Aspherical lens? at DVinfo.net

Go Back   DV Info Net > Special Interest Areas > Alternative Imaging Methods

Alternative Imaging Methods
DV Info Net is the birthplace of all 35mm adapters.


Reply
 
Thread Tools Search this Thread
Old January 25th, 2006, 03:23 AM   #1
Regular Crew
 
Join Date: Jan 2006
Location: Minneapolis, MN, USA
Posts: 44
Aspherical lens?

For a while I've been planning on just buying a 35mm adapter for my HD1, but as I've browsed around here I've come up with some new ideas that I think I'm going to incorporate into a static adapter of my own. Yes, it's probably dumb to make a static adapter for an HD camera, but I'm going to experiment with a 12,000 grit ground glass. I've seen extremely good DV results from 1,500 grit (I'm specifically thinking of Jimmy Hedberg's, which inspired me when I was lurking here before I even joined), so hopefully a well-built adapter using 12,000 grit can perform equally well.

My actual question is this: Has anyone tried using an aspherical lens for a condenser instead of a PCX? From my own (extremely limited) knowledge of optics and from Optosigma's website, it seems like it might actually be a better choice. Does anyone have any experience on this front?
Joel Kreisel is offline   Reply With Quote
Old January 25th, 2006, 10:24 AM   #2
Major Player
 
Join Date: Dec 2003
Location: Brooklyn, NY
Posts: 636
Where do you get 12,000 grit glass? That's roughly 1.5 micron grit... will you be hand-grinding it, or getting it pre-ground from somewhere?
__________________
Realism, anyway, is never exactly the same as reality, and in the cinema it is of necessity faked. -- J-L G

Last edited by Jim Lafferty; January 25th, 2006 at 11:36 AM.
Jim Lafferty is offline   Reply With Quote
Old January 25th, 2006, 11:54 AM   #3
Regular Crew
 
Join Date: Jan 2006
Location: Minneapolis, MN, USA
Posts: 44
I'll be hand-grinding it. If it works as well as I want it to, I'll probably also sell them (in small quantities, I'm no factory!) for roughly the same as the Optosigma GGs.
Joel Kreisel is offline   Reply With Quote
Old January 25th, 2006, 01:49 PM   #4
Major Player
 
Join Date: Dec 2003
Location: Brooklyn, NY
Posts: 636
What's your aluminum oxide source?
__________________
Realism, anyway, is never exactly the same as reality, and in the cinema it is of necessity faked. -- J-L G
Jim Lafferty is offline   Reply With Quote
Old January 25th, 2006, 02:47 PM   #5
Regular Crew
 
Join Date: Nov 2005
Location: Concord, CA
Posts: 118
don't aspherical lenses induce more light reduction? possibly the reason for manufactures limiting the amount of aspherical elements in their lenses.
__________________
Letus35 Flip on HC1
FCP5 w/ Intel Mac Mini and Powerbook Ti G4
R.P. Cuenco is offline   Reply With Quote
Old January 25th, 2006, 02:52 PM   #6
Regular Crew
 
Join Date: Jan 2006
Location: Minneapolis, MN, USA
Posts: 44
R.P.: According to Optosigma, aspherical lenses primary purpose is for condensing, which is what made me do a little research and start thinking of using it in place of a PCX.

Jim: I'd rather not say at this point since that would kind of decrease the marketability of the glass, but if I decide not to sell them then I'll definately make my source available to the community. (Or maybe even if I do.)
Joel Kreisel is offline   Reply With Quote
Old January 25th, 2006, 11:34 PM   #7
Regular Crew
 
Join Date: May 2005
Posts: 30
Aspherical lenses are generally used to reduce the optical aberrations that occur with higher power spherical lenses. One such effect is "pincushioning" that can occur toward the lateral edges of the lens.

A condensing lens is a type of spherical lens, usually spherical on one surface and plano (or spherical) on the other surface. From what I remember the aberrations start at around 8 diopters for a spherical lens (roughly a 2x magnification or a f 125mm condensing lens).

