I have just bought a used Canon EF 50mm lens, and it exhibits strong
blue halos around bright stars. This model of lens is reviewed as having no C.A. at all; could there be another explanation? It is absolutely
unmarked, so it seems unlikely that it has suffered a heavy fall.
raymo
When using these lenses for astro imaging, focus is CRITICAL, else they will exhibit C.A.
For terrestrial imaging its not as obvious.
Only a very few high end Canon EF lenses will not show C.A for astro, like the EF 300mm f/2.8.
The nifty fifty, the 50mm f/1.4 and the 50mm f/1.2, the 85mm f/1.2 and others. will show C.A around stars and as I said, perfect focus will minimise C.A but won't eliminate it.
Astro imaging is very demanding on these lenses and where they may be suitable for terrestrial not showing obvious C.A, for astro the pin point source of light does separate the men from the boys and will show and highlight the C.A in even the higher end lenses.
Yeah I stop mine down to f/4 to (try to) control the coma, but focus is really tricky as the focus ring is quite free moving. It can be a really sharp lens if you can nail the focus though, otherwise you get halos...
Perhaps I should elaborate a little. I did film astro for over 40 yrs before attempting the move to digital, and therefore am familiar with C.A. and
coma etc: etc: I was expecting some C.A. in these first images as I had
the lens wide open, but was shocked by the amount- far more than my
Tamron 18-200mm exhibits at any f number. Every review of this lens
that I have read [5] states that the C.A. of this lens is minimal, better
even than the f1.4 version [for terrestrial use, of course]. I was also expecting, and got, the coma.
I guess what I am really asking is whether dropping the lens could
dislodge any of the glass causing C.A. or whether in this digital world that I now inhabit, there could be any other reason for it.
The lens is the 50mm EF 11[plastic], and doesn't have a focusing distance scale, so I couldn't use infinity even if I were silly enough to
want to Pat. My focusing ring has some resistance, so stays where you put it quite well.
Thanks people
raymo
My 50mm focus ring is as loose as a corner worker. But to the matter of the CA it's entirely possible you've got a bad lens. It happens every now and then.
Electronic focus doesn't work for astro and the plastc lens's that come standard are pretty ordinary. I still have my old metal and glass pentax 35mm film lenses so I got an adaptor for my 1100d and use these for my wide field. The 50mm is f1.7 and a breeze to find a star to focus on. (I then stop it down a couple of stops to f4)
Thank you everyone; the C.A. has just about entirely disappeared in the
subs I have now got at f3.5, so that is o.k. All I have now are the usual
colour halos from saturation.
Sorry to have wasted people's time. I do however, as I discussed with
another member in another thread, have almost perfect representations
of the lens' 5 leaf aperture diaphragm on the outer side of the brightest stars in my Orion image. The other member thinks it might be a reflection caused by the clear sensor cover, which is apparently very highly reflective. Has anyone else had this effect?
raymo
I do however, as I discussed with
another member in another thread, have almost perfect representations
of the lens' 5 leaf aperture diaphragm on the outer side of the brightest stars in my Orion image. The other member thinks it might be a reflection caused by the clear sensor cover, which is apparently very highly reflective. Has anyone else had this effect?
raymo
To avoid the diffraction spikes caused by the aperture diaphragm requires the use of an aperture mask to stop down the lens rather than relying on the internal lens hardware. There are a number of lens filter step down rings available on eBay or you can make your own from sturdy plastic. I have successfully used this technique on my 200mm lens. I've had some success with my 50 and 100mm lens but I think I need to experiment a bit more with the diameter of the hole in the mask as I am still getting some coma.
To avoid the diffraction spikes caused by the aperture diaphragm requires the use of an aperture mask to stop down the lens rather than relying on the internal lens hardware.
This is correct.
You'll get these diffraction spikes on brighter stars if you stop down the lens using the internal leaf mechanism.
Are we talking about the same thing? It is a light grey pentagon, like a
pale shadow, with one of the five side hidden within the star's glare, no spikeiness [ I don't think that's a word] at all.
raymo
Are we talking about the same thing? It is a light grey pentagon, like a
pale shadow, with one of the five side hidden within the star's glare, no spikeiness [ I don't think that's a word] at all.
raymo
To avoid the diffraction spikes caused by the aperture diaphragm requires the use of an aperture mask to stop down the lens rather than relying on the internal lens hardware. There are a number of lens filter step down rings available on eBay or you can make your own from sturdy plastic. I have successfully used this technique on my 200mm lens. I've had some success with my 50 and 100mm lens but I think I need to experiment a bit more with the diameter of the hole in the mask as I am still getting some coma.
I'll have to give that a try. So if I cut the centre out of a lens cap, then make some cardboard inserts with different sized holes in the centre to find the "sweet spot".
Anyway back to the diffraction spikes here is a pic of m45 with a 135mm f2.8 lens stopped down to f5.6
I'll have to give that a try. So if I cut the centre out of a lens cap, then make some cardboard inserts with different sized holes in the centre to find the "sweet spot".
Anyway back to the diffraction spikes here is a pic of m45 with a 135mm f2.8 lens stopped down to f5.6
Yes, a perfect set of direction spikes Not everyone finds these offensive of course and I personally don't mind them but they can become an issue with reflections though when there is a particularly bright star in the field of view (such as Rigel or Sirius). It could still be that the issue that Raymo is having is due to reflections caused by the diffraction spikes from a bright star. It is worth experimenting with the aperture mask and seeing if you are happy with the results.
The other benefit from using an aperture mask is when attaching a Canon or Nikon lens to a CCD (a QSI683 in my case) as you cannot use the DSLR electronics to stop down the lens. It can be done with a bit of trickery but using the aperture mask makes things a little easier.
Not everyone finds these offensive of course and I personally don't mind them but they can become an issue with reflections though when there is a particularly bright star in the field of view (such as Rigel or Sirius). It could still be that the issue that Raymo is having is due to reflections caused by the diffraction spikes from a bright star. It is worth experimenting with the aperture mask and seeing if you are happy with the results.
