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Old Jul 3, 2006, 4:06 PM   #1
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hello, I have a 20D and when I shoot in auto mode with auto focus, parts of the picture is in focus and part is not. I have two subjects or multiple subjects standing next to each other, one or some would be in focus and the others will not. While pressing the shutter button halfway down, only some of the subjects get the rectangle around them. Can someone suggest what is wrong? Greatly appreciated.
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Old Jul 3, 2006, 6:10 PM   #2
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Short Answer:

You'll need to use another mode besides Auto.

An easier solution for someone not familiar with photography concepts like Aperture would probably be to use the A-DEP mode. It's designed to help you maximize depth of field (it will lean towards using smaller apertures so that you can get multiple points in focus).

Longer Answer:

The amount of a scene that is acceptably sharp as you get further away from your focus point is going to be dependent on your aperture selection.

Think of aperture as being like a pupil in your eye. The camera opens it up wider to let more light in. That allows it to get faster shutter speeds for any given lighting and ISO speed (which is how sensitive the sensor is to light).

The problem is that wider aperture openings allowing more light in (so that faster shutter speeds can be used to help reduce blur from camera shake or subject movement) give you a shallower depth of field (how much is acceptably sharp as you get closer to or further away from the camera's focus point).

So, the Auto Exposure Algorithms are a compromise (allowing enough light in to help reduce blur, which may not always be what you want).

You'll need to use smaller apertures (higher f/stop numbers) to get greater depth of field (more of the scene in focus as you get further away from the focus pont). That requires slower shutter speeds for any given lighting condition and ISO speed (which can lead to blur from subject movement or camera shake if you don't increase ISO speed). Of course, increasing ISO speed increases noise levels (similar to getting more grain from using higher speed film).

It's all a compromise and the Auto Exposure Algorithms don't know what you want. Some users may deliberately use a larger aperture to get less of a scene in focus (so that subjects stand out from distracting backgrounds by blurring them). In your case, you want a greater depth of field. The auto exposure algorithms are selecting something "in the middle" for exposure purposes.

To get a better idea of how aperture impacts depth of field, see this online calculator (plug in your camera model, focal length, focus distance and aperture to compute depth of field):

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

To get a better idea of the relationship between lighting, ISO speed (shown as film speed in the calculator), aperture, and shutter speeds needed for proper exposure, see this handy onilne exposure calculator:

http://www.robert-barrett.com/photo/...alculator.html

If you are interested in learning how to control a camera's behavior more, versus using the Auto modes, check out a book on basic photography at your local library. It doesn't need to be specific to digital (as the same concepts for things like depth of field and exposure will apply to both film and digital).


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Old Jul 3, 2006, 6:25 PM   #3
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thanks JimC for your reply. I understand the concept of the use of the aperture settings, depth of field, shutter speed, and also ISO. My problem is that the two subjects are standing right next to each other. Also since the camera is in auto mode, the aperture and shutter speed is automatically selected. My question is if the two subjects are next to each other thenwouldn't that mean that they both are the same distance away from the camera. How come one subject is in focus and one is not? There are times when i set my aperature at say f/18, one subject is in focus and one is not even when that are standing next to each other. Am I making sense? or is there something i don't understand. thanks for your help.
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Old Jul 3, 2006, 6:54 PM   #4
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If you're at f/18 and one subject is blurry and the other is not, I think you're more likely seeing a shutter speed issue (motion blur from subject movement).

Depending on your mode, you'll only see one active focus point at a time. That doesn't mean that everything else will be blurry. That's just letting you know what the camera is using for a focus point and that it's locked on that point. You can setup your camera to select the most likely focus point for you, or you can select it.

A-DEP allows you to select more than one (so that the camera picks an appropriate aperture to maximize depth of field to get everything you want in focus).

I'm not a Canon shooter. So, perhaps some of our forum members that use the 20D can elaborate. But, I doubt there is anything wrong with what you're seeing in the viewfinder.

You shouldn't need to use f/18 for very many subjects, unless you're "filling the frame" with a smaller subject. You'll end up with softer photos if you use aperture extremes (too large or too small), as most lenses are sharpest about 2 or 3 stops down from wide open apertures (not to mention the resulting motion blur from slower shutter speeds you can get trying to stop down the aperture to f/18 ).

Are the subjects standing next to each other differing in sharpness (one blurry, one not), or are you just concerned about what you're seeing in the viewfinder for your focus selection?

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Old Jul 3, 2006, 7:12 PM   #5
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Yes, the subjects standing next to each other are differing in sharpness (one blurry, one not) although both subjects are standing completely still. I'm not concern of how i see in the viewfinder, only concerned when viewing the images on my PC. This not only happens to subjects but also to still objects. For example, i took a picture of a television and some parts of the television was in focus and some was not. Obviously the television wasn't moving. I would understand if i set a aperature like f\5.6 or something and the trees in the background is blurry but i don't see why part of the object is blurry and part not. It seems that the camera only focuses part of the picture and the other parts don't get focused.:?:sad:
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Old Jul 3, 2006, 7:18 PM   #6
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I'd post an example of what you're referring to.

Some wider lenses don't have a very flat focus plain. So, at wider apertures, you may start seeing the lens focusing at points in front of subjects nearer to the edges of the lens versus what you see as sharp in the center of the lens.

Using a smaller aperture (higher f/stop number) can help that.

Also, some lenses are just a bit soft on the edges anyway, and this usually improves at smaller apertures. In low light, most cameras are going to select a wider aperture (smaller f/stop number) to help keep shuitter speeds fast enough. So, this kind of problems is more obvious there.

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Old Jul 3, 2006, 8:26 PM   #7
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Jim, somewhat related to the above discussions, I am planning on taking a shot of the Golden Gate bridge this summer which I would like to blow up to 13 by 9, maybe even bigger.

"PhotoSecrets" by Andrew Hudson suggests Hawk Hill at 3:00 p.m..I have a 20 D, will set the camera to take a large jpeg and a raw at the same time.I plan on taking a picture in Landscape Mode first (because I do not trust my camera savy), Program Mode second because I can fiddle with white balance,Manual Mode third with an aperture of f/11.

Any suggestions would be appreciated.

Thanks
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Old Jul 3, 2006, 8:44 PM   #8
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You'd probably be better off asking someone with better photography skills than I have. ;-)

You may want to take a look at some photos from others to get some ideas for composition.

A quick search on pbase found over 2000 photos in over 100 different albums contaning photos of the Golden Gate Bridge.

http://search.pbase.com/search?q=gol...hotos&c=sp

I'm not very familiar with the area, so I can't comment on vantage points. But, you can get an idea of the lens used (or at least the focal length used) by looking at the EXIF in many of these images (you'll see camera settings shown for many photos in the EXIF underneath them). But, I wouldn't assume that they weren't cropped or enhanced in some way (as these are just user albums on pbase).

Trying to maximize dynamic range can be dificult with digital, and shooting in raw can help (as can bracketing your photos so you've got some exposed a little darker and brighter compared to what the camera's metering thinks needs to be used). Checking your histogram for proper exposure would also help insure a greater chance of success.

A tripod can be a good thing to bring along, too. Even if you don't need one to help with steadying your camera for the sharpest possible image, it can help you think about composition.

You may also want to try the shots with and without a polarizer.


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