Is there a need for F1.4 lenses....

Started 4 months ago | Discussions thread
AiryDiscus Senior Member • Posts: 1,930
Re: Is there a need for F1.4 lenses....
1

Erik Kaffehr wrote:

fferreres wrote:

JimKasson wrote:

LoCA in the long wavelengths impacts MTF. Less so in the short ones.

Hi Jim. I eat my words here. Very tasty, and nice to be corrected. I just saw this article and it mentions the work you’ve done around some of these impacts. Not sure how accurate it is but it was very understandable to me:

https://www.strollswithmydog.com/chromatic-aberrations-mtf-mapped/

Found it great to understand a bit more. Can it be the case, it may either be too expensive, or that maybe trying to correct LoCA could result in an even worst trade off, and thus may be preferred for overall sharpness?

I now understand the LoCA (after article) as related to the question, “which color do you want more sharp?” (Usually would be whatever bandwidth are more clustered together for max overall sharpness). In general, best point to focus makes red a little more blurry. Based on what I read LaCA is easier to correct, and LoCA very hard.

Why do we still see this much LoCA in good lenses? To me it’s a mistery.

Hi,

Nice article you have found...
I would guess that the reason LoCA is not really well corrected is that it needs very expensive glass. Making a large aperture lens may need a large piece of expensive glass.
Just as an example, Brandon Dube mentioned somewhere that just the glass cost for the frontal element of the Otus 85/1.4 is around 500$US, before grinding, polishing, coating and mounting.
Some optical glass is also difficult to use. May be very brittle, difficult to coat, etc.
As far as I know, one of the lenses that are best corrected for the LoCA is the Voigtlander Apo Lanthar. It is affordable but limited to 65/2.0.
Personally, I would be quite happy with a lens that is fully usable at f/2 or even f/2.8.

From what I have read, written by folks who understand optics, much of what we call LoCA or axial chroma is coming from spherochromatism, which may be the color variation of spherical aberration.

Best regards

Erik

If you wish to do the math yourself - the slab price for anomalous partial dispersion materials is $150-$300/lb.

The density of optical glasses ranges from about 2 to 4 g/cm^3. You may start an approximation for the blank material with the enclosing cylinder, though it would be slightly larger.

So, if we have an element which is ~ 86 mm in diameter and ~ 8 mm thick (to keep the theme of 8s for a moment), we have 46,500 mm^3 = 46.5 cm^3 of material, about .4lbs ($20-120).

This assumes unity yield, and does not account for the cost of processing. If the blank started at 90 mm diameter and 15 mm thickness, the cost would be slightly more than double.

Also note that in coring your cylinder, you make less-than-ideal use of the rectangular slab, and there is some waste generated at that step.  You may request blanks of a certain dimension from the vendor, but this would be uncommon for anomalous partial materials (demand exceeds production capacity, no incentive to customize for you) and increases cost anyway.

Polishing a high precision sphere on an optic of that size would cost you circa $10,000 from a custom shop, about 100 hours of work. If we use a base rate of $10/hr for the amortization of equipment and developing market rate of $10, overhead included for the human capital, then there is $2,000 of work put into the polishing. If the one worker can monitor ten spindles, each working 7 of these optics, the cost of polishing works out to about $250/element. I doubt anyone would polish more than 7 on one spindle -- machines of such a size are uncommon.

These labor and amortization rates are somewhat artificially low. Approximately speaking, dropping four zeros gives you the hourly rate to run a machine, i.e., a quarter million dollar machine costs $25/hr after amortization, electrical, and so on.

Consider that these are operating costs, and that if you lack production of your own, you pay a margin on top of this rate, though I have no idea at all what sort of margin OEMs in Asia take.

This also does not account for the cost of coating, assembly, etc.

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