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TOKINA SZ-X 270II 28-70mm f3.9-4.8


guy_gervais

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<p>Greetings,</p>

<p>Does anyone have any info or any hands-on experience with this lens?</p>

<p>I'm looking for a 24/28-70/80 manual PKA zoom to use as a<br>

general purpose lens with 1:2/1:4 magnification ratio.<br>

I don't really care for or need auto focus.</p>

<p>Any suggestions or comments?</p>

<p>Thank you,</p>

<p>Guy</p>

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<p>The Tokina 28-70mm f/2.8-4.3 is a very nice lens, and reasonably fast given the modern zooms available nowadays. Furthermore, and unlike today's zooms, you can use this lens wide open at any aperture. You might want to check out <a href="../pentax-camera-forum/00OKZK">Hin's post</a> about it.</p>

<p>I owned it for some time (in fact, I still do) and only stopped using it when I got the Tamron 28-75mm f/2.8. You can probably find it for a silly cheap price on Ebay these days. While the P/KA mount is welcome, the only issue is that with its variable aperture the camera will only expose correctly at 28mm, unless you shoot wide open all the time.</p>

<p>It's one of those lenses that although I never use it anymore...I can not bring myself to sell :-)</p>

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<blockquote>

<p>Mis,<br>

Thanks for your reply but what do you mean by"the only issue is that with its variable aperture the camera will only expose correctly at 28mm,unless you shoot wide open all the time." and the aperture is supposed to be F3.9-4.8 which, I think, is kind of unusual...</p>

<p>Regards,<br>

G</p>

</blockquote>

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<p>I've explained this a number of times, but I can't find any posts to link to, so I'll have to go over it again. Here's the short version:</p>

<p>Until the F series lenses, zooms had no way of communicating their focal length to the camera body, or their aperture, through non-mechanical means. Because this zoom lens is variable aperture it means that wide open it's f/2.8 at 28mm, but f/4.3 at 70mm. The problem stems in the P/KA mount which tells your DSLR camera this is an f/2.8 lens. When you zoom to 70mm the camera still thinks it's at f/2.8 because it has no way of knowing the aperture has been reduced to f/4.3 because of the crippled K mount.</p>

<p>How this affects you: Say you set your camera at f/5.6 shooting in Av, and want to shoot at 28mm; the camera will meter the light coming in and say "if at f/2.8 I need 1/500s, at f/5.6 I'll need 2 stops more light, or 1/125." You will then get a correct exposure. Now zoom to 70mm and the camera will think exactly the same thing, except this time your image will be overexposed by 1.25 stops. Why? Because the camera thinks it's metering off a lens at f/2.8, when it's actually at f/4.3 (which is about 1.25 stops slower).</p>

<p>There is no problem when shooting wide open because the camera just meters off the light coming through the lens and doesn't adjust anything.</p>

<p>I hope this was clear!</p>

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<p>Guy, what body are you planning to use this on? (film, digital, autofocus?).</p>

<p>Mis, I don't think this is true. Exposure will be fine...it's just that the aperture indicated on the body (viewfinder, LCD, etc.) will only be correct at the short end of the zoom.</p>

<p>I'm assuming that since we're talking about PK/A, the lens will be set to 'A'; the following discussion also assumes we're talking about an SLR that can tell you what aperture is selected in the viewfinder or body--so basically any digital, autofocus, or A-series (Super Program/Super A/Program Plus/Program A) models.</p>

<p>The KA mount is limited into what apertures can be reported to the body, so it will most likely tell the body that your lens is either f/3.5 or more likely f/4 (this would be up to Tokina). The body will think that f/4 is available at all focal lengths, but this serves only as a base value used only for display in the VF/EXIF/LCD.</p>

