Bokeh in the background with a 50 mm lens. What am I doing wrong?

Started Apr 22, 2013 | Questions thread
Olaf Ulrich Contributing Member • Posts: 955
Re: Background blur with a 50 mm lens. What am I doing wrong?
RasiWick wrote:
... my question was NOT about composition.

Right—it wasn't. I still couldn't help noticing that the composition was ... poor (sorry). That adds to the impression that the picture is, umm, missing something. So I took the liberty to give some advice about composition. Obviously you aimed at your daughter's eyes for focusing and then fired the shutter without a further thought about image composition.

For a portrait to look good, it isn't utterly required for the person who is the picture's main subject to be entirely in focus. In fact, many fine portraits have very narrow depth-of-field, but as long as the eyes are in focus, it will look good to the beholder. It also helps when there is not only a sharp-blur contrast between subject and background but also a contrast in colour and/or tone, such as dark hair (like your daughter's) against a bright background, or blonde hair against a dark background.


RasiWick wrote:
In a 50 mm lens at the widest aperture (1.8), there is plenty of background blur.

No, not necessarily. It's a common misconception to think the aperture was the main tool to control depth-of-field and background blur. But it isn't. Instead, the most important factor is distance. And please note that two distances are involved—that between camera and subject (subject distance) and that between subject and background (background distance).


RasiWick wrote:
I was wondering whether there was a way to get the best of both worlds—i. e. still have most of my daughter in focus, but with a nice background blur in the distant background. I would assume that making the aperture smaller is the trick (like f/2.2 or f/4), but this clearly also has the untoward effect of making the background less blurry.

So you want to control depth-of-field and background blur independently of each other—increase the former but keep the background blurred. Yes, it is possible to do that ... but not just with the aperture alone. The trick is to change distances.

You took the picture of your daughter from approx. 2.5 m away at approx. f/2. If you shoot from, say, 1.5 m then a) the composition would be nicer and b) the background would be more blurred. So you can stop down a little more and still have the background blurred enough. That's what I said in my first reply—get closer and stop down just a little more (to, say, f/2.8 - f/4).

Generally, the trick is to have a short subject distance and a long background distance. Of course, you cannot push back the background scenery ... but 'short' and 'long' are relative. Shortening the subject distance automatically 'increases' background distance—not absolutely but in relation to the subject distance, and that's what counts.

Furthermore, a longer lens helps to separate the subject from the background ... as has been stated several times now. After all, you cannot always make the subject distance closer deliberately just in order to increase the relation the the background distance, because at too short a subject distance, you may not be able to fit your subject in the frame, or you may get an unfavourable perspective (that is, big nose and small ears in a portrait, for example). A longer focal length also effectively 'pushes back' the background even when you have to increase the subject distance due to the narrower angle-of-view. The relation between subject distance and background distance will be different with a longer lens (and still different with a wide-angle lens)—that's the main reason why there are different focal length lenses in the first place.

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