AboutScott Valentine Expertise Most anything about digital post processing (Photoshop, etc), effects, and image restoration, some technical information about digital imaging systems, light and electron microscopy, optical physics and image analysis. I can help with shooting basics, lighting setups, and getting the most out of your digital camera. I am also an 'expert' here for Photoshop (Computers/Technology > Software > Graphics Software > Photoshop).
Please - NO operating, trouble-shooting, or purchase questions on point-and-shoot or video cameras.
Experience Author, "Real World Compositing with Photoshop CS4" (available January 2009 from Peachpit.com). I've been an image analyst and advanced amateur photographer for over 6 years. I run an Adobe user group, focusing on digital media, and have lectured on digital image capture systems. Currently, I am an administrator for a rather large instructional Photoshop forum
Organizations Adobe User Groups, National Association of Photoshop Professionals, Advanced Imaging Specialists
Publications CommunityMX.com, Real World Compositing with Photoshop CS4 (Adobe Press)
Education/Credentials Bachelor of Physics, University of California, San Diego
Awards and Honors Winner: Best in Show and several category first place awards - Imaging Professionals of the Southwest Print Competition (multiple years)
Question Hello Scott,
I am interested in outdoor photography. I have a Canon EOS Rebel Xti digital camera. I want to know how to get the most effective blurry background from this camera. I have an 18-55 mm lens & 70-300 mm lens. I've tested the 50 mm lens with my family members, but the background can still be seen clearly. Which lens will take better pictures with a great amount of blurry background? Thank you so much and I hope to hear from you soon!
Answer Hi, and thanks for your question!
The term you are looking for to describe the blurred background is called "depth of field". It has to do with the physics of lenses and apertures, rather than the actual quality of the lens. In a camera lens, there is the actual glass that focuses the light, and at the back of the lens is a variable opening called an aperture. The aperture affects the depth of field based on the 'length' of the lens.
Your camera manual will have instructions on how to set the aperture manually, so go grab it and sit down with your Canon to get familiar with the settings. I highly recommend you set your camera to Aperture Priority mode so you can use the scroll wheel to adjust the aperture - the camera will automatically adjust the shutter speed and possibly even the ISO (sensitivity of the image sensor to light).
Aperture is measured in relationship to the length of the lens, as a fractional value. A setting of 1.8 means the diameter of the aperture is found by dividing the length of the lens by 1.8 (commonly seen as "f/1.8"). The bigger that value, the smaller the actual size of the aperture. But be careful; when someone says "larger aperture", they mean the actual diameter is large, but the value you set is small. So, a setting of f/22 means the aperture is very small, but you might talk about the setting as "22". It can be confusing, but just remember the relationship.
So, a large aperture (small aperture number) gives a "narrow" depth of field. That means the area of focus is very small in terms of distance; everything outside of the exact focal plane gets progressively more blurry. You can test this by taking a picture of meter stick or ruler, using a tripod. Set the tripod and camera up just above the level of a table, pointing slightly down. Set the ruler pointing away from the camera, as if you were measuring the distance from the lens to some object you want to shoot.
Now, start taking pictures without moving the camera or ruler. Focus on the closest part of the ruler that you can so you can read the numbers clearly. Set your camera to aperture priority mode with the smallest number (largest aperture), and start shooting. After every shot, change the aperture value by one increment. As you move from 1.8 (or whatever) to 22 or higher, you will see the change in depth of field. It will be important *not* to change the focus at all so you get a good feel for the settings.
Once you have taken your pictures, use a program on your computer that can read the meta data - the information that tells you what settings your camera used for each picture. Doing this, you can study exactly how much blur is associated with each aperture setting. You can then repeat the experiment changing other settings, like focal length (try moving the camera backwards after each shot, then zooming in slightly so the object or ruler stays the same size in the viewfinder).
If you keep experimenting and taking notes, you will learn much more about your camera, and how to imagine your photos before you actually take the picture.
You might want to consider taking an introductory class on photography to get the most out of your hobby. You can also check sites like dpreview.com for forums that let you interact directly with many other enthusiasts to compare notes and gain insight. I use these kinds of forums all the time!
I hope this helps you understand the concepts, which should get you on the road to taking exactly the picture you want. Remember - larger aperture (small number) gives a narrow depth of field (more blur). There are other details you will need to know about, like the relationship between aperture and shutter speed for proper exposure, but sticking with aperture priority at first will help you concentrate on depth of field.
Please let me know if I can help in any other way, or clarify anything above.
