EOFB - Is it Real? - with Poll

[Ruud]

Hi all,

EOFB stands for edge of field brightening and pertains to eyepieces. It seems that certain eyepieces get brighter toward the edge of the field. I've never seen this myself, but according to discussions elsewhere it's a real thing and it has nothing to do with either a bright source close by the field of view or light shinig into the eyepiece from behind, past the observer's eye. EOFB is supposed to be a property of the eyepiece itself, and is not something caused by external factors.

What's going on? Is EOFB a contrast effect caused by the blackness beyond the field stop? Or is it really centre of field darkening instead of EOFB? Is EOFB real at all, or is it an illusion like the canals on Mars which people would see if they stared long enough? 

Have you ever seen EOFB in any eyepiece that you own or have looked through? 

[michael.h.f.wilkinson]

EOFB can also show itself as a coloured ring (often red) during daylight use of UWA EPs (my 31T5 shows it). Not all design have the problem. That fact, and the red colour suggests it is a real effect rather than somethimg like a Mach-banding effect caused by the dark field stop. My Paragon 40mm and LVW42 do not seem to show any, the 31T5 as mentioned does. As the background is brighter in the 40 and 42mm EPs the contrast with the black of the field stop is higher, and I would expect a stronger effect than with the 31mm.

[Ruud]

Hi Michael,

I know those coloured rings. My Delos eyepieces each have a blue one. They're called Rings of Fire.

EOFB is different. It is just an increased brightness of the image toward the edge. Like inverted vignetting. Have you ever seen that?

[John]

I've not noticed EoFB in any eyepiece that I've used. That does not mean it was not there, I've just not noticed it. It's a relatively new term - up until round 18 months ago I'd not seen the issue discussed on forums.

[damnut]

Hi Ruud,

Interesting thread, have not noticed it, but to be frank I have not looked for it either. Have  a whole mish mash of ep's!!

[Damo636]

Haven't noticed it myself either, although to be honest never really looked for it.

[YKSE]

I'm not quite sure it's conclusive that EOFB is a property of eyepieces, there's a lengthy discussion about EOFB recently
together with other dicussions I've read, my impression is that it's more likely different factors involved here, the whole optical chain from telescope, eyepiece, our eyes, brains, eye placement and observing environment.

1. Telescope:
Same eyepieces show very different amount EOFB in different scopes
 

2. Eyepiece:
Some eyepieces show EOFB no matter which scope it's in, who ever is looking through it. lens edge blackning, barrel tightness, eyeguard etc, may be part of the reasons.

3. Our eyes:
Some people see EOFB in many more eyepieces than any other do, such as in Nikon 17.3SW (which I don't see any), T4 Naglers, 20ES 100deg, XWs, Delos etc, and not many reported EOFB in those eyepieces from what I've read.
Also, our eyes can see things differently than they actually are, as illustrated in Optical illusion
 

4. Our brains:
I read all about EOFB in Leica Zoom before I bought one, and in my first light
I took the vignetting in low mag as EOFB, just like some others in that EOFB thread.

5. Eye placement :
Different eyeplacement can have impact
 

6. Observing environment:
Even people see EOFB in a certain eyepiece, there's variation how visible EOFB is from day to day in their scopes.

There're certainly other factors I forgot to mention,

Just as John has pointed out, EOFB got more attention very lately, because some individuals have talked more about it on CN. There were only sporadic reports of EOFB before, with other designations, e.g.in Bill Paolini's classic "24-26 mm Eyepiece Comparison" where "lightening at field Stop" was used instead of EOFB. BTW, this is a great review for anyone who's interested in having better understanding of different properties of an eyepiece.

[michael.h.f.wilkinson]

Hi Michael,

I know those coloured rings. My Delos eyepieces each have a blue one. They're called Rings of Fire.

EOFB is different. It is just an increased brightness of the image toward the edge. Like inverted vignetting. Have you ever seen that?

I have seen that as well at night in the 31T5. It is not that obvious, but it is there if you look for it. I notice it to a lesser degree in the 22T4 (which does not show the ring of fire effect), especially when moving faint fuzzies through the field. The centre is slightly darker than the edge.

[michael.h.f.wilkinson]

I'm not quite sure it's conclusive that EOFB is a property of eyepieces, there's a lengthy discussion about EOFB recently

together with other dicussions I've read, my impression is that it's more likely different factors involved here, the whole optical chain from telescope, eyepiece, our eyes, brains, eye placement and observing environment.

1. Telescope:

Same eyepieces show very different amount EOFB in different scopes

2. Eyepiece:

Some eyepieces show EOFB no matter which scope it's in, who ever is looking through it. lens edge blackning, barrel tightness, eyeguard etc, may be part of the reasons.

3. Our eyes:

Some people see EOFB in many more eyepieces than any other do, such as in Nikon 17.3SW (which I don't see any), T4 Naglers, 20ES 100deg, XWs, Delos etc, and not many reported EOFB in those eyepieces from what I've read.

