Micro Four Thirds Focal Reducer Shootout

Started Feb 20, 2014 | Discussions thread
MOD Tom Caldwell Forum Pro • Posts: 33,643
Re: Hot spotting - the cause?

brian wrote:

Tom Caldwell wrote:


Thank you for making these tests it is appreciated.

I am always puzzled by the condition that creates these hot spots. I have no proper technical knowledge of what causes the but I have tried to explain them on forums to the best of my understanding. (For want of an alternative more knowledgeable source). It seems that the "spots" are shown and commented upon but their technical reason for occurring remains a mystery surronded in optical jargon.

If I repeat my understanding here then you might be able to comment and confirm or advise differently.

My understanding is that back in the film days the film stock was not as reflective as modern sensors and therefore the reflection of a strong light source from the film and further re-relection back off some part of the lens glass back on to the film was less of a problem. In fact some lens manufacturers did not particularly worry about it as inconsequential. Different legacy slr lenses are therefore more prone to sensor re-reflection spots/smudges than others.

Sensor reflection can be bounced back via some flatter glass surface in the form of a aperture delineated circle if from an element located in front of the aperture or as a smudge if after the aperture. According to some Canon literature even a flat filter lens surface can cause this reflection. Canon seem quite proud that they have a meniscus protective front lens to prevent this re-reflection "unlike some of their opposition". Canon's advice for those getting this hot spot on their esteemed telephoto range in extreme conditions was to try removing the lens filter.

I believe/think that good lens design can disperse this re-reflection before it is completely reflected back to the sensor surface. You are to be congratulated as the Metabones product seems to have been designed to reduce or eliminate this re-reflection that happens in more extreme circumstances very well.

I also have to make a disclaimer so that readers might understand: I share your surname, but we are not related. I have no commercial connection with Metabones nor any other Chinese focal reducer manufacturer. I buy my own focal reducers, including a Metabones Speed Booster and several other Chinese made focal reducers for the Sony E-mount. I have one RJ made M42 to M4/3 focal reducer adapter. I am not suitably qualified to make exacting tests, nor do I have any optical knowledge other than what I have read.

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Tom Caldwell

Hi Tom:

Central hotspots are a ghost image of the aperture stop. The visibility of the ghost image depends on the magnification of the ghost image (smaller means brighter), the amount of aberration in the ghost image (less aberration means a more pronounced ghost), and the efficiency of the lens coatings.

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Brian Caldwell

Thanks Brian, but do you endorse my stumbling attempt to try and describe what is happening or disagree with it?

I can understand the concept of bounced light of the sensor being bounced back as a ghost image off a lens element through the aperture which gives it the circular appearance. I think you once said that you could count the aperture blades in one example.

Do you say that the ghost can be cured by efficient lens coatings?  Would the aberration come from the sensor reflection through the adapter or in the lens itself? I mistakenly thought that the ghost image could be designed out by dispersing the rays.  But a little knowledge is a dangerous thing and I thought it truly heroic if such an adapter could be designed to deal with any old lens it might be connected to.  I can understand if you are unable to know in more than a general manner.

Alternatively the aberrations might be caused in the first pass as secondary rays and these concentrated into a second ghost shape in the form of a circle.  Sorry, I am just after an understanding of the cause.

You will remember that I hooked up a Zykkor glassed FD to EF adapter which works reasonably well in its intended use in combination with my Speed Booster and had some horrible hot spot problems using two different lenses. But I fully realised that this was coming from the Zykkor and not the Speed Booster and it was unfair to expect the Speed Booster to cope with anything thrown at it.  I have no real reason to use this combination which gave the problem in an unusual situation involving fluorescent light at night it was also only really bad at smallest aperture (at night).  In this case the spot was very bright and white at the smallest lens aperture and visibly diffused in a regular manner eventually into the full image circle.

I did not persist as I had no need to do so, but I should try this again in more normal light conditions.

None of this has diminished my respect for the Metabones product.

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Tom Caldwell

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