Thursday, March 19, 2009

photography 201 - working with shallow depth of field

I recently purchased a 50mm/f1.4 lens for my camera. I love the lens for portraits, especially since with the maximum aperture you can really blur any distractions in the background and achieve some fabulous light and colors in your photos.

But take a close look at this picture - this was taken with an f-stop of 2.5, and I'm about 4 feet away from my 3 year old. I was attempting to focus on her face, but only the sleeve of her sweatshirt is actually in focus (and maybe some of her hair). I seem to be having this problem now that I have a lens with an f-stop greater than 3.5, so I decided to do some research and see what the problem might be (assuming I know how to focus my camera, which I do. Most of the time).

The answer? Shallow depth of field. Shallow depth of field is where a very small portion of your image is actually sharp while the rest is blurred. If you are shooting a landscape photo, for example, you would want the widest depth of field you can achieve so that the entire image is sharp. But for portraits (which I take more frequently) I love images where the face, especially the eyes, are sharp but the rest of the image is blurred. FYI the quality of the blurry portion of the image is referred to as the "bokeh" in a photograph, and while there seems to be various debates on the quality of one's bokeh, we'll stay out of the debate for now.

A friend of mine asked me why when she would take a picture of 2 or 3 kids using her 50mm/f1.4 lens, one of the faces would be sharp while the others would be blurry or out of focus. The answer is too shallow depth of field, which allowed for only a razor-thin portion of the photograph to be in focus while the rest becomes more progressively blurry (see my picture above).

So how to fix this? You need to make adjustments to the following: (1) aperture setting, (2) focal length OR distance between you and your subject, and (3) ISO setting.

If you are using the automatic setting on your SLR, change to aperture priority setting (or manual setting) and increase your f-stop number (thereby decreasing the amount of light entering your lens/camera). For example, the aperture setting for my photo was f2.5 - pretty large. If I had used f3.5 or f4.2 I would have more of the image in focus, rather than just my daughter's sleeve.

Here are some images of various f-stop settings:


Another option, since I was using a fixed focal length lens, would have been for me to back up a little (although since she was climbing up something and I was by myself with her, I didn't want her to fall...). If you are using a variable length lens, try decreasing the focal length to see if your image is sharper. As a general rule, the longer the focal length the shallower the depth of field. For example, a 200mm focal length will produce a more shallow depth of field than a 50mm.

Also, the lower the ISO setting the greater depth of field you can achieve. So if you are shooting at 800 ISO, try lowering to 400 or 200 and see if your images become sharper.

Any questions? Leave me a comment.


Angela September 20, 2009 at 7:15 PM  

Just found this post. I thought I had read everything on your blog...goodness knows how I missed this one! I am just looking at buying a Canon 50mm f/1.8. Not sure why I think I need it but I think I do need it!!! I love the bokeh it gives and I am not getting that from my kit lens at all. Thanks for the info...I think I will get one as the majority of my photos are portrait.