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Archive 2009 · Alt Film Glass ... Digital Coatings

  
 
RustyBug
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p.1 #1 · p.1 #1 · Alt Film Glass ... Digital Coatings


Can an old lens designed for film have new coatings applied to make it more useful for shooting digital ... or ... will new coatings in a TC compensate for the film vs. digital issues (i.e. purple fringe)?


Aug 15, 2009 at 03:14 PM
dirb9
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p.1 #2 · p.1 #2 · Alt Film Glass ... Digital Coatings


Fringing is more the result of optical design than coatings (and many lenses that fringe on digital fringe on film as well, it is just not as noticeable). Coatings within the past 30 years are tailored to a specific element, you can't just randomly recoat them, especially since they are most likely multicoated. Not to mention the fact that a single run (like one would have to do for coating optics for a single lens) would most likely far outrun the cost of the lens itself. Older coated lenses from the 40s and 50s can be recoated as the coating degrades with cleaning easily, and pretty much only one type of mineral was used for coating. In addition, the idea of 'digital' coatings is complete bunk anyways, a well coated lens for film is a well coated lens for digital. A TC will not improve CA it pretty much can only make it worse.


Aug 15, 2009 at 05:08 PM
RustyBug
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p.1 #3 · p.1 #3 · Alt Film Glass ... Digital Coatings


Okay ... thanks.

I was just wondering if it was possible to take a good glass ... and make it even better



Aug 15, 2009 at 06:27 PM
jjlphoto
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p.1 #4 · p.1 #4 · Alt Film Glass ... Digital Coatings


All you can do with good glass is have it serviced, cleaned, and adjusted if necessary. Can't make it better than originally designed. Places like Zeiss in Oberkochen can re-coat the front element if the original coating is damaged.


Aug 15, 2009 at 08:49 PM
papageno
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p.1 #5 · p.1 #5 · Alt Film Glass ... Digital Coatings


Someone somewhere could possibly improve a lens in some way (recollimating, recoating, reassemblikng). That said, it would probably make no sense economically.


Aug 15, 2009 at 08:54 PM
olyacme
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p.1 #6 · p.1 #6 · Alt Film Glass ... Digital Coatings


RustyBug wrote:
I was just wondering if it was possible to take a good glass ... and make it even better


You can have an old coating polished off and replaced with a modern one, without significantly altering the element's figure, but it's rarely economical to do so. The polishing costs more than the coating. Due to the heat involved, lens groups must typically be decemented before coating and recentred and recemented afterwards, which also adds significantly to the cost.



Aug 15, 2009 at 09:02 PM
Mike Deep
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p.1 #7 · p.1 #7 · Alt Film Glass ... Digital Coatings


dirb9 wrote:
In addition, the idea of 'digital' coatings is complete bunk anyways, a well coated lens for film is a well coated lens for digital.

What about lenses like the Tamron 52B which have a prominent sensor flare at smaller apertures?



Aug 15, 2009 at 09:53 PM
olyacme
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p.1 #8 · p.1 #8 · Alt Film Glass ... Digital Coatings


Mike Deep wrote:
What about lenses like the Tamron 52B which have a prominent sensor flare at smaller apertures?


Efficient coatings for a given spectrum are always good, regardless of their age. Optimizing for digital implies using good coatings throughout the lens, and avoiding flat or nearly flat surfaces in the rear groups as these are more likely to reflect sensor flares back onto the sensor.



Aug 15, 2009 at 10:04 PM
Mike Deep
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p.1 #9 · p.1 #9 · Alt Film Glass ... Digital Coatings


olyacme wrote:
Efficient coatings for a given spectrum are always good, regardless of their age. Optimizing for digital implies using good coatings throughout the lens, and avoiding flat or nearly flat surfaces in the rear groups as these are more likely to reflect sensor flares back onto the sensor.

That makes sense. I learned something today.



Aug 15, 2009 at 10:09 PM
dirb9
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p.1 #10 · p.1 #10 · Alt Film Glass ... Digital Coatings


olyacme wrote:
Efficient coatings for a given spectrum are always good, regardless of their age. Optimizing for digital implies using good coatings throughout the lens, and avoiding flat or nearly flat surfaces in the rear groups as these are more likely to reflect sensor flares back onto the sensor.


Yep. The main difference in terms of optical design is that light rays pretty much have to arrive straight on to the sensor, whereas on film, it didn't matter whether they arrive at odd angles. Think of a sensor as a series of narrow buckets. Unless the light ray arrives close to vertically, it will just bounce off. This is also why some lenses have significant falloff (sometimes incorrectly referred to as vignetting) on digital and virtually none on film. In addition, CCDs and CMOSs are far more reflective than film, so flare from reflections from the sensor can present itself far more easily. Flare within a lens causes a loss of light transmission and reduced contrast, so it was always within the best interest of lens designers to minimize internal flare; starting with single coatings and painting the inside of the barrel and the edges of elements with matte black paint, and later on using multicoating to further increase transmission and reduce flare.

Based on the optical layout of the 52B, it appears to have an almost flat rear element, so that could lead to the flare you experienced.



Aug 15, 2009 at 10:26 PM
Greg Campbell
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p.1 #11 · p.1 #11 · Alt Film Glass ... Digital Coatings


Don't blame the lens for flare or high AOI light loss.

High end industrial/scientific CCD chips have been using anti-reflective coating for ages. I'd assumed that camera makers do too, but maybe not?!

Adding AR to the chip is one of those things they should have done ages ago - instead of pimping pixels.



Aug 16, 2009 at 02:24 AM
Toothwalker
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p.1 #12 · p.1 #12 · Alt Film Glass ... Digital Coatings


dirb9 wrote:
Yep. The main difference in terms of optical design is that light rays pretty much have to arrive straight on to the sensor, whereas on film, it didn't matter whether they arrive at odd angles. Think of a sensor as a series of narrow buckets. Unless the light ray arrives close to vertically, it will just bounce off. This is also why some lenses have significant falloff (sometimes incorrectly referred to as vignetting) on digital and virtually none on film. In addition, CCDs and CMOSs are far more reflective than film, so flare from reflections from the sensor can present
...Show more

I don't see fundamental differences between lens coatings for film and digital. Both media benefit from minimizing reflections wherever possible.

How much do CCDs and CMOSs reflect? The reflectance of undeveloped photographic film varies from 15% to 40%.

PS Do you have references on the origin of the word vignetting?






Aug 16, 2009 at 02:57 AM





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