Also, our eyes can see things differently than they actually are, as illustrated in Optical illusion

4. Our brains:

I read all about EOFB in Leica Zoom before I bought one, and in my first light

I took the vignetting in low mag as EOFB, just like some others in that EOFB thread.

5. Eye placement :

Different eyeplacement can have impact

6. Observing environment:

Even people see EOFB in a certain eyepiece, there's variation how visible EOFB is from day to day in their scopes.

There're certainly other factors I forgot to mention,

Just as John has pointed out, EOFB got more attention very lately, because some individuals have talked more about it on CN. There were only sporadic reports of EOFB before, with other designations, e.g.in Bill Paolini's classic "24-26 mm Eyepiece Comparison" where "lightening at field Stop" was used instead of EOFB. BTW, this is a great review for anyone who's interested in having better understanding of different properties of an eyepiece.

Very good points. Regarding the scope, some people might report EOFB and blame the EP, when in reality they are seeing the shadow of the secondary which can easily occur when the exit pupil is very large (especially when the eye is not fully dark adapted). That is not the only factor. I feel EOFB is slightly stronger in my F/6 APO than in my F/10 C8. There may be several reasons for that. First of all, like many other aberrations, EOFB might increase in faster scopes (not sure, but not far fetched as hypothesis). Secondly the brain might notice the EOFB more if the field is brighter anyway. Finally, the Mach-banding effect (which would certainly contribute) might be stronger if the contrast between field stop and field is higher. I certainly see more EOFB (such as it is) if my eye is fully dark adapted.

[SpaceWalker]

Yes, the EoFB is real. Some eyepieces are more prone to it than others; some don’t show it at all. For example, I could consistently see intensive EoFB when was using the 13mm AT AF70 (same as the TS Expanse or Celestron Ultima LX ) . The 13mm Hyperion is optically similar so I wasn’t surprised reading some reports about the EoFB in it. Another well-known example of the EoFB is the entire Celestron Luminos (and Axiom) line.  There are several lengthy discussions on CN as follows

link 2
link 2
link 3

But still nobody was able to track the source or explain this mysterious phenomenon.

[jetstream]

What puzzles me is that in the EOFB area I can still see faint objects as well as in the area not affected .

[John]

What puzzles me is that in the EOFB area I can still see faint objects as well as in the area not affected .

Maybe the effect causes everything close to the edge of the field to be brightened a little, deep sky objects included ?

One thing I'm interested in is the aggregated glass thickness across the field. We know that the individual lenses in eyepieces have various curves, some concave and some convex, some fat and some thin, some spherical and some perhaps parabolic ?.

To me this means that the combined thickness of glass that light has to pass through could well vary at different points across the field of view. Less at the centre and more at the edge perhaps ?. Maybe the other way around with some designs ?.

Perhaps this, combined with the focal ratio of the scope and even the characteristics of the eye, is responsble for the effect we call EoFB ?

[jetstream]

Great thoughts John, I never considered the glass thickness and design...excellent. Yes its strange that object are still seen the same in the EOFB are and you must be right on this too... at least when it comes to my EOFB ( my eye/brain interpretation). The barlowed LZ is so good on planets/lunar that "all is forgotten" so to speak- I rarely use it unbarlowed however... it is a VG EP.

[Chris]

Yes, the EoFB is real. Some eyepieces are more prone to it than others; some don’t show it at all. For example, I could consistently see intensive EoFB when was using the 13mm AT AF70 (same as the TS Expanse or Celestron Ultima LX ) . The 13mm Hyperion is optically similar so I wasn’t surprised reading some reports about the EoFB in it. Another well-known example of the EoFB is the entire Celestron Luminos (and Axiom) line.  There are several lengthy discussions on CN as follows

link 2

link 2

link 3

But still nobody was able to track the source or explain this mysterious phenomenon.

I've recently bought the 7mm and 15mm Luminos from ebay to specifically look at EOFB.

EOFB seems to be the only bad thing said about these eyepieces, apart from this the plus side seems to be a well built 82 degree EP with good eye relief for less than 100 pounds new. Some people seem to notice EOFB and some don't as discussed, so I would like to see which category I fall into.

I've had one outing with them so far with the ED100 f/9 and Skylight 60mm f/17, and I think I can see a slight brightening towards the edge, but only because I was looking for it, I'm not sure I would have seen it otherwise. I really need to check again because the Moon was close by so I can't say for sure.

One thing that did surprise me was how well the 7mm Luminos did on Saturn. I had my best view of the night with this EP, but I'm guessing this might have been down to it being the best mag for the conditions, maybe?

More testing required, sadly I can't test the edge of field sharpness as my fastest scope is f/9.

[jetstream]

I have the 7mm Luminos and to my eyes shows no EOFB, neither does the 10mm- strange. You are right about them being good- I have also had excellent planetary views with them.  The 10mm is excellent on DSO in my 10" f4.8... From what you describe -"slight lightening toward the edge..." you should see the EOFB in my Leica  its right there, in your face, no mistaking it- until you barlow it, then it disappears lol!

