Focal length and depth-of-field

As photographers we spend time considering how we want to compose an image. We often think about balance, leading lines, ‘rule of thirds’ and depth-of-field. Typically we associate aperture settings with depth-of-field. One of the factors we sometimes overlook is how lens focal length and depth-of-field are related.

First let’s consider how changing aperture affects depth-of-field and image separation. We’ll have a look at a series of images taken with a Nikon 1 J5 and the 1 Nikon 32mm f/1.2 prime lens at increasingly smaller apertures.

I focused on the horizontal petal on the left hand side of the image in each photograph in the series.

NOTE: Click on images to enlarge.

Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/2.8, 1/2500, ISO-800
Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/2.8, 1/2500, ISO-800

At f/2.8 I was able to get enough of the flower in focus for an acceptable image. As you look through the series notice how much of the flower and the background are in focus.

Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/4, 1/1250, ISO-800
Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/4, 1/1250, ISO-800

The above image was taken at f/4.

Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/5.6, 1/500, ISO-800
Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/5.6, 1/500, ISO-800

The image above is at f/5.6.

Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/8, 1/320, ISO-800
Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/8, 1/320, ISO-800

We’ve stopped down further for the image above, now at f/8.

Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/11, 1/160, ISO-800
Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/11, 1/160, ISO-800

The one above is at f/11. How are you finding potential softening from diffraction in the image?

Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/16, 1/60, ISO-800
Nikon 1 J5 + 1 Nikon 32mm f/1.2, f/16, 1/60, ISO-800

And the last one at f/16. At a shutter speed of only 1/60th on a windy day I had to wait for a reasonably still moment to capture the image. How is potential diffraction impact at f/16?

The images above illustrate a couple of things. The first is how much more of the subject and background are in focus with each change in aperture. The second is how dramatically the shutter speed used was affected by changes in aperture, going from 1/2500th in the first image, down to 1/60th in the sixth image.

Aperture does affect depth-of-field as you can see from the set of images above. The distance of the camera from the subject, and the distance of the subject from the background also impact depth-of-field.

Let’s look at four more sample photographs. These next images were taken with the same camera, using the same lens set at the same aperture. The lighting changed slightly for one of the images, but the shutter speeds are very similar.

Nikon 1 J5 + 1 Nikon 10-100mm f/4-5.6 @ 10mm, efov 27mm, f/5.6, 1/800, ISO-800
Nikon 1 J5 + 1 Nikon 10-100mm f/4-5.6 @ 10mm, efov 27mm, f/5.6, 1/800, ISO-800

In the image above you can clearly see that at a focal length of 10mm not only are the two blooms in focus, but much of the background is easily discernible and is distracting.

Now, let’s look at the same two blossoms shot with the same lens, at the same aperture, but at a longer focal length.

Nikon 1 J5 + 1 Nikon 10-100mm f/4-5.6 @ 100mm, efov 270mm, f/5.6, 1/800, ISO-800
Nikon 1 J5 + 1 Nikon 10-100mm f/4-5.6 @ 100mm, efov 270mm, f/5.6, 1/800, ISO-800

Given the windy conditions I did my best to try to capture the blossoms at roughly the same size in the frame. You can see how using a much longer focal length and having to move away from the subject slightly has dramatically changed the depth-of-field in the image. There is still a vertical fence board visible in the upper left corner but it is much softer and muted.

Let’s look at two more examples. The next one was captured at a 10mm focal length.

Nikon 1 J5 + 1 Nikon 10-100mm f/4-5.6 @ 10mm, efov 27mm, f/5.6, 1/800, ISO-800
Nikon 1 J5 + 1 Nikon 10-100mm f/4-5.6 @ 10mm, efov 27mm, f/5.6, 1/640, ISO-800

You’ll notice the same distracting effect from the background when a wide angle focal length was used.

Now, the same two blossoms captured at the same aperture, with the same lens, but this time at a focal length of 100mm.

Nikon 1 J5 + 1 Nikon 10-100mm f/4-5.6 @ 10mm, efov 27mm, f/5.6, 1/800, ISO-800
Nikon 1 J5 + 1 Nikon 10-100mm f/4-5.6 @ 100mm, efov 270mm, f/5.6, 1/800, ISO-800

These last four images demonstrate how using longer telephoto focal lengths can dramatically compress depth-of-field, and how using wider focal lengths give us increased depth-of-field.

Remembering these simple facts when composing images can help us dramatically change the overall look of our images.

If you’re like me you’ve likely heard photographers claim that you “must have” fast prime lenses if you want to create separation between the subject in your image and the background. While changing aperture does impact depth-of-field and image separation, you don’t “have to” spend money on fast primes to create this visual effect.

Personally, I would much rather change the focal length I’m using and adjust the distance from my camera to the subject, and the subject to the background to create separation in images. I feel this approach gives me more flexibility since I don’t have to adjust my exposure at all.

Using this approach is also a reason why I love using lenses like the 1 Nikon 10-100mm f/4-5.6 zoom lens. It has a comparatively short minimum focusing distance which helps to make it easier to create image separation and shallow depth-of-field.

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3 thoughts on “Focal length and depth-of-field”

  1. Would be interesting to compare out of focus blur, taken with 32mm and zoom 10-100mm at 32mm with same aperture f:5,6. Is there a difference?

    1. Hi Stanislaw,
      Yes, there are all kinds of possible lens comparisons that could be done! I almost never shoot still images with any of my 1 Nikon prime lenses as I much prefer using zoom lenses for still. I think the images for this article are the first still images I’ve shot with the 32mm in almost a year. My three 1 Nikon primes are my ‘go to’ lenses for client video work.
      Tom

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