77mm ltd vs FA 50/1.7 for auto focus speed?

[hnl_imaging]

<p>I am wondering if any one could compare the auto focus speed of the FA 77/1.8 and the FA 50/1.7. I have no hands on experience with the 77/1.8, but was able to look at a used 50/1.7 in a store yesterday. I had a very hard time getting it to lock focus on my 2 year old, even when she wasn't moving too much. It did seem to lock on ok when the subject was completely stationary. I am looking for something to photograph my girls with and they move... I have been thinking that the 77/1.8 or the 70/2.4 would be a good replacement for my smc-m 100/2.8, which I generally liked but was having a hard time photographing my girls with, I thought because of the focus (my focus...) speed. So I wanted to replace it with an auto focus lens.<br>

With what I saw yesterday, I may be better off going back to manual focus if the 77/1.8 is as difficult to work with as the 50/1.7. <br>

Any input or suggestions?<br>

Thanks</p>

[john_shriver]

<p>Fundamentally, the speed of autofocus depends on the camera for screw-drive lenses. How fast it can turn the motor, and how fast the electronics for the autofocus are.<br>

Both the FA 77/1.8 and FS 50/1.7 have 120 degrees of focusing throw. But the 50mm lens focuses a lot closer, so the amount of focus distance change for a given rotation is more. <br>

I just tried both on my K5, and I think this different "rate" makes the 50mm focus a bit faster than the 77mm. For a given screw-drive motor speed, you get there faster on the 50mm.<br>

But this will have nothing to do with ability to <em>lock</em> focus. That's just a matter of the camera, and how much light you have. Well, fast lenses make focus lock better -- but they also make accuracy more important when used wide-open.<br>

Forget about visual manual focus unless you replace the focusing screen with one intended for manual focus. The screens are optimized for brightness with the slow kit zoom, not for effective manual focusing.</p>

 

[hnl_imaging]

Thanks for the response. Not to argue with that last assessment that the camera could be to blame, but with my 20-40 ltd my k200 is

pretty fast. I even put that lens on in the store to compare and see if I was imagining things. The 20-40 was not only faster, but would

actually achieve focus lock. I guess what I am getting at is if the FA series lenses, even the top notch lenses, can be expected to behave

like this.

I am well aware of the manual focus issues, I just sold 5 legacy lenses in hopes of purchasing one really decent auto focus lens. I used

those lenses almost exclusively for almost 2 years... I know that, from a bit of experience with my manual lenses, that I can usually get a

decent shot off were as with the 100/ 2.8 wr macro and the 50/1.7 in a daily well lit store, I couldn't actuate the shutter. I wanted to know if

that was going to be an issue with the 77/ 1.8 as well.

My experience with canon gear has been very different in general, but it seems that manually focusing autofocus lenses is possible,

maybe better, but not really better than legacy lenses on pentax. Pentax seems about the same in this regard...

At any rate, during the middle of the day, in a store with large windows, iso=400, shutter=1/60, aperture=2.8, I should be able to focus

plenty easily.

My canon 20-35/2.8 would have, my 50/1.4 or 1.8 would have, the 24/2.8 usm would have, and my 20-40 does. With in some limits, my

legacy lenses did, but not really fast or always perfectly accurate. Can I expect the 77/1.8 to act like the 20-40 ltd or my canon gear, or is

there another equivalent lens option?

[john_shriver]

<p>I'd expect any SDM or DC motor lens to focus faster than a screw-drive lens. But that shouldn't affect lock.<br>

Don't understand why the 50/1.7 would not lock. Maybe it's EEPROM programming is messed up, has bad hysteresis data. (It has data about the slop in the drive screw that's particular to the lens.)<br>

Pentax AF just isn't as fast and solid as Canon or Nikon...</p>

 

[hnl_imaging]

John, thanks. That is what I was looking to find out.

[stemked]

<p>I have the 77 mmf1.8 and find it a very fast focusing lens. The image below was taken with a K10D and the lens set to max aperture. It was obviously able to get her on the swing.</p>

<p><img src="http://d6d2h4gfvy8t8.cloudfront.net/17929055-lg.jpg" alt="" width="432" height="645" /></p>

 

[michael_kuhne]

<p>I have both SDM (prime and zoom) and DC lenses, in addition to my screw drive ones. I do not have the 20-40mm LTD. </p>

<p>With my K5 IIs and my K-20D I have found that my larger-aperture lenses with screw drive acheive slightly quicker focus than those of self-motorized types, but of course with the little zzt noise. I too have the Pentax F 50mm f/1.7, a very fine lens. But the 77mm LTD is in a different class in many respects. Both are useful at their respective focal lengths. With the 77mm you would not have to get as close to your subject. Very good for candid shots. It features superb bokeh, has a built-in lens hood, and has a very fine manual-focus feel. Its build quality is on another level, though the 50mm is good. Since you already have the 50mm, you would have a greater difference in focal length by acquiring the 77mm over the 70mm, as well as a stop more of speed, which by providing more light helps AF. The 77mm is also amazingly small for its focal length and aperture. It tested as actually 77mm, while the 70mm came out as 65mm. The 77mm would be a very good kit addition to your 20-40mm LTD and 50mm. As you go more into telephoto, however, it does become increasingly trickier to "nail" your subject as you handle your equipment. If you can do well with a much larger say 200mm lens, the 77mm will be a breeze. Practice makes perfect.</p>

<p>I do think the upper line of Pentax professional style bodies have better AF mechanisms than do the types of the K-200D variety. </p>

[Javier Gutierre]

<p>Having owned both of those lenses, the 77 was the quicker locking of the two. In fact the 50 was quite slow. </p>

[snik75]

<p>The DA 70 focuses pretty fast, too. It's not quite as highly rated as the 77, and gives a very different look, I believe (I've not had a 77 before) but it is quite a bit cheaper. You can pick up an SMC version for under $300, which is amazing to me. Neither of those lenses focuses that closely, though, which gave me a little trouble when my kids were that age - can't zoom in just on the eyes or face. The fifties let you get a little closer.</p>

<p>But that particular 50/1.7 sounds like it may have had an issue. I don't think I would judge all F and FA lenses by that.</p>