"Focus" problems through CLS filter

[Nomis Elfactem]

Hi everyone

I have a problem and I'm stumped but I'm sure it's something obvious and I just don't know it yet.

This week I got a Astronomik CLS light pollution filter (EOS clip) and took it out for a test drive Friday night and last night... I soon found something wrong.  Below are two images (they are cropped parts of the actual image), one with the filter the other without.  They are exactly the same photo (10sec, f4.5 ISO1600 using canon 400d with a sigma 28-300 lens)... all I did was take out the filter before taking the second image - the image using the filter appears out of focus but it gets worse if I de-focus from infinite so this looks like it's the best it gets.

Does anyone know what I'm doing wrong or is it just a function of the lens I'm using (i.e you can't use the EOS clip with certain lenses due to internal reflection or something) ?

Simon


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[splog]

Hi Simon

This is an interesting problem as I also have had focus problems with this filter and a Canon 180mm F3.5L lens, sorry can't answer your question but will try the same or similar test

Steve

[the fordster]

I can only guess that inserting something in the focal path of the lens has changed the autofocus position - Where the lens is reporting to you that you are in focus, you are actually a wee bit out. Have you tried manual focus using the preview screen? Get focus close, take a snap, zoom in, make a miniature adjustment, take another snap and repeat until star sizes are as small as possible. There will be a place you can nail focus, mebbe just not where your lens thinks it is...

The other option of course (assuming that that is the tightest focus can get) is that you may be getting secondary reflections between the CCD coverslip and the back of the filter which is showing as enlarged stars - these usually appear as halos around a bright star...

[mcrossley]

It looks like the filter is increasing the optical path length sufficiently to make infinity focus with a camera lens impossible. The problem will be worse the shorter the focal length of the camera lens you use. Are these filters marketed to be used with camera lenses, or to be used with scopes?

[Nomis Elfactem]

It looks like the filter is increasing the optical path length sufficiently to make infinity focus with a camera lens impossible. The problem will be worse the shorter the focal length of the camera lens you use. Are these filters marketed to be used with camera lenses, or to be used with scopes?

That was my thought/question too - does anyone know, or know anyone who would ?

The images were taken using manual focus as it has always struggled taking pictures of the sky (even during the day) on autofocus.  I also did try adjusting it but it needs to go the other way (further anti-clockwise) so it looks like infinity focus isn't possible with this lens  :cry:

Hopefully we'll get a clear sky tonight so I'll give it a test drive using my 'scope and see if that works.

Thanks for help so far 
S.

[Nomis Elfactem]

Hi Simon

This is an interesting problem as I also have had focus problems with this filter and a Canon 180mm F3.5L lens, sorry can't answer your question but will try the same or similar test

Steve

Thanks Steve... I'll be very interested in your results... I'll try some different things too and report back.
Cheers
S.

[brianday]

Simon
It might be worth noting exactly whre the camera autofocusses during daylight on a sharp distant object both with and without the filter

then manually set to these positions and retake your star shots to see if its any better

I use maual setting always for star shots now

cheers
Brian

[Nomis Elfactem]

Simon
It might be worth noting exactly where the camera autofocusses during daylight on a sharp distant object both with and without the filter

then manually set to these positions and retake your star shots to see if its any better

I use manual setting always for star shots now

Hey Brian - Thanks, I'll give that a go and see if I have any luck.  Having said that I know the lens/camera does struggle to autofocus on the sky when I've tried taking photos of say a plane flying over head.

It also still leaves me with the question of why the images above are not the same where the only think that has changed is the inclusion of the clip CLS filter. 

It does appear that the filter is changing the focal point (which as it's a slice of glass isn't to be unexpected I guess) but that makes me wonder why infinite focus isn't so in all cases, so if I'm looking to get another lens what do I need to know in order to get one that "works" (if that makes any sense).

Cheers
Simon

[Nomis Elfactem]

Just been looking at this....

http://ukastroimaging.co.uk/forums/index.php?topic=36967.0

which is pretty much what I'm doing (but Kevin is using a different lens (Sigma EX DC 10-20mm), much longer exposure and I guess tracking/guiding)) and focus appears sound.... I'm even more confused now ???

S.

[mba007]

I use the clip-in filter in my modded-300D with manual Nikon lenses. I found that the focus point changed position when using the filter. I use a vernier scale on the side of the lens to accurately find the focus point which is measurable and repeatable so it seems a definite issue. The Nikon lenses also have a small metal tab which presses against the plastic surround of the filter so I thought that the lens position was slightly off. Now it seems others have similar issues.

I was surprised to find the infinity focus point had changed, but as I'm using manual focus this hasn't been a problem.

[bern]

Because of the laws of Physics the clip filters have to move the focal point slightly, by about 1/3rd  of a mm according to Astronomik.
However this should be easily taken care of, when using lenses, by a small movement of the focusing ring.
Astronomik are not aware of any issues using the filters with non EF-S lenses and would be interested to hear if there are any problems that can't be resolved, either contact them directly or through me.

bern

 
 

[Nomis Elfactem]

Astronomik are not aware of any issues using the filters with non EF-S lenses and would be interested to hear if there are any problems that can't be resolved, either contact them directly or through me.

bern
 

Hey Bern

That's great, thanks for that - I'll do a bit more investigation following some of the suggestions and get back to you - looks like it will have to wait as it's just started raining here so won't be tonight  :cry:. 

I'll try and get a different lens too, to try and narrow down by process of elimination whether it's the filter, lens or camera (i.e. if it's OK with a different lens then that answers the question).

Once again, thanks for all the help on this everyone... if nothing else I'm learning a lot about photons and glass which is great 

S.

[mcrossley]

The accommodation in focus would apply to auto-focus lenses that have a focus range which goes beyond infinity, but old manual focus lenses have the focus stop set at infinity for the correct back-focus distance and these may well not focus at all with a rear filter. Maybe people should be aware that manual lenses will probably struggle.

I know that my 8mm fish-eye lens only has a total focus movement of about 2mm (maybe less) and if I remove the rear filter that comes with the lens it will not get anywhere near focus. The opposite of this issue.

[splog]

Hi all

I just tried a fairly basic test, but after reading Marks post I'm not too sure how relavent it is ? Anyway here's a couple of pics taken with 180mm EF lens. First I used autofocus without the clip filter and took the shot, set the camera to manual focus   and then took the second with the filter in place.

There is a definite softening of the shot with the clip filter, however the camera was then able to focus accurately. so as far as I can tell my focus problem was probably down to the method I often use, which is to focus in daylight (without filter) and lock off the lens  :)

Thanks all at least now I think I know what's happening 

Steve

[splog]

Sorry, here's the pics

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