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FORUMS Photography Talk by Genre General Photography Talk 
Thread started 11 Apr 2012 (Wednesday) 20:01
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Aperture Priority Confusion!

 
cariad49
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Apr 11, 2012 20:01 |  #1

Hi, I have a Canon EOS 450D & used AV Priority thinking that it auto selects Shutter speed (read that in Manual). I shot indoors under lights in evening using f5.6 & ISO 400 and the shutter speed selected was 1/2 sec and it was all blurred! I switched to Auto and it selected f5.6, Shutter speed 1/60th sec & ISO 400 and the shot was perfect. I simply don't get it so can someone please explain or I will go Nuts trying to figure it out myself.
Cheers




  
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DAMphyne
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Apr 11, 2012 20:08 |  #2

Probably the Auto mode uses a multi-point metering, you probably have the AV set to spot metering. Just a guess.


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crn3371
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Apr 11, 2012 20:08 |  #3

And I bet the flash fired during that auto shot.




  
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mike_d
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Apr 11, 2012 20:11 |  #4

Av mode assumes no flash so it needs a shutter speed long enough to expose properly given the ambient light. This will usually result in very long shutter speeds while shooting indoors at night. I prefer to shoot manual mode indoors with the flash.




  
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StanNJ1
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Apr 11, 2012 20:12 |  #5

(AV) In aperture priority mode YOU choose the aperture and the camera will select the appropriate shutter speed.

(TV) In shutter priority mode YOU pick the shutter speed and the camera will select the appropriate aperture.

In the two examples you explain above something has changed between shots. Either the second shot had more ambient light or one of your settings must have changed such as the ISO or perhaps you had some exposure compensation dialed in without knowing it. Can you post your examples?


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crn3371
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Apr 11, 2012 20:15 |  #6

The only way you get that shutter speed without changes in iso or aperture is by having the flash fire, which auto does by default.




  
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sandpiper
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Apr 11, 2012 20:25 |  #7

crn3371 wrote in post #14250582 (external link)
The only way you get that shutter speed without changes in iso or aperture is by having the flash fire, which auto does by default.

Yeah, I agree. If your aperture and ISO caused the camera to select 1/2 second for correct exposure (which given your settings, indoors, sounds about right - normally indoors you need a faster lens than that and still have ISO up around 3200 at the very least) then putting it into auto would cause it to pop the flash up and use that to light up the room. In that case, nothing unusual is going on, you just had the wrong start point in Av, causing the camera to have to use areally slow shutter speed for correct exposure. Next time, just see what shutter speed the camera is giving you and if it is too slow, either open the aperture (if you can) or bump up the ISO. In this case you really needed to do both to get a sensible speed.

If you compare Av to full auto outdoors, in decent light, you should get the same exposure level (although not necessarily the exact same settings) as the camera won't need to use the flash.




  
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DAMphyne
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Apr 11, 2012 20:27 |  #8

On second thought, I agree with crn3371(funny name), examples would be helpfull.
Post the pics.


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Wilt
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Apr 11, 2012 20:31 |  #9

'Auto' mode (Green Box) on my 40D will

  • put the ISO setting at 400,
  • it will also turn on the flash if the ambient light is too low,
  • it will set the shutter speed to 1/60 for the flash and to be able to capture just a bit of ambient light,
  • and it will choose an aperture f/4.


The flash gave the photo enough light.

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itzcryptic
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Apr 11, 2012 22:18 |  #10

The camera will expose for the ambient light in Av mode and use the flash to expose your subject. This is called dragging the shutter and it allows you to have the background properly exposed.

If you don't like this behavior, you can lock the sync speed in Av mode to 1/250 to 1/60 of a second using your custom functions. Your background will come out dark, but you won't have the camera shake issues.




  
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tonylong
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Apr 12, 2012 00:16 |  #11

We haven't heard from the OP as to whether the flash was only fired in the Auto shot. One would think it would have been mentioned!


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satuman
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Apr 12, 2012 00:23 |  #12

tonylong with 41,903 posts cares to reply! hats off to you sir,




  
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cariad49
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Apr 12, 2012 01:48 |  #13

Hi, now I know this is a really DUMB question, but how do I attach the 2 photos as it asks me for MY URL & I don't have a Web Site. When you ave finished laughing, I really need to know!!
Cheers




  
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Tragic_Golfer
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Apr 12, 2012 02:30 |  #14

If you shot it out of the hand at 1/2 sec it would be blurry due to camera movement.1/2 second shutter speed is extremely slow. I may be wrong but it would be almost impossible to get it clear. 1/60th out of the hand is reasonable shutterspeed and would be appear much sharper. F5.6 at night under lights will probably not help. If you can open the aperture in manual mode and even increase the ISO it will allow you to speed up the shutter and eliminate camera shake. I am no expert, but thats how I see it.


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tzalman
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Apr 12, 2012 04:47 |  #15

cariad49 wrote in post #14251843 (external link)
Hi, now I know this is a really DUMB question, but how do I attach the 2 photos as it asks me for MY URL & I don't have a Web Site. When you ave finished laughing, I really need to know!!
Cheers

There are dozens of photo sharing sites on the web. I prefer http://imageevent.com/ (external link) , most people use Flickr. Posting on one of them will give you a URL (which should be in the form http://xxxxx.xxx.jpg (external link)). Or you can "Go Advanced" and select Attach Files/Manage Attachments and upload a file or two from your hard drive, so long as they are no more than 1024 pixels on the long side and less than 150 KB.


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