Scott Peden wrote:
> A bit over a week ago I lucked into a Sigma 600/8.
>
> I noted immediately that at 200 or so feet, infinity was to far of a focus, I
> had to back off a bit.
>
> Last weekend It was just barely focusing all the way over to infinity.
>
Two things here.
1. Many long lenses have a sort of an "infinity zone" because they will
focus slightly differently at different temperatures. Look at your lens.
If you turn it all the way to the stop, the red line is lined up with
the infinity symbol. To the right of that symbol, you will see a
horizontal line ending in a vertical line. The red line and the vertical
white line define the area in which you may find infinity focus.
You may have the idea that simply running the lens against the end stop
is infinity, but that isn't true of many long and/or zoom lenses and
that sideways, inverted "L" is the standard marking. You have to focus
with the viewfinder even for infinity.
2. 200 feet isn't infinity for a 600 mm lens I don't know the exact
numbers, but for a lens that long, 200 feet should focus closer than
infinity. You need more like half a mile to be certain of infinity.
> 2 days ago shooting Red-tailed hawks, I noted that it didn't seem to want to
> go far enough to focus.
>
> Tonight when trying to shoot the moon, all the way over just didn't seem in
> focus
In both cases, do you know for sure if it would focus, or just not sure?
On nights with atmospheric disturbance, exact focus may be hard to
determine. Did the shots end up out of focus?
Looking from the rear of the lens, 90 degrees CCW from the red focus
line, there is a break in the back end of the lens. Is there a plastic
tab in that hole? If so, pull it out and check if there is a small
filter mounted on it. The filter is part of the optical formula and a
neutral one should be in there when not using any other. It's possible
that without it and in cold weather, infinity focus can't quite be
achieved, although whether it would be in hot or cold weather I don't
know for sure.
I can assure you that a properly operating 600/8 with filter will focus
nicely on the moon; I've done it. Getting a sharp image is another
matter around here, as there is a lot of atmosphere wiggling around
between me and the moon.
> and with the Vivitar 2X Macro telefocusing teleconverter, there was no way
> to turn the dial far enough to get the moon into focus, just a bit shy.
>
- If there is a problem getting quite to infinity anyway, the 2x may
just be magnifying it.
- Or that teleconverter, which is primarily designed for much shorter
focal lengths, just doesn't get along with a long mirror lens.
> Am I loosing it, or is infinity closer than the moon and somewhere under 300
> feet?
>
I guess you already know the answer to that.
Moose
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