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Author Topic: Does a Digital camera need SLR?  (Read 6039 times)

zhgart

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Re: Does a Digital camera need SLR?
« Reply #15 on: November 02, 2011, 04:43:28 AM »
Review of Fujifilm's image sensor phase-detect AF is explained on dpreview so it can be done.  http://www.dpreview.com/news/1008/10080505fujifilmpd.asp

They claim AF speed of 0.158 seconds but obviously DSLR AF is much faster since shooting at 10 frames per second and autofocusing in between frames must be less than 0.1 s.   So at least in those fuji compacts, that system is not as fast.
Im afraind you may confuse DSLR with AF. they may be quit different concept.
To be frank, 0.16s is rather quick for a mechanism. but DSLR is a optical route reflected by prism. which can can get a accurate image positoin on the film through finder.

or say, ccd or cmos sensetivity is faster than mechanism  movement which drive lens to a fit position. one is electronic, another is mechanism.

however, lens has a large mass.

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Re: Does a Digital camera need SLR?
« Reply #15 on: November 02, 2011, 04:43:28 AM »

zhgart

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Re: Does a Digital camera need SLR?
« Reply #16 on: November 02, 2011, 04:44:22 AM »
Thank you all !

Edwin Herdman

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Re: Does a Digital camera need SLR?
« Reply #17 on: November 02, 2011, 05:29:20 AM »
Review of Fujifilm's image sensor phase-detect AF is explained on dpreview so it can be done.  http://www.dpreview.com/news/1008/10080505fujifilmpd.asp

They claim AF speed of 0.158 seconds but obviously DSLR AF is much faster since shooting at 10 frames per second and autofocusing in between frames must be less than 0.1 s.   So at least in those fuji compacts, that system is not as fast.
Im afraind you may confuse DSLR with AF. they may be quit different concept.
To be frank, 0.16s is rather quick for a mechanism. but DSLR is a optical route reflected by prism. which can can get a accurate image positoin on the film through finder.

or say, ccd or cmos sensetivity is faster than mechanism  movement which drive lens to a fit position. one is electronic, another is mechanism.

however, lens has a large mass.
I assure you, the poster Meh has not confused the two.

I somewhat understand your point, though.

Meh is correct (great link by the way!) that there is an essential problem for mirrorless cameras in phase-detection AF, perhaps.

My speculation is that the Nikon V1 and J1 could tilt the sensor to induce the phase shift.  Looking at the simple geometry of the tilted mirror, you'll see that substituting the actual image sensor for a mirror and a set of secondary sensors placed further away makes no real difference (assuming that the difference in distances compared to the slightly longer path of an SLR AF system is not a minimum required, and I can't see any reason to assume it must be so), except that distances and the length of the sensors will have to be computed slightly differently in setting up the system, and you'll have to apply those figures to the portion of the image sensor being sampled.

I rather hope they figured out a better way to do it, though.  The "hybrid" description, if true, suggests they're using the image sensor already to provide some contrast detection as well, so why not also sample a few pixels at a time and apply a phase detection routine to those as well?

Phase detection AF is inherently only slightly faster - in practice this may be irrelevant if your scene fools either system; phase detection may be fooled as well if there are two coincidental peaks in the frequencies hitting the phase sensors that don't correspond to the same point (though this seems to be a nearly freak situation, and I have a hard time envisioning how it could occur if the light hitting each phase detect AF sensor comes from the same portion of the image, except perhaps maybe at very close distances where some parallax error could occur) - and definitely not more accurate than contrast detection.

Both systems have to physically move the lens in order to get a second point for comparison purposes - if we aren't already focused on a sharp point that satisfies the camera's AF system (of any type).  As Meh's link demonstrates, phase detection doesn't work on an entire image at once, but instead is comparing two one-dimensional samples (two lines with different values strung along).

Software contrast (or better yet, fast edge-detection as seen in "focus peaking," seen here) on a full image, at least in theory, still allows the camera to immediately confirm correct focus without moving the lens elements at all, and even better gives the camera (assuming sufficient CPU resources and a fast enough EVF, OVF, or Live View display, as Cetalis mentioned) the ability to display this so you can see this in action yourself - whereas phase detect is a process hidden from the user (though that is often considered a good thing because it happens while you are viewing the scene through the viewfinder).  Of course, as the Stanford graphics resource link says - phase detect works immediately in practice, but there is a chance (as with contrast detection) the lens movement will initially be in the wrong direction or by the wrong amount (which I mentioned earlier).  An addition problem is presented, as we all know, when there isn't a physical phase detection sensor present where we want to focus.  A big potential benefit of contrast detection AF is that it can theoretically be applied to as many pixels from the image sensor as you can capture quickly - though of course reading a full image sensor quickly is nowhere near as fast as using a phase detection array.  That'd also help contrast detection overcome its potential to be misled by certain patterns in scenes.  Cost savings for manufacturers are probably a big draw of contrast detect AF, because no separate AF array is required, and thus eliminating one fixed cost and a complex CMOS (or CCD) circuit from production.

Looking at the phase detection chart, you'd think that it will work immediately and accurately every time, but it assumes that there is a bright (contrasty) point in the scene to lock focus onto - we all know from experience that even good AF systems coupled with good lenses can misfocus on a low-contrast target, so the essential problem for contrast detection holds true for phase detection.  This can be seen clearly in the Java applet - the lines hitting the sensor do not merely represent a "point of sharp focus," but a part of the visible spectrum that contrasts with the light hitting the other portions of the linear AF sensor.

And yes, contrast detection does guess at the direction and magnitude of corrections: link.

