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The great lens I have is a 50mm 1.8F G Nikon. My camera is a D3100.

I realized I need more blur in the background so I thought may be I should purchase an 85mm prime lens.

But I wonder since it is possible to blur the background through software, does it make sense for me to buy a 85mm prime lens?

Basically, yesterday, my child was playing with a sleeping dog, but I could not take a photo with the 50mm prime lens because the cars behind them weren't blurred even at a low aperture. I had to cover both of them in the photo so going too close wasn't an option.

Is there some reason I should consider buying an 85mm prime lens when I already have 50mm prime lens with the aim of blurring the background (which could also be blurred through software)?

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+100

TL;DR

You will get a bit more and smoother blur. If it's worth the Investment is up to you. But be adivsed that the smoothness of the blur is highly depending on the lens-construction itself, so the numbers only say so much

Calculating the DoF, and the Bokeh.

In this Screenshot of the Software cBlur (a german online version exists here), you can see the Sharpness-curves for a 50 and a respective 85mm lens, set at the same aperture (f/1.8) and focused on a distance which covers the approximate same object height in the image (190cm, which is roughly a full body portrait).

Circles of confusion for a 50mm and 85mm Prime focused to show (roughly) the same object height

As you can see, the circles of confusion are higher for the 85mm (blue line), which basically means more blur. In the online version, you can also get example image of the approximate blur. (Which does NOT equal a higher bokeh quality, because that also depends on contstruction details of the lens).

Personal experience

From my personal experience with a 35mm and a 56mm prime on a Camera with Crop Factor 1.5 (which equals your Nikon DX) set to the same aperture (1.4), for Portraits with only head or head and shoulders it's irrelevant (though the 56 has less perspective distortion and makes "nicer" faces), but on a full body portrait, the 56 has more blur. But you also need a lot more space, which may not always be available.

A note on artifical bokeh

Creating a Bokeh in Photoshop or "Blurring Backgrounds" is not an easy Task and requires Depth-Information, which you don't have in a 2D Image (that's why e. g. the iPhone 7 uses two cameras to create an artifical DOF effect, and event this doesn't work perfectly).

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In general, the difference between 50mm and 85mm focal length is not dramatic and the difference will be only incremental, but 85mm lenses are usually specifically designed for portraits, so their background blur may be smoother and they may be optimized for use wide open, which is rarely the case with lower end 50mm lenses. So if you are looking for smooth backgrounds, 85mm is a step in the right direction.

That said, it would make a better sense to look for something like 85/1.4 or a fast 100 - 135mm lens.

  • 85mm is a step in the right direction - Nikon's 85mm is also called the cream machine – fubo Oct 17 '16 at 6:57
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I'm not sure you've correctly understood how depth of field works. To a very good approximation, if you keep the same framing and the same aperture then the depth of field stays the same - as such an 85mm lens will not help you, as you'll just have to step further back. If you want a shallower depth of field, then you need to get a lens with a wider maximum aperture; the obvious option here would be a 50mm f/1.4 lens.

  • I'm sure we must have a question on the site somewhere about depth of field and framing, but I couldn't find it. If you can, please edit this answer to replace the off-site link to our question :-) – Philip Kendall Oct 14 '16 at 8:34
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    The answer at photo.stackexchange.com/a/5496/47528 asserts that you get more background blur at 55mm f/5.6 than at 18mm f/3.5 when you keep the subject size the same. It has many upvotes so it seems that a lot of people agree with it. So which one is correct? – Jules Oct 14 '16 at 13:26
  • @Jules I'm not sure that having the same DOF also means having the same amount of blur for those background elements that are out of focus. I'd love to see a treatment of that subject. – Mark Ransom Oct 14 '16 at 16:28
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    You get the same DOF, but different look/amount of the background blur. See A and B comparison here: toothwalker.org/optics/dof.html – MirekE Oct 15 '16 at 3:58
  • @Jules - they are both right. Amount of background blur is not exactly the same thing as depth of field. With longer lens the out of focus areas in the background are more enlarged and appear more blurred. – MirekE Oct 15 '16 at 4:14

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