Aperture

Emmanueluso

I'm getting my camera later next month. One thing I'd like to understand is this, if a high aperture (f/2.8) allows more light into the camera, won't it wash out any attempt at a small depth of field photograph with excess light?

ohyeahar

You're right. So you must find another way to reduce the amount of light captured by your sensor. That means shooting at a lower ISO and/or faster shutter speed.

Emmanueluso

How fast would the shutter speed need to be?

bruto

That really will depend on how much light there is, and how much room there is to compensate. If you are taking a picture indoors by available light, there will be plenty of room for increasing shutter speed or lowering ISO. If you are shooting outdoors on a bright snowy day, you may find that even at ISO 100 you run out of shutter speeds and will need either to stop down the lens or put a neutral density filter on to darken it.

If you use aperture priority, you will see immediately what shutter speed is chosen. Compose your picture at f/2.8 and ISO 100 and if the shutter speed is slower than 1/4000 you are within range. If it is 1/4000 it may not be. Close down the lens until the shutter speed changes. When the shutter speed goes to 1/3200 you have stopped your lens down 1/3 of a stop below the absolute maximum. If you open it up again by that 1/3 stop, you will be right at the known maximum aperture for the light you're in. Whatever aperture that is, you will get no better.

I should add to clarify that on the D3xxx and its ilk, the wheel varies aperture and shutter speed in steps of 1/3 stop, 1/3 for each click. Three clicks will halve or double exposure, equivalent to a stop. The ISO speed is varied by full stops only.

ohyeahar

@bruto gave a very detailed explanation. Let me try to give a slightly more simple one.

The basic idea behind balancing an exposure is very easy to understand. If you add light, you must take away the same amount of light to keep the exposure balanced.

Yes, if you open up the aperture, then that means you’ll need to lower your ISO and/or shoot at a faster shutter speed. How much to lower the ISO? How fast should the shutter speed be? Well, unless you’re an expert at metering light, you’re probably clueless, and that’s ok. Let your camera to its job.

In Aperture Priority mode, your camera will automatically use a faster or slower shutter speed depending on what aperture you set.
If you adjust your ISO, your camera will again automatically adjust the shutter speed.

Emmanueluso

Much thanks guys for taking time out to give me a detailed explanation. I really appreciate it.

avasqu

This I'm sure is a pretty shrilly question but since I don't know the answer, I'll ask. I have been trying to set my aperture to f/2.8 on the D3100 and am unable to? The least possible setting is f/5. What am I doing wrong?

bruto

The maximum aperture depends on the lens. The kit lens cannot achieve f/2.8. Its maximum aperture, depending on focal length, is listed on the lens, usually, 4 to 5.6 with the largest aperture (smallest number) applying to the shortest focal length. Large aperture zooms are quite expensive.