wide field bloated stars

[Lester]

G'day all,

Justs started to experiment with the Canon 20Da using 50mm f1.4 lens mounted piggy back on EQ mount. Took some shots last night Orion and Crux area tried 400 & 800 ISO f1.4 and f2 with exposures ranging from a few seconds to 4 minutes. I was happy with the nebula that was revealed eg Barnards Loop, Eta Carina But the stars on all except the short=12 second exposures were very bloated.

At first I had focus set on infinity and later I backed it off just a fraction but that made the images out of focus.

Light pollution is not an issue from my area. I don't believe focus is causing the bloated stars otherwise the short exposures would not show pinpoint images.

Is there a filter that can be used to minimise or elliminate bloated stars?

[Astroman]

To me they look way out of focus, try this, during the day set up the camera looking at a distant object (if you can see one with all this muck around) , such as a tower, look through the viewfinder and focus the distant object. Now check where the infinity line ends up, on many lenses the infinity focus is a little off from the manufacturers focus.

If you have already tried this, I have no idea :D I am assuming that your alignment is pretty well spot on (by looking at your equipment)

Anyway good luck.

[Dennis]

Hi Lester

You may be either out-of-focus or suffering severe lens aberration, or both.

Try stopping the lens down, in steps, to f8 taking individual exposures and see if the stars are any tighter. If they progressively get tighter from frame to frame, until they appear perfectly focused at say f5.6 of f8, then it looks like the lens cannot be used wide open.

This is probably okay, as an f1.4 lens will be severely tested using pin point star images, so it may not indicate a “bad” lens.

Cheers

Dennis

[ving]

yeah getting the same thing at f1.4 too... i'll try stopping it down :)

[Lester]

Hi Dennis & Astroman,

Thanks for your reply, just went outside and checked focus at objects about 4km away. I was very surprised to find that I had to back off the focus about 5mm of a turn from infinity. I will add the 2 shots. Obviously they don't make lenses like they use to; the price goes up and the quality comes down.

Thanks again for your reply.

[fringe_dweller]

the lens wasnt dewed up mate? yes infinity isnt what it used to be - in these days of macro lenses - damn flower people :P sounds like you might have it sussed anyway good luck! :)

[Gama]

If you have the 20Da, then why dont you use the focus screen on the rear, and enlarge the image enough to see it focusing.
I have the 20D, and all i do is manually focus it thru the viewfinder and snap away.
When i do higher quality work, i use a home made hartman screen over the fron lens, and use a short exposure and download it to the computer for focusing check.

[avandonk]

All modern good lenses do focus past correct point for infinity to accommodate change in focus due to any filters fitted to the lens. What lens is it and is it autofocus. Even being slightly off true focus causes stars to bloat.


Bert

[Lester]

G'day all,


Thanks again for your input, I have never had so much help in my 30+ years in this hobby since I joined IIS. It use to be one of the disadvantages of being in the country and isolated; but not any more.

The lens is a Canon EF 50mm f1.4 with auto and manual focus. Last night was the first time that I had used it on manual focus. Next time I will use the LCD screen to check focus. :astron:

[avandonk]

Believe it or not if a bright star is inside one of the focus points, the lens autofocus will work! The Canon F1.4 is a very good lens and at about f2.8 should give very good star images even to the edges. The best way of course is to use the LCD screen at high magnification.

Bert

[Merlin66]

Any success yet? I'd be interested in hearing about your latest trials?

[RB]

Lester I'd say that they're out of focus.
Do try the livefocus mode.
Watch as you glide through the focus point, there will be a distinct point where the stars look their tightest.
Don't spend too much time with the live focus preview screen on as it will heat up the sensor, creating noise.
Focus with the lens stopped down to f/2.8 or even f/3.5 and then use this to shoot at as well. If the stars look too dim to fucus on at f/3.5 use 1600ISO to focus then cut it back to 800ISO for shooting the frame.

Remember to switch off the preview screen after focusing and let the sensor cool down a few minutes.
It's recommended that you allow the sensor to cool down between shots too.

And try using in camera noise reduction too, but it will take twice as long to write the file, but well worth it.

Here's one I took using the 24mm lens at f/2.8
It's a 3 min exposure at 800 ISO using the 20Da.
http://www.iceinspace.com.au/forum/attachment.php?attachmentid=7931

Sorry for the long reply.

[Lester]

G'day,

Took some photos on the weekend Orion and Eta Carina with 50mm and 18-200mm lenses, focus was much better. I am still not happy with the Canon 50mm f1.4 lens, when I move it in my hand the part that slides in and out to focus rattles= its loose.

I am planning to fix a Eos T-Ring to the rear of one of my old Minolta f1.4 lenses. Its going to take me 3 weeks to get the T-Ring. I will post the results.