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Old Apr 3, 2012, 4:48 PM   #21
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It sounds to me like you need to do a lot more research. I'm not sure where you're getting most of this info, but speaking from experience, its wrong...
I think contrast detection can focus pretty much at any aperture, as long as there's some image coming from the sensor... at least in theory. In practice I'm sure the modern Canons can focus using contrast detection at F8.

The problem with Sony for telephoto is that you don't get the benefit of image stabilization in the viewfinder.
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Old Apr 3, 2012, 7:30 PM   #22
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I think contrast detection can focus pretty much at any aperture, as long as there's some image coming from the sensor... at least in theory. In practice I'm sure the modern Canons can focus using contrast detection at F8.

The problem with Sony for telephoto is that you don't get the benefit of image stabilization in the viewfinder.
You do in 'Live View', which is where you'd be if you were using contrast detection AF.
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Old Apr 4, 2012, 7:00 AM   #23
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BTW, Sony has jumped into its "Translucent Mirror" technology with both feet, which replaces the optical viewfinder with an electronic viewfinder. The image supplied to the electronic viewfinder comes from the main image sensor, which is stabilized, so the image that appers in the viewfinder of Sony's SLTs is stabilized.
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Old Apr 4, 2012, 8:17 AM   #24
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BTW, Sony has jumped into its "Translucent Mirror" technology with both feet, which replaces the optical viewfinder with an electronic viewfinder. The image supplied to the electronic viewfinder comes from the main image sensor, which is stabilized, so the image that appers in the viewfinder of Sony's SLTs is stabilized.
But wait... If the thing is fully electronic, then where does the mirror reflects to?

I mean, for phase detection you would only need tiny independent stuff at the very edges of the beam coming from lens. So why do you need a full-blown mirror?

BTW, I personally don't like this kind of technology that "undoes" all the progress to make super-clear glasses, and now they put a semi-transparent piece of glass that certainly degrades quality.

I think the future is mirrorless though, not like this, but the way the NEX series are going, but with a good, highres, large and comfortable electronic viewfinder. I definitely don't like to shoot using the LCD screen.

Also, considering something like 300 or 400mm, how much will the steadyshot stabilize in stops? I just read a review with a 300mm and the thing doesn't seem very useful... The guy was able to shoot in 1/200, but not more... The Sigmas can be soft and bad, but I was able to shoot at 1/60 on a 150-500 handheld. It's a pity after some time you begin to shake a lot because of its weight.

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Last edited by Electrolyte; Apr 4, 2012 at 8:19 AM.
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Old Apr 4, 2012, 9:11 AM   #25
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In conventional SLRs and dSLRs, a moveable mirror reflects about 70% of the light coming through the lens up to the focusing screen, pentaprism (or pentamirror), and the eyepiece. The other 30% of the light that passes through the mirror is reflected by another moveable mirror, down to the phase detection AF system in the base of the body. When you fully depress the shutter button, both mirrors flip out of the way, the shutter opens exposing the image sensor, the shutter closes, and the mirrors return.

In Sony's SLTs, the immoveable mirror reflects 30% of the light coming through the lens up to the phase detection AF system in what would be the pentaprism housing in a conventional SLR. The remaining 70% of the light passes through the mirror to the image sensor. So the PDAF system works constantly, even during exposures. That allows Sony's SLTs to use their PDAF systems to maintain focus while recording videos as well as high rate bursts of still images.

In general, Sony's Super SteadyShot sensor shift image stabilization system is good for about 2-3 stops, whatever the focal length. Some optically stabilized lenses can do better, but most can't.

The rule of thumb is that, generally, you shouldn't use an exposure time that is longer than the inverse of the focal length (i.e.: with a 300mm lens, you shouldn't use a shutter speed longer than 1/300 second.) But that's for 35mm film SLRs and 'Full Frame' dSLRs. For APS-C bodies, you should use the 35mm equivalent focal length (i.e.: with a 300mm lens, you shouldn't use a shutter speed longer than 1/450 second, or 1/480 on a Canon.) A 2-3 stop improvement would let you use a shutter speed that was 4 to 8 times longer.

Your performance may vary.

The only attempt to quantify the advantage of image stabilization is done by SLRGear.com. You can see the results of those tests for the Canon EF 70-200mm f/2.8L IS II USM (Tested), the Sony 70-200mm f/2.8 G SAL-70200G (Tested), and the Sigma 70-200mm f/2.8 EX DG OS HSM APO (Tested), by clicking on the IS Test tabs on the respective pages.

BTW, the mass of a lens adds its own stabilizing effect. Camera shake is caused by the muscles in your body in your attempt to stand upright. Newton's First Law of Motion, put simply, is that an object at rest tends to remain at rest, and an object in motion tends to remain in motion. The bigger and heavier a lens is, the harder it is to move it in one direction, stop it, and move it in the opposite direction. The harder it is for us to shake a lens, the less it shakes.
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Old Apr 4, 2012, 9:26 AM   #26
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Thank you for your detailed explanation!

I didn't know you had to reflect the entire thing in order to do phase detection...

70% seems too little . It must be good for filming and action, but the blinky viewfinder of conventional DSLRs is okay to me.

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The bigger and heavier a lens is, the harder it is to move it in one direction, stop it, and move it in the opposite direction. The harder it is for us to shake a lens, the less it shakes.
Tell this to my muscles after 15 mins trying to hold a 2kg lens steady .

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Old Apr 4, 2012, 9:48 AM   #27
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Quote:
Originally Posted by Electrolyte View Post
...and now they put a semi-transparent piece of glass that certainly degrades quality.
Quote:
Originally Posted by shutterbug1076 View Post
It sounds to me like you need to do a lot more research. I'm not sure where you're getting most of this info, but speaking from experience, its wrong...
'Nuff said!
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Old Apr 4, 2012, 9:52 AM   #28
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Originally Posted by Marawder View Post
'Nuff said!
It's 30% less light to the sensor it's certainly not better than 100% (or kinda).
Could you just ignore this very topic please?

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Old Apr 4, 2012, 10:07 AM   #29
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Originally Posted by Electrolyte View Post
I didn't know you had to reflect the entire thing in order to do phase detection...
No. Phase Detection AF only needs 30% of the light. In a conventional dSLR, the PDAF gets what's left after 70% is reflected up to the viewfinder. In Sony's SLTs, the PDAF gets 30% off the top.

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Originally Posted by Electrolyte View Post
70% seems too little . It must be good for filming and action, but the blinky viewfinder of conventional DSLRs is okay to me.
70% is a loss of about 1/3 stop of light. That's the difference between f/1.8 and f/2.0 or between 1/250 and 1/320.
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Last edited by TCav; Apr 4, 2012 at 10:16 AM.
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Old Apr 4, 2012, 10:18 AM   #30
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No. Phase Detection AF only needs 30% of the light.
By "the entire thing" I mean "the entire 'area'" of the image. I though you could just make some clever mechanism that only reflects the internal "borders" of the entire light beam.

Anyways...

The bad thing about contrast detection, beside the fact it's dead slow, is that when you most need it it always begins searching the focus in the wrong direction so it takes much more time.
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