focusing question

[sgazer]

just a question on focusing which has been at the back of my mind for a while since getting the 1000d with liveview;

I do my focusing with a bahtinov mask, but when I remove it and use liveview, there is usually some colouring around the star which I'm focused on, ie. the circumference is not white, it's usually redish or orange.

I can adjust the focus slightly, so that this colouration goes and the whole star is white. However when I put the mask back on, it then appears slightly out of focus, ie. the centre line in the pattern is not quite central.

So, do I go with the bahtinov mask and accept the colouration or ignore the mask and focus until there is no colouration?

[Psychobilly]

Are you using the mask in live-view as well?

[themos]

I'd say do NOT ignore the chromatic aberration as I still haven't found an efficient method to deal with it in image processing. Anybody?

[Psychobilly]

The reason I asked if the mask was being used with liveview as well is because there might be a slight discrepancy between the "focus" on the viewfinder and the sensor following the mod..

Peter...

[sgazer]

Are you using the mask in live-view as well?

yep, I used liveview to focus with the mask, then remove it and check liveview again.

I haven't found this chromatic abberation to be too much of an issue once the subs are stacked, but I was mainly concerned with whether it's correctly in focus with or without it.

Thanks

[Psychobilly]

If you have the camera tethered I prefer to use eos utils taking exposures of a few seconds at hi iso to check focus... you get a much better image to look at...I use a mask and Bahtinov Grabber app to check focus...

I only use live when the camera isnt being run from a computer... in which case I go for minimum star size with live view zoomed in x10 I tend to have the star about 1/2 way between the center and one of the corners of the frame...

Peter...

[Catanonia]

You will get some colouring due to the affect with the mask. Go with what the mask says and test it and see how it looks.

I normally with my 1000D take an image with the mask and you see colour difraction layers on the image with the mask.

[sgazer]

You will get some colouring due to the affect with the mask. Go with what the mask says and test it and see how it looks.

I normally with my 1000D take an image with the mask and you see colour difraction layers on the image with the mask.

Thanks, I've seen the colours with the mask on, quite nice actually, but it's after the mask is taken off, there is still some colour fringe around the star I've focused on.

[sgazer]

well there's now't like trying it. It was just clear enough last night for a quick test;

both using liveview at 10x zoom, I tried focusing with the Bahtinov mask, removing it and taking a shot, then focusing without the bahtinov mask, but adjusting the focus to remove the colour fringeing, then taking a shot. The result shows that the Bahtinov rules and gives the best focus of the two. Although the second method has no colour fringe during liveview, when taking the shot at a longer exposure than the liveview (8sec), then the colour fringeing is much more prominent than when focused with the Bahtinov, the very faint stars appear slightly out of focus and the spider vane diffraction pattern is split.

Here's a GIF comparing the two (you might have to click on it to get the GIF animation cycling);

[themos]

experiment rules.