Focusing RF vs SLR - why is it "harder" using fast lenses?

[PentHassyKon]

I've seen it noted here on the forum "..... it is harder to focus the ____ (Nokton, Nocti, Summilux, etc...) on the CL, or .58 M6, or Bessa, etc...."

I'm relatively new to RF, being weaned on SLRs (Petri FT, OM10, Nikon 8008, Hassy 503, now Pentax k200d).

Reason I ask this question is that with an SLR, one can "see" the difficulty in focusing a fast lens - I have an f1.2 for my Pentax and yes, although the aperture is larger, hence the finder is brighter it is relatively harder to get focus correct with it.
However, I'm having a hard time understanding the statement of why it would be harder to focus say the Nocti on a CL considering the rangefinder mechanism is somewhat independent of what the lens or one's eye "sees".

Having just bought a CL, and being used to SLR lenses with f1.4 or my f1.2 Pentax-A lens, I'm interested in understanding why this statement. I understand that due to the CL's shorter EBL that focus will not be spot-on especially considering the narrow depth of field at a large aperture. Is this due to the rangefinder coupling and short EBL that a small twist of the lens could equate to a "rough" translation of the lens groups? Then maybe the statement shouldn't be " harder to focus" but rather - " narrow depth of field will require smaller finer twist of the lens focusing ring". Or am I missing something?

 

[Doug]

With the SLR, focus ease/accuracy depends on the quality of the screen and the speed of the lens; a fast f/1.2 lens focused wide open gives better focus accuracy at, say f/4, than an f/2.8 lens.

Depth of field wide open with a fast lens always has its difficulties, due to movement, picking the right focus point, etc.

With an RF, focus accuracy as you note is not lens dependent so much as on the mechanical RF in the body. A fast lens shot wide open tests the collimation of the lens to the body and your eye's ability to distinguish when the overlapping RF images in the viewfinder are 100% aligned.

Further, the distance between the centers of the VF window and the smaller RF window affects how much movement there is of the RF spot in relation to the fixed VF view for a given focus difference. A long baseline means more movement of the spot image, easing and speeding focusing. That's where the CL suffers vs an M body or Zeiss Ikon for instance. I think you've got this aspect ok in your last paragraph.

FWIW, I too had a Petriflex V long ago, later a Nikon FG, still have an OM-G, many Pentaxes including a K20D.... no CL but two CLE's 🙂

 

[Chris101]

It is not more difficult to focus fast lenses with a range finder per se, but fast lenses are more prone to error by virtue of their thinner depth of field. If you focus, then recompose with a fast lens, you need to be sure the place you focussed on is exactly the same distance from the camera as in the intended plane of focus, and that small movements do not change the focus.

When using a stopped down lens, you can make focus errors with impunity by comparison.

 

[Pickett Wilson]

Assuming the SLR hasn't been damaged in such a way that the lens plane and film plane are no longer parallel (for example), the SLR depends more on visual acuity than mechanical precision to to properly focus the image. And the faster the lens, the easier it is to determine focus with the lens wide open on an SLR. The RF depends on a complex rangefinder system and coupling, precise lens assembly, along with EBL, to accurately focus a lens. With the SLR, if it looks in focus, it is in focus. The RF requires a bit of faith that the RF mechanism and the lens are operating properly, because unless it's a fairly gross error in focusing that makes you realize something is wrong mechanically, you aren't going to see it until you get your photos back.

Until the advent of AF, the design gave the RF an advantage over the SLR with wide lenses in low light because it was harder to determine precise focus with the SLR as the apparent DOF made everything look in focus. Focusing aids (split image in the finder) helped, but still didn't make up the difference.

There are just a lot more factors that determine focusing accuracy (or inaccuracy) with a rangefinder than an SLR, especially with longer or faster lenses.

 

[PentHassyKon]

With the SLR, focus ease/accuracy depends on the quality of the screen and the speed of the lens; .... snip .....

With an RF, focus accuracy as you note is not lens dependent so much as on the mechanical RF in the body. A fast lens shot wide open tests the collimation of the lens to the body and your eye's ability to distinguish when the overlapping RF images in the viewfinder are 100% aligned.



FWIW, I too had a Petriflex V long ago, later a Nikon FG, still have an OM-G, many Pentaxes including a K20D.... no CL but two CLE's 🙂

snip..... the SLR depends more on visual acuity than mechanical precision to to properly focus the image. And the faster the lens, the easier it is to determine focus with the lens wide open on an SLR......

Until the advent of AF, the design gave the RF an advantage over the SLR with wide lenses in low light because it was harder to determine precise focus with the SLR as the apparent DOF made everything look in focus. Focusing aids (split image in the finder) helped, but still didn't make up the difference.

.

hmmm....
collimation and AF.
I'm curious- do the current AF cameras use a similar concept to RF's to determine focus?

