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Hello and right away a question
#1
Hello. Just joined but have often looked around.  Love the lens tests and of course the forum.  I am a long time Olympus user in California who is transitioning to mostly micro-4/3 at this time.  I'm using my 4/3 lenses on the OM-D cameras (got the -5 and the -1) and even some old Zuikos.  Gotta say these cameras are the creme de la creme.  I'm also building a collection of m.Zuiko lenses for use without any adapter.  As of now I have the little 12-50 which came bundled with the E-M5, and when I bought the E-M1 I also purchased the 12-40 on the rebate deal and sprang also for the 75 F/1.8  Both these lenses are extremely well done.

Now my question concerns the 75 F/1.8  It is a truly luscious lens, beautiful and smooth to operate.  I haven't done much with it thus far but I did do some star images, star tests at F/2 and the results leave me a bit at a loss.  Most everything about the results are fantastic - sharpness across the field, NO Coma, great resolution etc.  However, there is a lot of chromatic aberation - purple mostly but some blue fringing around bright stars.  The tests herein show very little CA.  I also tested the 12-40 at 40mm and see virtually no CA with that zoom, which is in line with the tests results here. 

So I am wondering - do I perhaps have a bad sample of the lens?  I think I might be able to swap it for another as I bought direct from Olympus and they always treat me well.  But I don't want to send it in just yet.

I wonder if I could post the test image, might someone here give a look and an opinion?  I can say that the CA I saw with the 75 is on a par with my old Zuiko 90/2 macro in the same test and actually worse than an old 50/1.8 MC Japan lens but that one had abundant COMA so is fairly useless for this application.

I attached a crop of about half the 75mm image here [ATTACHMENT NOT FOUND] which should show the CA pretty well.  I also attached a tighter crop from the 12-40 at 40mm [ATTACHMENT NOT FOUND] showing appx the same area and little to no CA.

I'd be very interested in any comments and, of course, especially be interested in any interest from the lens testing folks.

In any case, glad to be here and will talk later.

#2
I do not think that these are LATERAL CAs in the image taken with the Oly 75. Lateral CAs would have different inbound/outbound colors from the image center (things are radial on lenses).

 

You mentioned that you used it at f/2 here ?I suspect that you are seeing purple fringing. Honestly I am not sure about the origins of this. I haven't seen any good explanations out there at least. The best bet points towards a blooming effect on the image sensor due to the angle of light at large aperture settings combined with a overly harsh contrast. You may notice that the effect is only visible around very bright stars thus around extreme contrasts but not around the fainted ones. The effect is also more pronounced on smaller sensors (pixel density ?).

I suspect that if you use the Oly 75 at f/2.8 and f/4 the effect will be gone or at least largely reduced. 

 

FWIW, I am aware of 3-4 CA types:

- Lateral CAs (which is what we test)

- Axial/Longituadal CAs

- Bokeh fringing (I am not 100% sure about the relationship of bokeh fringing and axial CAs - there seems to be a relationship but it doesn't seem to be identical). During earlier tests we referred to LoCA (axial CAs) but lately we are only using the term bokeh fringing.

- Purple Fringing (as discussed)
#3
Quote:I do not think that these are LATERAL CAs in the image taken with the Oly 75. Lateral CAs would have different inbound/outbound colors from the image center (things are radial on lenses).

 

You mentioned that you used it at f/2 here ?I suspect that you are seeing purple fringing. Honestly I am not sure about the origins of this. I haven't seen any good explanations out there at least. The best bet points towards a blooming effect on the image sensor due to the angle of light at large aperture settings combined with a overly harsh contrast.
No, it is lens dependent (PF that is). So it can't be that elusive "blooming effect".

Hs to do with light reflecting from the sensor back to the lens. In the past it was a much bigger problem than nowadays, as new coatings of back elements take care of it mostly. PF is not a CA, in that sense, more something like sun flares and veiling.

Quote:You may notice that the effect is only visible around very bright stars thus around extreme contrasts but not around the fainted ones. The effect is also more pronounced on smaller sensors (pixel density ?).
PF is more pronounced with compact digitals which lack the proper lens coatings. Especially older compact digitals (8 or so years ago or more) can show heavy PF. PF is light intensity dependent, that is why stopping down reduces/clears it.

