lars_bergquist Posted June 22, 2012 Share #21  Posted June 22, 2012 Advertisement (gone after registration) John, you are basically right on theory. When Ernst Abbe introduced the term 'apochromat' in the late 1880's, he meant a (microscope) lens that focused blue, green and red light in the same focal plane. Abbe however – the residing genius at Carl Zeiss – was mainly interested in microscopy, which was immediately relevant to the rapidly expanding scientific departments of German universities and large companies, and actively supported by a Reich that saw science as a means of obtaining bigger bangs. So he was concerned mainly with performance on the optical axis or in the near paraxial area, because microscopic observation at that time was visual. The final image was a synthetic one, built up in the microscopist's brain and on his sketch pad. You could always give the object glass a slight nudge and bring an unsharp detail into the sharp axial area.  This did not do for photography however (but remember that blue-sensitive-only film was still called 'ordinary' as late as in the 1930's, while panchromatic emulsions were still regarded with suspicion at that time). The image had to be fully colour-corrected off the axis too – but how well, and how far off it?  Aye, there's the rub – for there is no ISO standard to define apochromaticity. So manufacturers (or rather their marketing departments) define it as they please. Many define lenses without much special merit in colour correction as apo lenses, while other are more strict in their usage. Leica e.g. So the confusion is ultimately not in the viewer's (your) eye, but in the industry.  The old man from the Achromatic Age Link to post Share on other sites More sharing options...
jaapv Posted June 22, 2012 Share #22 Â Posted June 22, 2012 a problem with apochromatic correcting is that it is defined as coincidence of three specific frequencies of light. Obviously this does not guaranty coincidence of the whole spectrum or even any other frequency, so other colors may be (and are) substantially not-coincidental. An apochromat will often have blue and violet CA. Link to post Share on other sites More sharing options...
farnz Posted June 22, 2012 Share #23 Â Posted June 22, 2012 a problem with apochromatic correcting is that it is defined as coincidence of three specific frequencies of light. Obviously this does not guaranty coincidence of the whole spectrum or even any other frequency, so other colors may be (and are) substantially not-coincidental. An apochromat will often have blue and violet CA. A fair point, Jaap, but since apochromatic lenses are correcting for red, green, and blue then they're effectively covering the full visible spectrum because it would be a rare formula of glass with a peculiar non-linearity that focussed intermediate wavelengths between red and blue (that roughly represent the boundaries of the 'visible envelope') in a significantly different plane. Blue, indigo, and violet and are of course at the same end of the visible spectrum so it seems to me that it's a degree of under-correction at the blue end because there is no fringing (that you've mentioned) towards the red end of the waveband. Â Pete. Link to post Share on other sites More sharing options...
bpalme Posted June 22, 2012 Share #24 Â Posted June 22, 2012 I think you guys are getting way too deep here. Apochromatic corrects for spherical and chromatic aberrations. It helps correct a little blur and purple fringing. Link to post Share on other sites More sharing options...
farnz Posted June 22, 2012 Share #25 Â Posted June 22, 2012 I think you guys are getting way too deep here. Apochromatic corrects for spherical and chromatic aberrations. It helps correct a little blur and purple fringing. Brian, Â You'll have to help me with how an APO lens corrects for spherical aberration.<frowning, scratches head and cradles chin> Â Pete. Link to post Share on other sites More sharing options...
SJP Posted June 23, 2012 Share #26 Â Posted June 23, 2012 Wikipedia I think as always gives a pretty decent definition of the various salient points, see here Apochromat - Wikipedia, the free encyclopedia Link to post Share on other sites More sharing options...
bpalme Posted June 24, 2012 Share #27  Posted June 24, 2012 Advertisement (gone after registration) Brian, You'll have to help me with how an APO lens corrects for spherical aberration.<frowning, scratches head and cradles chin>  Pete. I'm addressing the OP who I think is asking for a simple answer. If you disagree then let's hear your explanation of APO. Link to post Share on other sites More sharing options...