I've never really been able to determine focus accurately on my SLR w/o use of a split image - isn't this the same concept used on an RF?

Doug- do you use a split image prism (like Katz Eye) in your K20d when using MF (I assume you use MF lenses). Anyways, with my k200d, I depend mostly on the camera's AF sensors to tell me when I have focus but it is not easy and definitely not fast. I've been meaning to install a split image screen on it but have not had the chance to do so yet.

Just starting to use RF's more and I find using the RF patch much easier to use compared to my k200d's AF sensors in MF.

 

[Doug]

Doug- do you use a split image prism (like Katz Eye) in your K20d when using MF (I assume you use MF lenses). Anyways, with my k200d, I depend mostly on the camera's AF sensors to tell me when I have focus but it is not easy and definitely not fast. I've been meaning to install a split image screen on it but have not had the chance to do so yet.

Just starting to use RF's more and I find using the RF patch much easier to use compared to my k200d's AF sensors in MF.

Hi... I'm satisfied with the green in-focus indicator in my Pentax with manual focus lenses. And I'm concerned about reported loss of metering accuracy with aftermarket screens.

RF cameras with sharply-defined focus spots, like Leicas, can be focused using that sharp top/bottom edge in a split-image method. The split-prism in the SLR focusing screen has a similar visual effect but the effective base-length there is the apparent diameter of the lens aperture, a shorter distance than most modern RFs.

 

[ZeissFan]

The issue is two-fold.

Some of it has to do with physics -- that is, the rangefinder mirrors/prisms are too close together. Couple this with a reduction factor in the viewfinder, and it becomes too difficult.

An extreme example would be to take a pair of binoculars, turn then around and then use as the eyepiece for your SLR viewfinder. As I said, that's taking things too far, but it gives you an idea of why a reduced viewfinder makes it tougher to focus accurately.

 

[peterm1]

I assume this refers to focussing when shooting wide open. If so, is it not just the case that with a shallower depth of field focus is more critical. Most cameras struggle to provide the focussing accuracy - even some rangefinders. But if you stop the lens down....different story of course. In general though I find it easier to focus with a split image RF (and with a leica M camera) than with any other kind of camera. Even an electronic pseudo RF like the Panasonic L1 (Leica DigiLux 3) - where I find that even though it has electronic focus confirmation in the viewfinder when using manual lenses, it is still very hit and miss to get correct focus especially when shooting wide open.

 

[splitimageview]

I've seen it noted here on the forum "..... it is harder to focus the ____ (Nokton, Nocti, Summilux, etc...) on the CL, or .58 M6, or Bessa, etc...."

...

However, I'm having a hard time understanding the statement of why it would be harder to focus say the Nocti on a CL considering the rangefinder mechanism is somewhat independent of what the lens or one's eye "sees".

Cameras like the CL have short rangefinder base length which makes them ok for slow lenses with more DOF (which is why the 90/4 was ok but not a 90/2.8). For more info visit this thread, although some of the links are now broken...

http://www.rangefinderforum.com/forums/showthread.php?t=55500

 

[ampguy]

I think the SLR might have an advantage

I think the SLR might have an advantage

In my experience. But I have read some good arguments for RF's, when aligned and collimated, etc. being more accurate within certain focal lengths, I think this topic is referenced in the Gunther Leica M book.

For differences in focusing accuracy among RF's, there is a useful table by Roland and LCT here:

http://www.rangefinderforum.com/forums/showthread.php?t=55500&page=3

My experience with say 2 typical camera systems give slight preference to the SLR for focusing wide open and min. focusing distance with a lens with focus shift:

Nikon F3 with 43-86 zoom has focus shift, but you can easily see it and correct for it you change apertures.

My Yashica GSN has slight focus shift between 1.7 and 2.0. I can't see it in the RF, only after developing the film can I see it, and there aren't any collimation issues since this is a fixed lens RF.

I've seen it noted here on the forum "..... it is harder to focus the ____ (Nokton, Nocti, Summilux, etc...) on the CL, or .58 M6, or Bessa, etc...."

I'm relatively new to RF, being weaned on SLRs (Petri FT, OM10, Nikon 8008, Hassy 503, now Pentax k200d).

Reason I ask this question is that with an SLR, one can "see" the difficulty in focusing a fast lens - I have an f1.2 for my Pentax and yes, although the aperture is larger, hence the finder is brighter it is relatively harder to get focus correct with it.
However, I'm having a hard time understanding the statement of why it would be harder to focus say the Nocti on a CL considering the rangefinder mechanism is somewhat independent of what the lens or one's eye "sees".

Having just bought a CL, and being used to SLR lenses with f1.4 or my f1.2 Pentax-A lens, I'm interested in understanding why this statement. I understand that due to the CL's shorter EBL that focus will not be spot-on especially considering the narrow depth of field at a large aperture. Is this due to the rangefinder coupling and short EBL that a small twist of the lens could equate to a "rough" translation of the lens groups? Then maybe the statement shouldn't be " harder to focus" but rather - " narrow depth of field will require smaller finer twist of the lens focusing ring". Or am I missing something?