Meh

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Re: Does a Digital camera need SLR?
« Reply #18 on: November 02, 2011, 10:51:54 AM »
Review of Fujifilm's image sensor phase-detect AF is explained on dpreview so it can be done.  http://www.dpreview.com/news/1008/10080505fujifilmpd.asp

They claim AF speed of 0.158 seconds but obviously DSLR AF is much faster since shooting at 10 frames per second and autofocusing in between frames must be less than 0.1 s.   So at least in those fuji compacts, that system is not as fast.
Im afraind you may confuse DSLR with AF. they may be quit different concept.
To be frank, 0.16s is rather quick for a mechanism. but DSLR is a optical route reflected by prism. which can can get a accurate image positoin on the film through finder.

or say, ccd or cmos sensetivity is faster than mechanism  movement which drive lens to a fit position. one is electronic, another is mechanism.

however, lens has a large mass.

I'm not confusing the two, but I can see why the way I wrote my comment may be unclear...   The Fuji is a mirrorless camera and is therefore not a DSLR.  So, my comment was comparing the 'phase-detect AF built into the image sensor of the Fuji' to the 'phase detect AF implemented in separate AF sensor of a DSLR'

Unfortunately, I'm not sure what your other comments are getting at but perhaps you also misunderstood why I referred to the fps of a DSLR.   I did that to demonstrate that the phase-detect AF in a DSLR using a separate sensor is much faster than the Fuji system... if a DSLR can shoot at 10fps then the AF must be able to focus in less than 0.1 s because it is happening in between each flip of the mirror.

Hopefully that's clearer than my shorter original comment.

Mt Spokane Photography

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Re: Does a Digital camera need SLR?
« Reply #19 on: November 02, 2011, 09:21:27 PM »
if a DSLR can shoot at 10fps then the AF must be able to focus in less than 0.1 s because it is happening in between each flip of the mirror.

Hopefully that's clearer than my shorter original comment.

10 FPS cameras like the 1D series predict where the subject will be for the next frame and focus there, and do not AF each frame at 10FPS. 
you can force a 1D camera to focus each image before closing the shutter, but you will not get 10 FPS if you do that.

Mirrorless cameras do something similar.  The new Nikon may improve on this with much faster focusing.  The previous mirrorless cameras are hampered by a lack of computing power, but the new Nikon seems to have plenty of it.  Sony also has upped the stakes with their latest mirrorless cameras, but there are still lots of problems to solve before they get to professional levels, and the viewfinder is one of the biggest.

AprilForever

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Re: Does a Digital camera need SLR?
« Reply #20 on: November 02, 2011, 11:17:01 PM »
Probably, there will be SLRs manufactured and used for the next hundred years or more. For example, the rangefinder camera is still well in use in its circle of users...

No matter how good an electric monitor may look, nothing at all beats looking at the real thing as it happens through my OVF. Long live the prism!
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Meh

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Re: Does a Digital camera need SLR?
« Reply #21 on: November 02, 2011, 11:17:21 PM »
10 FPS cameras like the 1D series predict where the subject will be for the next frame and focus there, and do not AF each frame at 10FPS. 

Sorry had to delete and edit the reply I just made if anyone started reading that:  Yes, in "one shot" mode the AF locks and then the camera will fire away without refocusing between frames.

You're referring to predictive AF and that is what my comments are relative to.  With predictive AF, the camera will fire away at whatever the frame rate is and will take an AF reading between every frame and adjust focus between frames.   And you're right, it also measures the speed of the subject and predicts where the subject will be in about 60 milliseconds (or whatever the shutter lag for the camera is) in the future when the exposure will be taken and focus there rather than focus where the subject is at that moment.  Between each frame AF measurements are made, a focus position is predicted, and the focus element is moved to that position.  It's essentially the same process whether in one-shot or predictive mode the difference being whether it focuses where the subject is now or where it will be 60 ms later.  And in predictive AF mode, if it can do that between frames then it is doing it in less than 0.1s (for a 10fps camera).

Mirrorless cameras do something similar.  The new Nikon may improve on this with much faster focusing.  The previous mirrorless cameras are hampered by a lack of computing power, but the new Nikon seems to have plenty of it.  Sony also has upped the stakes with their latest mirrorless cameras, but there are still lots of problems to solve before they get to professional levels, and the viewfinder is one of the biggest.

Depends which mirrorless cameras you are referring to.  My comments were referring to the differences between contrast AF and phase-detect AF.  The new Nikons V1 and J1 (if that's what you're referring to) claim to have a hybrid AF that uses contrast and phase-detect and I'm not sure how they implemented it.  The Fuji camera I mentioned has phase-detect built into the image sensor but it is slower (0.158s) and masks off some photosites so technically that affects the image and has some other issues I believe.

The Sony SLT (if that's what you're referring to) are not true mirrorless... they just use a translucent prism in place of the mirror that doesn't flip.  It stays in place and splits the light up so that 70% goes to the image sensor and 30% to the EVF and AF sensors.  The downside to this system is of course that you lose 30% of the light (about a half stop) from the image and also that the image may be slightly degraded (not much though) because the light used for the image must pass through that translucent mirror.  In a flipping mirror camera, when the mirror flips up 100% of the light is used for the image and does not pass through an additional optical elements.

Feel free to correct anything I'm not getting quite right.
« Last Edit: November 02, 2011, 11:20:30 PM by Meh »

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Re: Does a Digital camera need SLR?
« Reply #21 on: November 02, 2011, 11:17:21 PM »

49616E

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Re: Does a Digital camera need SLR?
« Reply #22 on: November 03, 2011, 01:48:31 AM »
Some photographers may not like having to use the lcd all the time with the power that it consumes. Especially the wilderness sort that do not have a convenient way to charge batteries or want to carry around a bunch of them.

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Re: Does a Digital camera need SLR?
« Reply #22 on: November 03, 2011, 01:48:31 AM »