Sticking Aperture?

[JimFox]

In a timelapse sequence from out at Death Valley a few weeks ago, I noticed some images that were darker then the others. At first glance it was like the exposure was flickering, going a bit darker, but then going back to the set exposure. But at a closer look, it's only the edges that are getting darker, like vignetting, where the middle of the photo, the exposure stays correct when compared to other frames.

So my first thought was a sticking shutter, that would partially stick some of the time, but these images don't really look like it.

The lens is a f2.8 lens, because it was so bright I set it to f4. But I wondered if the aperture isn't opening all the way sometimes, and that would cause the vignetting? Since the lens vignettes at f2.8 more then it does at f4.

The camera was set to complete Manual. Manual Shutter, ISO and f-stop. Though the f-stop is electronically controlled. But I manually had it set to f4.

Here are 3 samples.

 

[Kurt Harrigan]

You might want to check your menus to make sure the ISO, Noise removal, etc are all turned off. That's the first place I'd look.

 

[JimFox]

You might want to check your menus to make sure the ISO, Noise removal, etc are all turned off. That's the first place I'd look.

Thanks Kurt for the ideas.

But I haven't changed any settings, it's the same settings I have been using for years. There is no AutoIso on, it would display on the top menu. Noise Reduction wouldn't do this, not have 100 frames perfectly exposed, and then all of a sudden it starts to vignette. and then the vignetting goes away. That to me smells of a mechanical issue, something sticking and then freeing back up.

The camera was on 100% Manual.

 

[DES]

When I measured the (same) spot in the 3 samples using the Mac digital colormeter, the red channel measured 112, 98, & 94. I'm not seeing much difference on my monitor, I'd say no more than 1/3 stop. Could this simply be due to variations in the metering, especially if it's spot metering? This probably isn't much help -- nice photos.

 

[JimFox]

When I measured the (same) spot in the 3 samples using the Mac digital colormeter, the red channel measured 112, 98, & 94. I'm not seeing much difference on my monitor, I'd say no more than 1/3 stop. Could this simply be due to variations in the metering, especially if it's spot metering? This probably isn't much help -- nice photos.

Thanks, I appreciate your ideas.

The issue with this is it's in a timelapse, so that even if it's not much of a change, even just one frame being off, it will cause a flicker.

There is no metering, the camera was totally on Manual. So there is no metering.

I will post some more extreme examples.

 

[JimFox]

Here is another sequence.

 

[DES]

The variation is more apparent in this sequence. Perhaps someone else has a thought. I would suggest cloud motion, but I notice there is variation at the distant skyline which would seem to eliminate effects of cloud motion. BTW, the clouds are a key feature in the photos that are very attractive.

 

[JimFox]

The variation is more apparent in this sequence. Perhaps someone else has a thought. I would suggest cloud motion, but I notice there is variation at the distant skyline which would seem to eliminate effects of cloud motion. BTW, the clouds are a key feature in the photos that are very attractive.

Thanks. Even though everything is in Manual, and I chose f4 for the aperture. I believe that the aperture moves from open to f4 as each image is initiated. So if the motor that runs the aperture, or the mechanism itself is sticky, that could account for it. I think.... Next time I go out, I will try a different lens, and see if at any point in the night it does it again.

I have heard so many people who won't even photography at night if there are clouds because they want an uninterrupted view of the Milky Way and stars. But I found out years ago, that clouds at night, (as long as the whole sky isn't covered) can add a real sense of mystery to the photos at times, and can add visual interest as well.

PS. I hope to have the timelapse put out in the next day or two. I am doing some final editing on it.