Quote:I suspect that if you use the Oly 75 at f/2.8 and f/4 the effect will be gone or at least largely reduced. 

 

FWIW, I am aware of 3-4 CA types:

- Lateral CAs (which is what we test)

- Axial/Longituadal CAs

- Bokeh fringing (I am not 100% sure about the relationship of bokeh fringing and axial CAs - there seems to be a relationship but it doesn't seem to be identical). During earlier tests we referred to LoCA (axial CAs) but lately we are only using the term bokeh fringing.
LoCA and "bokeh fringing" are the same, aren't they? How are they not identical?

Quote:- Purple Fringing (as discussed)
#4
Quote:Now my question concerns the 75 F/1.8  It is a truly luscious lens, beautiful and smooth to operate.  I haven't done much with it thus far but I did do some star images, star tests at F/2 and the results leave me a bit at a loss.  Most everything about the results are fantastic - sharpness across the field, NO Coma, great resolution etc.  However, there is a lot of chromatic aberation - purple mostly but some blue fringing around bright stars.  The tests herein show very little CA.  I also tested the 12-40 at 40mm and see virtually no CA with that zoom, which is in line with the tests results here. 

So I am wondering - do I perhaps have a bad sample of the lens?  I think I might be able to swap it for another as I bought direct from Olympus and they always treat me well.  But I don't want to send it in just yet.

I wonder if I could post the test image, might someone here give a look and an opinion?  I can say that the CA I saw with the 75 is on a par with my old Zuiko 90/2 macro in the same test and actually worse than an old 50/1.8 MC Japan lens but that one had abundant COMA so is fairly useless for this application.
Like Klaus points out, the CA measurements are about lateral CA. Lenses refract light of different wave lengths to varying degrees. This can result on some colours being projected larger than others, which leads to for instance the red channel being "bigger" in the projection than the red and green channel. This most visible in the resulting coloured edges around contrasty areas, near the borders of the image. They colours always coma in pairs (green/magenta, yellow/blue, red/green-blue).

 

This is not what you are seeing. 

 

There is another kind of CA, LoCA. This happens when in OOF areas light lets "focussed" on different planes. Usually one gets magenta in front and green behind the focal plane. This probably is also not what you are seeing.

 

Most likely you are seeing purple fringing. PF happens when a lot of light hits the sensor, and part of it reflects back. When you stop down, it will lessen and disappear. PF is lens dependent, as you have noticed (your 75mm f1.8 and 90mm f2 show more than your 50mm f1.8). This probably has to do with the shapes of the lens elements, the 50mm f1.8 will diverge the reflected light more and less will end up concentrated. Lens manufacturers have developed coatings for lenses to reduce PF, Sigma started to call lenses with the new coatings "DG" (digital generation) and later when they introduced APS-C lenses "DC". And Tamron started to call lenses with the new coatings "Di" (digital integrated or something like that) and "Di II" for APS-C.

 

You can't do anything about camera/lens combinations which show PF, other than get a lens which is designed not to show any PF problems. I am not aware of any lens test site which does test for PF showing up, so it will have to be trial end error...
#5
Hello and many thanks for the responses.  Purple fringing yes.  CA no.  Is my take.  I will certainly do some testing by stopping down. Of course, this kinda defeats the purpose of using a fast lens for astro shots but nothing is perfect.  Probably that $1500 nocton lens would be no better.  There also are some pshop actions that claim to remove purple fringing.  I will try them as well.  Shouldn't require a rocket scientist to write such an action.

Kinda disappointed that this otherwise fantastic lens has this issue while the 12-40 zoom does not. Could a different coating have fixed the problem in the 75?  Wonder how the upcoming 40-150 Pro zoom will fare?  Course, both zooms are over a stop slower.  More engineering radeoffs I imagine.

Lastly, I read somewhere, and will test, that the E-M1 camera auto-removes CA but the E-M5 does not.  I can vouch for the latter but not yet the former. Can anyone confirm this?

Thanks again.

