Mirror lenses & 2× Converters

It is unfortunately not possible to use the Schneider-Kreuznach converter with the three mirror lenses, as the front element of the converter protrudes forward of the converter mount into the back of the lens used with it, and there is not space for this at the back of the Rubinars and the 3M-3B.  For this reason, with these three lenses the next-best 2× converter was used: the one from Arsenal in Kiev.

Rubinar f/5.6 500mm mirror lens with Arsenal 2× converter

The first thing to say is that with this combination the subject is magnified much more than when the 1000mm mirror lens is used.  In other words, this confirms the conclusion in the 500mm-600mm test that this “500mm” mirror lens is in fact nearer to 600mm.  That was the “good news”, assuming that “the longer, the better”.  Now to the “bad news”.

Coverage
To avoid any possibility of the lens hood (“shade”) being blamed for vignetting, this lens was tested with and without the hood.  Of course, with a 2× converter in place, only the middle area of the image is being used anyway, so even if the hood were to be blamed for the vignetting when the lens is used on its own, it couldn’t be the cause of any vignetting how.  But anyway, we can see if the hood improves contrast.

The central “hot spot” reported here is still visible, although now of course it occupies a much larger portion of the image, reaching nearly – but not quite! – to the centre point of each of the four edges.  Put another way, a larger area of the image has reasonably even illumination, before the vignetting cuts in.  This is to be expected, as we are only using the middle area of the image produced by the lens.  After the vignetting, which is obvious but may not be disastrous in all situations, come the deep shadows in the corners, particularly at the top, which we recognise from when we used the Arsenal 2× converter with the Arsat APO 500mm lens.

[C430-4] “500”mm f/5.6 Rubinar with Arsenal 2× converter 1/250 (effective f/11) No shade

[C430-5/6] “500”mm f/5.6 Rubinar with Arsenal 2× converter 1/250 (effective f/11) With shade

Lens hoods, or shades, can be crucial in improving contrast when there is a danger that the main light source might fall on the front surface of the lens, but with the sun here well away to one side and high above us, no difference is really obvious in these two shots in consequence of using the shade.

Resolution
The resolution with this combination is very good on the in-focus parts of the image.  It does not of course match the resolution achieved with the Pentacon or Arsat APO 500mm lenses when stopped down to f/11 and used in combination with the Joseph Schneider 2× converter, or even the resolution of the APO lens at f/11 with the Arsenal 2× converter, but it is very similar to what is obtained at f/5.6 with those lenses and a converter.  The “with shade” shot is another negative that suffers from fingerprints from the processing lab.  I have endeavoured to clone these out, but with only partial success.  As indicated in the 500-600mm review, I wrote to the lab.  Their response was unsatisfactory, and so I have changed lab.  The new processor has so far developed films and produced prints without any of these problems.
 

[C430-4r] “500”mm f/5.6 Rubinar with Arsenal 2× converter 1/250 (effective f/11) No shade

[C430-5/6r] “500”mm f/5.6 Rubinar with Arsenal 2× converter 1/250 (effective f/11) With shade

Colour Fringeing
We saw with the 500-600mm tests that mirror lenses do not give rise to colour fringeing.  Of course, here we have a mirror lens image magnified 2× by a refrective lens – the Arsenal 2× converter that gave us massive chromatic aberrations when used in conjuction with the Arsat 500mm APO lens.  When these images are magnified enormously, a tiny amount of chromatic aberration or colour fringeing is just visible as one approaches the corners of the image.  At normal degrees of enlargement it would not be visible at all.
 

[C430-4cf] “500”mm f/5.6 Rubinar with Arsenal 2× converter 1/250 (effective f/11) No shade From bottom left-hand corner

[C430-4cf2] “500”mm f/5.6 Rubinar with Arsenal 2× converter 1/250 (effective f/11) No shade There is no image detail in the top right-hand corner with this shot, so I have taken this detail from very near to the top left-hand corner. Vignetting is clearly more of an issue than chromatic aberration here.

Rubinar f/8 500mm mirror lens with Arsenal 2× converter

The lighting with the tests of this lens is slightly different, and so is the film: Fuji NPH 400 instead of the Fuji NPC 160 used with most of the other tests.  It was necessary to re-shoot the original test for this combination, as the results were not sharp.  This just re-emphasises the principal difficulty when working with such long lenses: getting the focus spot-on.

Coverage
Coverage is better than with the f/5.6 Rubinar, with slight image darkening near the corners, plus the usual cut-off caused by the Arsenal 2× converte much nearer to the corners.  In prints or mounted slides the cut-off caused by the converter is scarcely visible at the bottom of the image, but it is clear at the top, especially at the top left-hand corner.

[C437-13] “500”mm f/8 Rubinar with Arsenal 2× converter 1/250 (effective f/16) No shade

[C437-14/15] “500”mm f/8 Rubinar with Arsenal 2× converter 1/250 (effective f/16) With shade

Resolution
Resolution is very good on the in-focus areas.  With both images it is clear that critical focus is on the rear crane.

As with the f/5.6 Rubinar, adding or removing the lens hood has not made any difference to the images, with the direction of lighting prevailing at the time.  Naturally, there could be many other occasions when using the lens hood would improve the image.  It clearly is not the cause of the vignetting that is observed.
 

[C437-13r] “500”mm f/8 Rubinar with Arsenal 2× converter 1/250 (effective f/16) No shade  Critical focus is clearly on the rear crane – or somewhere between the two!

[C437-14/15r] “500”mm f/8 Rubinar with Arsenal 2× converter 1/250 (effective f/16) With shade

Colour Fringeing
There is a negligible amount of colour fringeing at the very edges of the frame.  This is very good, while not quite up to the result when the lens was used on its own.
 

