100-400 mk I tightness ring

[Tsleel2811]

Does anyone know if the **bleep** tightness ring can be moved without turning the focus too? Whoever designed this lens should be taken into a dark room and beaten

[Waddizzle · Accepted solution]

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@Tsleel2811 wrote:
I appreciate it alot but none of those are my problem. My problem is as follows: when the lens it at 100mm it's when I'm carrying it in my bag. I'm going to focus on the moon a few days prior to the eclipse. So when I focus on the moon the lens is at 400. I have to tighten and loosen the tension ring accordingly. And that in turn takes me in and out of focus. I'm not saying it's a defect I'm just trying to find a way to able to keep the focus. I think I'll just mark it with a marker. Because none of the tips seem to be good ones for my need. Especially the rubber band. But I appreciate the input. And yes I'm using a solar filter and manual everything

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Attempting to pre-focus your lens days in advance is an exercise in futility, IMHO.  Why?  Because of small differences in focusing as the temperature of the lens changes.  

You're focusing on he Moon at night, but plan on shooting the eclipse in the middle of a summer day.  A fair guess says that is at least a 10 degree difference, and it could be significantly more.

Practicing with the Moon is worthwhile.  It will get you in the ballpark of where your focus could be.  But, do not look at it as a fixed setting.  Your focus at midnight can vary slightly from the focus at high noon, just enough to make your photos a little soft.

[RobertTheFat]

I'm going to be a bit of a contrarian here. The main reason for not using auto-focus for objects at infinity is that there's apt to be very little contrast. Common examples are seascapes and the night sky. But the partial phase of a solar eclipse is about as high-contrast an object as you'll ever see. So I'd think that one could use AF and not have to worry about presetting it.

[Tsleel2811]

I'm not going to use auto focus. This is the whole reason for the discussion that the tightness ring moves the lens out of focus when manually setting focus. Just FYI auto focus doesn't work with a solar filter. So it needs to be manual

[Waddizzle]

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@Tsleel2811 wrote:
I'm not going to use auto focus. This is the whole reason for the discussion that the tightness ring moves the lens out of focus when manually setting focus. Just FYI auto focus doesn't work with a solar filter. So it needs to be manual

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Your focus is likely to drift over the course of a few hours because the temperature of the lens will most likely change.  You will need to find a way to check it as the hours pass.  If you expect to "set it, and forget it" you are mistaken.  I made that mistake with the lunar eclipse last year.

[kvbarkley]

Worked fine on my camera several years ago with the Sigma 500mm.

I used manual focus yesterday, and it was fine. The problem I had was the rapidly changing cloud conditions, when behind a cloud I had to take the filter off, and then a few moments later had to put it back on again, argghh!

[jrhoffman75]

Xume magnetic filter system.

[kvbarkley]

I don't think that will work with my thousand oaks optical solar filter, but since I only use it every few years, I can live with the hassle.

[jrhoffman75]

My TO filter was threaded 77mm.

[kvbarkley]

Mine fits over the lens/ lens hood and attaches with threaded screws or masking tape!

[TCampbell]

Atmospheric conditions (what astronomers refer to as "seeing conditions") will also affect your ability to achieve good focus on celestial objects.  

The other day, I used my camera with the same Canon 100-400mm Mk II lens to shoot some pre-eclipse test shots of the Sun and managed to get tack-sharp sunspots without too much fuss.  On the morning of the eclipse, seeing conditions had degraded.  The sky was clear, but the upper atmosphere was less stable.  This tends to blur the image somewhat.  The best I could do (after fussing for an extremely long period of time) was probably good enough as long as nobody did any pixel peeping... but not anywhere near what I had done a few days before (with the same gear).  There's nothing you can do about "seeing" conditions other than choose a different time to shoot or move to a better location (if you're within roughly 200 miles of either (1) a warm-front, (2) a cold-front, or (3) the jet stream, then you'll likely have below average "seeing" conditions.)

Temperature swings are a real thing (and part of why Canon's big lenses are "white").  In astronomy, you can buy a device for the telescope (typically purchased only by those who do hard-core astrophotography) called a temperature-compensating focuser.  You have to calibrate it at several different temperatures and it learns your optical tube's expansion & contraction properties at various temperatures.  From there, as the temperature changes over the hours, it is silently tweaking focus to keep things tack-sharp.  Most imagers don't use such a device ... but they do have to re-check focus every hour or so and update it.  

You can also get telescopes with carbon fiber optical tubes.  Unlike most uses of carbon fiber where it's chosen as a material to save weight... astronomers choose it because it doesn't expand & contract as the temperature changes.  Basically focus will be good all night long regardless of how much the temperature changes.