Panasonic 7-14 - Usage tips for UWA neophyte

Started Apr 19, 2014 | Discussions thread
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Anders W
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Re: Panasonic 7-14 - Usage tips for UWA neophyte
In reply to richarddd, Apr 22, 2014

richarddd wrote:

Anders W wrote:

Florida Nature Photographer wrote:

Anders W wrote:

Subject motion. For that shot, I wanted the stars and the clouds to remain stars and clouds rather than star trails and cloud trails. Of course I tried both though. This one is at ISO 400 and 520 s (pulled back nearly four stops in PP for the scene as a whole and a bit more than that in the sky but with the shadows slider at 100).

Another fantastic tip I hadn't thought of. I could have used that Sunday night. That is one thing I'm not too found of with that shot , the stars aren't spot on.

If you want to have the stars look like stars rather than something else, I'd say the max shutter speed is about 30 s. But that's on the border line and you might want to go down to about half that, as I did in the ISO 1600 shot you asked about.

A formula I've seen to get stars rather than star trails is:

Max Exposure Time = 600 / (focal length * sensor crop factor)

Some use 500 rather than 600.

I haven't tested.

Thanks! I have tested though regrettably not in advance. Had do to some trial and error out there in the desert.

For the shot I posted (7 mm on MFT), the formulas would give us 600 / (7 x 2) = 43 s or 500 / (7 x 2) = 36 s, where the latter value comes pretty close to the 30-second rule of thumb I gave Florida.

Now let's look at the evidence produced by some of my trials, the first at 15 s, the second at 30 s, and the third at one minute. These images are downsampled to about 2.6 MP but still reasonably large.

One problem that you can readily see is that the UWA distortion makes things worse toward the edges than in the center. That distortion makes the stars a bit oblong even without any motion. At 15 s, I would say things still look pretty much OK unless you look to close and there are no problems at all in the center. At 30 s, we are at the border line, and at one minute we have clear signs of trails even in the center.

So by and large, the formula you provided seems to work pretty well. However, UWA stretching towards the edges complicates things a bit and may give us reason to be a bit more conservative.

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