RE: [APML] Re: Need Help Focusing Telephoto Lens

[Jon Kolb <jkolb@mindport.com>]

Kevin,

> Aberration: I assume that this would show up more toward the 
> edges than the
> center. When I'm out of focus with this lens, I'm out of 
> focus all the way
> from center to edge. And I have successfully taken some sharp 
> photos with
> this lens. Just not consistently. So I don't think it's aberration.

I agree.  Donuts are not lens aberrations, in my experience anyway.
 
> Flexure: Am I correct that flexure would show up as elongated 
> stars, rather
> than donuts? My stars are showing up as tiny circles with 
> dark centers. I
> don't think this is a symptom of flexure, am I wrong about that?

I agree again.  Donuts are not caused by flexure, unless the flexure goes
round and round, but it doesn't really do that.  When I get tiny circles
with dark centers, it's due to bad focus, but I have the "advantage" of
knowing this is true because I use a reflector and can see, at high
magnification with a loupe, doubled refraction spikes on the brightest stars
- always a sure indicator of bad focus.  Unfortunately, I've seen a lot of
this over the years.
 
> I don't have a magnifier for the OM-1 viewfinder, but that 
> certainly seems
> like a good suggestion for getting good focus. I'll have to 
> look into that.
> I hadn't considered making a Hartmann mask for the telephoto 
> lens either,
> but for about $0.25 in cardboard or construction paper, I 
> could certainly
> give that a try.

Certainly the inexpensive solution is the Hartmann mask, but I question
whether it will work on stars.  A more expensive, but absolutely foolproof
method (important to fools like me), is to get the Mitsuboshi KE focuser
from Hutech.  It replaces the camera, so you don't have to worry about
whether the film is loaded or not.  Once you use this, you won't want to try
any other solution.  I once owned an OM2-s and Olympus Varimagnifinder (the
device you are referring to above).  It works after a fashion, but the view
is small and dim, and I still got too high a percentage of out-of-focus
shots.  One night with the Mitsuboshi, and the cost will evaporate into
success.  I eventually switched to Nikon, and the DW-4 finder on an F3 works
a hundred times better than the Varimagnifinder, but it still can't beat a
KE (although it's damn close).
 
> My latest attempts were 60 min exposures of M31. I haven't 
> yet finished
> scanning the negatives, just enough to tell they were out of focus. I
> usually don't lock up the mirror when taking piggybacked 
> photos with this
> lens. The STV reported good tracking, with average guide 
> errors around 2
> arc-seconds. I don't think that small an error would show up 
> with a 300mm
> lens. I had the proper parameters entered in the STV for the 
> e-finder (1"
> diameter objective, 4" focal length).

You are right - STV errors of 2 arc-seconds will never be noticeable in an
image taken on 35mm film with a 300mm lens - at least at any resolution you
would ever post on the web or even print at reasonable sizes.  Always lock
up the mirror if you can - what's to lose if you do?
 
> I think temperature shifts may be a big contributor to my 
> problems. I was
> focusing on distant terrestrial objects at about 50F, and 
> photographing at
> about 25F. That could certainly throw off my focus, I suppose.

Despite the other excellent opinions you've received, I suspect that
focusing on a terrestrial object is the real problem here.  Unless the
object is really, really far away, I think there's room for error.  I've had
this problem myself.  In my case, I mounted a new Intenscreen in a Pentax
67, and calibrated it by focusing on a mountain top some 6 miles distant
with a 200mm lens - it wasn't enough.  Subsequent KE focus proved this.
 
> So, with a KE focuser for the OM-1, I would need two OM-1 
> camera bodies, one
> for focusing and one holding the film? Do I understand that 
> correctly? (The
> costs of this hobby never cease to amaze me.)

That's one way, but the Mitsuboshi is another, and might be competitive in
cost (depending on what you have to pay for a second body).  It will
certainly be more accurate out of the box, and more consistent in the long
run.

Sincerely,
Jon Kolb
Adventures in Astrophotography
http://home.datawest.net/jkolb/
jkolb@mindport.com


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