[Note: I’m bringing this post ‘back to the front’ because it’s on my list of essential-that-you-know-this stuff for beginning shooters. Originally published in March of 2010, the information about aperture and depth of field is all still up-to-date and relevant. –/DiscerningPhotog]
Aperture: Now there’s a real ‘Photography’ word! Do you understand what ‘aperture’ refers to and how it affects your photography? Here I’ll explain exactly what it is and why you should care.
Simply stated, “aperture” is the hole in your lens that allows light to pass through to your CCD (charge coupled device) during an actual digital camera exposure. The size of the hole has a big impact upon your photos.
So what does this mean for your shooting?
Let’s start with how aperture works in your lens/camera setup.
If you’ve ever taken a lens off of a DSLR, held it up to a light and looked through it, the ‘default’ position that the lens will be in is at the largest, maximum aperture(sometimes even larger than the maximum aperture). If you’re looking through a 200mm f4 lens, for instance, then the wide-open hole will correlate with the ‘f4’ position on the camera. This f4 position will let the most light in, and also be the position with the shallowest depth of field. This just means that except for your point of focus, most of the other elements within your composition will tend to be out of focus. (‘Bokeh’ photography is all about this type of shallow depth of field shooting.)
Higher f-stop numbers correspond to smaller aperture holes: f2.8 or f4 will be a relatively large aperture (hole), while f11 or f16 will be a much smaller hole.
With a modern DSLR, you can see this by stopping your lens down to f8 or f11, then peering through the front element and pushing in the depth-of-field preview button. You’ll see the wide-open aperture hole suddenly contract down to a much smaller octagonal shape.
Try different f-stop numbers and you’ll quickly get a sense of how this works.
How does this affect your images? As you stop the lens down, say to f8 or f11, the hole gets smaller, and more things in front and behind your point of focus will appear sharp in your photograph. So large apertures = shallow depth of field; small apertures equal larger depth of field in your photograph. This can be a good thing or a bad thing, depending upon the photo you’re trying to shoot. The trick is learning to be in control of this phenomenon.
To really start to see how this can work, I suggest you operate your camera in a fully manual mode. As you change the aperture, making it larger or smaller, you’ll need to make a corresponding change to the shutter speed you’re using, since these two adjustments are dynamic and always work together. For instance, if your exposure is 1/125 sec @ f 8 and you want a shallower depth of field, you’ll need to open up the aperture, say to f2.8 or f4. Since you’re now letting all of that extra light into the exposure, you’ll need to increase the shutter speed to compensate and keep the overall exposure correct. I’ll address the creative and critical importance of shutter speed in another post, but for now, just use your camera meter to make the correct adjustment when changing the aperture.
I have some examples here that illustrate how important this can be. In this first set of photographs, the image on the left was shot at 200mm with an aperture of 2.8. The background is thrown completely out of focus, allowing your eyes to immediately go straight to the faces and butterflies. In the second image, the focal length is still 200mm, but now the aperture is f14. Suddenly all of the distracting detail in the background comes more into focus, in effect competing for your attention with the important foreground detail. So in this case, all of that depth of field would be a mistake, ruining an otherwise decent image.
In the second set of images, the opposite is the case. In the driftwood still life, more depth of field is a good thing.
It helps to know that every time you move your aperture a full f-stop number, you are doubling or halving the amount of light the exposure will use. On older camera lenses, this was easy because the f-stops were the clicks on the lens barrel:
F1.4/f2.0/f2.8/f4/f5.6/f8/f11/f16/f22 was a normal sequence of f-stop numbers. So changing from f8 to f5.6 doubled the size of the aperture, allowing twice as much light in.
This is trickier with modern digital cameras, since most of them provide third-stop increments and there are no more clicks on a lens barrel: between f5.6 and f8, you’ll also have options for f6.3 and f7.1, for instance. Don’t worry about the actual, weird numbers, just know that these are third-stop increments on the f-stop continuum.
So go out and experiment with this yourself! Find images that benefit from the shallowest depth of field (sports action shots and wildlife are two that come to mind) and shoot them at the widest aperture possible. Now stop your lens way down to say f11 or f16, adjust your shutter speed accordingly, and shoot away. What happened? Were your new images better or worse than the originals? Which version worked? Try the same thing with something that needs lots of depth of field: a landscape, a field of flowers, maybe a still life. Shoot some images both with and without lots of depth of field. Which ones work better? As you experiment and play around with aperture and depth of field, you’ll start to see how fundamental this is to all of your shooting.
Now get out and make some great images!
Hi, I’m Andrew Boyd, a.k.a. The Discerning Photographer, and I hope this post has been interesting and informative. Please leave me a comment about it, let me know what you’d like to see more of on the site! You can also sign up for email delivery of all future articles or my RSS feed. Thanks!–DiscerningPhotog
Related articles on the web:
Aperture, Shutter Speed & ISO at The Discerning Photographer
Depth of Field tutorial at CambidgeinColour.com
Six Tips for Controlling Depth of Field at Epic Edits
Nothing Could Be Finer at Serious Amateur Photography
Aperture, Shutter Speed and ISO at YourPhotoTips