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Does subject isolation only depend on subject distance and focal length, aperture and sensor size remaining equal?

From my reading of a question on depth of field, if I take a photo of a subject at 4m away, using a 50mm lens at f/2.8 I should have the same depth of field as if I took the same photo on a 100mm lens at f/2.8 at 8m. Is this correct?

However, I don't always care about depth of field. Usually I just want my subject to pop out from a blurry background. So even if depth of field remains constant, will my "subject isolation" be identical? Obviously subject isolation is a somewhat fuzzily defined term (pun intended), but are there any rules of thumb in terms of measuring it?

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Let me know if this is unclear by the way. I probably lack the terminology to ask this question correctly. –  fmark Jun 12 '11 at 8:02

3 Answers 3

up vote 8 down vote accepted

The short answer: It depends how you define “subject isolation”, but more telephoto is probably what you want.

For comparison, I present two pictures, one taken at 100mm f/2.8, the other taken at 50mm f/2.8: (At ISO 400, 20s on a 1.5x crop sensor)

100mm f/2.8 ISO 400 20s 50mm f/2.8 ISO 400 20s

Since subject size and relative aperture (f-stop) are the same, depth of field should be pretty much the same between the pictures. This depth of field calculator says that at 10 feet, 100mm, f/2.8, I should get 0.33 feet, and at 5 feet, 50mm, f/2.8, I should get 0.33 feet. If depth of field is the same, why do the lights in the background produce larger circles in the 100mm picture?

The answer is that, while the circles may look bigger, that's a product of the extra magnification given by the 100mm lens. If we take the background from the 50mm image, blow it up, and compare it to a similar crop from the 100mm image,

enter image description here enter image description here

we see that they look nearly identical.

Depending on how you look at it, you can take two conclusions from this:

Conclusion 1: A telephoto lens produces larger circles from points of light in the background because of its larger magnification.

Conclusion 2: The circles made by points of light in the background are the same size, relative to the size of the details in the background. You could say that the background is just as "out of focus" in both -- that is, if you had a sensor with infinite resolution, you wouldn't get any more information about the background out of either image.

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5  
Conclusion 3: The 17-55 I used to take the 50mm shots has pretty ugly bokeh. –  Evan Krall Jun 12 '11 at 12:00
    
Thanks for clearing that up with an example :) –  fmark Jun 13 '11 at 13:46

There are other ways to isolate subject in addition to thin DoF. Off the top of my head, some other tricks you could use:

  • lighting to bring more attention the subject (underexpose background for more isolation)
  • slow sync and deliberately move the camera during no flash
  • tilt to rotate focal plane so the background is blurrier
  • panning with a moving subject (use longer shutter time for more isolation)
  • tighter perspective to have less distracting objects in background (use longer focal length from a further distance for more isolation)
  • a better background
  • a special lens, like a Lensbaby or a lens with Defocus Control
  • techniques in post production, such as using masks to sharpen the subject and blur the background

Of course, not all of those options are usable in all situations.

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Another big piece of subject isolation is distance to the background. The further away your background, the more out of focus.

In the field, worry less about the exact math of it and more about the factors themselves - if you want to increase the isolation you do one of the following :

  • Increase aperture (lower f number)

  • Move closer to the subject

  • Move subject further from background

  • Get a longer focal length, but the 'isolation' starts to taper off quick here.

The first three should really be your tools for isolation control, IMO - unless you're shooting really wide and move to a telephoto, moving from like 150 to 200mm doesn't appear to make that much of a deal in many photos.

A note on subject isolation: Remember also, subject isolation is good for dramatic effect, but don't miss the story around the person. There's a million shots of people with blurry backgrounds, but you've got one opportunity to capture the entire moment and emotion of the scene. Think, is the scene ALL about the person or is it the external influences on the person? (It may indeed be all about the subject and a perfectly isolated subject is correct - I'm just advocating that thin DoF and subject isolation should be one of many creative tools in your bag).

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