Most expensive precision equipment that use condensing lenses use aspherical condensing lenses. Someone earlier in the thread mentioned that asphericals cause light reduction (transmittance), but this is not the case. Transmittance is more a function of the materials the lenses are made from as well as reflection from the internal and external lens surfaces. In better lenses the surfaces will have AR coating to reduce the reflection and thus increase the transmittance.

The reason that manufacturers may use less aspherical elements has to do with cost. They are more expensive. Also, the more aspherical elements, the more engineering required for the lens system.

Barry Oppenheim
Barry Oppenheim is offline   Reply With Quote
Old January 25th, 2006, 11:45 PM   #8
Regular Crew
 
Join Date: Jan 2006
Location: Minneapolis, MN, USA
Posts: 44
Thanks for the info, Barry! So it seems like from what you're saying it would at least be worth it to experiment with one and see what I come up with?
Joel Kreisel is offline   Reply With Quote
Old January 26th, 2006, 11:07 AM   #9
Regular Crew
 
Join Date: May 2005
Posts: 30
Quote:
Originally Posted by Joel Kreisel
Thanks for the info, Barry! So it seems like from what you're saying it would at least be worth it to experiment with one and see what I come up with?
If you had the time (and the cash) it would be a good experiment to compare the Beatie Fresnel condenser to an aspheric lens. You might want to try an aspheric lens with an AR coating to increase the transmittance of light.

The Beatie screens are trying to accomplish the same thing you are: condense/gather light while minimizing optical aberrations. The Fresnel lens that is part of the Beatie screen is serving as a condenser lens.

Fresnel lenses are frequently used to reduce optical aberrations that thicker or higher power lenses can induce. The are basically a cheap way of getting aspherical performance and can be mass produced easily. Unfortunately, they do have drawbacks compared to aspherical lens.

Depending on how good the Fresnel lens is you will have some loss of light. Also, unless the Fresnel is using extremely close and small grooves, the focus might not be as good as the better aspheric lens.
Barry Oppenheim is offline   Reply With Quote
Old January 26th, 2006, 11:21 AM   #10
Regular Crew
 
Join Date: Jan 2005
Location: LA
Posts: 130
Some aspherical lenses cannot be coated because of their steep angle of incidence. The anti-reflective coating would keep light out at the more accute angled areas.

Fresnel lenses, due to their thickness, are usually considered to have less light loss than thicker lenses.
Alain Bellon is offline   Reply With Quote
Old January 26th, 2006, 11:43 AM   #11
Regular Crew
 
Join Date: May 2005
Posts: 30
Quote:
Originally Posted by Alain Bellon
Some aspherical lenses cannot be coated because of their steep angle of incidence. The anti-reflective coating would keep light out at the more accute (sic) angled areas.
A steeper/higher angle of incidence will actually cause more reflection (and conversely less transmission). An AR coating should actually be more beneficial for this type of lens.

Quote:
Originally Posted by Alain Bellon
Fresnel lenses, due to their thickness, are usually considered to have less light loss than thicker lenses.
Agree.

Resolution/focus though, might still not be as good with a Fresnel lens compared to a good aspheric.

Don't get me wrong. Well made Fresnel lenses definitely have a role to play.
Barry Oppenheim is offline   Reply With Quote
Old January 26th, 2006, 11:58 AM   #12
Regular Crew
 
Join Date: Jan 2005
Location: LA
Posts: 130
Quote:
Originally Posted by Barry Oppenheim
A steeper/higher angle of incidence will actually cause more reflection (and conversely less transmission). An AR coating should actually be more beneficial for this type of lens.
I am not sure I follow the logic here. Could you explain it a bit?

It seems to me that, at steep angles of incidence, the AR coating will prevent more light from entering the lens (or leaving the lens in the case of internal reflection). The aspherical lenses usually have very steep angles of incidence, so I don't see how that is beneficial for this type of lens.

Quote:
Resolution/focus though, might still not be as good with a Fresnel lens compared to a good aspheric.

Don't get me wrong. Well made Fresnel lenses definitely have a role to play.
Absolutely.
Alain Bellon is offline   Reply With Quote
Old January 26th, 2006, 01:06 PM   #13
Regular Crew
 
Join Date: May 2005
Posts: 30
Ugh. Making me drag out physical optics :-)

OK.

