Where have all the Focal-reducers gone? (with apologies)

Started Feb 3, 2014 | Discussions thread
OP Tom Caldwell Forum Pro • Posts: 34,901
Re: speaking of blue dots...

forpetessake wrote:

Tom Caldwell wrote:

greypixelz wrote:

Hi Tom,

I bought a second focal reducer, this time the RJ variety, for EOS mount. The first one I got was the Mitakon for Nikon F mount.

I thought the blue dot would be less visible on this second adapter, however, it seems to be slightly worse.

Thanks for your addition to the knowledge pool, I presume that it was the same lens set?

Having looked at some of those Chinese clones, I suspect they all utilize the same optical assembly from the same supplier, maybe it's Mitakon.

As far as I know Mitakon is the only serious lens manufacturer in theis field in China, but I am surely wrong.

I recollect others complaining about the RJ and Nikon lenses specifically. If the blue dots appear on both adapters it may indicate a similar lens element configuration but also highlight that Nikon company was not over-concerned with ghost reflections off film stock when the lenses were first made. Presumably the reflections off film were so inconsequential the the Nikon designers either did not notice them or decided to ignore them.

It has nothing to do with the lenses, only with the adapters. As I demonstrated before, there is ablue spot even without any lens attached. And since I switched to Metabones, all my Nikon lenses work fine without any blue spots.

This is interesting - namesake Brian said that he could could count the aperture blades in a ghost reflection.  My (layman) reading on the subject says that this can only happen if a sensor re-reflection goes back through the aperture bounce off something and be reflected as a ghost on the image.  The aperture blades can be seen on the defined roundish circle that shows up.  If the reflection is from behind the aperture then the blue spot becomes a smudge and could posisbly be a reflection from anywhere behind the aperture  in the lens to from within the  focal reducer itself.

Not saying that you are wrong  in any way, just commenting.

It also seems that ghost reflections can be dispersed by good lens design but tthis might be much harder when working with multiple lens sources.  Therefore it might be easier to do this for a single range of lenses such as Canon EF where the manufacturer has already taken considerable pains to try and eliminate ghost reflections than for (say) M42 mount where there are a host of manufactured sources.  Canon in its own literature talk about these ghost reflections on their telephoto lenses brag a little that they use a meniscus "protector" front element to obviate  the problem and carefully suggest that those who find their images so afflicted should try removing their lens filters which have a flat glass reflecting surface.

All kudos to Brian for designing focal reducer elements that can disperse ghost re-reflections.  If the Chinesee versions have not fully figured out how to do this then a least they mostly work fine even if the "blue spot" does happpen in some situations.  I could only force a blue smudge with Takumar lenses by severely trying to make this happen.  Presumably the Takumars are themself very well sorted out to resolve ghost reflections and the smudge I forced may well have been from the focal reducer adapter I was using.  But blue spots and smudges have not worried me much in practice.

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Tom Caldwell

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