George

#6
Quote:Like Klaus points out, the CA measurements are about lateral CA. Lenses refract light of different wave lengths to varying degrees. This can result on some colours being projected larger than others, which leads to for instance the red channel being "bigger" in the projection than the red and green channel. This most visible in the resulting coloured edges around contrasty areas, near the borders of the image. They colours always coma in pairs (green/magenta, yellow/blue, red/green-blue).

 

This is not what you are seeing. 

 

There is another kind of CA, LoCA. This happens when in OOF areas light lets "focussed" on different planes. Usually one gets magenta in front and green behind the focal plane. This probably is also not what you are seeing.

 

Most likely you are seeing purple fringing. PF happens when a lot of light hits the sensor, and part of it reflects back. When you stop down, it will lessen and disappear. PF is lens dependent, as you have noticed (your 75mm f1.8 and 90mm f2 show more than your 50mm f1.8). This probably has to do with the shapes of the lens elements, the 50mm f1.8 will diverge the reflected light more and less will end up concentrated. Lens manufacturers have developed coatings for lenses to reduce PF, Sigma started to call lenses with the new coatings "DG" (digital generation) and later when they introduced APS-C lenses "DC". And Tamron started to call lenses with the new coatings "Di" (digital integrated or something like that) and "Di II" for APS-C.

 

You can't do anything about camera/lens combinations which show PF, other than get a lens which is designed not to show any PF problems. I am not aware of any lens test site which does test for PF showing up, so it will have to be trial end error...
 

 

I do not think (as opposed to know) that PF is a reflection effect caused by the lens. It feels highly unlikely that reflections create a halo effect. (Re-)Reflections would be distributed across the image field - essentially re-reflections are a freak effect.

 

If I had to GUESS, this may be caused by the micro-lenses that sit in front of the photodiodes but not by the lens. These micro-lenses are certainly NOT coated. It feels viable that sensor-surfaces reflections are distributed to adjacent micro-lenses. This would also explain the halo effect.

 

Just guessing.
#7
"LoCA and "bokeh fringing" are the same, aren't they? How are they not identical?"

 

A while ago I had a chat with a physicist. He said that LoCAs are somewhat different from bokeh fringing.

When looking at our usual test target, the fringing is primarily visible in the blur portions. 

However, just like lateral CAs, axial CAs are actually located at the focus point not beyond.

While he agreed that there seems to be a certain dependency, the "bokeh fringing" has at least an additional cause.

He wasn't too sure about it either though.

#8
Quote:I do not think (as opposed to know) that PF is a reflection effect caused by the lens. It feels highly unlikely that reflections create a halo effect. (Re-)Reflections would be distributed across the image field - essentially re-reflections are a freak effect.

 

If I had to GUESS, this may be caused by the micro-lenses that sit in front of the photodiodes but not by the lens. These micro-lenses are certainly NOT coated. It feels viable that sensor-surfaces reflections are distributed to adjacent micro-lenses. This would also explain the halo effect.

 

Just guessing.
But is that were the case, one would see similar with every lens with a similar f-ratio. And we know that is not the case... Even the OP notes a difference between his 75mm f1.8 and his 50mm f1.8.

 

Do you have any ideas about how to do a quick and easy PF test? Then I could conduct some experiments.

#9
Quote:But is that were the case, one would see similar with every lens with a similar f-ratio. And we know that is not the case... Even the OP notes a difference between his 75mm f1.8 and his 50mm f1.8.

 

Do you have any ideas about how to do a quick and easy PF test? Then I could conduct some experiments.
 

There seems to be a relation to the angle of the light.

e.g. Fisheyes are especially prone to PF.

I am pretty sure that if a lens is prone to PF, it can be reduced by stopping down (at least on a fast tele prime).

 

A typical scene that pushes PF would be a something with sparkling water - e.g. a fountain or white water in bright sunlight.

Or stars ;-)
#10
Quote:There seems to be a relation to the angle of the light.
The OP notes strong PF with the 75mm lens, not so with the wider 50mm lens....

Quote:e.g. Fisheyes are especially prone to PF.

I am pretty sure that if a lens is prone to PF, it can be reduced by stopping down (at least on a fast tele prime).
Yes, it will be reduced. 

Quote:A typical scene that pushes PF would be a something with sparkling water - e.g. a fountain or white water in bright sunlight.

Or stars ;-)
So no experimenting until summer, I guess?
  


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