R2a/3a EBL questions

[Way]

Hi all,

Another new member to this great forum!
I'm currently using a .72 MP with a 50/1.4 and the 90/2 and would like to get a VC for the AE. Thinking of getting an R2a or 3a to use with a 90 Summicron. I saw that in an earlier thread that focusing accurately might be difficult to be at f2 or so. Could someone explain this? I guess I don't quite understand what the EBL does. I thought that the 1:1 vf of the R3a would make it easier to focus the 90?

Thanks!
Way

 

[iMacfan]

The EBL comes from the multiplication of the VF magnification and the distance between the VF and RF patch windows. These are quite close together on the Bessa, so your MP still wins out - in fact even the .58 has a bit longer EBL than the R3a.

David

 

[Huck Finn]

Way, the physical length of teh base line on the MP is .69 meters - almost twice as long as the 37 mm long base line on the R3A. Since the 2 images in the rangefinder are converging over a longer physical distance, it is easier to be precise with the longer base line, all other things being equal, i.e. you can fine tune your focus more easily & quickly because you're not converging the images as far with each little turn of the lens on the longer base line. Higher magnification doesn't change this. Higher magnification means you can see what you're doing better, you can see more accurately as the 2 images converge, see as 2 parts are aligned. It expands the limits of the human eye. "Effective base length" is just a convenient short hand for expressing the comparative capabilities of different combinations of baseline length & viewfinder magnification.

You can read more about this at www.imx.nl/

Click "Leica Pages," then "Engineering." then "Rangefinder Accuracy."

You should be able to focus the 90 Summicron just fine on the F3A at f/2.8 & narrower apertures. However, as you get closer to f/2, it will be increasingly more difficult. Even f/2.8 will be difficult on the R2A, while f/4 will be no problem. In either case, the MP will be easier & more precise at all apertures. It's just that as you increase the depth of field with narrower apertures, precision becomes less important.

 

[Way]

Thank you for the great info. Erwin's write up is very informative. Since I will be doing mostly available, low-light work, perhaps it would be best to use the 90 exclusively on the MP. Tough decision!

Way