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Old Dec 26, 2007, 12:17 PM   #1
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Hello, i am fairly new to photography and i need some help. Thanks in advance. I have a canon 20d with an 18-55mm sigma lens. I am having trouble getting my pictures to focus right. Below is a picture, can someone tell me why the subject on the left is focused and the other two is not? How would i get all of them focused? Alot of my pictures come out this way, with some subjects focused and some are not. It is very frustrating and don't know why this is happening. How do i get all the subjects focused? This was shot in auto mode.
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Old Dec 26, 2007, 1:16 PM   #2
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You have to take Depth of Field into consideration (how much is in focus as you get further away from your focus point).

The wider the aperture (lower f/stop settings), and the closer the focus distance, and the longer the focal length, the shallower your depth of field will be.

That was taken zoomed into 50mm, with the aperture wide open at f/5.6

See this Depth of Field Calculator. Pick your camera, plug in 50mm, f/5.6, and the focus distance to see Depth of Field.

http://www.dofmaster.com/dofjs.html

Don't fill the frame so much. Go for fuller length shots versus head and shoulder shots of groups at close range.

Back up (or don't zoom in as much) to get more depth of field indoors where you may want to use wider apertures.

You could also shoot in Av (Aperture Priority) mode and stop down the aperture a tad (for example, use f/8 or f/11) for group shots to get more depth of field.

But, don't stop it down too much (as the built in flash is fairly weak and using a smaller aperture (represented by a higher f stop number) will lower your maximum flash range, as well as let in less ambient light when using the flash.

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Old Dec 26, 2007, 3:41 PM   #3
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I thought the depth of fied only comes into consideration when the subjects are at different distances? The subjects in the picture are pretty all at the same distance. Why would some not be in focused and some are? I am still not understanding.

Now if i set the camera at f/8 or f/11 my shutter speed would slow down significantly because of lack of adequate lighting and would cause blurry pictures. How would i deal with that situation if i cannot get adequate light and am not using a tripod. Now if i use a shoe mount flash, would the camera know and not slow down my shutter speed too much? Also, what do you mean when don't lower your f/ stop too much because it will lower your flash range?
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Old Dec 26, 2007, 4:12 PM   #4
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calitran wrote:
Quote:
I thought the depth of fied only comes into consideration when the subjects are at different distances? The subjects in the picture are pretty all at the same distance. Why would some not be in focused and some are? I am still not understanding.
Go to the depth of field calculator I posted a link to and enter in your camera model, f/5.6 and 50mm (the settings used for that photo), and you will probably get a better understanding of how it works.

For example, you would have less than 3 inches of total depth of field with those settings if your focus distance was around 3 feet away, before the subjects further away from that distance would start to become blurry (or less than 1 1/2 inches in front of, or behind where you were focusing).

Experiment with the calculator so you'll have a better understanding of how Depth of Field is influenced by Focus Distance, Aperture and Focal Length.

Quote:
Now if i set the camera at f/8 or f/11 my shutter speed would slow down significantly because of lack of adequate lighting and would cause blurry pictures.
Most cameras will keep the shutter speed set at around 1/60 second with flash (as your camera did above) by default. A shutter speed that fast is fine in lower light conditions using a flash at lower ISO speeds.

That's because the photo would be underexposed without a flash. So, the flash itself can freeze movement (because the flash burst length is typically around 1/1000 second, and the subject is only illuminated during the flash burst).

Most cameras models use a default shutter speed in low light with flash. If yours exhibits different behavior in some modes, just use manual exposure instead (setting both the aperture and shutter speed).

Quote:
How would i deal with that situation if i cannot get adequate light and am not using a tripod.
See my last response, or just use manual exposure instead (set the aperture to where you want it, and set the shutter speed to where you want it, letting the camera vary the flash burst length for proper exposure).

Quote:
Now if i use a shoe mount flash, would the camera know and not slow down my shutter speed too much?
Buy one and find out. ;-) With a dedicated flash, it's usually going to treat the external flash in the same way that it treats the internal flash.

Quote:
Also, what do you mean when don't lower your f/ stop too much because it will lower your flash range?
That's not what I said. This is what I said:

Quote:
But, don't stop it down too much (as the built in flash is fairly weak and using a smaller aperture (represented by a higher f stop number) will lower your maximum flash range, as well as let in less ambient light when using the flash.
If you use a higher f/stop number (I did not say "lower your f/stop"), you can lower your maximum [key word is maximum] flash range. Note that a higher f/stop number represents a smaller aperture opening.

A given flash will have a GN (Guide Number) assigned. Maxiumum flash range will be GN / aperture at ISO 100.

For example, your EOS-20D has a Guide Number of 13 Meters or 43 feet at ISO 100.

So, if you are shooting at f/11, maximum flash range will be 3.9 feet at ISO 100

43 (your GN in feet) / 11 (your aperture as expressed by f/stop) = 3.9 feet

Then, each time you double the ISO speed, your flash range will increase by 40%



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