farnz Posted June 24, 2012 Share #28 Â Posted June 24, 2012 I'm addressing the OP who I think is asking for a simple answer. If you disagree then let's hear your explanation of APO. Brian, Â Please refer to post number 4 for my explanation of APO. Spherical aberration is corrected with an aspherical element; apochromatic correction doesn't correct for spherical aberration. Â Pete. Link to post Share on other sites More sharing options...
bpalme Posted June 24, 2012 Share #29 Â Posted June 24, 2012 I read your post and don't see anything I disagree with. Here's my reference. An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses. Â Apochromat - Wikipedia, the free encyclopedia Link to post Share on other sites More sharing options...
farnz Posted June 24, 2012 Share #30 Â Posted June 24, 2012 I read your post and don't see anything I disagree with. Here's my reference. An apochromat, or apochromatic lens (apo), is a photographic or other lens that has better correction of chromatic and spherical aberration than the much more common achromat lenses. Â Apochromat - Wikipedia, the free encyclopedia Okay I think I see where Wiki is going with this; it's using the premise that by all 3 (RGB) wavelengths being dispersed by a greater amount in an APO lens than wavelengths are in a non-APO lens then spherical aberration must be by definition better corrected. This effect is a by-product of apochromaticity rather than a deliberate correction and here's why. Â Spherical aberration affects all wavelengths in the visible waveband by a similar amount but Chromatic aberration affects different wavelengths (colours) by different amounts. So while some rays that form the image in an APO lens will appear to be partially corrected for spherical aberration others won't depending on what their colour constituents are. Â The question to ask is: if APO lenses fully corrected for spherical aberration why would Leica go to the effort and expense of including aspherical elements in APO lenses, for example, the 50/2 APO-Summicron ASPH, the 75/2 APO-Summicron ASPH, and the 90/2 APO-Summicron ASPH? Â References: Leica M Advanced Photo School (Second English language edition), pp104-109, Gunter Osterloh, Stirling Publishing, ISBN 978-1-4547-0069-2 Optics - The Technique of Definition, pp 103-110, Arthur Cox, The Focal Press, 1961. Link to post Share on other sites More sharing options...
masjah Posted June 24, 2012 Share #31 Â Posted June 24, 2012 The Wiki article states, inter alia: Â "Apochromats are also corrected for spherical aberration at two wavelengths, rather than one as in an achromat" Â That I did not know. I suppose that it is possible that aspherical elements are needed to accomplish this, or at least to accomplish it more efficiently, which might address Pete's last question? Link to post Share on other sites More sharing options...
farnz Posted June 24, 2012 Share #32 Â Posted June 24, 2012 John, Â I think you're largely right but I sense that there's more to it than the Wiki article implies. Gunter Osterloh indicates that there are different 'degrees' of apochromatic correction and Leica has taken a more stringent approach than others towards correction of chromatic and spherical aberrations in order to achieve optimum image quality. Â Pete. Link to post Share on other sites More sharing options...
bpalme Posted June 24, 2012 Share #33 Â Posted June 24, 2012 Sounds like you guys know more about it than I. I think I'm at the limit of my knowledge on this subject so I'll just leave it at that. Smiley Link to post Share on other sites More sharing options...
masjah Posted June 24, 2012 Share #34  Posted June 24, 2012 Sounds like you guys know more about it than I. I think I'm at the limit of my knowledge on this subject so I'll just leave it at that. Smiley  I suspect your puzzlement is akin to my puzzlement, and I suspect that Lars was in turn right about its cause when he said:  Aye, there's the rub – for there is no ISO standard to define apochromaticity. So manufacturers (or rather their marketing departments) define it as they please. Many define lenses without much special merit in colour correction as apo lenses, while other are more strict in their usage. Leica e.g. So the confusion is ultimately not in the viewer's (your) eye, but in the industry.   Link to post Share on other sites More sharing options...
farnz Posted June 24, 2012 Share #35 Â Posted June 24, 2012 Sounds like you guys know more about it than I. I think I'm at the limit of my knowledge on this subject so I'll just leave it at that. Smiley I wouldn't claim that at all but I like your attitude and I'll also leave it at that with relief and a smiley and hope that the OP's question has been answered. Â Pete. Link to post Share on other sites More sharing options...
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