testing RF alignment in the city

[ryan_pasia]

<p>I'd like to test the RF alignment of the Bessa cameras in my small collection. Problem is, as much as I'd like to conduct the tests by focusing on something really far away (like a full moon on a clear night), I'm in NYC where my field of view is compromised by trees, buildings or haze.</p>

<p>Would it be possible to conduct a reasonable RF alignment test by focusing on buildings eight or nine city blocks away?</p>

<p>Assuming the answer is "no" to the above question, can you please suggest accessible alternatives? I can think of two options right now (the horizon at Coney Island Beach or buildings on the New Jersey side from Battery Park downtown).</p>

[jean_yves_mead]

Eight blocks should be more than far enough for your purposes.

[Rob F.]

<p>Yes I think that is adequate. The usual advice is to use a target at least 2 miles away, but I haven't found any real need for that much distance. One of my tricks is to use a TV tower in the distance, since they are very easy to focus on precisely. But when the ideal focus target is not available, I think 2 blocks will do.</p>

[ryan_pasia]

<p>Thank you, all. I read somewhere it had to be several miles away--and your comments cleared it up for me.</p>

[kelly_flanigan1]

<p>Hi Ryan:<br>

How far away depends on the RF's slop; your eyes; the baseline length of the RF; whether the RF's optics are magnified.</p>

<p>Lets us assume one can with fair eyes resolve 1 part in 1000. Ie see 1mm at 1meter; 1 meter at one KM; see 5.28 feet at 1 mile. This is fair; better eyes can resolve more.</p>

<p>Since we are aligning something and using a reference; let us make it 10 times tighter to cover our rears. Thus now we have a 1 part in 10,000 criteria. too tight; but it will give us some conservative numbers.</p>

<p>With a dinky 1 inch baseline you then would need an object 10,000 inches away or 833 feet or only about 2 city blocks for 12 block per mile.</p>

<p>With a 2 inch baseline one would need 4 city blocks.</p>

<p>For a WW2 rangefinder from WILD that has a 1 meter baseline; one would have 10km.</p>

<p>There are a mess of variables; using a longer distance is conservative.</p>

<p>A mechanism can have backlash; one can have poorer eyes. The whole alignemnt process has some non repeatable "slop" thus there are practical limits to optical RF's.</p>

[ryan_pasia]

<p>Now that makes sense, Kelly. I was wondering how the recommended distances were derived.</p>

<p>FWIW, I focused on a building about 2900 ft away for my Bessas, so if I understood your math right, the distance was more than enough.</p>

<p>Sadly, that just confirmed the RF was misaligned on one of them.</p>

[john_shriver]

<p>The moon is handy for much of the month if there's no clouds.</p>

[jody_stowitts]

<p>I'd use full aperture because at a much smaller apertures your DOF will be so large at those distances you'll never know if focus is spot on (unless you're using a tele). Also be apprised of "focus shift", where spot-on focus @ f2 may not coincide with spot-on focus @ f2.8, et.al. apertures.</p>