I can see the lower loop in M42 with the Leica in the EOFB zone and this is pretty faint really. I'm going to check some other faint nebs with it this season for something to do.

[Physopto]

I suppose it could be some sort of constructive interference effect.

[Chris]

I have the 7mm Luminos and to my eyes shows no EOFB, neither does the 10mm- strange. You are right about them being good- I have also had excellent planetary views with them.  The 10mm is excellent on DSO in my 10" f4.8... From what you describe -"slight lightening toward the edge..." you should see the EOFB in my Leica  its right there, in your face, no mistaking it- until you barlow it, then it disappears lol!

I can see the lower loop in M42 with the Leica in the EOFB zone and this is pretty faint really. I'm going to check some other faint nebs with it this season for something to do.

Well if you can see EOFB in your Leica, I think we can forgive any in the Luminos

[Ruud]

COFD (centre of field darkening) would be much easier to explain. For instance like John says: peripheral rays travel through less glass, which makes the edge of the field brighter than the centre. Who knows, this might be enough to be perceptible. At least we should be able to measure it. EOFB might be a misnomer.

If it is not a misnomer EOFB could still exist, but becomes very difficult to explain as the extra light must come from somewhere. Unless EOFB is caused by a drop of magnification toward the edge of the field (angular magnification distortion - amd). Less magnification means a brighter image causing real EOFB.

If EOFB is real it can be measured. I guess I'll be waiting for that.

[michael.h.f.wilkinson]

I am not sure the thickness of the glass is to blame. The path through the optical system is complex, and not all rays ultimately on the edge of the field needs to pass through more glass (especially in EPs with Smyth lenses). I have heard that one problem in ultra-wide EPs is that some light starts to act as surface waves in the coatings of the lenses (which can act as a kind of wave guide) to be released at some point along the curved lens surface. This occurs when the light rays travel at an acute angle to the optical surface, if I recall correctly. Not sure if this causes EOFB, but it might explain why EOFB in the Leica was not observed with the Barlow, as the range of angles entering the optics is narrower due to the narrower light cone of the slower system. This makes it somewhat less likely for the effect to occur.

[John]

I'm not sure either Michael - I just stuck it up as a possibility.

Glass does do some strange things when light enters and exits it at high incidence. I guess Al Nagler would be able to contribute something very interesting on this.

[michael.h.f.wilkinson]

I'm not sure either Michael - I just stuck it up as a possibility. Glass does do some strange things when light enters and exits it at high incidence. I guess Al Nagler would be able to contribute something very interesting on this.

Very true. I think we are also getting very spoiled with the currently available EPs. Many aberrations are corrected to an extremely high degree over fields of view seemingly impossible a few decades ago. Visionaries like Al Nagler have given us unprecedented windows on the universe,and here we are niggling over something that would never occurred to Charles Messier or William Herschel to complain about, mainly because the effect would be masked by a donkey's load of aberrations long before they got close to our FOV

[Chris]

I take it we can rule out any physiological effect to do with the eye if EOFB is shown in some EP's but not others? I would have suggested the arrangement of the rods and cones at the back of the eye could have something to do with it? Our eyes are more sensitive to dark at the periphery which is of course why we use averted vision when we observe. Maybe this could translate into some people noticing the edge of the eyepieces FOV being brighter?

If it wasn't for the fact that some EP's seem to show it and some don't, I would have put money on this being the cause.  

[michael.h.f.wilkinson]

I take it we can rule out any physiological effect to do with the eye if EOFB is shown in some EP's but not others? I would have suggested the arrangement of the rods and cones at the back of the eye could have something to do with it? Our eyes are more sensitive to dark at the periphery which is of course why we use averted vision when we observe. Maybe this could translate into some people noticing the edge of the eyepieces FOV being brighter?

If it wasn't for the fact that some EP's seem to show it and some don't, I would have put money on this being the cause.  

It might contribute. However, I do think the edge looks a touch brighter if I look at it directly, compared to the centre.

[Chris]

It might contribute. However, I do think the edge looks a touch brighter if I look at it directly, compared to the centre.

In that case, perhaps that further rules this out. 

How about reflections inside the barrel if not properly blackened? If there were reflective areas in an eyepiece barrel, and the light entering the eyepiece is almost parallel to the reflective area, it could illuminate the outer regions of the lenses perhaps? 

[michael.h.f.wilkinson]

In that case, perhaps that further rules this out. 

How about reflections inside the barrel if not properly blackened? If there were reflective areas in an eyepiece barrel, and the light entering the eyepiece is almost parallel to the reflective area, it could illuminate the outer regions of the lenses perhaps? 

I have heard about studies showing lens edge blackening doesn't do much. I have seen EOFB in TV EPs and they do not show much in the way of internal reflections. I would expect such reflections to depend on exact eye position, and be more prominent with bright objects in the FOV. If anything, I see less EOFB when a bright object is in the FOV, although that might be caused by loss of dark adaptation