Investigating Blurred/Bokeh Background.

To shoot a subject with a blurred background, it's important to understand what's Depth Of Field (DOF), so the title of this post might as well have been 'Investigating Depth Of Field'. The trick is to have small / shallow DOF, so that only your subject is sharp and in focus, while the background is out of focus and blurred.

79.4mm (447mm), F3.8, 1/100s, ISO400

According to the experts, generally, in order to have a shallow DOF, you would have to increase your aperture. The bigger your aperture, the smaller the DOF. Once you have a small DOF, you'll be able keep your subject sharp while the background is blurred.

With that, I zoomed out my FZ28 to as wide as possible, when only then it'll let me set it to the widest aperture possible at F2.8. So with such a setting I'll be able to shoot my blurred backgrounds, right? Well, as I found out, only occasionally, but not the rest of the time. Why is this? What did I do wrong?

To get the answers, I decided that it's time to have a long look at the mysteries of DOF for the FZ28. I started at DOFMaster.com, probably the only DOF calculator / site you'll ever need. Through their tables and equations page, I found out that you can actually calculate (pretty accurately) the near distance (listed as 'Near' in the illustration below) of acceptable sharpness and the far distance of the same. Once you have the near and the far, you can then get your DOF width. Armed with these information, you can then find out what settings will get you the smallest DOF.

It turns out that DOF is not just a function of the aperture alone, but is also a function of the focal length and subject distance. With focal length, the FZ28 starts at 4.8mm (27mm @ 35mm equivalent) and as you zoom, goes all the way out to 86.4mm (18x, 486mm @ 35mm). This means that the more zoom you have, the higher the focal length.

With subject distance, the closer a subject, the smaller the DOF (focal length & aperture remains constant). Because we are only interested in getting a small DOF, we shall only look at minimal subject distances. The FZ28, does impose minimal subject distance requirements, meaning the FZ28 is unable to focus on anything closer. If you zoom using the FZ28, you'll see the zoom magnification and minimum focus distance on the LCD, and these are the minimum subject distances we will be using. Bear in mind that if you press the macro mode button, the minimum focus/subject distance will change, but it'll not be covered for now.

Focal Length mm 4.8 9.6 14.4 19.2 24.0 28.8 33.6 38.4 43.2 48.0 52.8 57.6 62.4 67.2 72.0 76.8 81.6 86.4
35 mm 27 54 81 108 135 162 189 216 243 270 297 324 351 378 405 432 459 486
Zoom 1x 2x 3x 4x 5x 6x 7x 8x 9x 10x 11x 12x 13x 14x 15x 16x 17x 18x
Subject
Distance (m)
0.3 0.3 0.4 1.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.0
f
Stop
F2.8 0.11
F3.2 0.13 0.03
F3.4 0.14 0.03 0.03
F3.6 0.15 0.03 0.03 0.10 0.25
F3.7 0.15 0.04 0.03 0.10 0.25 0.18 0.13 0.10 0.08 0.06 0.05 0.04 0.04 0.03
F3.8 0.16 0.04 0.03 0.10 0.26 0.18 0.13 0.10 0.08 0.06 0.05 0.04 0.04 0.03 0.03 0.02
F4.0 0.16 0.04 0.03 0.11 0.28 0.19 0.14 0.11 0.08 0.07 0.06 0.05 0.04 0.03 0.03 0.03 0.02
F4.4 0.18 0.04 0.03 0.12 0.30 0.21 0.15 0.12 0.09 0.07 0.06 0.05 0.04 0.04 0.03 0.03 0.03 0.02
F5.6 0.25 0.05 0.04 0.15 0.39 0.27 0.20 0.15 0.12 0.09 0.08 0.07 0.06 0.05 0.04 0.04 0.03 0.03
F8.0 0.42 0.08 0.06 0.22 0.56 0.38 0.28 0.21 0.17 0.14 0.11 0.09 0.08 0.07 0.06 0.05 0.05 0.04
  • The black cells shows DOF (width).
  • FZ28 increases the minimum f stop you can use as you zoom in (e.g. at 86.4mm minimum f stop is F4.0)
Convert to inch

As can be seen from the table, the smallest DOF is not at max wide (4.8mm) and max aperture (F2.8), rather, if you zoom out slight at roughly 2x or 3x, you'll get much smaller DOF, with roughly the same subject distance and as wide an aperture as the FZ28 allows (F3.4).

You will also notice that the FZ28's minimum focus / subject distance goes in steps, 0.3-0.4 meters for 1-3x, then 1.0 meters for 4x, and after that 2.0 meters for everything else. In each step, the smallest DOF can be had when you have the largest zoom and therefore largest focal length. If you look at the values between 6x zoom and 14x zoom, at F3.7, the smallest DOF is at 14x zoom. In simple words, with all else being the same, a larger focal length (higher zoom) will give you a smaller DOF.

Also notice that a change in aperture actually brings only a small reduction in DOF, while a change in focal length will bring a large reduction in DOF. If you look at 4.8mm focal length column, changing from F3.8 to F3.2 causes a 0.03m (small) change in DOF. But if you zoom in to 9.6mm focal length (2x), you can have roughly 0.1m (large) change in DOF.

How to Shoot Blurred / Bokeh Background

  • Use aperture priority mode and set to as big an aperture as allowed by the camera. Big aperture means lowest f stop values, and remember the bigger the aperture, the smaller the DOF.
  • Place your subject as close to your camera as is focusable by your camera. The closer a subject is to your camera, the smaller the DOF.
  • Zoom in as much as possible to have the highest possible focal length. Remember that higher focal lengths results in smaller DOF.
  • If your subject is less than 2m away, your best bet is to place your subject at 0.3m to 0.4m away, zoom in at 2x or 3x, but not 4x, and then have as large an aperture as the camera allows.
  • If your subject is 2m or more, get your subject to as near to 2m as possible, zoom in as much as your composition allows with the biggest aperture.

Armed with these knowledge, I went out to see if my calculations and understanding can be translated into the real world:

Pic A: 4.8mm (27mm) 1x, F2.8 Pic B: 17.4mm (98mm) 3x, F3.6 Pic C: 86.4mm (486mm) 18x, F4.4

If you look at Pic A, blurring in the background is minimal and DOF is still rather wide. With Pic B, there is obvious blurring but it's not as nice as Pic C where the background blurring gives focus to your subject. All pictures were taken at minimum allowable distance, namely 0.3m, 0.3m and 2m respectively. All were cropped to get roughly the same view and auto focused. As you can see, the FZ28 works roughly as expected.

1 comment:

  1. nice write up. Convert image to Blurry Background using GIMP.

    ReplyDelete