Is it focus or depth of field? [duplicate]

Question

I was trying to shoot a group of people standing clumped but at different distances, quite close to me, think disorganized portrait. I have a Nikon d750, and was not able to get everyone into focus. If I brought the people closer to me in focus, then the background were blurred and vice versa. I pushed the aperture all the way to f18 or so and it didn't bring the whole scene into focus. Was I shooting from too close .... Or this makes me wonder is this about AF-S vs AF-A instead of aperture and the camera choosing one point to focus upon instead of the area? How would you compose a group shot like this to all be in focus? Thanks!

Answer 1

It sounds like depth of field. If (with an APS crop sensor, 30 mm lens, f/4), if you focus at say 6 feet you might have about 2 feet of DOF span, like from 5 feet to 7 feet (coarse approximations). If your subject is distributed at say 6 to 8 feet, this 5-7 DOF zone does not include the far ones. If you focus far, you miss the near ones. Which is your description.

If you focus on the near ones, or on the far ones, you have wasted half of your DOF range in empty space where there is no one. There are DOF calculators which compute these numbers.

Normal procedure would be to focus more near the middle depth of the group (or slightly in front of the middle), to put the zone more centered on your group. So yes, you do chose your point of focus too.

And of course, stopping down the f/stop, like from f/4 to f/8 or f/11, could greatly increase the span of DOF, so that the zone size is double or more.

DOF is rather vague, and is NOT a critically precise number. If the calculator say DOF is 5 to 7 feet, then 7.02 feet is no different than 6.98 feet, both are at the limit of acceptability. These 5 to 7 feet numbers are considered the extremes of acceptability, and the actual focused distance will of course always be the sharpest point.

Answer 2

If parts of the scene looked out of focus while you were composing it, it could be that the lens had not yet stopped down to the selected aperture. The D750 will have a depth of field preview function - this will make everything through the viewfinder darker, but it should show what will be in focus. Most SLRs keep the aperture wide open for composition, and only stop down for the exposure.

If parts are out of focus in the photograph, it's probably down to the focal length of the lens.

Answer 3

I pushed the aperture all the way to f18 or so and it didn't bring the whole scene into focus. Was I shooting from too close

Yes. Depth of field increases as you focus farther away. Taking just a few steps back from the group would have given you far more depth of field than closing the aperture a couple of stops.

Play with a depth of field calculator to get a sense of how much difference a little distance can make. For example, a 30mm lens set to f/8 and a distance to subject of 2 feet gets 0.4 feet of depth of field. Switching all the way up to f/16 only gets you 0.81 feet of DOF. If you instead step back to a distance to subject of 4 feet, you get 3.83 feet of DOF, and at 6 feet you get 11.6 feet of DOF. Plug in your own numbers and compare changes to aperture to changes in distance and you'll quickly see that just a step or two would've fixed the problem.

Answer 4

Depth-of-field is that zone, before and after the point focused upon, that is rendered acceptably sharp. This zone is not split down the middle. It extends about 2/3 behind the point focused upon and 1/3 from the point of focus, back towards the camera. If your subjects are spaced at various distances, best if you shift your point of focus, forward of the middle. This takes advantage of the fact that depth-of-field extends further behind than in front.

Also, you should lookup “hyperfocal distance”. We can set the aperture and the distance focused upon so that depth-of-field carries to the far background (infinity symbol ∞ “as far and the eye can see”. This is how the old Brownie camera of grandfather’s time was set. If you set the camera to the “hyperfocal distance” all can be in focus from about 3 feet (1 meter) to infinity ∞.

A little study on this subject will go a long way as to improving shots with mixed subject distances.

Answer 5

Remember that depth of field is about twice as large behind the point of focus as it is in front. So for a scene like this, where you want the largest usable depth of field, you should focus as close to the front of the scene as possible (if you use multi-point autofocus your camera might be choosing the wrong point).

If focusing in front made the people further back blurred, then my only suggestion is that the first person was ridiculously close to your lens or your focal length was too long. What lens were you using? Otherwise I don't see any reason why focusing on the closest person and using an aperture of F/18 will cause anything to be blurred (for comparison I often shoot at F/8 or F/11 and get everything in focus).

Answer 6

Based on the rather vague description you've given us (...a group of people standing clumped but at different distances, quite close to me...) it seems likely your subjects were too close to you to allow them all to fit within the depth of field of the focal length and aperture of your lens. For us to be certain, we need to know the camera's format (sensor size), focal length, aperture value, and subject distance. So far you've only given us f/18 for the aperture, which means that the effects of diffraction could be part of the issue as well.

You can only focus the lens at a single distance. Everything in front of or behind that distance will be blurry to one degree or another. The greater the distance (actually the greater the ratio of the two distances being compared), the greater the blur will be.

If more than one AF point is active and showing as in focus, that only means that each of them is indicating proper focus of a portion of the scene that is the same distance from the camera as the other portions of the scene that are being indicated as in focus.

For the same focal length and aperture, the closer the focus distance is, the more shallow the depth of field will be. Get close enough and use a wide enough aperture and you can't even get both eyes of a single person sitting at a 45 degree angle to the camera in focus at the same time!

What we call Depth of Field is the distance in front of or behind the point of focus that still appears acceptably sharp to our eyes when we view an image. DoF is dependent upon two main things: magnification and aperture. The focal length of the lens, the focus distance, and the size and distance of the image when we view the image affect magnification. You can take the same captured image and view it from a specific distance at a small size, say 8x10, and a much larger size, say 32x40, and things that look sharp in the smaller size will appear blurry in the larger one.

In the end, DoF is just an illusion, albeit a very convincing one.