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I know when for example camera is on f2.8 it means an area in front and back of the place that focus is, will be in focus and areas closer or farther will be out of focus, but if it is for example f22.0 then that area in focus will be much larger. What I can't understand is that is there a mathematical way of calculating how many meters is in focus depending on f number and the lens ( for example 35mm, 24mm, etc) ? For example last night I was about one meter away from a dancer who was also one meter away from the trumpet player and on a f2.8 24-70mm lens I wanted to be able to set it in a way that the dancer is in focus and the trumpet player behind her is in bokeh. How should I have approached this?

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There's only one plane of focus. Things further away or closer to the camera are out of focus.

However, objects close to that plane of focus are not as blurry as objects far away from it. What you can do is to define a threshold of blurriness for yourself and thus derive an area around the plane of focus to be acceptable for you as "close enough to being in focus".

Even with the formula, you still need to figure out what your threshold is, calculate the result and apply it. It's not practical to know the formula.

What is practical however are the parameters of the formula, which leads to the question of "how to maximize background blur for subject separation in the foreground".

In short:

  • Use the most open aperture possible. That is the smallest f-number. In your case, f2.8.
  • Use the longest focal length, in your case 70mm.
  • Get as close to the subject as possible.
  • In theory, a larger sensor also helps, but this is usually impossible to change unless you brought two different bodies.

Take a look at this question with excellent answers: What exactly determines depth of field?

Which one of these lenses [85mm f/1.8 | 24-70mm f/2.8 | 35mm f/1.8] would be a good choice for this kind of shot?

  • The 85 will no doubt blow out the background very much. Maybe a bit too much. I think you included the trumpet player intentionally. A dancer alone can look awkward in an image due to the frozen motion. Adding the trumpet player in the background tells a lot more about the situation. It's also a rather long lens, which squeezes the elements into a tighter space. But a dancer is everything but something that can live in a tight space. He/she moves around, making it hard for you to get both people into the frame. Don't get me wrong, if you can get everything into the frame, the 85 will produce stunning images. But with the situation of a moving subject in a confined space, it won't be easy. Like a portrait session with the subject jumping around, basically.

  • 35 mm allows you to get more in the frame. This allows for more variation in the composition (think both behind each other, side by side, ...). It would be less of a "portrait of the dancer" approach but more of a "two people picture, with emphasis on the dancer". If your goal is to show the interaction of both, the 35 would be a better choice over the 85.

  • With the zoom, the separation is reduced, but you can adjust to the movement of the dancer best. I do not own such a lens, but I guess it should provide enough separation to draw the attention of the viewer towards the focused dancer. If your goal is to blur the trumpet player to the degree that he/she just appears as a mush color in the background, this lens is not the best choice. But again, I don't think this is your goal as you could simply compose the frame without the trumpet player in it. I think I'd go with this lens. It will produce a lot of keepers as it has the flexibility of an adjustable focal length allowing your to "dance along" with your subject.

The aperture advantage of the primes allows you to use shorter shutter speeds, which in turn freeze the motion better. It depends on the light situation, how far you can push your iso and how fast the movement is, but it can be helpful to get a few more stops of light that you can invest in shorter shutter speeds.

On the other hand, a slow shutter speed with a 2nd curtain flash can do wonders to an image containing motion. But that's a different story.

  • Ok thanks, for the situation I described do you think actually having a 85mm, f/1.8 does the job better than using this 24-70mm f/2.8 ? I also have a 35mm, f/1.8 but looks like you say longer focal length is better. – Brandon Jan 16 '15 at 20:19
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    @Blake I added some thought regarding the lens choice to my post. – null Jan 16 '15 at 21:48
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How can I know how many meters is in focus area?

Lens markings

In the days when cameras and lenses had manual controls, lenses used to have a handy guide to depth of focus ...

enter image description here

At f32 everything from ~1.3 to ~7m should be in focus.

Stop-down preview

Most cameras display a viewfinder (or LCD) image of the scene at the maximum aperture so that what you are viewing is bright. Some SLR and DSLR bodies had a stop-down preview, this let you see exactly what was in focus (though probably very dimly if at f32).

is there a mathematical way of calculating how many meters is in focus?

There must be, but it may not be something you can calculate easily in your head without resorting to pen and paper, calculators or apps on a phone/tablet/etc. Having a facility built into the camera (see above) is what I would prefer.

How should I have approached this?

I couldn't say. I would have used aperture priority and taken a few test shots.

  • "Most cameras display a viewfinder (or LCD) image of the scene at the maximum aperture so that what you are viewing is bright" Oh Oh! did not know that. So it means I can't trust what I see in ViewFiner for bokeh effect, is that right? I now remembered there is a button on my camera (Nikon D-610) that when I hold it , it actually closes the aperutre to the real number I have set so I can see the real image that is going to be taken, I had mostly used it to see if I am over or under exposing tho, hadn't noticed it for bokeh test – Brandon Jan 16 '15 at 21:15
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    @Blake eyes adjust to darkness, sensors don't. Estimating the exposure from just the aperture will not work because there are two other factors (shutter speed and iso) that play an equally important role. – null Jan 16 '15 at 22:08

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