Through a not very clean window

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I am wondering how bad it is (when it too cold or wet to get the shot outside) to shoot through a window that isn't terribly clean. More importantly what would I look at when I examine the result on a computer that would tell me how much worse the image is as a result of the window?

I've only had a lens with an f2.8 widest aperture for a short time. Experimenting with that lens, I lined up a shot with a small obstacle a very short distance away blocking part of a very distant object that I focussed on. The small object was entirely missing from the image. It was so out of focus that it vanished entirely and blocked no part of the distant object. That gave me the idea that with a wide aperture and a fairly distant subject and a dirty window closer than the lens even could focus, each spec of dirt might distribute so well over the image that it is just a tiny reduction in total brightness, rather than a more serious issue. In test photos I don't see evidence of the window (unlike results I would get with my older camera). But I'm not sure I'm evaluating the result correctly.
 
I am wondering how bad it is (when it too cold or wet to get the shot outside) to shoot through a window that isn't terribly clean. More importantly what would I look at when I examine the result on a computer that would tell me how much worse the image is as a result of the window?

I've only had a lens with an f2.8 widest aperture for a short time. Experimenting with that lens, I lined up a shot with a small obstacle a very short distance away blocking part of a very distant object that I focussed on. The small object was entirely missing from the image. It was so out of focus that it vanished entirely and blocked no part of the distant object. That gave me the idea that with a wide aperture and a fairly distant subject and a dirty window closer than the lens even could focus, each spec of dirt might distribute so well over the image that it is just a tiny reduction in total brightness, rather than a more serious issue. In test photos I don't see evidence of the window (unlike results I would get with my older camera). But I'm not sure I'm evaluating the result correctly.
You've figured it out very well - not much different than photographing through a chain-link fence at a ball game or the zoo with the lens nearly against the fence. (Post a picture if you wish to get silly, but of course it could prove nothing!)
To see the difference, take a shot through a dirty window and a shot not through a window and compare them side by side on your monitor. There should be little difference unless the window is very dirty.
 
The small object was entirely missing from the image.

Yes, for sure because light from the scene was coming in all around it
at wider aperture. A colleague was having the dirtiest front element I've e-
ver seen and when I noticed he said: "…doesn't matter!" He was right to
a certain point.

A dirty front element or window may impair focussing (M or A). "Marginally
and not always." he said. Personally, I would prefer never.

In the same conditions, one can be sure that the dirt WAS recorded but in
"homeopathic" proportions — read almost insignificant.

The most important place to avoid dirt is on the sensor.
 
For my first test photos, I used a shrub in direct sunlight, so the parts that had the right angle to reflect light back to the camera were completely blown, while I used the rest of the photo to look for evidence of problems due to the window, which I didn't find. I didn't want to post that, so I tried again this morning without direct sunlight. Several tries and the AF got it badly wrong each time and I don't have the skill for MF. I don't know whether that is the window or I'm doing something else wrong. I'll test more later. Since the request here for a sample was contradicted, I won't post one unless a later test brings up a more interesting question.

My Canon SX530 always had the problem that AF for moderate to long distances worked great in direct sunlight and failed otherwise. I hadn't see that with my Sony a7 iii before this test. Hopefully that is not what is happening.

Edit: I really overlooked something obvious.

When it was a little brighter outside, the AF was perfect. The low depth of field (which I generally want, especially through a window) was a bit too low for the subject. So I started to do some testing in my continuing difficulty learning to do focus stacking with a non focus stacking camera. That effort is ongoing, but as I moved my tripod in the limited space I noticed I was suddenly looking through a reflection of the room lights, wrecking that angle. Metaphorical head slap (I really should know better). The lights were on earlier when AF failed. I wasn't looking through a reflection of the lights then, but when the outside light level was a little lower, the general level of brightness inside was very likely the cause of AF failing. All I had to do was turn the lights off (but not seeing any reflections in the glass caused me to forget that).
 
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If it looks good, it is good
 

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