<p>Metering will be correct as it is done through the lens, wide open. Stopping down essentially applies an offset from wide-open. If you are shooting aperture priority and tell it to shoot f/8, it will stop down by two stops. The viewfinder and EXIF will say f/8 but by stopping down two stops, at the short end of that zoom light will be effectively ~f/7.8 and at the long end will be effectively ~f/9.5.</p>

<p>This is also basically true when you stop down using aperture rings on variable aperture zooms--the ring-indicated aperture is basically the true f/stop only at the short end of the zoom.</p>

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<p>Should work fine, generally-speaking. Possibly the biggest drawback on K10D is that because it doesn't report focus length you would need to enter that yourself for SR, maybe even changing it when zooming for best results.</p>

<p>One other place where you can have problems with manual-focus variable-aperture zooms is when not using TTL metering; either with an external light meter, or when using Auto flash (those auto flashes have a built-in lightmeter). You would probably want to compensate 1/3 to 2/3 stop depending on your zoom setting.</p>

<p>As for the particular Tokina, I don't know how good it is, I think it offers 1:4 magnification. A manual focus lens like this I like is the <a href="http://kmp.bdimitrov.de/lenses/zooms/medium/A35-70f4.html">Pentax-A 35-70/4</a> , which offers better than 1:3 mag (0.37x) but obviously starts at 35mm rather than 28.</p>

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<blockquote>

<p>Mis, I don't think this is true. Exposure will be fine...</p>

</blockquote>

<p>Oh Andrew, it's very true, and exposure is only fine (i.e., "correct") at the short end. Remember I have this lens (the Tokina 28-70mm f/2.8-4.3 ) and have shot extensively with it. Because Guy's lens has a smaller aperture difference between the short and long end (~0.6 stops), he may hardly notice the difference, but I'm trying to be thorough and precise here :-)</p>

<p>By the way, the Tokina I mentioned has a 1:3 macro at the 70mm end.</p>

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<p>Mis, unfortunately I have no equipment I can use to verify this right now as all my A-zooms are constant aperture. I can't think of a technical reason for what you describe, perhaps you can find something on <a href="http://kmp.bdimitrov.de/technology/K-mount/Ka.html">this page</a> that could explain what you're seeing. One excerpt:</p>

<blockquote>

<p>"Variable-aperture zooms have fixed contact patterns, and can therefore only indicate a fixed aperture range. In this case, the incorrect f-stop may be displayed in the viewfinder, and I guess that because of this, Pentax left the feature completely out. N<em>ote that due to the TTL exposure metering, the film will be exposed properly regardless of the fact that the f-stop's numerical value may be calculated incorrectly</em> ."</p>

</blockquote>

<p>I'm wondering if there's another explanation, particular to your lens or lens model. I believe there are a few forum members here with A35-70/3.5-4.5 but I've never heard anyone complain of this problem.</p>

<p>I tried to figure a way to try this but haven't come up yet with a good way to block the KAF info contact (making an 'F' an 'A'), and between my two crappy 28-210/3.5-5.6 variable aperture A-zooms (Vivitar, Promaster) one has a ricoh pin (so can't mount on AF body) and the other is missing the retractable A pin (which makes testing this problematic).</p>

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<blockquote>

<p>I can't think of a technical reason for what you describe,</p>

</blockquote>

<p>Andrew, the technical reason is simple: The lens does not transmit varying aperture information through the pins while it's being zoomed. The pins are set up to say "f/2.8" on my lens, and that's it. Maybe Pentax-A lenses were smarter? Hin had exactly the same problem as I did with this same lens, and others using variable aperture P/KA lenses have reported the same issue.</p>

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<p>I don't think the Pentax-branded KA lenses were any smarter. I think the design is supposed to account for this (otherwise how would these lenses have worked on the bodies they were designed for (A-series, P-series)? I'll have to give some more thought on how I can better express my understanding of how this is supposed to work, possibly figure out a way to test my theory. Since you have the lens and say it has exposure problems, I'm pretty sure my hypothesizing isn't going to convince you otherwise. :-)</p>
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