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Old Sep 22, 2010, 6:05 PM   #1
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Default AF in aperture priority

I'm new to manual controls, having never before used even a point-and-shoot that had PASM modes. I'm using an Olympus E-420 (with 14-42mm kit lens) loaned to me by a friend, and I've encountered a confusing issue involving AF in aperture priority mode.

I noticed that with autofocus enabled, it was as if I was in program mode - it doesn't matter what I set the aperture to, autofocus completely overrides it, and will set the aperture on the opposite end of the focus range as I set it on the camera, if necessary to focus. I did a little experimentation, setting the aperture to f22 and pointing the camera at an object only a few inches away from the camera. The camera refocused, but the display still quoted f22 with a shutter speed of 4 seconds. So I press the shutter, it snaps for a fraction of a second, and I get a clear-focused shot of the object only a few inches away from the lens. Even more confusing, I switched to playback mode, and viewed the picture info - it still said f22, 4-second exposure time.

I also did a test in reverse, zooming to 42mm, setting the aperture to f5.6 (maximum available for that focal length), and focused on an area several hundred feet to several miles away. Again, autofocus took over, focused on the distant objects, but the display (both in-viewfinder and LCD) quoted f5.6 with a fraction of a second, and so on.

Is this normal? If so, what's the difference between Av mode and Program mode?
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Old Sep 22, 2010, 7:36 PM   #2
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In Av mode, you set the aperture, and the camera selects a shutter speed to get the correct exposure, and it sounds as if this is what was happening. In the case of the 4s exposure, are you sure that is what it said? Most Dslr cameras use whole numbers instead of fractions, and then use a suffix for exposures over a second, so if you saw 4 in the VF, that would mean 1/4s, which sounds consistent with what you got.
The autofocus should have nothing to do with the exposure setting.

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Old Sep 22, 2010, 8:08 PM   #3
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The autofocus system and the focus distance do not affect exposure. The Aperture Priority exposure mode allows you to select the aperture, and the camera will select an appropriate shutter speed that will obtain a properly exposed image, depending on the ISO setting and the aperture you selected. What you describe is the exposure system selecting the shutter speed that's appropriate for the aperture you selected, based on the amount of light that was coming into the lens. Not that the exposure system would choose those settings whether or not the light coming into the lens was in focus or not. That's the autofocus system's job, not the exposure system's.
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Old Sep 23, 2010, 1:05 AM   #4
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My point is that at f22, an object 5 inches away from the lens should be completely out-of-focus, no? And at maximum aperture (5.6) at maximum focal length, very distant objects would also be a complete blur. But when I set the aperture value to f22 and pointed at the object 5 inches away, AF struggled a moment, then took it to the wide end, to focus properly, yet it still said the photo was taken at f22, at 4" exposure (this was low-light). And again, when I set the aperture wide-open, then pointed at far-away mountains, it again struggled, then stopped down to the closed-end to focus properly, and yet the aperture value was still recorded as 5.6.

I know the autofocus doesn't have anything to do with the exposure system, but aperture has as much to do with focus as it does with exposure. What I'm saying is that autofocus seems to be overriding the aperture I set entirely, even setting it at the exact opposite end of the range as I set it, as if it were in Program mode.

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Old Sep 23, 2010, 2:53 AM   #5
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I'm afraid you've firmly grasped the wrong end of the stick here.

Aperture is the size of the hole at the front of the lens.

It has NOTHING to do with where the lens is focussed.

If you set the aperture to say f8, you can focus at distances of 1m, 2m, 3m, ... 1000m, ... infinity. The same is true for all apertures. All lenses have a minimum distance at which they are able to focus, but between that distance and infinity, they are all able to focus at any distance at any aperture.

You're probably confused about DOF, but that's something entirely different.
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Old Sep 23, 2010, 3:25 AM   #6
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Quote:
Originally Posted by peripatetic View Post
I'm afraid you've firmly grasped the wrong end of the stick here.

Aperture is the size of the hole at the front of the lens.

It has NOTHING to do with where the lens is focussed.

If you set the aperture to say f8, you can focus at distances of 1m, 2m, 3m, ... 1000m, ... infinity. The same is true for all apertures. All lenses have a minimum distance at which they are able to focus, but between that distance and infinity, they are all able to focus at any distance at any aperture.

