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10-20-2009, 05:46 PM   #1
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70 vs 77 auto-focus speed

I've read in a number of places that the Pentax 70 limited auto-focuses much faster than the rather more expensive Pentax 77. Is this true? What accounts for this? I would have thought that the larger max aperture of the 77 would make focusing easier, and thus faster.

TIA,

Will

10-20-2009, 05:59 PM   #2
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Hi Will,

I believe AF speed is not related to the max. aperture, but rather low-light AF efficiency would be.
Many claim the FA ltds are slouches in AF, but really it ain't that bad - a few milliseconds slower perhaps...

I don't know what the focus ring thread is like on the DA 70, but the 77 ltd has a rather long focal throw, making AF take a little longer going from min to max focal distance (and vice versa).
10-20-2009, 06:12 PM   #3
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I don't have either lens, but it could be that the 77mm was developed for film and thus has better manual focus throw, but costs a bit of AF speed due to the extra rotation needed.
10-20-2009, 10:06 PM   #4
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Longer focus throw and greater weight for the FA77 both mean the AF motor has to work harder - more turns of the screw, and each turn is more difficult. Plus the 77 isn't as sharp wide open as the 70, so the AF system also might have a harder time for that reason.

10-21-2009, 01:47 AM   #5
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I have DA70, and used FA77 for one afternoon.

Yes, DA70 is a lot faster. I think lighter lens element and shorter focus throw helps.

For low light focusing, I know AF needs good contrast to lock. In my experience, the DA lens typically has higher contrast wide open than some FA lens.. like 24, 43 and 50.
10-21-2009, 03:29 AM   #6
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Don't know about the 77, but the 70 is the fastest focusing lens that I own. Half push ... zip and its done.
10-21-2009, 05:01 AM   #7
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Can't speak to the 77mm but the DA 70/2.4 is very fast focusing. Just another of its many charms.

Tom G
10-21-2009, 07:12 AM   #8
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Original Poster
QuoteOriginally posted by Marc Sabatella Quote
Longer focus throw and greater weight for the FA77 both mean the AF motor has to work harder - more turns of the screw, and each turn is more difficult. Plus the 77 isn't as sharp wide open as the 70, so the AF system also might have a harder time for that reason.
Thanks to Marc and everybody else for the quick responses. I think my original questions have been well answered.

Here's why I ask. (This veers the thread in a slightly different direction but it's my thread, so I hope that's okay.)

I've been agonizing for weeks over whether to get the 77 or the 70. For a while, I thought I'd decided to get the 77. It's about a full stop faster than the 70, and apparently the image quality of the 77 is just about as good as it gets.

The only real problem with the 77 is that it's considerably more expensive. And when it came down to placing an order, I guess I chickened out, and I bought the 70. It arrived yesterday. Now, of course, I'm second-guessing myself. I do this all the time, I'm afraid; it's my major character flaw.

The 70 from B&H was $250 less than the 77 would have been. That's good. The 70 is very compact, and that's nice, too. Auto-focus speed is a plus and I gather that the image quality is excellent, if not quite up to the 77's. I haven't had a chance to test it yet. And now Marc adds that the 77 is not as sharp wide open as the 70.

But that extra stop in the aperture sounds awfully good to me. The slightly longer reach of the 77 is also a good thing. And then there's the issue of image quality. I haven't used the 70 yet at all and it's not too late to change my mind and exchange it for the more expensive 77. If anybody wants to tell me what to do, I'm willing to listen! :-)

Thanks again,

Will

10-21-2009, 07:19 AM   #9
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QuoteOriginally posted by 8540tomg Quote
Can't speak to the 77mm but the DA 70/2.4 is very fast focusing. Just another of its many charms.

Tom G
I use both. The 70 focuses faster than the 77.
10-21-2009, 07:30 AM   #10
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I have both. 70 is not only faster, but also much more quiet.
But 77 is beautiful wide open
10-21-2009, 08:26 AM   #11
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I don't have the DA70 but have had the FA77 for several months. I love the images. Even wide open they seem perfect for portraits, which was my primary purpose anyway.

Since my subjects don't usually move or run away too fast, very high AF hasn't been an issue in my case.
10-21-2009, 08:42 AM   #12
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QuoteOriginally posted by EsBee Quote
I don't have the DA70 but have had the FA77 for several months. I love the images. Even wide open they seem perfect for portraits, which was my primary purpose anyway.

