Again about the 17-70

[mggm59]

<p>I have been playing around with my lazy 17-70 and came up with a few findings I'd like some advice about.</p>

<p>1) the diaphragm reamins slightly closed at the wider settings (up to 50mm). Is this normal? Is this a cheap way to get the constant aperture throughout the range while getting decent quality at the widest settings "wide open"? Since it has no aperture ring, I just pushed the lever near the bayonet while zooming and looking from the back)</p>

<p>2) The front lens block (the frontmost tube) is slightly wobbly when I try to move it sideways. Is this common too?</p>

<p>3) Optical quality is actually quite good at infinity, contrary to my first tests at close range. However, it's very difficult to get focus right. A minute rotationa at infinity makes a huge difference, which means it is very difficult to use manually.</p>

<p>4) Since I am going to LA, is there any reliable and fast repair center there that could do the repair while I am there (a week)? Any idea of the price to replace the motor? Are the newer SDM reliable, or I am going to have the same problem in a few years?</p>

[andrewg_ny]

<p>1. Yes. I think this is part of the optical design for constant f/4. I don't know about cheap -- if anything, it means that some of the focal length range could gather more light than f/4 but for the sake of constant-ness they even it out by automatically stopping down slightly.<br>

2. Mine can wobble a little. I'd actually say the front-most extension only wobbles a very tiny amount, but the combination of that and the second (middle) extension wobbles a bit more. This has never bothered me, I've had worse, actually think this lens is fairly well constructed. Thanks to internal focus none of this moves when focusing.<br>

3. Agree, the short focus path makes manual focus very touchy. Hadn't particularly though the lens weak at close range -- how close are we talking about? And what are you comparing it to?</p>

<p>4. Not sure why you're so convinced that motor needs replacement. I sometimes have a little trouble obtaining AF lock, especially at the long end. When this happens, usually if I let a finger drag on the focus ring to add just a little resistence, it will behave better and lock. I don't particularly care for this behavior but since lens has been like this since I bought it new I never assumed it was a motor problem. I have been considering sending lens to Pentax with my camera to see if it can be calibrated properly. It may be that lens motor is actually adjusted slightly too <strong>strong</strong> such that tiny AF adjustments slightly overshoot their target, and when it tries to reverse that goes too far too, overshooting in the other direction.</p>

<p>I don't think we have any information reliable enough to act on that says there are improved SDM parts available for old lenses.</p>

<p>I don't know if you've tried DA 16-45/4 or not. It doesn't feel as solid but it's manual focus is a bit better and you won't be worrying about a future involving SDM repairs. If you don't need the wide end, the FA24-90/3.5-4.5 is excellent, better than the DA17-70 at 70mm. Construction is typically (for FA zooms) not as nice-feeling as the more recent DA series.</p>

[ariel_s1]

<p>1) It's not a cheap way to get constant aperture. A lens is limited by its longest focal length, as aperture is a ratio of the pupil diameter to the focal length. So, since the lens is designed to be f/4 at 70mm, it could be much more wide open at 17mm. However, this would involve improving the optics quite a bit. Pentax probably tested this lens, and decided that it would be prohibitive to try and give it that kind of aperture performance at the wide end, because the image quality wouldn't be up to their standards, and the reputation of the lens would suffer. By the way, overall this is true for every constant aperture lens. It's just, especially in today's internet-linked world, a lens would completely tank if it had vignetting noticeable even to non-photographic people and horrible border performance. As an example, look at all of the various m4/3 users adapting C-mount lenses; the 17-70mm's performance wider than f/4 would probably be like that. Found some examples on the C-mount flickr group:<br>

http://www.flickr.com/groups/c-mount_/<br>

<p>2) A little wobble is normal.</p>

<p>3) Yeah, either rely on the AF system (as current lenses don't have infinity hard stop), or if you want to do some critical work, use a depth of field calculator, plus live view or your focus scale, to focus to your hyperfocal distance.</p>

<p>4) There is no preventative maintenance to do on a lens. I have a full range of SDM lenses, and they are all going fine. Or am I misunderstanding you, and your AF motor doesn't work? If so, repair centers aren't like grocery stores, you can't just get in, get service, and get out. The shop will need to check out the lens, potentially order parts, and then spend a significant amount of time repairing and calibrating the lens. If you need repair, your best bet is to send it off, as the lens won't be ready before your trip is likely over.</p>

[andrewg_ny]

<blockquote>

<p><em>"It's not a cheap way to get constant aperture"</em></p>

</blockquote>

<p>As I suggested before, I agree with this -- just how many 4.1x zoom constant aperture lens designs don't have to resort to some clever measures so that exposure is consistent across the zoom range? I seem to remember my A35-70/4 (only 2x) employed a similar technique.</p>

<p>Without it, the lens might have been f/3.2-4 with less satisfactory wide-angle performance...if this is what you prefer, consider the Sigma 17-70 which has been available in two versions, f/2.8-4.5 and f/2.8-4. Both versions had larger front elements requiring 72mm instead of 67mm filters. The newer version probably has a more reliable HSM as well.</p>

[mggm59]

Thanks everybody for the exhaustive answers!

 

 

About aperture, this behaviour is new to me because all my constant aperture zooms (Sigma 70-200 f2.8 Ex and

Pentax 28-70 f2.8) don't behave like that, that's why I wondered if this was a malfunction and if it was (not in the

negative sense it sometimes has in English, maybe low cost sounds better) a cheap way of achieving constant

aperture.

 

 

I am also relieved wobbliness is not an issue, and I am positive that the motor has problems as it seems to struggle to

move the focus elements.

 

 

I am very interested in comments on quality of other lenses, since I bought this lens to test its quality at low cost to

replace the 28-80, a great lens which misses something on the wide side. Any more detail on the ranking between the

mentione lenses (16-45, 17-70, 24-90, 28-70 etc) has best quaity.

 

About short range tests the distance was about one meter but it was a quick one done when I did not appreciate the criticality of focus ring excursio, I intend to do more short range tests as I did for infinity.