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Old Jan 28, 2009, 9:28 AM   #1
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I am a beginner to DSLR photography....ofcourse i spent some time before with my olympus p&s for about 3 yrs now....
I purchased a canon rebel XS with 75-300mm III lens (combo deal) for 500$. I had Rebel Xsi in mind but went with Xs as I was getting this 75-300 mm zoom lens as well for about 150$ less than XSi....i purchased the canon 50mm f1.8 II lens with the money i saved...donno if it was a good choice......

I am fascinated abt the blur effect in the photographs...I use the 50mm f1.8 often for that reason...

my question is: Is rebel XSi better then XS with the 50mm f1.8 lens. I thought both xsi and xs were similar but for the 12 mp and 10 mp difference and the 9 point AF vs 7 point AutoFocussing in xs...but I am now told by my friend (who has an XSi) that XSi has a better focussing system with fast lens ... he showed me the line in the xsi specification missing in the rebel xs spec...

• 1 cross-type - F2.8 in rebel XSi
• 1 cross-type - F5.6 in rebel XS

Can anyboody throw more light on this pls? Was my thinking correct when I purchased Rebel Xs or should I have opted for XSi?


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Old Jan 28, 2009, 11:54 AM   #2
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Canon does a poor job of explaining this. Both the XS and XSi have a cross type sensor at the center focus point. That cross-type behavior will be available on both cameras for all f5.6 or better lenses (note: if the lens isn't f5.6 or better the camera won't auto-focus anyway - ignore zoom lenses that are rated at 6.3 it will focus with them too).

What the XSi has that the XS does not is the additional high precision center point. That means the focus tolerance is tighter on the XSi when using an f2.8or better lens.

So, the cross-type means that point can detect contrast differences along both horizontal and vertical axis. The high precision point on the XSi is supposed to guarantee a more accurate focus when using an f2.8 or better lens.

Also in regards to focus there is debate over whetherthe XS still uses the same focus algorithms as the earlier XTi (the last camera to have the 7 point system). Those algorithms were not as advanced as the ones used in the XSi.

What is known is the XS has better focus performance than the XTi. But some attribute that to the DIGIC III processor which it shares with the XSI. So, the supposition by these folks is the XS is better than the XTi because of the digic III but not as good as the XSi because of the lack of high precision and the lesser focusing algorithms.

The high precision is a known difference. The digic III is a known shared component. What no one knows and what Canon does not say is whether the focusing algorithms are different.

The algorithms will mostly affect AI-Servo focusing mode since it relies on "predictive focus" algorithms which calculate where the subject should be next and get a "jump start" on focusing there.

Did you make a mistake? That's a tough call. If you were shooting action I would say yes you made a mistake. For other purposes, the XS is a fine camera. I will suggest though that the 75-300 is not a very good lens on any camera. That's why so many shops give it a way in packages.

The 50mm 1.8 however is a fantastic bang-for-the-buck lens. It really is. Sure it's not on the same level as the L series primes (a noticable difference with the better primes is the quality of the blurred background improves) but hey for $80 it's a fantastic bargain and thus IMO a very good buy.
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Old Jan 29, 2009, 1:56 AM   #3
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Great explanation, thanks John.
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Old Feb 2, 2009, 10:57 PM   #4
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JohnG,... just to add some similar questions,... Since I have an XSi, and shoot mostly still shots, should my Camera be set to One Shot AF or to AI Focus AF. The later says it will switch to AI Servo if it detects movement. Is this really stable or just sitting of the edge waiting to switch between One Shot and AI Servo AF. Most of my problems with the XSi are always with accurate focus and I've tried both end settings. I have found that if the subject contrast is not excellent, it will not focus correctly in One Shot AF. If it has excellent light and good contrast, of course it works perfect.

My other problem is with Manual Focus on the 18-55 IS AF kit lens. I think it is a terrible design and I can't believe they let this on the market. At the 55mm setting the manual focus ring has a total rotation from close to infinity of 50 degrees and at 18 mm this is reduced to only 30 degrees. When you are trying to focus on something like a flower at 4' the total movement of the ring is less than 5 degrees to take in from front to back focus. I lose half of my shots because I don't get it right in manual focus. This is rediculous for any lens.

I would appreciate you comments
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Old Feb 3, 2009, 2:10 AM   #5
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Well which do you want - fast AF or accurate MF? You can't have both. Think about it.

The only lenses which tread the line with a nice balanced approach to using either AF or MF are Macro lenses. And most people complain that they have very slow AF.
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Old Feb 3, 2009, 9:14 AM   #6
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peripatetic is right!

IMO the issue with trigger1937 is not one of one-shot (or AI-servo), but because his lens is not an f/2.8 hence the camera can not achieve its higher focus precision. Most new lenses now are very fast in focus because of this very short throw in design - My Tokina 16-50 f/2.8 does the same thing (45 degree arc from close to infinity) but is extremely fast without any ultrasonic drive...
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