[C437-14-15cf] “500”mm f/8 Rubinar with Arsenal 2× converter 1/250 (effective f/16) With shade From bottom of image, about 1/4 of the way in from the left. At this degree of magnification, it is clear that the image is not critically sharp on the front crane.

[C437-14-15cf2] “500”mm f/8 Rubinar with Arsenal 2× converter 1/250 (effective f/16) With shade From very near to the top right-hand corner. A tiny amount of faint green fringeing can be seen where the diagonal beam in the sunlight crosses the beam behind it in the shadow. This is not visible at normal degrees of enlargement.

Arsat 3M – 3B f/8 600mm mirror lens with Arsenal 2× converter

For these tests, the Hasselblad compendium shade no. 40726 as described here was mounted on the lens.  When this shot was taken, the top crane had rotated slightly towards the taking position, as will be observed from the picture, moving the front of the crane boom arm away from the plane of sharpest focus.

Coverage
The tests of the 3M – 3B f/8 600mm mirror lens on its own showed even illumination across the whole frame.  As discovered in these new tests, the Arsenal 2× converter has introduced severe vignetting when used with other good lenses, and the current combination is no exception.  There is very marked darkening (almost to black!) of all four corners of the image.  At the bottom of the picture this does not extend very far, and on standard prints or mounted slides is not particularly noticeable, but the darkening of the top corners, especially the top-left, extends considerably further into the image and is very obvious.

Resolution
Another frame from the batch of films that came back from the processing lab with fingerprints, the scan of this negative has required careful re-touching, especially in the sky areas.  Success has been partial, but I have been careful not to invalidate the test by more extensive re-touching.

Focus is not totally spot-on, but sharpness is excellent, restricted principally by the extremely shallow depth of field.

Colour Fringeing
When using this lens on its own we discovered a total lack of chromatic aberrations or colour fringeing, as is to be expected with mirror lenses.  When the Arsenal 2× converter is added, some colour fringeing appears at the edges and especially approaching the corners of the image.  This confirms our experience when using this converter with other lenses, above.

[C430-10cf] 3M – 3B f/8 600mm mirror lens with Arsenal 2× converter (effective f/16) 1/125 with Hasselblad compendium shade fully extended From the bottom of the image, less than a quarter in from the left-hand corner.

[C430-10cf2] 3M – 3B f/8 600mm mirror lens with Arsenal 2× converter (effective f/16) 1/125 with Hasselblad compendium shade fully extended From the top of the image, very close to the top left-hand corner.  The grain of this ultra-fine-grain film is again obvious in this massive enlargement.  A tiny amount of colour fringeing is noticeable, and this must be due to the Arsenal 2× converter, which is also the cause of the considerable vignetting that is obvious here.

Conclusion

Judged by coverage alone, the worst results came from using the Arsat APO lens with the Arsenal 2× converter.  The best results were from the same Arsat APO lens with the Schneider 2× converter.  The 1000mm Carl Zeiss Spiegelobjektiv is between these two extremes, not far behind the APO/Schneider result.

Judged by image resolution, the sharpest image of the red crane was obtained with the 1000mm Carl Zeiss Jena mirror lens on its own.  However, this is easily matched by the Arsat APO 500mm lens with the Schneider 2× converter and by the Pentacon 500mm lens with the Schneider 2× converter, when the lens is stopped down to f/11.  Crucial to sharpness with these extreme focal lengths is critically accurate focus, and it might be better to use the magnifier head on the camera, which magnifies the prism image 4×, or the magnifying attachment (also known as the  “Focussing Telescope”) on the back of the metering prism.  (See these two accessories here and here.)

Looking at colour fringeing (or the lack of it), the 1000mm Carl Zeiss Jena mirror lens is top.  The other mirror lenses also show an absence of colour fringeing when used on their own, but in the combination used here, with the Arsenal 2× converter, some very minor colour fringeing is introduced.  The mirror lenses are followed closely by the 500mm Pentacon and 500mm Arsat APO lenses, when used with the Schneider Kreuznach 2× converter.

Unfortunately, coverage problems with the two Rubinars virtually eliminate them from the running, in my opinion.

All lens designs are the result of a compromise: a balance between performance, size, weight and cost.  In many cases, the absolutely peak performance would result in a lens that was too heavy to carry or too expensive for most people to buy.  Going for this option might be right for a scientific astronomical observatory, but not for ordinary photographers – or even most professionals.  If you get the ultimate lens, but it is so heavy that you rarely take it with you, it won’t be there for that rare and special shot.

My personal conclusion is that the best way for me to achieve 1000mm in most circumstances is with the Arsat APO 500mm lens used in combination with the Schneider-Kreuznach 2× converter, combining high quality with portability, with the additional benefit of increased depth of field when stopping down to f/11 on the taking lens (equivalent to f/22 as regards light reaching the film, when used with the converter) – an option that is not available with mirror lenses.  The Pentacon 500mm lens with the same converter gives results that are virtually indistinguishable from the above combination, but with a larger, heavier lens, and loss of automatic aperture operation.  The advantages, however, are that the Pentacon lens is much more readily available, and is also a lot cheaper than the rare Arsat APO lens.

As a source for the 500mm APO or the 600mm 3M – 3B mirror lens, I recommend Zhanna in Kiev, who trades on eBay as “grizzly33bear” – though you may have to wait for a while for an APO lens to turn up.
 

To read a report on a sturdier tripod for these longer lenses, click here.

I have been asked, “What about 2000mm?”
To go on to the next section, the results of my 2000mm tests, click here.

To go back to the beginning of the lens tests, click below and then choose the focal length that you want to read about.
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© TRA November 2007, January 2013