You're probably confused about DOF, but that's something entirely different.
I'm a noob.

I always thought aperture controlled both DOF and focus. Thank you peripatetic, that was the answer I needed.

OK, to sound even more incompetent here... how does focus work?
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Old Sep 23, 2010, 3:30 AM   #7
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There's no reason to expect that some object would be in focus or out of focus, depending on the aperture you selected.

Your lens is capable of focusing on anything that is at a distance of from 10 inches to infinity. It also has a maximum aperture of from f/3.5 to f/5.6 (depending onthe focal length in use) and a minimum aperture of f/22 (watever the focal length.) The aperture and the focal length are entirely independent of one another. That is, the camera is capable of any focus distance (within its available range) and any aperture (within its available range.)

If the exposure system says that it needs to use a 4 second shutter speed to obtain a proper exposure because you selected an aperture of f/22, and the photo was properly exposed, why would you suspect that it used some other aperture?

Aperture is the size of the hole that light passes through to get to the image sensor. Changing the aperture does not affect the autofocus system, which does all its work while the aperture is at its widest (f/3.5 to f/5.6) before you actually take the shot. When you half-press the shutter button, the AF system determins the proper focus. Once you fully depress the shutter button, the AF system stops what it's doing, the camera closes the aperture to the setting you selected and opens the shutter, exposing the image sensor for the amount of time determined by the exposure system, depending on the aperture and ISO you selected.

So, ...

Quote:
Originally Posted by jWest View Post
My point is that at f22, an object 5 inches away from the lens should be completely out-of-focus, no? ...
No.

Quote:
Originally Posted by jWest View Post
... And at maximum aperture (5.6) at maximum focal length, very distant objects would also be a complete blur. ...
No.

Quote:
Originally Posted by jWest View Post
But when I set the aperture value to f22 and pointed at the object 5 inches away, AF struggled a moment, then took it to the wide end, to focus properly, yet it still said the photo was taken at f22, at 4" exposure (this was low-light). And again, when I set the aperture wide-open, then pointed at far-away mountains, it again struggled, then stopped down to the closed-end to focus properly, and yet the aperture value was still recorded as 5.6.
... which is exactly what I would expect.

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I know the autofocus doesn't have anything to do with the exposure system, but aperture has as much to do with focus as it does with exposure. ...
No. Aperture affects Depth of Field, but not focus distance. Your lens is capable of focusing on anything from 10 inces (from the focal plane, approximately the rear of the camera body) to infinity, and the aperture does not affect the lens' ability to do so.

I think you may be confusing focus distance and Depth of Field.
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Old Sep 23, 2010, 3:41 AM   #8
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Quote:
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OK, to sound even more incompetent here... how does focus work?
There are two autofocus systems in your E-420. There's the phase detection system that uses multiple sensors to sample different places in the frame. (This AF system is used when 'Live View' is not in use.) And there's the contrast measuring system that samples the contrast in the image that's projected onto the image sensor. (This AF system is used when 'Live View' is in use.)

Whichever system is used, AF is performed immediately prior to the exposure, while the aperture is wide open, and stops working once the exposure starts and the aperture may or may not be smaller.
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Old Sep 23, 2010, 3:45 AM   #9
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From wikipedia


Focus

Until recent years focusing of a camera lens to achieve a sharp image on the film plane was achieved by means of a very shallow helical thread in the lens mount through which the lens could be rotated moving it closer or further from the film plane. This arrangement whilst simple to design and construct has some limitations not least the rotation of the greater part of the lens assembly including the front element. This could be problematical if devices such as polarising filters were in use that require to maintain an accurate vertical orientation irrespective of focus distance.
Later developments adopted designs in which internal elements were moved to achieve focus without affecting the outer barrel of the lens or the orientation of the front element.
Many modern cameras now use automatic focusing mechanisms which use ultrasonic motors to move internal elements in the lens to achieve optimum focus.
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Old Sep 23, 2010, 3:50 AM   #10
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Hi just to put it simply as I can large number = small hole large DOF, small number = large hole small DOF.
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