Question about Canon EF 50mm f/1.8 II?

Question

i recently got a new lens for my Canon Rebel XT. The lens is a Canon EF 50mm f/1.8 II .. It arrived today, no problems.. i took off the regular kit lens that came with the camera.. and put on the new 50mm..how come in my view finder.. i see where it says 1.8, but once i take a photo.. it jumps to 4.0. this is in auto mode.. should i be in manual mode?.. I have used the AV setting and it seems to stay in that.. i have to shoot some pics of an outdoor concert next week for my friends fanzine..it will be nighttime.. so i want to take the best pics possible.. can someone help?

Answer 1

In full auto mode your camera is just trying to guess the proper exposure that you want. It meters the light at gives an average reading. f/4.0 is not a bad thing, it's just what your camera needs at that moment with your shutter speed and ISO to create a well exposed image.

I say guess because it doesnt know what you really have in mind. Do you want everything in focus or just the subject with everything else blurred (bokeh). Putting it in Av mode gives some more control, and you should venture there when your comfortable with it. f/1.8 will give a shallow depth of field and anything smaller than that (bigger number) will increase the depth of field. Keep an eye on your shutter speed. You will want to go faster than your lens. i.e. you have a 50mm lens so you want to go faster than 1/50th of a second, if you had a 300mm lens you would want to go faster than 1/300th of a second. Just a general rule of thumb.

Answer 2

In auto mode, the camera will decide which aperture to use (you will have no control over it)... if you want it to stay on f/1.8, then you will need to set your camera to aperture priority (Av) or manual (M).

Remember that shooting at f/1.8 will allow more light to come in, but it will also give you a shallower depth of field.