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

Started 10 months ago | Discussions thread
Tom Caldwell
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Re: speaking of blue dots...
In reply to greypixelz, 10 months ago

greypixelz wrote:

Hi Tom and co.,

With the Mitakon Nikon to NEX focal reducer I had seen blue spots with the 1.8/50E, a lens know to be less capable than the regular AI-S, and the 2/40Voigt. However, with the 2/28AI-S not so much, in fact, none what so ever.

With the RJ EOS to NEX focal reducer, onto which I mount another Fujica M42 to EOS ring, I had only tried the 1.4/50EBC Fujinon on it and had seen biggerand more colorful blue spots than what I was used to with the Mitakon focal reducer.

Another thing is that with the Mitakon, the blue spot issue only became apparent starting with f/5.6-8. On the RJ focal reducer, however, the blue spots were visible starting with 1.4 on the Fujinon (truth be told, I was framing directly into the light bulb).

I will have to try some wide angles to see whether it is more pronounced on the RJ compared to the Mitakon.

Nevertheless, I am clueless as to what actually causes this behavior. My inital and best guess was the coatings or rather lack of in the focal reducer optical unit.

I am not an optical expert, just someone with a curious mind, but I will explain what I believe is happening.

Sensors are much more reflective than film stock. Light bouncing back off the sensor into the focal reducer and lens can be re-reflected off one of the lens surfaces back on to the sensor as a round object if it came from before the aperture or as an ill defined smudge otherwise. This is more likely to happen if the reflecting surface is flat. Apparently even a lens filter can cause this.

I believe that it might be possible to design a focal reducer that could disperse this re-reflection but it would be very hard to universally cope with the vast number of lenses from many manufacturers. Especially since some manufacturers may have decided that the ghost reflections off film stock were inconsequential. Add to this the propensity for focal reducers to enhance both good and bad points in any lens.

Note that it seems possible that a blue smudge can come from a re-reflection from inside the focal reducer itself, but in this case the smudges generated should be consistently generated across all lenses used with the same focal reducer.

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Tom Caldwell

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