if you are in Manual mode, to change the aperture, just use the front scroll wheel (without pressing the +/- button.
In Manual mode Nikon uses Exposure Compensation as information only, and does not change the actual exposure of the photo. It is up to the photographer to use the information presented and make the adjustments as necessary.
If exposure compensation is set when using automatic Digital and Film SLRs in the manual exposure mode, the actual aperture and shutter speed settings do not change as they do if one of the automatic exposure modes is selected. Instead, the metering scale in the camera's LCD readouts will be shifted by the amount of compensation set, and the photographer must then readjust the aperture and/or shutter speed setting to bring the meter back to the "0" indication to actually achieve the compensated exposure.
To illustrate this, assume you have set f/11 and 1/125 second and the meter scale indicates 0 for correct exposure. If you then set +1 EV compensation, the scale will move 1 EV to the negative side (because the current aperture and shutter settings are now one stop below your desired exposure). Therefore, to achieve the compensation in the actual exposure, you will need to open the aperture to f/8 or reduce the shutter speed to 1/60 second to bring the meter scale back to 0.
Although it is possible to use the exposure compensation feature in the manual mode, it is more practical to use it with the P, S, or A modes because the camera will make the exposure adjustment automatically. In the manual mode, it is much easier to simply shift the aperture or shutter speed by the desired amount directly if you wish to deviate from the metered exposure.
You need to zoom your lens back out to a much lower number (say for example you're using a small 50mm lens then zoom the lens out to it's even smaller at 18mm), and then you can adjust your F-stop to a lower one (I got mine down to 3.5 from 5.6) and open up your aperture. I've been practising with this camera and while I was zoomed in on a subject to get a really shallow depth of field, I couldn't get down to the f-stop lower, until I zoomed out. I just had to move in closer to the object to get my ideal shallow depth of field. I guess if you don't want to zoom out and depth of field isn't important, but you want more to let more light in then you'll have to just slow down your shutter speed (i'd use a tripod to lesson blur) and increase your ISO. I personally don't like increasing the ISO above 200 if I want sharpness in my image. I'm just learning myself, but this is what i've picked up when I had the same issue with the same model camera. Best of luck.