 

[PentHassyKon]

http://www.rangefinderforum.com/forums/showthread.php?t=55500&page=3

Looks like I have to read that thread further. I note an attached chart in post 62 in the above thread along with a link to a spreadsheet which unfortunately is no longer being supported (..."This site has been suspended").
I have further questions but probably best I post in the RF accuracy thread noted.

 

[splitimageview]

Here is the spreadsheet although the Red/Green colors did not translate into google docs.

http://bit.ly/8jtpR2

 

[BillBingham2]

In the Leica School of Photography my father went to years back Leica said that RFs have the advantage of focusing wide angles to normal lenses. Normal is where it's about the same and the advantage goes to SLRs.

As stated above the CL is a bit short EBL when you look at it in comparison to say a Nikon S3 or a Leica M3 or an M2. The CL was designed for compactness. One issue some older RFs have is that the VF and/or RF patch dims over time. This can impact your ability to focus quickly. While not as sturdy in many was as a CL the Bessa R product line has a wonderfully bright viewfinder and snappy rangefinder. While it to is somewhat hampered by a shorter EBL (when compared to other RFs) it works very well.

You might take a look here for a bit too much information on EBLs and RF.

http://www.imx.nl/photo/technique/technique/page49.html

another interesting chart can be found here:

http://www.cameraquest.com/voigtchart2.htm

You might note that the R4 has a very short EBL but this is fine as it was developed to be used with 21-35mm lenses and stretch to a 50mm.

Hope this helps.

B2 (;->

 

[ferider]

For me personally, fast manual lenses are easier to focus on RFs for 50mm and shorter (on the M3 in particular), and on SLRs (using split image screens) when longer. Of course that depends on the RF, collimation, etc. A 90/2 still works well on the M3, but I bracket short focus distance photos, if I can.

All my current SLRs (OM, Nikon, Hasselblad) and RFs (Leicas) came to me used. In my experience there are as often mirror alignment/accuracy issues with used SLRs, as there are RF alignment/accuracy issues with used RFs. Just for SLRs it's less obvious, and therefore people complain less.

Roland.

 

[rjbuzzclick]

hmmm....

Doug- do you use a split image prism (like Katz Eye) in your K20d when using MF (I assume you use MF lenses). Anyways, with my k200d, I depend mostly on the camera's AF sensors to tell me when I have focus but it is not easy and definitely not fast. I've been meaning to install a split image screen on it but have not had the chance to do so yet.

Sorry for the slight OT tangent here, but I wanted to respond to PentHassyKon: I installed the Katz Eye split image focus screen, with their ultra-bright treatment or whatever it's called, in my K110D earlier this year and it made a big difference for me in focus accuracy. I mostly use shorter MF lenses, including a 50mm f1.4. One issue, it has that's worth keeping in mind is that he prism does darken a bit with longer zooms that start at f4,/f5.6 or so, but it's still usable. It gets worse at smaller apertures. The screen also has a microprism collar around the split image which helps in low light too. There is a cheaper Chinese screen out there too, but I didn't investigate it much. FWIW, despite the cost, I'm happy with the screen.

I wanted to add that I've not noticed any change in metering accuracy with regards to the Katz-Eye screen, but I do notice that my meter is about one stop off when using my one AUTO manual focus lens in auto aperture mode (a Pentax-A 50mm f1.7). It seems I have to set the camera to over expose a stop or so. This was the case before I installed the Katz-Eye. I think it has to do with the older lenses being designed for a larger frame than the 1.6 crop factor sensor in my K110D. I use the stop-down metering method with all of my manual lenses and get accurate results with no compensation. Likewise, I also get accurate meter results when using the DA kit lens with the screen and no compensation.

 

[PentHassyKon]

You might take a look here for a bit too much information on EBLs and RF.

http://www.imx.nl/photo/technique/technique/page49.html

...............

Hope this helps.

B2 (;->

Oh my!, My head hurts! Interesting read, one that I should do with dedicated attention to it in order to grasp and apply the knowledge presented.

..... There is a cheaper Chinese screen out there too, but I didn't investigate it much. FWIW, despite the cost, I'm happy with the screen.

I wanted to add that I've not noticed any change in metering accuracy with regards to the Katz-Eye screen, but I do notice that my meter is about one stop off when using my one AUTO manual focus lens in auto aperture mode (a Pentax-A 50mm f1.7). ............

rjbuzzclick- thx for the info. I understand that the screen could cause some darkening with some of the slower lenses. Probably won't be an issue for me as my MF lenses are quite fast, while my AF lenses are the slow ones (DA 55-300 being slowest at 300mm).