For the past eight years, I’ve taught a class titled : Introduction to Digital Photography. The emphasis being on understanding camera controls and how they effect the photo. The goal of the class is for the student to understand how to get the best possible photo in the camera. I don’t want students relying on Photoshop or other image editing software to correct their mistakes that can be avoided by using the correct camera settings.
Most students taking my class have DSLR or Mirrorless cameras with an APS-C sensor. These cameras often come with an 18-55mm f3.5-5.6 zoom lens or similar lens. Some students have difficulty adjusting their cameras to accomplish a blurred background. Blurring the background allows the subject to stand out – making for a better composition. Blurring the background can be more easily accomplished with a 50mm lens that has a large aperture in the range of f/1.2 to f/1.8. Lenses with these ranges of aperture cause an effect of very soft out of focus background called Bokeh.
However, you don’t necessarily need a lens with f/1.2 to f/1.8 aperture to accomplish blurred backgrounds, if you understand how the lens focal length, aperture and distance to the subject contributes to achieving a blurred background. It’s possible to get a soft focus background with an 18-55mm, f/3.5-5.6 zoom lens. It just takes a little practice and knowledge about your camera.
In my blog post on depth of field (DoF), I discussed in detail how your camera‘s settings: lens focal length, aperture and distance to the subject effect depth of field.
Here’s a Quick Review:
Depth of Field is defined as the area in front and behind the subject that is in focus.
– The larger aperture, the smaller the DoF for any given focal length lens and subject distance.
– The longer focal length, the smaller the DoF for any given aperture and subject distance.
– The closer you are to the subject the smaller the DoF for any given aperture and focal length lens.
In this blog I worked with three cameras and the kit lens most often purchased:
(1) Olympus E-5 DSLR with a MTF 4/3 sensor with a 14-42mm f/3.5-5.6 lens
(2) Nikon D3300 DSLR with an APS-C sensor with a 18 -55mm,f/3.5-5.6 lens
(3) Nikon Z6ii with a full frame sensor with a 24 -70mm f/4 lens
First, I’d like to get a little technical. Sorry, it will pay off with a better understanding of your camera. The majority of my students have cameras with one of these three sensors: Full Frame, APS-C, and Micro Four/Thirds (MFT). The illustration below shows the relative size of the sensors.
Note: Canon and Nikon use slightly different-sized APS-C sensors. The difference between the Canon and Nikon is small and doesn’t have an appreciable effect on the resulting photo DoF.
Let’s look at the Nikon D3300. This camera has 18-55mm f/3.5-5.6 zoom lens and APS-C sensor. The aperture varies from f/3.5 at 18mm to f/5.6 at 55mm. However, when using a sensor such as an APS-C the effective focal length will be longer than what is printed on the barrel of the lens. Why? The smaller sensor has a narrower field of view than what the lens is capable of viewing. Cameras with sensors smaller than a full frame sensor (35mm film size) are referred to have Cropped Sensors
Crop Sensor vs Full Frame (smaller angle of view)
A Crop factor is used to calculate the actual focal length for sensors smaller than Full Frame (35mm film size). Do you remember from high school geometry the Pythagorean Theorem, a2+b2=c2? That formula enables you to calculate the hypotenuse, i.e. long side of a right triangle (diagonal).
So for a Full Frame sensor: 36mm2 + 24mm2= 1872= c2, so, c=square root of 1872 = 43.3mm. To calculate the equivalent focal length, multiply the focal length used by the crop factor associated with your camera’s sensor. (See table below)
Note: With a smaller diagonal the effective focal length is longer. Equivalent focal length will be noted throughout this article by dimension with (##mm) such as 18mm (27mm) for a 1.5 crop factor.
Let’s take a look at six photos below: two from each of the test cameras. Each camera was set up 4 feet from the subject (larger flower vase) using the largest aperture and shortest focal length available from the lens. The back wall was approximately 10 feet from the camera. Looking at the top row of photos they are very similar. When we compare the cropped images at the bottom we see that the background is blurred approximately the same for Full Frame, Nikon APS-C, and Olympus 4/3. So all three cameras yield about the same resulting photo for the setting noted in each photo.
Note: The DoF graphs below were calculated using a Depth of Field cellphone app. There are many available for Apple and Android phones, as well for computers on the web.
Let’s look at the three graphs above, each shows the near and far focal distance for each of the test cameras at the shortest focal length and largest aperture available with the kit lens. The orange line is the point at which the background starts to become blurred, so at 4 ft. subject distance (as was used in the photos above) the Full Frame, APS-C, and MTF 4/3 sensor all have values of just over 5ft. We can see that the background is at least somewhat blurry for all three. The blue line is the point in front of the subject that is sharp. The depth of field is the vertical distance between the two lines for any given distance to the subject. Also, it’s important to note that for smaller sensors the orange line rises more quickly. Compare the orange line at 8ft. to the subject, for each of the sensors: full frame=16 ft., APS-C=18 ft., and MTF=26 ft. We can conclude that with a smaller sensor, it’s more difficult to blur the background because the depth of field gets wider as you move further from the subject.
Now let’s look at DoF Far Focal length curves for the Nikon D3300 DSLR with an APS-C sensor with an 18 -55mm,f/3.5-5.6 lens.
The above chart shows the relationship between the distance to the subject and the distance behind the subject where the background begins to be blurred. Each line is represents the largest aperture available at each focal length. If we look at the blue line, 18mm (27mm) f/3.5, we see at a distance of 10 feet to the subject anything within 32 feet from the camera is sharp; for the orange line 24mm (36mm) within 17ft, gray line 35mm (52.5mm) within 14ft, yellow line 45mm (67.5mm) within 12ft and light blue line 55mm (82.5mm) 12ft. By using a longer focal length 55mm (82.5mm) the background is blurred at a distance closer to the camera.
Blurring the background, as I mentioned earlier allows for the separation of the subject from the background. This is most often used in portrait photography. Most portrait photographers use a lens with focal lengths, between 70mm-135mm. If you have an APS-C sensor camera with a maximum 50-55 mm focal length setting on your lens you have the equivalent focal length of 75 mm to 83.5 mm. This is at the lower end of the range preferred by portrait photographers.
Now here’s the trick to help compose your photo:
1) Compose you photo using 50-55mm focal length.
2) Review the photo taken on your camera LCD screen.
3) If the background is not blurred to your liking have your subject move closer to you while you move away from a subject while maintaining the same distance between you and the subject. By doing this you are increasing the subject’s distance from the background thus blurring the background more. You may need to repeat step 2 and 3 again to get the desired effect.
(Note: If part of your subject is slightly out of focus you can select a smaller aperture which increases the DoF slightly.)
Now look at the two photos below, taken with an APS-C sensor, Photo (1) was taken f/5 .6 focal length 50 mm (75mm) 5 feet from the subject. Photo (2) was taken with the exact same settings except the subject was moved 3 feet further from the background. Note that the window is blurred more in the photo (2)
I hope this post helps you to better understand DoF and how to blur the background.
References for Future Study:
http://Understanding Crop Factor | B&H Explora (bhphotovideo.com)
https://photographylife.com/what-is-depth-of-field
photos available for purchase at FineArt America
© Tom Wiggins Photography 2015-2023