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Let’s say that perhaps you’ve been taking photos for a while now. You’ve gotten yourself a good DSLR camera and have recognized that the standard 18-55mm kit lens that comes with your camera is nice, but just doesn’t give you the shots that you are looking for.
So you plunk down your money on the ever-popular 50mm f/1.8 prime lens that everybody is talking about, mount in on your camera, change your aperture to its widest (f/1.8) setting and start shooting. You spend all day shooting with this wonderful little lens and then you get home and put them on your computer and realize that 80% of your shots are out of focus.
In the past, when this used to happen to me, I would reason that shooting wide open was just not possible, because I ended up with too many shots that were out of focus. I incorrectly reasoned that I always needed to close down my aperture when shooting portrait subjects, or they would end up out of focus because the shallow depth of field was just too unusable wide open. For a while, I only used my 50mm 1.8 lens at f/4 because it was the widest aperture that I trusted to get the shot in focus. Crazy yes, I know. But then I figured out something that has changed my use of wide-aperture lenses forever.
Before we continue, let’s break down the meaning of “wide open” and “fast prime lenses”. To shoot “wide open” means that you are choosing to photograph at your lens’ widest aperture setting or f-stop. On a lot of lenses, the widest aperture is listed somewhere on the lens itself with Canon usually listing it on the front of the lens, and Nikon listing this information on the body of the lens. Generally the ration looks something like this: 1:2.8 or 1:1.8. (See photos)
A “fast prime lens” is one that has one focal length (does not zoom) and has “fast” light-gathering ability (due to its wider apertures). Most photographers consider a fast lens to be one with an aperture number of f/2.8 or wider (the smaller the number, the wider the aperture). Two of the most popular features of fast prime lenses are their ability to obtain beautiful out of focus backgrounds and shallow depth of field, as well as their ability to handle low-light conditions because of the aforementioned large apertures.
Let me let you in on a little secret about shooting wide open – it’s about the distance to your subject. Most people learn that wide-aperture lenses blur the background and let in more light, but they never understand that the really neat shallow depth of field created by their lens is also affected by another factor; how close they are to the subject.
You won’t find many manuals on subject to camera distance. It’s kind of an assumed topic that doesn’t get enough attention. Let’s look at it as simply as possible: the closer you are to your subject, the shallower the depth of field is relative to your chosen aperture. In other words, if you are shooting at f/1.8 and you are 20 feet away from your subject, you will have MORE depth of field than you will if you are shooting 2 feet away from your subject.
To get more mathematical, if you’re using a 50mm lens at f/1.8 and photographing something at 4 feet, your depth of field will be around 1.5 inches deep. But if you photograph that same subject from 10 feet, you will have a depth of field of just under 10 inches deep.
The right image cropped to similar framing as the left. Notice the increase in depth of field on the hair and ears, and also the reduction in lens distortion.
With this information, it is also very important that you get to really know your lens and its abilities. For instance, if you happen to know that you shoot a lot of portraiture close to your subjects, be aware of how much depth of field your lens gives you at three feet, four feet, and so on, when shooting wide open. In time, with experience, you will be able to immediately predict the depth of field your lens will give you based on the distance you are away from your subject.
The depth of field does increase slightly in the right image, but not as dramatically as the 35mm lens due to the 85mm longer focal length.
In conclusion, you can see that the reason your photos might be coming out blurry would be because of your distance to your subject when shooting wide open. So the next time you find yourself frustrated at your results shooting with that wide-aperture lens at its widest aperture, take a step or two back. You might like the results.