I am a new photographer and am slowly trying to learn the ropes. I have previously had no experience what so ever with photography when I was moved by Eli Reeds work. That was when I decided I wanted to become a photojournalist and work my way into the field. So after several months of reading and research I jumped in and bought a Canon 40D...I LOVE the camera. But it has been a very steep learning curve. I am currently using an EF 28-135mm lens. I am having difficulty understanding the AV mode in terms of Focus. With my limited experience I have been having difficulty with getting the "sweet spot" or focus exactly where I want it. I have messed with the AF points and what not, but the band of focus just really doesnt do the job sometimes. Occasionally i get all of the subject in focus with the nice blur behind, but most of the time I cant repeat this. I was hoping someone would point me in the right direction, and explain to me or send me a link that would tell me what effect the size of the aperture had on the band of focus or am i just limited by my lens. Thanks in advance for any help or advice.

In a nutshell, the higher your F-number, the more you will have in focus. I'd really recomment a "Photography for dummies" book from your library to get you started (no offense).

Ha no offense taken, I have been reading several books on the subject, but for some reason the Aperture and the numbers just is not sinking in. I recently purchased Understanding Exposure and the Photographers Eye which have been helpful.

APERTURE Most lenses have an adjustable opening near the pupil named the aperture. The aperture regulates the amount of light that reaches the film and can be increased or decreased as required for exposure of the film. On older cameras the aperture was a board placed before the film that was cut to a certain size to restrict the amount of light reaching the film. Now the aperture is similar to the earlier leaf style shutters and is adjusted by moving a ring located on the end of the lens that is nearest the camera body to slide thin blades in a circular motion. Moving this ring to the left will decrease the aperture’s size or ‘stop the lens down’. Moving it to the right will increase it or ‘open the lens up’. The term ‘stop’ refers to an increment of adjustment to exposure. An f-stop number is a fractional representation of a ratio between a lens’ focal length (f = focal length) and a lens’ aperture diameter. The following equation provides the framework for determining f numbers using the lens’s focal length and the aperture’s physical diameter.. av = f / d (where av = f number, f = lens’s focal length and d = physical aperture diameter) For instance, if the aperture on a 50mm lens has a diameter of 12.5mm then with the preceding equation you can determine that the f number would be 4 (4 = 50/12.5). A 100mm lens’ f/4 setting has an aperture diameter of 25mm (100/4=25). Though the 50mm f/4 aperture diameter is one half that of the 100mm, the 100mm lens requires the light to travel through twice as much lens as the 50. Both lenses set at this setting allow the same amount of light to reach the film because both of these lenses has an opening that the light passes through that is one fourth of the focal length of each lens. The aperture primarily controls the amount of a subject that is in focus. Only a certain depth of space in front of a lens will be in focus. Before and behind this area will become more gradually out of focus. This in-focus area is referred to as depth of field. A larger aperture diameter will gather more light and hence will have a shallower depth of field in relation to a smaller aperture diameter. Smaller apertures such as f/22 or f/16 can have the capability of rendering everything from seven feet to infinity in at least relatively sharp focus. Larger apertures such as f/2.8 or f/4 can produce dramatic images by blurring the background and emphasizing or visually isolating the subject by rendering distracting clutter virtually unrecognizable. Larger apertures also come in handy when you have a low light situation but need to maintain a higher shutter speed. Aperture numeric series (f - numbers) f/1 f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32 f/45 f/64 f/90 Upon looking at this table you may begin to see a pattern, a repetition from one number to the next. Now, photography is replete with series of numbers that indicate either a doubling or halving of an increment of exposure. This simple ratio makes it much simpler to maintain equal adjustments in exposure. This series of f-numbers is geometric which means that this series of numbers is formed through either multiplication or division. The constant in this series is the square root of 2, or 1.414. Each f-number is the product of the previous value multiplied by 1.414. But most importantly, each numeric increase in this series is a halving in size of the preceding aperture’s geometric area. Conversely, each numeric decrease is a doubling in size of the preceding aperture’s geometric area (see the aperture geometric area table). Try removing your lens from the camera and looking into the end of the lens that mounts up against the camera body. You can then observe the change in aperture size as the aperture ring is turned. Why does a larger aperture have a smaller number and a smaller aperture have a larger number? Try to remember that the f-number is simply the number of aperture diameters in the lens’ focal length. There are 2-22’s in a 45mm lens so an aperture setting on this lens with a wide aperture diameter of 22mm would be f/2. There are 11-4’s in a 45mm lens so an aperture setting on this lens with a small aperture diameter of 4mm would be f/11. Remember, the smaller the number, the larger the aperture and the larger the number, the smaller the aperture. HOPE IT HELPS.

Although you have the 40D camera, but this tutorial should give you some idea. Go to chapter 5 http://www.usa.canon.com/content/rebelxt_tutorial/rebelxtlessons.htm

I found it a hard concept at first, too. Try putting your camera on a tripod and work your way from one end of the aperature spectrum to the other (do it in AV mode) taking a picture of several objects at varying distances (3' to 20'). Then, look at them on the computer to see the difference. Eventually you get a feel for it. After that, try it outside on more distant objects using 135mm. You'll find that more objects stay in focus at a greater distance.

the larger the f-stop number the smaller the aperture will be, and therefore the greater the depth of field will be. Pretty much just remember that the bigger the number the bigger the depth of field, and the smaller the number the shorter the depth of field. I even have to stop and think like that sometimes

Wow Walrath, that really broke it down for me. Thank you everyone for taking the time to explain it to me...I think I have the basic idea now, and have an idea of what I have been doing wrong. Thanks again, Saltface I will do your suggestion with the Tripod tomorrow. Thanks so much...

Remember on the 40D you have 'Depth of Field Preview Button' located on the front of the camera, just below the lens release button. Try looking through your viewfinder with the aperture wide open, and hold it down. Notice how much of the image is in focus. Then step the aperture down a notch or two, and hold the DoF button down once more. Your viewfinder will get darker, but you should be able to see that the out-of-focus areas of your shot have just gained some clarity. Hope this helps.

I feel like an idiot. I shoot film but should have remembered that the DOF preview will be there on a DSLR, too.