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Old Apr 5, 2010, 9:28 PM   #1
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Default Help With Focus

Hey guys--I am going to request a bit of patience here as I can be a bit dense at times, especially when it comes to the technicalities of photography. I love taking outdoor nature and travel photography and I feel I've become quite good at it, but when it comes to taking portraits of people I really stink and I often leave disappointed and frustrated. I know an external flash and a better portrait lens would be helpful (I just have a cheap 50mm prime and an 80-200mm), but apart from that I am having trouble understanding how I can change the depth-of-field and it's relationship to the photo.

I wanted a photo of two friends walking down a path holdings hands (full body), in focus, with the entire background out of focus. If I had taken a close-up this would have been simple but the wider the picture the less control I have over focus. I often see a lot of wedding/engagement photography where this is achieved. Is it through computer alteration afterwards? How could I have achieved this? I understand that changing aperture has something to do with this, but this is where I get confused. Can someone please explain to my poor thick head the relationship between aperture and focus?
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Old Apr 6, 2010, 5:40 AM   #2
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Depth of field, the area where the image appears to be in sharpest focus, is controlled by three factors:

Distance from subject, the farther away from the camera the greater the depth of field.

The focal length of the lens, the shorter the focal length the greater the depth of field.

The aperture of the lens, the more "stopped down" (larger f/ number) the greater the depth of field.

In your case I would experiment with a longer focal length, not too far away and a smaller f/ number.
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Old Apr 7, 2010, 9:00 AM   #3
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To get use to it, hopefully your 50 prime in 'old school' where they included the depth of field scale on the lens. That will give you an idea of the focus range as you change f stop. Unfortunately, newer lens did away with that.

When you understand that concept, put in "pre-view" mode, look through and note changes as you change the aperature. Now put the zoom on and do the same at different zoom points. Obviously, when taking those types of photos, either full manual or aperature priority.

Depending on model, not all DSLR's (entry level in particular) have "depth of field preview." You may have to look in the manual to see how to engage it for your camera.
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Old Apr 7, 2010, 9:31 AM   #4
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The best chance you have for this in full length is shoot in portrait orientation, use the 80-200mm at f2.8 and frame quite tightly to them. You can also do similar with the 50mm at f1.8 but the DOF is going to be slightly wider but you can be a lot closer to them and further from the background so have a play with that. You might find that f1.8 isn't sharp enough for the couple so poss f2 or f2.2 might be better. It's a balance with sharp, getting enough DOF for the couple in focus and getting the background out of focus enough.

To say what Bob said in another way (good to cover all bases), to reduce the depth of field (DOF) to get the background more out of focus, you want: longer focal length, wider aperture (smaller f number), closer to the subject. Also having the background as far away from them as possible also helps.
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