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09-28-2010, 03:50 PM   #16
Lowell Goudge's Avatar

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QuoteOriginally posted by telfish Quote
I have a sf10 and the 1.7 af converter will that do?
Do what? Focus with an F4 or slower lens?

It is also specified at +2 to +18 EV so if it can focus with F4 lenses there goes the theory all together.

09-28-2010, 06:27 PM   #17
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QuoteOriginally posted by Lowell Goudge Quote
Do what? Focus with an F4 or slower lens?

It is also specified at +2 to +18 EV so if it can focus with F4 lenses there goes the theory all together.
It looks not. I fitted my DA*300 and it would not. I then fitted my Sigma 17-70 2.8-4.5 and it would focus almost to 70mm (4.5) any lens 2.8 and faster works fine. Was trying under tungsten light and will give it a go in daylight in the morning.
09-28-2010, 10:26 PM   #18

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QuoteOriginally posted by JohnBee Quote
This has been on my mind for awhile now.

If the Pentax SMC-AF 1.7x TC was limited to f/4 lenses on average due to AF limitations, does this mean that the new K-5 AF will raise this threshold?

You thoughts...
Hi John,

Here's something I posted to another forum about 14 months ago -- not a formal test, nor a scientific one, but I think it might be add some real life data that might be relevant to the original question:

"Hi All,

Got a K7 yesterday and had a chance to play with it for a couple of hours both yesterday and today. I'll hold off on most impressions until I've had more of a chance to get settled in with it, but I will illustrate one of the main reasons why I wanted to upgrade from the K20 -- that the lower light AF sensitivity would allow me to use either more magnification via TCs or that it would allow me to use at least my 1.7x AFA with slower lenses.

Most look at low light AF sensitivity as just that -- focus in the dark with very fast lenses. I don't shoot too much in the dark, so I concern myself with better AF performance (or any at all) at slower max apertures -- pretty much the same thing in terms of the amount of light entering the AF sensor.

With the DS through the K20, f4.5 max at the lens was about the limit for AF ability with the 1.7x AFA, which was understandable. f8 max is a pretty common limit for AF systems, and most systems automatically default to MF when it senses f8 as the max aperture of the lens. Luckily Pentax doesn't have that limitation. My FA* 300/4.5 + AFA comes out as @ f7.7 (4.5 x 1.7) and this works well until the light drops to medium dark overcast -- that's why I started looking into 300/2.8s -- to allow me to shoot in less than optimal conditions.

Anyway, after playing indoors last night with the K7, I decided to see how far I could push it today. It started out as partly cloudy, and quickly became medium dark overcast. I found a young adult Black Crowned Night Heron hanging out at one of the local lakes, so I grabbed a couple of TCs and started shooting with the FA*300/4.5 and the 1.7x AFA (510mm, f7.7). Then I added the Sigma EX 1.4x APO TC for 714mm f10.7. I had tried this before with the K20 on a bright day and the cam just hunted lock to lock, no matter the target. As I had hoped, the AF system locked focus -- it was slow, but it locked. I shot a bunch handheld, sitting on the ground with my elbows propped up on my knees. I finally remembered to set the FL to 700 for the SR after about 20 shots. Here are some, all cropped and PP'd to taste -- mostly local sharpening, brightening and adding contrast.

K7, FA*300/4.5 + Sigma 1.4x APO TC + P F 1.7x AFA, f8 (actually f11 Av or wide open at the lens), 1/200, ISO 400, SR set to 500mm

This is a full height crop from a landscape frame

K7, same lens combo and settings, except ISO 500, SR set to 700mm

This is about the same crop as above.

The BCNH was pretty much ignoring me, so I went to the car, grabbed the FA*300/2.8, the tripod, and my Tamron F AF 1.4x TC, set everything up, and got some more shots at 1050mm f9.3. It was quickly getting darker, so I had to up the ISO to 1250 to get @ 1/200

K7, FA*300/2.8, Sigma 1.4x, Tamron 1.4x, 1.7x AFA, 1/200, f4.5 (actual f10.7 Av -- wide open at the lens), ISO 1250, SR off, tripod mounted.

This is just an 8x10 crop from the full frame, so it's just cropped some on the sides.
AF was very slow with this combo, but it did lock accurately -- and it had gotten pretty dim.

Bouyed up by these results, I tried a few more lenses when I got home -- the Tokina 150-500 f5.6 AT-X SD which wouldn't AF with the AFA and K20 at noon in June with a cloudless sky -- it AF'd with the K7 with the totally overcast sky (no shadows cast) -- pretty quickly too. I really had hopes for the Tamron SP 500/8 mirror, as this would be a killer minature 850mm f 13.6 AF lens, but it wouldn't lock on anything but the highest contrast subjects -- I still have hopes that it might be acceptable on a bright day, but I'm not going to hold my breath. . . The Tokina 80-400 f4.5-5.6 locks focus pretty easily, even though the light is getting pretty low. It needs some contrast to lock in though, but should give good results with more light. This would give me a viable 136-680 f7.7-9.5 AF zoom that's very compact and light. I'll test these for IQ on a sunny day.

This all is very encouraging to me, and should make all the 70-300, 55-300, and Bigma shooters happy as their lenses might now AF reasonably with a 1.4x TC in good light, making these lenses that much more useful."

Remember the subject is whether any added sensitivity in the K-5's AF sensor might extend it's functionality to slower lenses, not whether stacked TCs (or even one TC by itself) might be a good idea nor to what degree stacking TCs might degrade IQ. . . The point of posting the images is really only that the images were reasonably well focused.

You can draw your own conclusions. . .

It must be said that I really didn't use these extreme TC combos throughout the season -- just didn't need the reach enough to justify the tradeoff in focusing speed. . .


Last edited by snostorm; 09-28-2010 at 10:46 PM.

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