Canon XS on the way, question on Aperture

Discussion in 'Digital Photography' started by duncanapple, Dec 9, 2008.

  1. duncanapple macrumors 6502

    Jun 12, 2008
    Hi all -

    I finally took the plunge into the DSLR world. After getting tired of dark and blurry p&s photos, I decided I wanted to not only learn more about the manual settings and what not, but also step up to a little more serious camera. I read countless reviews and ended up with the Rebel XS (Got it on amazon for what I thought was a reasonable $459). Its supposed to be here next week and I am already reading up on what various settings mean, future lenses, etc. I think I found myself a more expensive hobby than my mac fetish lol.

    Anyway, to my point. I understand a lot of what the tutorials are saying about lenses and aperture. A lower aperture number means a more wide open lens that lets more light in. Since all a photo is is light, more light is for the most part better. I also see lenses have various aperture ratings (for example, the canon 17-55 IS USM has a fixed aperture of 2.8).

    However I get lost on this part... I read you can change the aperture on the camera itself - a lower number would focus on less (such as a persons face with a blurry background) and a higher number would focus on more (the face and the background). Can someone explain how this ties to the amount of light the lens lets in and the aperture rating of the lens itself?

    And finally, how does aperture relate to ISO? Does a lower aperture number just let you get away with a lower ISO sensitivity? Ie if I was shooting in a darker church, I could set a lower ISO number with a lens that had a 2.8 aperture than a lens with a 3.5 aperture?

    Sorry for the litany of questions - I have been doing a lot of reading and I am a little confused on a couple points, and would just like confirmation on a couple as well.

    I am really (really) looking forward to getting into this hobby. I must say, between all of my horribly blurry/dark photos I have taken over the years, and watching the "photo of the day" threads here at MR, I cant wait for my canon to show up!

    - Chris
  2. SayCheese macrumors 6502a

    Jun 14, 2007
    Thame, Oxfordshire, England

    First of all congratulations on the camera. It should serve you well.

    I think that you have pretty much answered your own question.
    The aperture is simply a hole through which the lens allows light to pass. In doing so as you correctly state a small aperture number (also known as an f stop), is the size of that hole wide open and therefore a lot of light being let in. A large number, or f stop, is a small hole and therefore a little bit of light gets through.
    Lenses can be set at varying f stops throughout the range. A lens normally only states the smallest number in the lens description. Therefore in the case of your 17-55 IS USM, the aperture range is in fact anywhere between f2.8 and probably about f22.

    Generally most people for most shots use an ISO somewhere between 100 & 400. The higher the ISO number the less light you need to take an image. An ISO number is directly related to the f stop and shutter speed. Therefore an image taken at ISO 100, shutter speed of 1/125 sec and f8 will look exactly the same in brightness as an image taken at ISO 100 1/250 sec and f5.6.
    This is because the f stop is a smaller number so there is more light coming through the aperture. Therefore you are compensating for that with a faster shutter speed. However because you have a smaller f stop and therefore a larger aperture opening you are going to get less in focus in front and behind the point of focus.

    So if you were to take a shot with the same camera and the same f stop (5.6) but at ISO 200 (remember a faster film so less light required) then you would need a shutter speed of 1/500 sec.

    The faster the shutter speed the faster the action you can freeze frame. It's basically one huge balancing act to see what works for you. If you want to photograph football and freeze the action but blur the background while keeping the players in focus you would maybe use ISO 400 1/500 f4. However if you are photographing a landscape and want to keep the whole scene in focus then you need a large f stop, say around f22. However this will mean that little light is hitting the sensor at the same 1/500 sec you used for the football. Therefore you would need to use a longer shutter speed such as 1/2 sec.

    I would suggest that as your camera is digital that you take loads and loads of pictures using the manual mode. Set one variable (f stop, shutter speed or ISO) and play around with the others. Download them all to the computer and see what you like the look of, which ones work and which ones don't.

    You can always look at the exif data in your shots to see what the settings were at the time of shooting.

    Hope this helps.
  3. duncanapple thread starter macrumors 6502

    Jun 12, 2008
    Thanks - that actually helps a lot. It also shows me that the more I learn, the less I realize I know! Yeah, I figured I would have to just mess around quite a bit to get started, but I wanted to have a rough idea of what I was doing before I started.

    - Chris
  4. AlaskaMoose macrumors 65816

    Apr 26, 2008
    I will add the following: I imagine that the XS, like a lot of other Canon cameras, have a few automatic modes, and you switch from one to the next by rotating a wheel or knob on the left-top corner of the camera. Some of the symbols are obvious. For example, the one with a flower is used for macro/close-up photography, the one with a skier is used for sports (moving subjects), and so forth. Another is a green color rectangle. This one is a do all mode, and fully automatic as all other modes below it. In these modes the camera does everything for you.

    After the green rectangle, there is the P mode. The "Creative" modes begin with P, and this mode is almost full auto, except that you can change a few of the settings manually. For example, WB, ISO, the flash only pops up when you press the flash button (it pops up automatically in the auto modes), etc. In this mode, the camera does everything automatically, except for what you change.

    I would recommend that you use the creative modes as soon as possible, to make the camera work for you they way you want, not automatically. One of my favorites is Av. This mode is nothing more that "aperture priority," and while you change the lens aperture with a wheel near the shutter button, the camera sets a matching shutter speed to match the lens aperture you have chosen. The Av mode is very useful when you want to blur the subjects background (for example), which also translates to decreasing the depth-of-field. Of course, you can also use this mode to increase the depth of field such as when taking landscapes photos, since you want the foreground as well as the background in focus.

    Then there is another very useful mode, and this is Tv (shutter priority). This is the opposite of Av, in that you choose a shooter speed, and the camera choses a matching lens aperture.
    There are other modes, including M (manual), but those two above are very handy and will save you a lot of time, specially on action shots when you set the camera to AI Servo and burst mode.

Share This Page