Back Focus distance for DSLR lenses?

[Psychobilly]

What sort of focal lenghts are you after Steve?

I wish I hadn't started trawlign the bay of E as i am now watching 4 lenses

Peter....

[SteveL]

I'd like to do the much wider field than I normally do. As the system will be piggybacked onto the normal, I`ll be imaging with the C11/190MN/FLT98/etc, and have this wide field rig pointing somewhere in the sky at the same time.

I`d like a fisheye (or close to it) at some point

i`m guessing the primes tend to be better because they are not compromised for zoom use.

[PAC]

Hi Steve,

I have a Mogg adaptor which I use with my Canon lenses and Atik CCD. It will hold a filter but it's not easy to change filters as you have to remove the lens from the adaptor to do so.

So far I've only tried my 135mm f/2L lens with it. Focusing was tricky as you don't have any fine focus control. I used the lens wide open. At f/2, light pollution saturates the CCD quite quickly. The obvious solution is to use a narrowband filter but these are not designed to function correctly with super fast lenses (e.g. at f/2). I don't know why this is but they don't. Camera lenses also contain multiple lens elements and I found that internal reflection even with my pro quality lens was quite high. This resulted in 'solid' discs of light behind the brightest stars.

I tried to capture M45 at f/2 with a astronomik blue filter. I was hoping to capture the blue gas clouds but the results weren't great as light pollution/sky glow were just too strong.

More experiments are clearly needed. Would suggest stopping the lens down a little to avoid making it too fast. With my Canon lenses this is achieved by mounting the lens on the DSLR and pressing the depth of field preview, whilst at the same time removing the lens from the camera. I'm assured by pro photographers that this doesn't do any harm. The removed lens retains the stopped down aperture setting.

Ok by doing this you lose some lens speed but you still retain the widefield and don't forget that most lenses are at their sharpest when stopped down a little (my 135mm f/2L is sharpest at f/2.8 to f/5.6).

Hope this helps.

[SteveL]

is to use a narrowband filter but these are not designed to function correctly with super fast lenses (e.g. at f/2). I don't know why this is but they don't

Thats because the light cone angle cones in too steep for the inteference based filters to cope with. As an ex-hyperstar user (f/2), I`ve been there Not sure I`d be happy going faster than f/2.8-3.0 for exactly this reason, maybe even f/3.8-4.0 (fastest my current two lenses go)... will have to investigate some more with this to see what happens.

The Nikon lenses (AF and AI) dont need the camera body to keep the aperture settings, you can change the settings and see the blades open and close.

I`ll be making a mini-baltinov mask for my lenses, just I know the focus is spot on. That will be a mission in itself!

[PAC]

Thanks for clearing that one up, Steve.

I'm currently thinking about making an adaptor to allow Canon lenses to be attached to my Atik filter wheel. Hopefully I can make it thin enough to allow the lens to reach infinity focus on the CCD. This will make filter changing much easier than with the Mogg adaptor.

[MartinB]

I wonder if Gerd would do a retro fit for my drawer? Does the astronomic filter just replace the nosepiece? Wish I was going to Lucksall to have a look at this!

[SteveL]

Why dont you ask Gerd, he can only say no

As for the T2 filter, yes. I just removed the standard QHY8 nosepiece, and screwed in the UVIR T2 filter.

[Coco]

Another good choice is the Tamron SP range.

Adaptall-2.com

Interestingly my post last night about a blue cast with the CLS filter was probably due to using an EF 135/2 L lens wide open.

Glad I found this link and I should have posted what lens I had used.

Cheers

Guy

[Psychobilly]

Guy,

You'll get the color cast nevermind what aperture you use the lens at...

Peter....