500cm Body focus error?

<p>Hi Edward,</p>

<p>Yes, the distance from the lens bayonet to the film would be exactly the same as the distance from the bayonet to the bottom surface of the focusing screen. That's the goal.</p>

<p>Using that logic, it should suffice to adjust the focusing screen support points to a measured value, and that's it. The secondary operation involving the light source with the colored lines should not be needed at all.</p>

<p>Remember that you're meking two separate measurements on the body using different tools, each of which has its own tolerance. The body length is 71.40 mm, while the distance from the bayonet mount to the focus screen is 74.90 mm.</p>

<p>The point of my already too-lengthy posts is that we're dealing with tolerances and human perception. Whenever you get a person involved in a measurement procedure you introduce errors.</p>

<p>I don't know how to improve the tool. It appears to project the lines at different angles, such that when the screen is at the correct position the lines are parallel and the green one is centered between the red ones. This check is done after the mirror is accurately set at 45° to the lens axis.</p>

<p>It's the adjustment that's difficult. The screw pitch is 0.35 mm, roughly 0.001mm per degree of rotation. So turning it 30 degrees takes up the entire .03 mm of tolerance. And you have to move all four screws the same amount.</p>

<p>I'm willing to bet that few technicians will mess with the factory adjustments. The requisite equipment is VERY expensive (probably well over $20k now). Unless the body has sustained significant damage it's unlikely that the screen position would be off in the first place. On the other hand, an error in the position of the mirror itself is not uncommon.</p>

<p>Just trying to focus the discussion on potential sources of error. ;-)</p>

<p>- Leigh</p>

<p> </p>

<p><em>"Just trying to focus the discussion on potential sources of error. ;-)"</em></p>

<p>Hmm... You sure do an excellent job giving the impression that you are distracting from potential sources of error.</p>

<p>There's nothing wrong with the way these cameras are adjusted. It works perfectly fine.<br>

The OP's question how much error would be "normal" has been answered.</p>

<p>Maybe just send your 500cm to David Odess to have it fixed so you don't have to worry about it anymore.</p>

<p>As another poster mentioned, this could also be a problem with the magazine, or with the body parts to which the magazine attaches.</p>

<p>Best to send the camera + magazine for evaluation.</p>

<p>- Leigh</p>

 

<p>Got the Acute Matte screen in the camera now, what a difference it makes! I got the one with the split circle in the center which is great. I actually have two backs with this camera, both have new light seals installed by me (from OEM Hasselblad parts). I would send it out to David Odess but I am going on a road trip next week and he already told me a week ago he couldn't get it back to me that quickly. So at this point it could be the body, it could be the back. I'm just going to shoot with it and hope for the best. I'll be primarily using the 60mm Distagon lens and shooting at f8 or f11 for max depth of field. The tiny focusing error with the 80mm lens focused at 3 ft with the Proxar filter shouldn't really matter, at least I hope so. When I get back I'll probably send the body and one back to him for repair. A third back that I have has all 9 screws on too tight for me to remove the back and replace the seals, so it will go out to him for repair.<br>

Thanks again for all your replies. I have 20 rolls of color film and 3 rolls of black and white I'll take with me, though I may not shoot all of it.</p>

<p>That's great, Dave.</p>

<p>I expect the considerable depth of field with the 60 will compensate for any actual focusing errors.</p>

<p>Have a great trip. Hope you're happy with the pix.</p>

<p>- Leigh</p>

<p>

<p >I have been using Hasselblad since 1966 professionally………..you guys have me totally confused, I’m at a complete loss…..much too technical for me……I hope Victor is following this thread……I bet he has the answer………………sorry !!!</p>

</p>

<p>I think there is some confusion appearing here regarding what "precision" means. Precision is not the same thing as "accuracy".</p>

<p>To take a good illustration of the difference, and quoting from a google search because I don't hold this in memory(!): regarding attempts to calculate the age of the Earth, "Dr. John Lightfoot, (1602 - 1675), an Anglican clergyman, rabbinical scholar, and Vice-Chancellor of the University of Cambridge had already arrived at an estimate of 4004-OCT-23 BCE, at 9 AM".</p>

<p>Lightfoot's value was remarkably <em>precise</em>: he gave it to the nearest hour. But it wasn't remotely <em>accurate</em>, because science now estimates the age as 4.5 billion years, not 4 thousand years!</p>

<p>I find this example to be very useful for correcting students' misconceptions when I'm teaching physics labs. Each lab experiment requires them to quote errors with their values, and they sometimes mistake the precision of the measuring equipment for the accuracy (error analysis) of their results. Precision can be a guide to random errors, but other approaches are required to take account of systematic errors. (Lightfoot, of course, took account of neither!).</p>

<p>The two terms are also nicely distinguished here (1 page PDF file): http://tinyurl.com/3acortr<br>

<strong><br /></strong></p>

 

<p>Ray,</p>

<p>Just so that your informative post also becomes relevant to this thread, where in the thread do you find that confusion?</p>

<p>For some people, Hasselblad screens give them problems. It can be easy to focus your eye on the top surface of the focus screen instead of the botom surface of the screen. Or you can "look-thru" the screen and focus on any aerial image in-front or behind the screen. This is a hard habit to break but once you learn to focus your eyes to the right place the camera can do the same. It drove one of my studio mates nuts. Once he got it, the problem was forever solved.</p>

<p>I think the <em>focus</em> of this thread was lost days ago. Nobody ever answered my simple question of is this focus error normal or not? All I wanted was a yes or no answer...</p>

<p>Dave, Dave, Dave...</p>

<p>You have been given both a "no" answer and precise and accurate figures of how big the error could be.</p>

<p>Dave,</p>

<p>You did not receive a direct answer because no such answer exists.</p>

<p>Have you stopped beating your wife?</p>

<p>Yours was not a "simple" question, so there was no "simple" answer.</p>

<p>We don't always get what we ask for in this world.</p>

<p>- Leigh</p>

<p>Before despair sets in completely, i'll repeat: the precise and exact answer has been provided.<br>

It <em>was</em> a simple question, and (despite the sidetracks that were explored) a simple answer <em>was</em> given.</p>

<blockquote>

<p>Just so that your informative post also becomes relevant to this thread, where in the thread do you find that confusion?</p>

</blockquote>

<p>Where? Almost all the way through it. The original poster has a problem with focus (in)accuracy. In the responses before mine, I noticed that there were very few mentions of "accuracy" or "accurate" and a great many mentions of "precision" or "precise" (I counted them just now: 3 of the former and 15 of the latter, not counting quotation repeats!). It was emerging as a sufficiently general issue that I was motivated to chip in, hoping to gently nudge the thread back on topic to the real problem, which was one of accuracy.</p>

<p>Now that I've been "outed" as a college prof by Kelly's recent ramblings (hah!), I am more likely to use illustrations from my field of expertise. So here's another: an effective teaching method is to set a task for a class; then when they've finished, one outlines "here is how you should, or at least could, have done it". You don't identify those who got aspects of it wrong - certainly, not publically. They self-identify themselves in their own minds, by reflection on their work. This reflective learning technique only really works with adults; and we're all adults here, are we not? So: some people did use the term "precision" or "precise" correctly; some did not. I'm not going to name names; that would be silly and unfair. People can reflect on what they've written themselves, if they so please. Maybe something's been learned by someone, which is all I can hope for. And BTW, I certainly learned something too: about the calibration procedure and the tolerances involved. So, thanks guys.</p>

 

<p>What a tedious thread.</p>