I'm shooting with a Sony a77. What would produce a more shallow depth of field for portraits: a Minolta 50mm 1.7 wide open or a Minolta 70-210mm f/4 wide open?

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    If you type in "depth of field calculator" to your favorite search engine, you will find many options to calculate any scenario you desire. – dpollitt Feb 16 '16 at 1:20
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    At what focal length for the zoom? At what shooting distance for either lens? At what aperture setting for either lens? All of those variables will affect the outcome. – Michael C Feb 16 '16 at 1:31

Depth of field calculation depends on multiple factors:

  • captor size
  • focal length
  • aperture
  • and last but not least: subject distance

Using a Depth of field simulator (http://www.dofmaster.com/dofjs.html) with your camera, you will get the following result:

  1. Distance to subject: 5 meters

    • 50mm f/1.7 => Depth of Field of 0.67 m
    • 70 mm f/4 => Depth of Field of 0.81 m
    • 150 mm f/4 => Depth of Field of 0.17 m
    • 210mm f/4 => Depth of Field of 0.09 m
  2. Distance to subject: 10 meters

    • 50mm f/1.7 => Depth of Field of 2.73 m
    • 70 mm f/4 => Depth of Field of 3.3 m
    • 150 mm f/4 => Depth of Field of 0.70 m
    • 210mm f/4 => Depth of Field of 0.36 m

I suggest you try the simulator with your conditions. You might want to take a look at those questions :

  • Your comparison isn't quite right. The OP is talking about portraits, and it seems a reasonable guess that the subject should fill about the same amount of the frame regardless of whether the OP chooses the 50mm or 200mm lens. To get the same magnification with the 200mm that you do with the 50mm, the camera should be 4x farther away. So you should really compare the 50mm at some distance with the 200mm at 4x that distance. The 50mm still wins for shallow DOF, but without an apples:apples comparison the reasoning it's so solid. – Caleb Feb 16 '16 at 3:01
  • I totally agree with you Caleb, that's why I didn't made any conclusion regarding the "best" lens for the OP usage. He is the only one to know how he wants to use his lens. I have added a link to field of view to help him. – Olivier Feb 16 '16 at 4:19
  • The Dimensional Field of View calculator here could be used to figure out the subject distance to compare like framing, for someone curious: tawbaware.com/maxlyons/calc.htm – Lee Saxon Feb 16 '16 at 5:52
  • @Olivier I think you've made a good start objectively answering which has the shallower DOF -- I'm just saying that it'd be more relevant to compare, say, the 50mm at f/1.7 and 10 feet to the 200mm at f/4 and 40 feet. The subject will be the same size at those respective distances. – Caleb Feb 16 '16 at 6:21

The 1.7 is really going to have a shallow depth of field. Likely you won't want to use this on a portrait as you will have a thin slice in focus. Distance to subject plays a part here as well.

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