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Old Feb 17, 2006, 5:28 PM   #1
nacoya's Avatar
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just got the d50 + 18-70mm lens.

I'm struggling a little with aperture and dof.

Is the dof increase linear from f/3.5 to f/22 if the focal length remains the same ?

I have tried a few shots at f/3.5 but at a distance of 2m from the subject with the lens at 18mm i get very little bokeh.eg i took a pic of my dog and managed to get his nose and face in focus but his ears weren't. It is hard to check quickly and accurately on the lcd and sometimes when the shot has one it's gone.

However if i move back and zoom in to say 50mm retaining the same fov the bokeh is much better.

What would be the best setting in normal daylight for an evenly focused shot. Would everything be in focus at f/22 or is there a sweet point at another aperture setting ?

Is there a rule of thumb or is it pure and simple a case of trial and error and experience.

sorry for so many questions and being a bit garbled.

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Old Feb 17, 2006, 7:06 PM   #2
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The wider the zoom ie 17mm the more depth of focus you have at all apertures. The more telephoto you have you also have less depth of field and things can become somewhat compressed. Macro you need to have middle range f stops or smaller to enable the best DOF you can get becuase in macro, it "just ain't there!"

on a small aperture you will have less available light and will have to compensate with longer shuuter time.. prepare for using a tripod or monopod for less than 1/60th second shots.

What's a "bokeh"?:-)
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Old Feb 17, 2006, 8:13 PM   #3
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I believe you have the term bokeh confused with DOF. The definition of Bokeh is:

When you use a lens at a wide aperture (ƒ1 - ƒ5.6), 'Bokeh' refers to the out of focus portions of the image - in particular its visually pleasing nature or lack thereof.

Things to look for? An absence of "double-vertical" lines, perfectly circular highlights and a gentle gradation between the in-focus region and OOF parts.

Unfortunately Bokeh is competely subjective.

At larger aperatures you have less DOF.... a smaller portion of the picture is in focus. As you get closer to the subject, or increase the focal length the DOF becomes less and less as you have found. By increasing the distance from the subject or by stopping down you can increase the DOF.

Your smallest aperature will yield the greatest DOF....however image quality begins to suffer at smaller aperatures due to diffraction of light. You will also need longer shutter speeds to achieve proper exposure, increasing the chance of subject blur or camera shake. Shooting at f/8 or f/11 at moderate telephoto should yield sufficient DOF without the distortion, and still seperate the subject from the background.

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Old Feb 17, 2006, 9:14 PM   #4
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Here is a handy online Depth of Field calculator that can give you a better idea of the relationship between focal length, focus distance and aperture.


Plug in your camera model (any Nikon DSLR model will work, since they all have the same size sensor), select a focal length, aperture and focus distance, and it will calculate the depth of field for you.

Keep in mind that just because you may be outside the range of "acceptable sharpness", as calculated by this calculator (using typical circle of confusion settings for the sensor or film size), you may not have the desired impact (not out of focus enough to help your subject stand out from distracting backgrounds).

The larger your aperture (smaller f/stop numbers), the shallower your depth of field. Ditto for closer focusing (the closer you are, the shallower your depth of field), and focal length (the longer the focal length, the shallower the depth of field).

But, keep in mind that if you use a focal length that's twice as long, you'll need to be twice as far away from your subject to achieve the same framing. So, that cancels out the DOF difference at most focus distances you'll shoot at (making true depth of field identical if your subject occupies the same percentage of the frame and you're shooting at the same aperture, regardless of focal length at most shooting distances you'd be concerned with). As you get closer to infinity this can change.

However, perspective changes with focus distance. Shooting from closer distances makes foreground and background elements seem more separated. Shooting from further distances makes them appear more compressed and closer together.

So, even though DOF is technically the same at most shooting distances you'll be concerned about it with if you have the same framing and aperture, despite focal length differences (since you'd need to shoot from further away if you increased focal length for your subject to occupy the same percentage of the frame), shooting from further with a longer focal length gives an illusion of a shallower depth of field, just because of perspective, making the out of focus areas more obvious when elements in the scene are more compressed.

Using wider lenses and shooting from closer distances can also cause some perspective distortion. For example, a nose closer to the camera may seem too large in proportion to the rest of a subject's face. You can use perspective creatively by shooting at different distances to your subject with different focal lengths, outside of depth of field considerations, for more or less background compression, increasing or decreasing the apparent separation of foreground and background elements in the photo.

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