Legacy lens distance scale on m4/3

[jamesmck]

In using a legacy lens (Leica thread mount) with distance markings on a m4/3 camera, I assume that the markings will not be accurate due to the thickness of the adapter. If this is so, might there be any way to calculate how much of a difference the adapter will make? Would this difference factor be constant over the range of actual distances? The lens in question is a Snapshot Skopar 25mm f4, and the adapter is a Fotasy from EBay.

 

[Roger Hicks]

Dear James,

They should be accurate. It wouldn't focus on infinity otherwise.

Cheers,

R.

 

[retinax]

Hey, it might however focus beyond infinity, in that case the markings would be off. The difference factor would be constant with regard to the movement of the focus ring, not to the distance. So one could theoretically put another index on it to read the distance scale by. More elegantly, one could move the distance scale and infinity stop on the lens if it is to be used only on that camera. Ok, maybe not an option for this lens with click-stopped focus. Anyway if the adapter is too far off, looking for a differnt one might be better if you need to use the scale.

 

[Dwig]

Hey, it might however focus beyond infinity, in that case the markings would be off. ...

Correct.

If infinity is off, the distance scale will still be correct BUT the index mark won't. The simple fix is to manually focus to infinity and then put a new index mark across from the infinity mark on the distance scale.

 

[peterm1]

I must admit I had never considered this. I presume you are only talking of those adapters that allow a lens to focus past infinity?

But in any case for me its a bit academic as I seldom if ever scale focus - I use the LCD or more often the viewfinder to focus by eye, also using focus peaking as a guide if the camera has it.

 

[jamesmck]

Legacy lens distance scale on m4/3

Thanks, everyone.

 

[plummerl]

All the adapter is doing is putting the focus point at the exact legacy distance from the sensor. By doing that, the distance scale should maintain accuracy. As far as infinity, I have a couple of lenses that do diverge at infinity, but they are ones that exhibit the same behavior with the legacy system. In particular, my Oly OM 500mm mirror lens. In this case, I believe that it is to allow for thermal effects.

 

[kb244]

Usually I don't see the distance being "infinity and beyond" on lens other than screw mount lens like LTM/L39. In which case Fotodiox ones I've used tend to go a tad beyond infinity making it unusable for my Canon Serenar 35/2.8 (something at 50ft, shows as 6 feet on the lens), where as a fotasy works fine with that.

I haven't seen much of an issue with non-screw mounts, as they should be machined more precisely.

The depth of field scale though will be a bit more conservative as the camera's sensor size will record a greater DoF than what's shown on the lens.

 

[jamesmck]

The depth of field scale though will be a bit more conservative as the camera's sensor size will record a greater DoF than what's shown on the lens.

Thanks. I am aware that the DOF scale will differ from that shown on the lens.

 

[shimokita]

The depth of field scale though will be a bit more conservative as the camera's sensor size will record a greater DoF than what's shown on the lens.

I want to check my logic... it's my understanding (and I could be wrong) that the DoF markings on a FF lens are "incorrect" when used with a crop sensor camera because we normalize the CoC (e.g. 0.03 for FF and 0.019 for a 1.6x crop) based on an "industry standard" print size, viewing distance, and corrected eyesight.

 

[Scrambler]

Pretty much. Even the DoF marking originally were based on broad assumptions, and if critical focussing on an enlargement were considered the DoF would not be as marked. Similarly nearly everthing is in focus on a small on-screen image, even though everything is out of focus when you enlarge it.

Even on film most people say use 1-2 marking in (f4 instead of the f8 you are using, for example).

 

[Dwig]

... As far as infinity, I have a couple of lenses that do diverge at infinity, but they are ones that exhibit the same behavior with the legacy system. In particular, my Oly OM 500mm mirror lens. In this case, I believe that it is to allow for thermal effects.

Many lenses need to focus past infinity at their reference "normal" temperature so that they will still be able to reach infinity at other temperatures at which they are expected to be used. Mirror lenses are one type which are universally built to focus past infinity at 20deg C. Nikon's classic ED glass long teles also need the extra travel so that will be usable at a range of temps.

Most of the moderately priced to inexpensive lens mount adapters are spec'd to a target extension somewhat shy of the theoretically correct amount. This seems to be done to allow looser machining tolerances (read: less expensive machining and QC) without risking having a sample that fails to allow lenses to focus to infinity.

 

[Dante_Stella]

You can address the "too thin" problem with some self-adhesive copper foil tape under the flange. It's about 0.05mm, if I recall, and 1-2 layers should fix most adapters.