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FORUMS Canon Cameras, Lenses & Accessories Canon EOS Digital Cameras 
Thread started 18 Jun 2008 (Wednesday) 13:22
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Sxi-Using Full Auto

 
neil5054
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Jun 18, 2008 13:22 |  #1

Years ago I used film SLR's and just got an XSI and am rusty. I know that I should re learn use of TV or AV modes, but if I use Auto/P setting, how does the Canon determine the average f-stop and speed using full auto or the P setting ??




  
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neil5054
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Jun 18, 2008 13:25 |  #2

Sorry! I meant "XSI" !




  
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apersson850
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Jun 18, 2008 15:00 as a reply to  @ neil5054's post |  #3

The camera selects combinations of shutter speed and aperture according to a predetermined relationship. It depends upon what ISO setting you have, which lens you use, what focal length you've zoomed to and so on, so it's not possible to give a definite answer.


Anders

  
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neil5054
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Jun 18, 2008 15:07 |  #4

apersson850 wrote in post #5746855 (external link)
The camera selects combinations of shutter speed and aperture according to a predetermined relationship. It depends upon what ISO setting you have, which lens you use, what focal length you've zoomed to and so on, so it's not possible to give a definite answer.

That's what I thought, and understand that if I used a priority mode, I could set depth of field and/or speed. Just wondering whether the Auto/P mode would be average of these choices, ie., if I'm shooting with plenty of light outside with 100 or 200 ISO, the auto mode could give any number of combos, with larger or smaller depth of field. I guess the advice is just to see the result and shoot in priority mode if the auto isn't giving enough depth or speed...




  
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fxk
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Jun 19, 2008 17:29 |  #5

neil5054 wrote in post #5746910 (external link)
That's what I thought, and understand that if I used a priority mode, I could set depth of field and/or speed. Just wondering whether the Auto/P mode would be average of these choices, ie., if I'm shooting with plenty of light outside with 100 or 200 ISO, the auto mode could give any number of combos, with larger or smaller depth of field. I guess the advice is just to see the result and shoot in priority mode if the auto isn't giving enough depth or speed...

I think apersson850 has it right - maybe I can fill in a little more using an example.

Say you're outside in bright light, ISO 200, 100mm f2.8 macro lens. In general, lenses will perform best one or two stops off the maximum aperture. It also "knows" that one shouldn't try to hand-hold any less than 1/focal length of the lens, expressed in fractions of a second. Outside, sunny day, ISO 200, the "sunny 16" rule says 1/200 of a second at f/16.

F/16 is a usable aperture, but close to diffraction limits - a point where the lens becomes softer because of how the light "adheres" to the sides of the hole - Google lens diffraction limits to get a better explanation - trust me here - it is real.

Maybe f8 or f5.6 might be closer to the sweet spot of the lens. The Sweet spot is the point where as the lens is stopped down, the visible defects of the lens design are minimized, and the lens becomes sharper - that is until diffraction becomes a factor.

1/200 of a second is better than the 1/focal length rule (in this case 1/100) - but not that much. If we open the lens to f/8, we gain two stops of light - so we have to speed the shutter up by two stops to reduct the light again hitting the sensor - so it goes from 1/200 second to 1/800 of a second. So now, we have more protection from camera motion (1/800 sec vs 1/200 sec), and we're closer, if not at, the sweet spot of the lens (f/8.0).

So in Program mode (which, by the way, is the same as the fully-automatic Green Basic mode, except the "guard rails" have been removed), that type of decision tree is made - how much light? Maximum aperture of the lens? Focal length of the lens? yada, yada. It will always try to get to the sweet spot of the lens (most lenses are best f/5.6 or f/8.0), and keep the shutter speed above the handhold rule. Simple, huh? Now, if the light is lacking, the camera generally will sacrifice the lens over the shutter - using a larger aperture before falling below the 1/focal length rule.

By choosing Av or Tv, you've done half the work for the camera - all it has to do is speed up, slow down, open up, or close down to get the right exposure.

Some cameras also allow "auto ISO" - now there are three variables to juggle in Program mode - noise vs shutter speed vs best performing aperture. again, a fairly simple decision tree.




