Can you only change aperture in A mode?

I'm new photography and have bought a nikon d3100, i was wondering if you can lets say change the aperture in portrait mode or does the camera take charge on what it'll be?

Sincerely, you need to ignore any of those silly scene modes. Learn proper photography skills and you will have no need to rely on such nonsense.

You adjust the aperture in either A priority or in full M manual mode.

Any other type of "scene" nonsense I have no idea what the camera will do, and neither will you. That is why it is important for you to learn what needs to be done so you are in control of your photo.

M is manual mode where you chose the best aperture to match your selected shutter speed and ISO.

Av or A is aperture priority where you select the aperture you want and it automatically selects the matching shutter speed.

Sv or S is shutter priority where you select the shutter speed and the camera sets the rest as above.

Portrait Mode selects a shallow depth of field (low aperture #) that is appropriate for portraits as it blurs out distracting backgrounds. This is an Automatic mode. You can more easily do the same thing in the manual modes because you will have 100% control.

Steve P is absolutely correct. The only mode you need is manual ( M ) everything else is just bells and whistles that are likely to ruin your picture. Just learn to use manual and when you know how you will find you never use the other modes.

In Scene Modes the camera makes all the decisions for you - in Portrait mode it'll pick a wider aperture most likely.

If you are going to use a DSLR in scene modes you might as well have bought a point & shoot.

Learn to shoot in M (manual) and until then, there's always aperture or shutter priority (A & S modes) which are semi -auto. At Least in those you are making half the decision.

Aperture Priority Mode (A or Av) is the most used Exposure Mode.

You set the Aperture for the depth of field you want (more critical when close to the subject)

You set ISO as low as possible.

Half press the shutter and the camera will tell you what Shutter Speed it has to select to get a decent exposure. If the Shutter Speed is too slow increase the ISO and/or open the Aperture.

Metering from the camera position is not perfect (it makes no allowance for the tone of the subject, it will under-expose light toned subjects (e.g.snow scenes) and over-expose dark toned subjects) which is why there's Exposure Compensation easily accessible. It's how cameras are designed to be used. If you shoot in Raw this correction can often be made in Post Processing, but it's better to get it right in camera.

Manual Mode is for when you want to blend Ambient Light and Light from a Strobe, it can also be used with an Incident light meter to get spot on exposures and is sometimes necessary when the camera is used with a Microscope or Telescope, it's not for everyday exposures, shooting digital is not like shooting with film, there's little (if any) exposure tolerance with digital.

All cameras are just image recording devices, with digital you can get spot on exposures every time by using the Histogram and 'exposing to the right' (ETTR), this will maximise the data recorded, 1/3rd of a stop can make a visible difference. When shooting, especially in low light, ETTR can visibly reduce noise.

http://schewephoto.com/ETTR/index.html

Normally you expose to maximise data you alter exposure for effect in Post Processing, this ensures a cleaner less noisy image.

No.

You can change in any mode. Read your manual to learn how to do it.

You have three modes where you have a degree of manual control over the aperture.

"M"anual, where you are fully responsible for all settings, meaning you can under or over expose if you require.

"A"perture mode, where you set the aperture and the camera will select an appropriate shutter speed.

"P"rogram mode runs aperture and shutter speed against each other.

On the top of your camera near the shutter there's a little button with a + and a - symbol, hold it and change it like that.