To set aperture on a D5100 switch to A and use the command dial.
(Shooting in Manual mode is simply torturing yourself unnecessarily if you don't know exactly what you're doing. In Aperture priority — or Program shift — setting aperture is simple and intuitive.)
My D7000 is more straightforward since it has a separate dial for this and no need to half depress the shutter release. (I assume you can also change the settings using the LCD, but that's of no use if you're looking through the viewfinder and trying to set manual exposure.)
I started off with P&S cameras and before I knew the mechanics of cameras I was frustrated with the automatic exposures; once I discovered I was missing "manual mode" I never turned back.
Sure, from time to time I'll use the priority or program modes when the shot is more important than the learning experience, but after 4 years of hobbyist photography I still find myself with much to learn and I enjoy the challenge of manual exposures.
Edit: A mode on Canons is Av mode.
While I agree that constant aperture lenses are substantially costly than their variable aperture counterparts, cameras like Canon 5D Mark III is coming with a 24-70 F/2.8L II lens as a kit.
I am only expecting people who actually have a DSLR with either a variable or constant speed lens but still shoot auto will come and be able to try their hands out on a same scene with similar type of lighting but different shutter speed aperture combinations and later on be motivated enough to try it out on their real camera.
Prolific shooters usually know what to expect from their camera, so there's no need to experiment with settings all of the time, and if they see a P mode choice they don't like, they can program shift or switch to A/S/M mode.
I come from an age when SLR cameras did not even have batteries, leave aside digital sensors. So probably it is hard wired into me that I need to use my eyes and brain and leave aside the cameras' when shooting.
They may not use matrix metering (I definitely don't, I use center weighted, and I'm just a hobbyist) but you also have to remember the dynamic range of a full-frame RAW file is pretty good and that you can get good shadow/highlight recovery out of most situations.
When the photographer is chimping the image post-shot, if they see that the exposure is way off, you can still program-shift the settings in P-mode or use the EV dial to make adjustments and reshoot.
In any case, the need to have fixed fast aperture lenses is diminishing with IS and high ISO performance. I'd rather carry a fixed F4 than a fixed F2.8 zoom with a 2012 camera. It's going to be a much smaller lens and just as effective for most use cases.
It's important to note a distinction.
The faster aperture tends to necessitate better glass, but the idea that only fast lenses have good glass would not necessarily be true. There are plenty of "slow" lenses that have very good optical quality.
If it is indeed a lens with just a single aperture setting, can you tell me the model number? I'm quite curious now. Thx.
I bought a Nikon D5100 last summer, my first SLR since a Pentax K1000 I had for decades. The lack of a DoF preview button was a little hard for me to adjust to.
My girlfriend just got her first DSLR and was struggling with changing the aperture. While she understood the concepts, she wasn't familiar with working with a DSLR alone and since I wasn't physically present, it took me a while to realise over the phone she'd been changing the aperture by changing the focal length of the lens. (The D5100 has a pretty crappy and unintuitive control combination for setting aperture.) Had I been able to point her to a simulation like this it would have been clear to her that she needed a distinct method of setting aperture.