  
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apersson850
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Jun 19, 2008 17:38 as a reply to  @ fxk's post |  #6

You should also remember that you can adjust the combination of shutter speed and aperture the P mode comes up with as you like. Just turn the dial after metering, and you'll see 1/125 @f/8 change to 1/250 @f/5.6 or 1/60 @f/11, for example.
But this is for the current shot only, so you have to redo the program shift again for the next image, if you want to. The Canon T90 had seven different program settings, where you could adjust this relationship on a permanent basis, as long as you liked it to last. On the other hand you couldn't tweak the selection like you can on the EOS cameras.

If you consistenly want a specific aperture or shutter speed, use Av or Tv. If you all the time want the same combination, use M.


Anders

  
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prime80
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Jun 19, 2008 18:49 |  #7

From the way you're talking, you're not so rusty that you've forgotten the basic rules. They're still the same, except you no longer have to change the film to change the ISO. Slap it in Av and get back into the swing of things!


John
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neil5054
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Jun 20, 2008 09:10 |  #8

apersson850 wrote in post #5754773 (external link)
You should also remember that you can adjust the combination of shutter speed and aperture the P mode comes up with as you like. Just turn the dial after metering, and you'll see 1/125 @f/8 change to 1/250 @f/5.6 or 1/60 @f/11, for example.
But this is for the current shot only, so you have to redo the program shift again for the next image, if you want to. The Canon T90 had seven different program settings, where you could adjust this relationship on a permanent basis, as long as you liked it to last. On the other hand you couldn't tweak the selection like you can on the EOS cameras.

If you consistenly want a specific aperture or shutter speed, use Av or Tv. If you all the time want the same combination, use M.

As this is my first DSLR (coming from a Powershot where it wasn't as easy adjusting in P mode), I will try the wheel adjustment in P mode and see how it reacts. Good intermediate step before Tv or Av modes. Not sure if the camera makes this adjustment starting with Tv or Av priority in the P mode, but will shoot some and test it. Seems like using P mode and manual wheel adjustment is similar to just using the Tv or Av mode, but, from what fxk wrote, perhaps the Camera's computer in P mode attempts a combination closer to the optimal sweet spots of the lens and condition?




  
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ddphotography
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Jun 20, 2008 15:27 |  #9

Neil...I just got a Xsi over the weekend and I adore it. I have been shooting a sx100is powershot. I always shot it in AV. It is my personal preference. I have tried P mode, but it just doesn't do for me. I say, it's digital...put it on AV, set your ISO on 100-200 and go outside. This camera (Xsi) is very good and easy to work with. The lens kit has IS. Turn it on...shoot for an hour, delete what you don't like.

I "dare" you to go AV for an hour. Post your best shots here. I can't wait to see. You will have so much fun and you'll see just how easy it is to get back into AV.

......macro rocks, too BTW!




  
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neil5054
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Jun 20, 2008 15:54 as a reply to  @ ddphotography's post |  #10

I college I had a Pentax Super which was not fully auto but could shoot in Tv or Av modes, so I got used to that for a long time. that was 20 years ago, so I'm a bit rusty with judging DOF, but well up to the challenge!! I am traveling to Italy and Greece in August do expect a lot of landscapes. From what I'm reading here I need to be sure I don't overdue the extremes of DOF for the lens.




  
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ddphotography
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Jun 20, 2008 16:24 |  #11

for landscapes I shoot wide open. For flowers, etc. I am partial to f/11 ish. I also like f/5.6 for some nice close ups.




  
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neil5054
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Jun 20, 2008 16:28 as a reply to  @ ddphotography's post |  #12

Ok, so I'm really rusty but when you say wide open, I thought wide opened meant big apeture, thus small fstop and short DOF? wouldn't landscapes be sharper in f11 or higher?




  
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ddphotography
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Jun 20, 2008 16:35 |  #13

yeah, shoot landscapes in f/18-22




  
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Hermeto
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Jun 20, 2008 16:38 |  #14
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ddphotography wrote in post #5760834 (external link)
for landscapes I shoot wide open. For flowers, etc. I am partial to f/11 ish. I also like f/5.6 for some nice close ups.

neil5054 wrote in post #5760857 (external link)
Ok, so I'm really rusty but when you say wide open, I thought wide opened meant big apeture, thus small fstop and short DOF? wouldn't landscapes be sharper in f11 or higher?

Rather than guessing, take a look here guys:

http://web.canon.jp/im​aging/enjoydslr/ (external link)


What we see depends mainly on what we look for.

  
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