Sorry... I'm bored and depressed and in a posting frenzy...
And after mentioning my Turner & Reich in a previous post, it reminded me of an odd thing I've been meaning to ask about...

My first shot with the T&R (convertible) was interesting. Made two major mistakes. I forgot to open the window I was shooting through, and forgot to refocus after stopping down.
The result was interesting because of how it was focussed. I noticed first on the white cross bars of a window in an old barn. The vertical bar was perfectly focussed and the horizontal bar was very fuzzy. I first blamed this on shooting through the window.

Second test. I opened the window Then I decided to use that barn window as the focus object because of the first print. That's when I noticed this oddity. At perfect focus, both horizontal and vertical are truly focussed and sharp. Move the lens a tiny bit forward or back and either the vertical _or_ horizontal goes out of focus, but not both! Of course more movement brings everything out of focus. But there's that point where you have to be very careful about looking at both horizontal and vertical lines to make sure you really are focussed...
I haven't gotten around to testing my other lenses yet (things have remained pretty hectic here), but I think I would have remembered such a thing, like the donut out-of-focus of a cat lens. Just wouldn't forget, if I noticed in the first place anyway...

I've read through View Camera Techniques with a great section of lens problems and some other books. Couldn't find any explanation that would cover this. Is there a term for this observance? Is it common to all lenses and I didn't notice? Is this the reason people hate T&R's?

Just a curiosity question. If this has a name, at least I could tell someone what it is and sound like I know what I'm talking about

Quote:

On 2003-04-18 21:16, RichS wrote: Sorry... I'm bored and depressed and in a posting frenzy... <snip> Just a curiosity question. If this has a name, at least I could tell someone what it is and sound like I know what I'm talking about

Astigmatism, but I'm not sure why it gets really bad when the lens is slightly defocused. Years ago I had a dud Celestron C-90 that showed the effect, and badly, when in the best focus possible.

Cheers,

Dan

Quote:

On 2003-04-19 05:51, Dan Fromm wrote: Quote: On 2003-04-18 21:16, RichS wrote: Sorry... I'm bored and depressed and in a posting frenzy... <snip> Just a curiosity question. If this has a name, at least I could tell someone what it is and sound like I know what I'm talking about Astigmatism, but I'm not sure why it gets really bad when the lens is slightly defocused. Years ago I had a dud Celestron C-90 that showed the effect, and badly, when in the best focus possible. Cheers, Dan

Thanks Dan. I actualy did think of astigmatism, but thought that would only affect one plane. This switches 90 degrees when the lens is move through center focus. Maybe that's normal for astigmatism? And what's even odder (now, is that a word?) is that if the lens is in the center focus position, objects that would be out of focus, either more near or far, do not show the odd focus problem. What's the difference if an object is closer than where it's focused or if the lens is moved just off focus from that object?
Way past my bedtime and I don't think that made as much sense as it should?