Could someone explain what's wrong in my picture?

[Gonzo]

Make - Panasonic

Model - DMC-GF1

ExposureTime - 2 seconds

ExposureProgram - Aperture priority

ISOSpeedRatings - 800

MaxApertureValue - F 1.00

MeteringMode - Multi-segment

LightSource - Auto

ColorSpace - sRGB

SensingMethod - One-chip color area sensor

ExposureMode - Auto

White Balance - Manual

FocalLengthIn35mmFilm - 0 mm

Image Quality - Raw

I took these pictures a couple of nights ago while on holidays in Germany.

The issue I'm having in circled in red (click on the pictures for a better view).

I was using my Lumix GF1 wirth a Minolta 50mm f1.7 lense and a af-m4/3 adapter, the only thing I can remember when I took the pictures is that I realised that my aperture ring was not fully opened.

Between the two pictures, the first one (left) the star appear dimm but very nice, the aperture ring on the af-m4/3 adapter was not fully opened.

And on the second picture (right), the star appear very bright but has this ring around it (I had opened up the aperture ring to the max at that time)

Nothing was changed as in focus nor camera settings between the two pictures.

Some help would be greatly appreciated. thanks

[lukebl]

I'd say that was coma. Inevitable with fast lenses. I think you can reduce it by stopping the lens down (i.e, a higher f-number), but that will of course require longer exposures

[Gonzo]

oh that's coma, heard about this but I didn't know what it was.

thanks for the explanation, I'll do some test with the aperture ring whenever we have a cloud free night....

I'd say that was coma. Inevitable with fast lenses. I think you can reduce it by stopping the lens down (i.e, a higher f-number), but that will of course require longer exposures

[rfdesigner]

It's a known fact that camera lenses should be stopped down a tiny bit to get best performance, sharpness wise.

Assume 1.5 f-stops and you won't be far off.

see Canon 50 mm review

even top notch lenses benefit.

Derek

[Gonzo]

Thanks for the link.

[ollypenrice]

Rather than use the internal diaphragm which will create diffration spikes around bright stars, you can stop down from the front with a simple aperture mask. You can also use stepper rings to do this. Just calculate the desired aperture based on the focal length of the lens and the F ratio you want to try.

(The short wavelength blue light is the hardest for any lens to control hence the coma is much worse in that.)

Olly