Since my subjects don't usually move or run away too fast, very high AF hasn't been an issue in my case.
Thanks. That's my case, too. I don't do street photography and instantaneous auto-focus is not really a requirement for me. I'd be using the 77 for portraits and weddings and similar events. Seems to me that the 77 would have two great advantages:

1, the faster max aperture would be great for portraits, giving me the option to get very shallow depth of field. NOTE that when I'm shooting portraits, I usually have all the light I need, either because I'm shooting during the day with good available light or because I'm using flash or supplemental light.

2, the faster max aperture would be really useful in weddings, etc., when light is low and I am unable to use flash. I'm not usually very close to the subjects in this situation, so even shooting at f/1.8 or f/2, I think I'll get a couple feet of depth of field and that should be sufficient. And I might be able to take a shot a ISO 800 rather than ISO 1600.

These are significant advantages. But the 70 has its own advantages - quite different ones, of course.

Guess since it's here, I'll take some test shots today with the 70 and make an executive decision. Thanks for the input.

Will
10-21-2009, 12:01 PM   #13
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QuoteOriginally posted by WMBP Quote
1, the faster max aperture would be great for portraits, giving me the option to get very shallow depth of field.
Have you checked to see what DOF is like at 70mm and f/2.4 at portrait distances? I think that's already about as shallow as I'd ever want. And if you consider that the FA77 isn't quite so sharp wide open (check photozone.de MTF scores), you might find yourself stopping down just a bit, largely closing that gap.

QuoteQuote:
2, the faster max aperture would be really useful in weddings, etc., when light is low and I am unable to use flash. I'm not usually very close to the subjects in this situation, so even shooting at f/1.8 or f/2, I think I'll get a couple feet of depth of field and that should be sufficient. And I might be able to take a shot a ISO 800 rather than ISO 1600.
True in theory, but again, between sharpness and DOF issues, I'd guess you'd be tending to stop down the FA77 more than you think.

Realistically, most people I know that own or have used both give the nod to the FA77, which acknowledging that the DA70 runs a very close second and is still a wonderful lens. Me, I was swayed primarily by price and quick shift - I don't know that I'd buy an AF lens without it. Being half the weight helped too (size difference is not as significant), but I'm funny that way. But I know at least one photographer who had both and ended up selling the FA77. So it's really going to be subjective.

Oh, also in my case, I preferred the shorter focal length, because I wanted something with a bit more differentiation from the M100/2.8 that I really love. That's really the main reason I didn't get an M85/2.
10-21-2009, 07:10 PM   #14
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Aside from Marc's story and a rare few other's that I've heard almost no one switches from the 77 to the 70. If you've got a hard on for quick shift, have a limited budget, or need the ultimate in small lenses then your choice is pretty clear from the get-go...and this conversation wouldn't be continuing.

The 77 is what the 77 is...it's images are amazing in every way. It focuses quickly for an FA lens, it is more than usable at f/1.8 and gets even better as you move towards f/4 (I've never used it higher than this). There's no quick shift, its heavier (it's still very small), and it focuses slower than the 70 (longer throw) - all true.

As for 77 not being as sharp wide open....that may be true, but the 70 isn't even in the picture at f/1.8...so that point's moot. By f/2.4 or f/2.8 they are pretty much equal in centre sharpness.

Only you can decide whether the 77 is better for you than the 70, however I can almost guarantee that you will love it...and when you do you won't think twice about the 70 ever again. Ever. Can't say the same for the 70 (which I went through...and you are now going through).

Not meaning to sway your decision...just offer some opinion from someone who has used both and felt the same as you after owning the 70.

c[_]
10-21-2009, 10:41 PM   #15
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I have the 77 and decided on that one for 3 additional reasons not yet mentioned on this thread:

1. It is FF compatible, meaning I use it also on the beautiful black MX i just landed;

2. Lens hood is integrated. Once you add the hood to the 70, it looses its size advantage.

3. It is "prettier": reminiscent of some of my old Leica M lenses in shape, size and heft.

On AF speed: AF is "fast enough" for my photography, but it is true that K7+FA ltd lenses is far from being the fastest focusing DSLR outfit out there. I can see why people would want faster and the DA ltd line could well be the only way for Pentaxians to get anywhere near the sort of reactivity D300 users (for example) are accustomed to.

I can see a case for owning both. For that matter, I can see a case for owning absolutely all the ltd lenses....
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