We've all been wowed by the lovely depth of field (DOF) in photos - in fact a lot of you probably bought a DSLR for that very reason, am I right? So how come your photos don't quite look like you want them to? How come the dreamy blur always ends up on the wrong part of the shot? How can you get great depth of field in your shots?
There are 3 basic concepts that you have to understand to master the depth of field.
I know you've heard about Aperture and DOF before - that's the reason why you have an 'A' setting on your camera, right? When first using a DSLR it's so tempting to open that aperture as wide as it goes (choose the smallest number) and hope for dreamy background blur.
Let me show you a little diagram about how the aperture controls the range of blur in an image:
The key here, is understanding that there is a certain amount of light which is close enough to the 'true point' of focus that it will still be perceived as clear (these rays fall between the green lines in the diagram). A small aperture means that a greater proportion of the light's rays are close to the true point of focus and therefore a greater proportion of the image's depth appears clear. A larger aperture, however, will mean that a larger proportion of the light's rays are further from the true point of focus. This results in a greater proportion of the image's depth appearing blurry.
The thing to remember with aperture is that you don't always want to open it up as wide as you can go, but you can use it to control the amount of blur and the amount of focus you have in your image.
2. The point of focus.
So, now that you understand the distance over which an aperture will focus, you can appreciate how important it is to focus correctly in the first place. The point of focus is so important when composing your image, and should be something that you are aware of, and alter accordingly, every single shot.
One way to do this is to set your camera on a central auto-focus point. Place that point over the part of the image that you want in focus, half press your shutter, re-compose the shot, and then fully press the shutter.
This will work for images where the depth of focus is quite large, however with a wide aperture, even small movements of the camera can be your undoing. In these cases, it is better to manually set the focus point in your camera to lie over the point in your image that you need to be clear. It takes some practice for this to become second nature, but I assure you it's worth it!
If you are leaving your camera on automatic focus point selection, it is very difficult to get well composed, well focused images. You need to take control.
Bonus tip: With portraits, always focus on the persons eye which is closest to you.
3. The focal length/distance from the subject.
If you have played around with prime lenses (lenses with a fixed focal length and no zoom capability), you may have noticed how the depth of field seems to vary from one focal length to the next. The reason for this is more to do with the distance you are forced to stand from the subject with different lenses rather than the focal lens itself.
A shorter focal length (eg a 35mm lens) will give you a wide field of view due to lower magnification. This means that you will fit a lot into your shot, and if you wanted to take a photo of a person standing in a field you would be able to keep a lot of the shot in focus from a fairly close distance.
If you then set up the same shot with a 200mm telephoto lens, you would need to move much further away to have a composition which matched the original 35mm shot. The range of focus, or depth of field, decreases as the magnification of a lens increases. As a result, the telephoto lens will have a much smaller range of the shot in focus than the 35mm, and will give you that creamy background blur which makes a foreground subject stand out. Conversely, the short focal length lens will allow great clarity throughout the depth of the shot.
If you have been frustrated with the results of your attempts to create the perfect depth of field in your shots, I hope this has given you some pointers to focus on with your next images.
Allow your aperture and lens choice to work together to control your depth of focus, and remember to focus purposefully!
Emma Anderson. Creating, learning, growing and taking photos of it all!
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