On Using Focal Reducers...

[Norten]

I am using a "normal" DSLR as my main camera, but I also play around with a mirrorless, so this issue is only so I can get a better understanding of the influence of focal reducers on lenses.
I am using a Sony NEX 5R with a variety of legacy lenses and seem to have ended up with a lot of PK mount glassware. So after reading an article or two and some forums, I got myself a focal reducer to experiment with. It's not a high end product so I wasn't expecting miracles optically, but suffice to say that I am moderately surprised by the performance.
Apart from reading and experiencing the "wider" angles and wider apertures which is great for evening photography in the city there is something that I need to get my head around.
From the gradations on the lens barrel for focus distances it became aparent that these are now mere numbers and not relevant to anything. Futhermore it is also variable on zoom lenses. There is a point (usually around mid-zoom range) where it is more or less in line with the markings but at the wide and tele end it all over the place.
Also the nearest focus point to the camera is much further from the lens than usual.
Is this to be expeced or may this just be because the focal reducer I have is an entry level (generic?) unit?

 

[fmw]

It is to be expected with any add-on lens because of the laws of optics.

 

[Norten]

Dear fmw.

I gathered as much, but my photography books deal with only just that... photography, and not much on the physics. So I was hoping for a more in depth expansion to understand how this is happening?

 

[fmw]

I'm not a physicist. Minimum focal point is part of the lens design. With an add on lens you have changed the effective focal length without changing the ability of internal elements to move more to compensate. This is a minor problem, however, compared to the reduced image quality these add ons deliver.

 

[petrochemist]

I don't think mine has any effect on the focusing distance scale, any more than most teleconverters do. Well designed teleconverters & focal reducers sit between the lens & the sensor & are made to a precise length which takes into account the change in focal length. A TC leaves the lens further from the sensor, while with a reducer the lens has to be mounted closer - it will be shorter than a basic adapter for the same lens type.

If your reducer doesn't focus at the listed distance it suggests the length is not quite right for the power of the optics. It may be a deliberate decision during the design to allow for manufacturing variation. Users get annoyed if their adapter can't focus to infinity, so many adapters are made slightly short to ensure the lens will reach infinity.

Unlike TCs (or front mounted wise angle adapters) the focal reducer effectively improves image quality, by compressing the lenses image circle into a smaller area.