Aspherical lens? at DVinfo.net

Joel Kreisel · January 25th, 2006, 03:23 AM

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?

Jim Lafferty · January 25th, 2006, 10:24 AM

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?

Joel Kreisel · January 25th, 2006, 11:54 AM

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.

Jim Lafferty · January 25th, 2006, 01:49 PM

What's your aluminum oxide source?

R.P. Cuenco · January 25th, 2006, 02:47 PM

don't aspherical lenses induce more light reduction? possibly the reason for manufactures limiting the amount of aspherical elements in their lenses.

Joel Kreisel · January 25th, 2006, 02:52 PM

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.)

Barry Oppenheim · January 25th, 2006, 11:34 PM

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

Joel Kreisel · January 25th, 2006, 11:45 PM

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?

Barry Oppenheim · January 26th, 2006, 11:07 AM

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.

Alain Bellon · January 26th, 2006, 11:21 AM

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.

Barry Oppenheim · January 26th, 2006, 11:43 AM

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.

Alain Bellon · January 26th, 2006, 11:58 AM

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.

Barry Oppenheim · January 26th, 2006, 01:06 PM

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).

Alain Bellon · January 26th, 2006, 01:35 PM

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?

Barry Oppenheim · January 26th, 2006, 02:42 PM

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%."

January 25th, 2006, 03:23 AM	#1
Joel Kreisel 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?

January 25th, 2006, 10:24 AM	#2
Jim Lafferty 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.

January 25th, 2006, 01:49 PM	#4
Jim Lafferty 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

January 25th, 2006, 02:47 PM	#5
R.P. Cuenco 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

January 25th, 2006, 11:54 AM	#3
Joel Kreisel 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.

January 25th, 2006, 02:52 PM	#6
Joel Kreisel 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.)

January 25th, 2006, 11:34 PM	#7
Barry Oppenheim 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

January 25th, 2006, 11:45 PM	#8
Joel Kreisel 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?

January 26th, 2006, 11:21 AM	#10
Alain Bellon 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.

January 26th, 2006, 01:06 PM	#13
Barry Oppenheim 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).