Maximum Extension: Schneider Kreuznach APO Componon 40/2.8 Hm

Given my measuring is subject to a degree of error I would guess the SK 40/2.8 has about the same working distance( at high magnification).




This is probably at the heart of my not getting my head around the complex technique you and rmpsrmps are using.

First to answer your question( followed by an elaboration of that last sentence): I use focus peaking in combination with the cameras manual focus assist( 7X-12X magnification of part of the image), as a double check. It is the best practical tool available to me. Even without the manual focus assist I can still get an indication of the degree of curvature( or tilt) as at this magnification level if I use an objective with a curved field of focus only the center of the subject, when in focus will be high lighted if I then adjust the focus( by moving the camera and bellows ) slightly either I loose the focus highlighting or I see a ring of focus highlighting( depends on the direction of movement and the shape of the curvature). If part of the system is tilted( either the bellows camera... or the target or a combination of the two) I see a line of focus highlighting which moves across the target as I change the focus.

To elaborate on my not getting my head around the method you use. I understand the concept behind it, but have great difficulty with the practical application. As I imagine it, it is dependent on an extremely well tuned macro rig. My rig is vibration resistant, sits on a very stable table which has been "accurately" leveled. Likewise the rig has been leveled. I can also adjust the positioning of the camera-bellows set up, in the X and Y axis, for level. the big weakness is that I only use a spirit level for this. With all that effort and knowledge of how my rig operates(including its faults)I still doubt that I have the necessary accuracy in the set up to trust the method of measuring in the manner outlined. Simply put: for the time, effort and required equipment, the margin of error is still so high that in my mind it would provide no practical advantage over the method I use. Not every one has the ability to adjust the focus in the micron range as we each appear to have. Although I have that ability it just seems too much work for indicative results.

There are issues with focus peaking... it relies on contrast detection so it only gives an indication of where the focus is.. however I don't need optical lab accuracy. I find it hard to see how any one using less than 20X magnification would need optical lab levels of accuracy.

For those wondering what focus peaking is:



Better tutorial



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Yes it is simple. But why do it? Other than putting a figure to the curvature what is the practical advantage you are gaining? (The two questions are the same question)

I know from my experience of needing to use huge numbers of photographs to get a good stack when using my cheap 10X objective the lack of a flat field is annoying. The SK 40/2.8 requires less than a third of the photographs the 10X objective requires to produce a sharp final image. All I need to know is which of my lenses are suited for each purpose. I now know the SK has a much flatter field and if necessary can do a better job than the 10X objective that is now going to gather dust on the shelf. For me that is all I need to know ( with my current understanding).

It may well be that a Mitutoyo has a much flatter field, but with a second hand SK40'/2.8 at around US$400.00 against a Mitutoyo M plan APO 10X plus tube lens and associated adapters dissolving most of €1000.00 I am quite happy taking a few extra shots in a stack. As I understand it stacking makes a flat field lens a "nice to have"

Edit. Yes I could buy a second hand Mitutoyo however you have to be an expert to tell whether the objective is OK and more often than not( from what I have read) second hand mitutoyo objectives can be less than satisfactory performers( it doesn't take much to destroy them)

I see we are looking at two different types of objectives. I have absolutely no relevant experience with microscope objects. I have three very cheap generic microscope objectives, and really get nothing out of them. I have much more experience with Legacy lenses and a small range of enlarger lenses so my comments really relate to that experience. my best lenses generally cast a much bigger image circle than my sensor... so Flat field is a relative term as the sensor( even full frame) is at lower magnifications not using light from the edges of the lens optics. The standard image circle of the 40/2.8 is just over 43mm in diameter. I am happy to be corrected as I could be wrong==>I believe the vignetting that starts at just over 9.5 times and is clearly evident at 12 times is due to the length of the extension.

I can really see the need to assess curvature is greater for microscope objectives.

I had the same experience with a Schnieder Kreuznach componon S 210/5.6. which is designed for producing large format prints I only noticed the vignetting with the 210 when I migrated to a full frame camera.

I have already disassembled the combined bellows but given each bellows has a maximum extension of 210 mm and I added in two extension ring sets plus an adapter of 40mm I would guesstimate a total extension of 500mm-600mm.

What I believe is happening is the long extension is acting like an aperture and restricting the full image circle, effectively decreasing it. However the image that hits the sensor is only from the center of the lens not the edges. the center of lenses is always the best performing (optically) and tends to produce a flatter field of focus. Any testing of such a lens under such extension has to be interpreted in that light.
To explain my thoughts further.
Take the Agfa repromaster 80mm F4 ( or F 4.5) as an example. Its designed to produce an image circle that covers at least a letter sized page( at the designed 1-1 or 2-1 Magnification range). On an APSc or full frame sensor a very small proportion of the image circle( in reality image hemisphere) covers the sensor, and that portion of the image has been projected from the best performing part of the lens( the center) It could well be the case that the Repromaster has a curved field but for an APSc or FF sensor the image that is recorded comes from the flattest area of the image circle/hemisphere.

Perhaps... or it is the choke point created by the joining of the two bellows. The extension tube elements as well as camera and lens mount points could be well causing the issue. The multiphot bellows does not have such a choke point. I believe I may well discuss with the Uk firm that makes bellows the option of making a new bellows that provides 400- 450mm of extension. I can get an arca rail to which I can attach two bellows rails( I suspect it would be cheaper than obtaining a multiphot bellows

LOL I have the necessary coupling ring in the post already ( 10 days and counting). I have tried using the PN105 mm with the SK and it was disappointing as the working distance was so close as to be useless( for me). While you (rmpsrmps) were responding I had re-assembled the long extension and checked the two extension ring sets. The Hama Minolta SR extension ring set was the offender; its aperture is 34mm. I then realized I had a minolta compact bellows. So in reassembling my version of the Loch Ness monster I put the Hama extension rings at the front where they would do the least damage. I used the Minolta extension ring set ( silver in the first photo)as the coupling between the two bellows and then at the top I added the Minolta compact bellows( great piece of kit). I can now say that in the last shots at 12 times I was using 500 mm of extension. with the compact bellows its 600mm. What amazed me I have not yet hit empty magnification. You can see that the vignetting is improved but not gone. I now have a second original Minolta extension ring set on my hunt list. At 600mm its getting a little cumbersome, so I hope that coupling with the SK 210 will work( a 2X convertor is a great idea)

So here is my "Loch Ness monster"




The resultant magnification is 14.4 X




Unfortunately the APO designation of the lens is shown to be a little bit of a marketing stretch with the following 100% crop





A month ago I would have laughed if any one had told me I could get this SK 40/2.8 to produce 14X images. I hope some one on this forum got the SK 40/2.8 I posted in the el cheapo thread. Even with the CA, in the right hands it will perform very well. I will be impatiently awaiting the arrival coupling ring.

For the teleconverter, I highly recommend the Vivitar 2x Macro Teleconverter. It is the best teleconverter I've tried, plus it includes an extra focusing helicoid with 18mm of adjustable extension. It's available in MC/MD mount quite inexpensively.

The D mintmark looks very good. I suspect with small lighting changes it will be excellent.

Stacked lenses like this do give larger effective aperture than extensions.

With extensions, the Feff = Finf * (M+1)

With a tube lens, which acts as a teleconverter, the Feff = Finf * M. This assumes your tube lens is set for infinity focus.

So for your f2.8 lens, at 3:1, the Feff = 11.2. With the tube lens Feff = 8.4.