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02-14-2012, 07:04 AM   #61
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QuoteOriginally posted by falconeye Quote
The K-5 can auto focus a manual Sigma 500/4.5 with the 1.7AF TC. So, it should be able to do the same with a 560/5.6 + 1.4x TC combo.
I don't understand this concept of AF sensors being usable down to a particular f-stop because of the amount of light. If I used an f/8 lens in bright daylight, surely more light would get through then using an f/4 lens in a scarsely lit room? Can anyone explain how this works?

02-14-2012, 07:15 AM   #62
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QuoteOriginally posted by starbase218 Quote
I don't understand this concept of AF sensors being usable down to a particular f-stop because of the amount of light. If I used an f/8 lens in bright daylight, surely more light would get through then using an f/4 lens in a scarsely lit room? Can anyone explain how this works?
It is simply more difficult for the camera to determine focus as it also needs to be able to see the target. The green lamp on the front of the camera can help in a scarcely lit room but it's not much help at the distance to subject that can occur using a telephoto. This issue isn't unique to Pentax.
02-14-2012, 07:18 AM   #63
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QuoteOriginally posted by starbase218 Quote
Can anyone explain how this works?
Please google for phase detect auto focus for the full story.

To simplify, think of a split screen where the center blacks out if you stop down. Or think of the AF looking thru two distinct "virtual circles" on the glass on opposite sides of the lens front lens. And if you stop down (or the lens is too slow), the AF will see the aperture blades or lens tube and fails. The angular distance between the two circles is called the AF sensor's F-Stop. It is widely believed that all Pentax AF sensors are F5.6 sensors.

The interesting part in this story is that the K-5's AF system uses a larger aperture to gather more like to work better in low light. A side effect is that the two "virtual circles" mentioned above are rather large. This means that they don't blacken out immediately if the aperture closes beyond F5.6. But eventually, it will.

The success with 500/4.5+1.7xTC means that some light still reaches the AF sensors at F7.7. I don't think it blackens out entirely before F8. But it will need daylight levels of luminosity to still work properly.
02-14-2012, 07:30 AM   #64
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QuoteOriginally posted by starbase218 Quote
I don't understand this concept of AF sensors being usable down to a particular f-stop because of the amount of light. If I used an f/8 lens in bright daylight, surely more light would get through then using an f/4 lens in a scarsely lit room? Can anyone explain how this works?
Here's a flash applet that shows nicely how phase detect autofocus works:
Autofocus: phase detection

Now you can place micro lens 1 and micro lens 2 wide apart or close to each other. If you place them wide apart, accuracy increases, but it can only be used with fast lenses, because otherwise, the light rays won't hit both micro lenses simultaneously (the red and the green rays will be close to each other). So if you place them close to each other, they are usable with slower lenses - but then they're not as accurate.

Because of this, cameras with a center "f/2.8" sensor (i.e. a highly accurate center sensor) often have an additional f/5.6 or f/8 center sensor in order to make the center point work with slower lenses too.

02-15-2012, 12:28 AM   #65
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OK, I think I understand. Thanks!
02-15-2012, 12:43 AM   #66
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Now that I think about it, there's no way this thing telescopes...photo 1 here is the normal view, photo 2 is with the hood extended.

https://www.pentaxforums.com/forums/pentax-news-rumors/175093-pentax-560mm-f5...ml#post1821895

The first knob is to loosen the hood, and the second to loosen the tripod collar.

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02-15-2012, 01:07 AM   #67
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Pentax has never had any lens with a knob to loosen/fix the hood. And I don't see how the hood would be fixed with the first knob. If that was meant to fix the hood, then the knob should be on the hood and not on the lens barrel.

I still think this whole thing telescopes to make it more compact.
02-15-2012, 04:30 AM   #68
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QuoteOriginally posted by Asahiflex Quote
Pentax has never had any lens with a knob to loosen/fix the hood. .
Wrong. The A* 300/2.8 had a knob for just that (and I'll guess the same is true for the other A* super telephotos).

02-15-2012, 04:32 AM   #69
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QuoteOriginally posted by Pål Jensen Quote
Wrong. The A* 300/2.8 had a knob for just that (and I'll guess the same is true for the other A* super telephotos).
Sorry, I was wrong indeed (I don't see the A*'s that often). But the knob was never part of the lens barrel but always part of the hood.

IMHO the ribbed pattern around the ring on which the first knob sits makes clear that something can be done with that part. And I think it's telescoping indeed.
02-15-2012, 04:54 AM   #70
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Photoshoped for sure.

QuoteOriginally posted by RXrenesis8 Quote
Well, I made the following image in photoshop so... Here's your disclaimer!
There's no seem where the hood meets the tube in the picture with the hood extended. The way that lens is constructed in the shot where the hood is extended would make it impossible to retract the hood as in the other shot. I doubt we can take any value from these pictures at all after closer inspection.
02-15-2012, 04:59 AM   #71
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QuoteOriginally posted by Adam Quote
Now that I think about it, there's no way this thing telescopes...photo 1 here is the normal view, photo 2 is with the hood extended.

https://www.pentaxforums.com/forums/pentax-news-rumors/175093-pentax-560mm-f5...ml#post1821895

The first knob is to loosen the hood, and the second to loosen the tripod collar.
First knob isnt for loosen the hood for sure.
The knob would be placed on the hood, not on the barrel. Second knob is for tripod collar, thats right.
f
02-15-2012, 07:19 AM   #72
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It seems odd that Pentax would display a lens with cosmetic imperfections to the barrel, i.e. the mark in front of the first knob near the tripod mount.

The mark is either wear caused by telescoping or by something else. It is hard to come up with reasonable explanations that don't involve telescoping.

If it is telescoping I presume the general consensus is that the lens is merely collapsable - not two different useable focal lengths?
02-15-2012, 07:32 AM   #73
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Maybe a Pentax manager got angry, picked up the lens and started to hit one of his employees with it.
02-15-2012, 08:05 AM   #74
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Yeah, these pre-prod lenses suffer a hard life. A pre-prod DA*60-250/4 once passed through my hands. Boy had it suffered before! The images I took with it were close to the worst I ever saw from a telezoom. I know now it was that particular copy, but it had me worried about lens quality there for a while. Hence I understand they don't want everyone to handle such products (as they should never have let me handle that DA*60-250). Which doesn't forgive Pentax's presenting this damaged DA560/5.6, unless they wanted to provoke this speculation and thus get much more internet presence compared to when all would have been clear? Viral marketing?

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02-15-2012, 08:16 AM   #75
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QuoteOriginally posted by Ishpuini Quote
Yeah, these pre-prod lenses suffer a hard life. A pre-prod DA*60-250/4 once passed through my hands. Boy had it suffered before! The images I took with it were close to the worst I ever saw from a telezoom. I know now it was that particular copy, but it had me worried about lens quality there for a while. Hence I understand they don't want everyone to handle such products (as they should never have let me handle that DA*60-250). Which doesn't forgive Pentax's presenting this damaged DA560/5.6, unless they wanted to provoke this speculation and thus get much more internet presence compared to when all would have been clear? Viral marketing?
That's exactly what I was wondering.

I can understand pre-production models getting a bit worn as they must get handled a lot as part of the design process. But the nature and positon of that mark just seems a little hard to explain any other way. Of course the fact that it's hard to explain doesn't mean the lens is telescoping at all.

One reason to leave the mark on might be to provoke speculation and raise hype.

Alternatively the actual finish on the lens could be very poor and the mark just happened as a result of that, and this could be their only prototype at this level of maturity.

But still.
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