(light out or transmittance) = (light in) minus (reflected light).

Steeper lenses means higher angles of incidence and more reflectance. Lowering reflected light within a lens system increases transmittance.

As you correctly state an AR coating causes reflections. An AR coating on a lens produces reflections at both the air-coating interface and at the coating-lens interface. However, the AR coating is applied at a thickness of ~1/4 of the wavelength. This produces reflections at the air-coating interface that are destructive with the reflections from the coating-lens interface. The destructive waves "cancel out" the reflected light, thus increasing the transmittance (per the relationship above).
Barry Oppenheim is offline   Reply With Quote
Old January 26th, 2006, 01:35 PM   #14
Regular Crew
 
Join Date: Jan 2005
Location: LA
Posts: 130
Quote:
Originally Posted by Barry Oppenheim
Ugh. Making me drag out physical optics :-)

OK.

(light out or transmittance) = (light in) minus (reflected light).

Steeper lenses means higher angles of incidence and more reflectance. Lowering reflected light within a lens system increases transmittance.

As you correctly state an AR coating causes reflections. An AR coating on a lens produces reflections at both the air-coating interface and at the coating-lens interface. However, the AR coating is applied at a thickness of ~1/4 of the wavelength. This produces reflections at the air-coating interface that are destructive with the reflections from the coating-lens interface. The destructive waves "cancel out" the reflected light, thus increasing the transmittance (per the relationship above).
Yes, but the cancelling out is independent of angle of incidence, so it doesn't differentiate a spherical lens from a steep aspherical one. This just relates to the interfaces and the coating thickness.

So my question still is, how would an AR coating be more of a benefit for a lens that has steep angles of incidence, when a steeper angle means more reflectance (thus less transmittance)?

My only guess for an AR coating being a good idea on an aspherical lens is if the benefit from the destructive interference is larger than the loss from the reflections caused by the steep incidence angle.

(BTW, I just checked at Optosigma, the place mentioned earlier on the thread, and they don't recommend coating aspherical lenses)

Or am I totally off track?
Alain Bellon is offline   Reply With Quote
Old January 26th, 2006, 02:42 PM   #15
Regular Crew
 
Join Date: May 2005
Posts: 30
Quote:
Originally Posted by Alain Bellon
My only guess for an AR coating being a good idea on an aspherical lens is if the benefit from the destructive interference is larger than the loss from the reflections caused by the steep incidence angle.
I'm not sure I totally understand your question. Are you comparing a spherical lens with given incidence angle with an AR coated aspherical lens with a steeper angle of incidence?


Duane's 2006, Chapter 31, Physical Optics

"As a specific example, a ray impinging on the surface of an uncoated aspheric lens (refractive index, 1.523) near its edge at an angle of incidence of 58 would have a reflection loss of 8.3%. Lenses with steep front surfaces tend to produce larger angles of incidence and thus produce stronger reflections. However, by the addition of a single-layer MgF2 antireflection coating designed for normal incidence, the reflection loss is reduced by more than one half, to 3.6%. Only at angles of incidence larger than 60 does the reflectance of such a coated surface exceed 4%."
Barry Oppenheim is offline   Reply
Reply

DV Info Net refers all where-to-buy and where-to-rent questions exclusively to these trusted full line dealers and rental houses...

Professional Video
(800) 833-4801
Portland, OR

B&H Photo Video
(866) 521-7381
New York, NY

Z.G.C.
(973) 335-4460
Mountain Lakes, NJ

Abel Cine Tech
(888) 700-4416
N.Y. NY & L.A. CA

Precision Camera
(800) 677-1023
Austin, TX

DV Info Net also encourages you to support local businesses and buy from an authorized dealer in your neighborhood.
  You are here: DV Info Net > Special Interest Areas > Alternative Imaging Methods

Thread Tools Search this Thread
Search this Thread:

Advanced Search

 



Google
 

All times are GMT -6. The time now is 05:44 PM.


DV Info Net -- Real Names, Real People, Real Info!
1998-2017 The Digital Video Information Network