K7 focus

[RaSoFe]

Maybe the f/2.8 sensors require some technological wherewithal that Pentax > doesn't yet possess? Or perhaps it is more expensive than it would at first > seem. .....

Hi Greg,

I have a guess (but just a guess):

The f/2.8 system requires a bigger secondary mirror (the one which redirects part of the light towards AF sensor) - see Fig. 6 in http://doug.kerr.home.att.net/pumpkin/Split_Prism.pdf for illustration.

Could be that to implement f72.8 Pentax would have to redesign the whole mirror assembly.

Best,
Andrey.

 

[Tan68]

depth of field is affected by aperture, but it is also affected by focal length. even using the same aperture, the 200mm lens will offer less DOF than the 135.

is the difference significant enough to make the test entirely invalid ? maybe. but, the difference is significant enough that it is poor form to draw a conclusion without making account of the effects the different focal lengths have on DOF. in a review, he should have pointed out the apple-to-orange (or maybe orange to tangelo) aspect of his comparison.

subject = 100 feet
135mm, f/8, dof = 57 feet, starts @ 79
200mm, f/8, dof = 25 feet, starts @ 89

PS

boogity. i like to use the little squiggly (on the key left of "1") to mean approximate as in "approximately 79". apparently, it don't work that way, anymore ! oh, well, i will let it ride.

and, maybe the Nikon would perform better.
it was just poor procedure to come to the conclusion the way he did.

IMO, the fact that he tested with a constant f/2.8 lens doesn't mean that he shot using an aperture of f/2.8, therefore saying that he had issues with the shallow DOF is not accurate.

 

[wy2lam]

Roland, I guess you cannot help but be defensive due to the many bash posts. But did I ever imply there's a problem?

I'm just voicing my dislike of increasing torque alone (or at all) to improve AF like they did on the K10 and K20. (those improvements were not real at all).

Looks like the K-7 (and to a lesser extent, the K-m) finally has some genuine way to improve low light AF, that's why I also preordered.

Already the K-m has new AF algorithms which makes K-m clearly faster and more reliable in low light than the K20D. The K-7 definately has those new algorithms, but on top of the new algorithms the K-7 also has a stronger AF motor plus a colour sensitive AF sensor plus AF assist lamp.

The K-m has the fastest autofocus of all entry level cameras according to tests in swedish magazines, leaving Canon and Nikon entry level offerings eating dust. The K-7 is improved even further.

So what's the problem really?

You know what? I'm sort of hoping that the K-7 AF improvement would be > totally due to new sensors and algorithms, and have NOTHING to do with battery > or torque or any cheap "brute-force" techniques, because it'll mean that the > improvements may fade away when the battery is near dry, and these > improvements are always minor.

I'm very slightly disappointed to see "with more torque", but still I hope other parts of AF improvements will make this up.

--
Take care
R
http://www.flickr.com/photos/raphaelmabo

 

[Robgo2]

Yes you're right That's only my belief based on few opinions on pre-production K7's. I will wait for reviews! I like the K7 design a lot and I hope that Pentax can offer competitive cameras for Pentax lens owners.

You can believe what you like, but I prefer to see in the 'metal', then I'll confirm or not your comments.

My guess is that the K-7's AF will be so close to the D300's that in practical terms, there will be no significant difference. Pentax knew what their competition was when they designed the K-7 and set their goals accordingly. But, yes, we will just have to wait and see.

Rob

 

[OPTMEKX7]

Yes you're right That's only my belief based on few opinions on pre-production K7's. I will wait for reviews! I like the K7 design a lot and I hope that Pentax can offer competitive cameras for Pentax lens owners.

You can believe what you like, but I prefer to see in the 'metal', then I'll confirm or not your comments.

Atb ;-)
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Jose Rocha

My gallery:
http://olhares.aeiou.pt/jplacebo

I don't know what you call this, but check it out. A thorough hands on report of a newly acquired K7.

cheers,

Rene

 

[Zuseher]

very interesting, see this link.

http://www.hh-muc.de/4i/categories.php?cat_id=191

or http://www.hh-muc.de/4i/categories.php?cat_id=193

or http://www.hh-muc.de/4i/categories.php?cat_id=199

here are a video (K-7) with 35-lens to see the real speed of the mountainbike-race

http://www.hh-muc.de/k-7/video/K7HH6315b.avi (30 mb)

Am a pre-K-7-tester.

 

[OPTMEKX7]

thanks for the link Zuseher, my link to the other forum was blocked.

cheers,

Rene

 

[Bart Hickman]

Without getting too technical, the answer is "no". AF light pickoff (small prisms in the mirror) do not see brighter light on a brighter lens. They see a constant brightness until the aperture gets too small, then they see nothing at all.

I have an old manual minolta SLR with a split-prism focusing screen that demonstrates this really well. You can manually raise the F-number on the lens and watch as the viewfinder image gradually gets darker, but the split prism section stays constant brightness until I go past about F/5 and then the prism section suddenly turns black.

Bart

I wonder. Since focus happens with the lens wide open, wouldn't an f/2.8 lens, with its wide-open larger aperture, have more light to work with in auto-focusing, whereas the f/4 wide open would have a good deal less light to work with?

I'm no specialist, just wondering.

--
http://bhimages.zenfolio.com

 

[mutleybird]

Already the K-m has new AF algorithms which makes K-m clearly faster and more reliable in low light than the K20D. The K-7 definately has those new algorithms,

So, they learned something about algorithms and can apply it to different cameras. So come up with a firmware update for the K10D with better algorithms.

The K-m has the fastest autofocus of all entry level cameras. The K-7 is improved even further.

So what's the problem really?

The K-7 price is close to old-technology Nikon D300. The K-7 AF should be at least as good as that camera, or better by now. That's the problem.

At this point there's no reason for explanations.