EBL, silly question

[Sparrow]

EBL, silly question

Why is a longer EBL better?
In practise if you look through a Contax finder or an R2’s you see the same thing.

 

[FrankS]

The longer/larger the EBL is, the more accurate the focussing is. This is important with long lenses, close up, and wide apertures, all creating shallow DOF conditions. For example, a Leica CL would have a hard time accurately focussing a Noctilux lens.

RF focussing is based on triangulation. The longer the base of the triangle, the more precise the result.

 

[Huck Finn]

EBL has 2 factors. One is the magnification. The higher the magnification, the easier it is to see. The second is the length of the rangefinder's base line itself, meaning that the 2 windows, which are receiving separate images are physically farther apart. This means that each adjustment of the focus collar is moving the images a longer distance than it would with a rangefinder of shorter base line. Therefore, it is easier to make fine adjustments with shorter movements of the focus collar & therefore to be more precise in choosing a setting.

 

[Sparrow]

The longer/larger the EBL is, the more accurate the focussing is. This is important with long lenses, close up, and wide apertures, all creating shallow DOF conditions. For example, a Leica CL would have a hard time accurately focussing a Noctilux lens.

RF focussing is based on triangulation. The longer the base of the triangle, the more precise the result.

Hi Frank, yes that’s what I’ve always understood to be the case, what I’m asking is how is that manifested by what we see in the finder

 

[FrankS]

With a camera whose EBL is not up to the precision required by a situation with very shallow DOF, the camera's RF may be showing you that the image is in focus, when really it's not. RF accuracy also requires tight mechanical tolerances and lack of "slop" in the RF system. With shorter EBL cameras, the mechanics simply can't be tight enough for it to be accurate enough to deal with shallow DOF.

 

[Sparrow]

EBL has 2 factors. One is the magnification. The higher the magnification, the easier it is to see. The second is the length of the rangefinder's base line itself, meaning that the 2 windows, which are receiving separate images are physically farther apart. This means that each adjustment of the focus collar is moving the images a longer distance than it would with a rangefinder of shorter base line. Therefore, it is easier to make fine adjustments with shorter movements of the focus collar & therefore to be more precise in choosing a setting.

I can see higher magnification is better, but you can have high magnification and still have a short EBL
The lens moves through the same number of degrees from max to minimum regardless of the EBL

 

[Sparrow]

With a camera whose EBL is not up to the precision required by a situation with very shallow DOF, the camera's RF may be showing you that the image is in focus, when really it's not. RF accuracy also requires tight mechanical tolerances and lack of "slop" in the RF system. With shorter EBL cameras, the mechanics simply can't be tight enough for it to be accurate enough to deal with shallow DOF.

Yes, that one stands up, so the only advantage is it allows for tolerances in the manufacturing process? they can build in some backlash without it having too big an effect

 

[laptoprob]

In other words:
With a bigger EBL the baselength of the triangle for distance measuring is bigger, therefore the fcussing is more precise.
In the finder you might only notice a bigger difference in the two images: perspective is slightly different.
Higher magnification is like reading with a loupe. You can definetly see more detail.

 

[ferider]

Yes, that one stands up, so the only advantage is it allows for tolerances in the manufacturing process? they can build in some backlash without it having too big an effect

I would agree that this and the magnification (to prevent "human backlash") are the most important factors.

EBL by itself is less important.

In other words a shorter EBL camera can still focus accurately a longer lens when the tolerances
are tighter. This is obvious from Canon users using a 100/2 successfully, for instance.

Another thing that is often mistaken, is that the size of the glass, as impressive as it might
be is not so important. For example: an expensive Noctilux is as hard or even easier to focus than a cheap
Jupiter 85/2.

Roland.

 

[BillBingham2]

Within manufacturing (including quality control) is one area, but also in long term use. If you drive the manufacturing/QC to be so tight a small EBL might work with a longer faster lens, you run the risk of keeping it properly adjusted in the field. At that level hard bumps migtht take you out of allignment.

B2 (;->

 

[40oz]

Yes, that one stands up, so the only advantage is it allows for tolerances in the manufacturing process? they can build in some backlash without it having too big an effect

I would say there is more to it than that. Consider that the image you see in the rangefinder spot is coming from the rangefinder window. The farther that is from the viewfinder, the more obvious the difference between "in focus" and out of focus. We are all relying on our eyes to tell us if the images are lined up. A rangefinder mechanism that makes that easier is going to make you more accurate with focus.

Consider a target 30 feet away. When the rangefinder is focused at 3 feet, the rf spot is obviously not lined up with the finder image. The longer the effective rangefinder base, the farther apart the edges of the rf spot and finder image. You turn the lens barrel to line up the images, and as you get closer, the difference between the rf spot and finder image decreases. If you are focussed to 29 feet with a longer effective rf base, you will see a greater difference between the rf spot and finder image than with a shorter effective base rf. It has nothing to do with build tolerance or slop at this point. It has to do with the amount of difference between in focus and slightly out of focus. A higher magnification finder will magnify the difference between the rf and finder, similar to what you would see with a rf mechanism with a longer base.

The practical difference between a longer base and a shorter base with higher magnification is tolerance and "slop." With a short base, the rf mechanism must have less slop, as each minute movement of the lens has to translate to precise even more minute movement of the rf mechanism. The angle from the rf window to the target is very close to the angle from the finder to target, so from minimum distance to infinity, there is very little movement of the mechanism. Even a tiny amount of play in the mechanism can make the image falsely appear to be in focus or out of focus. A longer base mechanism requires a gross amount of play, comparatively, for a similar error. With a longer base rf mechanism, the rf window is further from the finder window and it moves through a wider arc from minimum focus to infinity. So if both mechanisms are built to the same tolerance, the longer base will simply be more accurate. At this point, the magnification of the finder is irrelevant since the error is in the mechanism, not in the user's vision. Magnification is only helpful for identifying the difference between the rf spot and finder image. It does nothing when it comes to rf error.

I would always prefer a longer real base than a shorter base with high magnification for focusing a fast longer lens. But it isn't always necessary. I do prefer the longer effective base because it is easier to see focus/out of focus.

 

[Paul T.]

The advantage to having a longer baselength, and lower magnification, is that you can also have the framelines for wider lenses in the viewfinder. This is one of the advantages of the Zeiss Ikon, which had nearly as long an EBL as the M3, but also has easy--to-see 28mm framelines, like a .58 mag M6.

I suspect that the tolerances involved in using a long RF baselenght are more accurate too, as suggested. Altho my Kiev 2 has a pretty dim RF patch, I've always found it easy to focus, it's much easier to see the second image sweep into alignment than on, say, my CL...