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#11
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The difference in enlarging lenses
In article t, "Nicholas
O. Lindan" wrote: 1) Attracts dust: helps keep the rest of your darkroom dust free Nope. A large, charged ABS plastic sink attracts the dust. Just wipe it clean once a day. It's like magic. 2) Creates Newton's rings: Adds color and pattern to your pictures Never had Newton's rings with the Leitz Focomat IIa enlarger. 3) Decreases contrast: Keeps those pesky highlights under control Nice try, but not true. 4) Alters light path: Helps achieve that sought-after 'soft focus' look Nope! 5) Shatters when dropped: Maintains full employment in the glass industry Now you are frightening me. No more coffee before printing! |
#12
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The difference in enlarging lenses
jjs wrote:
In article , Bob Salomon wrote: [... ] In a glass negative carrier as the lenses are designed to be used with? Bob, are you saying that the glass carrier is part of the optical formula? Does a glass carrier do more than simply keep the negative flat? Tell me it does so that I have some rationalization for the dust I put up with. John, Search this ng's archives for articles by Michael Gudzinowicz about enlarging lenses. He gives the optical formulas for depth of field at the negative stage. Doing the computations for your situation will most likely convince you that a glass carrier and a "perfectly" aligned enlarger are necessary for the best results your system can deliver. Bob |
#13
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The difference in enlarging lenses
HypoBob wrote: jjs wrote: In article , Bob Salomon wrote: [... ] In a glass negative carrier as the lenses are designed to be used with? Bob, are you saying that the glass carrier is part of the optical formula? Does a glass carrier do more than simply keep the negative flat? Tell me it does so that I have some rationalization for the dust I put up with. John, Search this ng's archives for articles by Michael Gudzinowicz about enlarging lenses. He gives the optical formulas for depth of field at the negative stage. Doing the computations for your situation will most likely convince you that a glass carrier and a "perfectly" aligned enlarger are necessary for the best results your system can deliver. Bob Glass between the negative and the lens affects the focus slightly, depending on the thickness of the glass. It does that by refraction, of course, which means that the effect varies with thickness of the glass as well as color of the light. Some lenses are designed to be used with a glass carrier of certain thickness. Whether any of us mortals could tell the difference is another matter. In any case, you can see how glass might affect chromatic aberration and curvature of the field for better or worse, depending on how the lens is designed. |
#14
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The difference in enlarging lenses
In article ,
Patrick Gainer wrote: HypoBob wrote: jjs wrote: In article , Bob Salomon wrote: [... ] In a glass negative carrier as the lenses are designed to be used with? Bob, are you saying that the glass carrier is part of the optical formula? Does a glass carrier do more than simply keep the negative flat? Tell me it does so that I have some rationalization for the dust I put up with. John, Search this ng's archives for articles by Michael Gudzinowicz about enlarging lenses. He gives the optical formulas for depth of field at the negative stage. Doing the computations for your situation will most likely convince you that a glass carrier and a "perfectly" aligned enlarger are necessary for the best results your system can deliver. Bob Glass between the negative and the lens affects the focus slightly, depending on the thickness of the glass. It does that by refraction, of course, which means that the effect varies with thickness of the glass as well as color of the light. Some lenses are designed to be used with a glass carrier of certain thickness. Whether any of us mortals could tell the difference is another matter. In any case, you can see how glass might affect chromatic aberration and curvature of the field for better or worse, depending on how the lens is designed. Any shift in focus would be compensated for when the enlarger is focused. It is a non-issue. -- To reply no_ HPMarketing Corp. |
#15
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The difference in enlarging lenses
HypoBob wrote:
Glass between the negative and the lens affects the focus slightly, depending on the thickness of the glass. It does that by refraction, of course, which means that the effect varies with thickness of the glass as well as color of the light. [...] That reads like impressionistic keyboard engineering. I've never seen it IRL. |
#16
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The difference in enlarging lenses
"jjs" wrote in message ... In article m, Bob Salomon wrote: [... ] In a glass negative carrier as the lenses are designed to be used with? Bob, are you saying that the glass carrier is part of the optical formula? Does a glass carrier do more than simply keep the negative flat? Tell me it does so that I have some rationalization for the dust I put up with. The glass is not part of the formula and has insignificant effect on the light in the optical path. What Bob is getting at is that high quality enlarging lenses are designed to image a flat object onto a flat surface. If the negative bows, as it will in a glassless holder, the corner performance of the lens is compromised. Despite the extra bother of the glass sandwich holder it does increase sharpness, particularly at the corners, and eliminates motion of the negative during exposure. This last can be the explanation for the occasional complaint of getting not quite sharp prints from sharp negatives even though the lenses are of excellent quality. The choice is whether these advantages are worth the considerable extra effort of keeping four glass surfaces absolutely clean and free of blemishes. For some printing, particularly where a great amount of magnification is involved, I think the glass carriers are a must. There are lenses designed to work from curved surfaces. Some slide projector lenses, for instance, the Kodak Ektanar series, are so designed, intended to work with cardboard mounted slides. The problem is that many times these slides are much closer to being flat than was assumed in the lens design so a flat field lens will give better pictures. Slides are another case where glass, in this case glass sandwich slide mounts, has advantages in consistent sharpness despite extra effort being needed to keep everything clean. -- --- Richard Knoppow Los Angeles, CA, USA |
#17
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The difference in enlarging lenses
"Patrick Gainer" wrote in message ... HypoBob wrote: jjs wrote: In article m, Bob Salomon wrote: [... ] In a glass negative carrier as the lenses are designed to be used with? Bob, are you saying that the glass carrier is part of the optical formula? Does a glass carrier do more than simply keep the negative flat? Tell me it does so that I have some rationalization for the dust I put up with. John, Search this ng's archives for articles by Michael Gudzinowicz about enlarging lenses. He gives the optical formulas for depth of field at the negative stage. Doing the computations for your situation will most likely convince you that a glass carrier and a "perfectly" aligned enlarger are necessary for the best results your system can deliver. Bob Glass between the negative and the lens affects the focus slightly, depending on the thickness of the glass. It does that by refraction, of course, which means that the effect varies with thickness of the glass as well as color of the light. Some lenses are designed to be used with a glass carrier of certain thickness. Whether any of us mortals could tell the difference is another matter. In any case, you can see how glass might affect chromatic aberration and curvature of the field for better or worse, depending on how the lens is designed. Because the light at the negative is essentially collimated the glass has little if any effect on the image forming path other than increasing it by approximately 1/3rd the thickness of the glass. If the glass is not homogeneous it can creat some problems because any differences in index will show up as shadows. This is really of academic more than practical interest because most glass now is pretty uniform. I think the main problem with glass type holders is dust. That is controlable but takes some effort. I've crossed swords with Bob Salomon here more than once but must agree with him on this one. -- --- Richard Knoppow Los Angeles, CA, USA |
#18
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The difference in enlarging lenses
"jjs" wrote in message ... HypoBob wrote: Glass between the negative and the lens affects the focus slightly, depending on the thickness of the glass. It does that by refraction, of course, which means that the effect varies with thickness of the glass as well as color of the light. [...] That reads like impressionistic keyboard engineering. I've never seen it IRL. A plane parallel glass sheet will have an effect on image forming light passing through it. The amount of effect depends on the "vergence" of the light and the properties of the glass or other material. If the light is collimated, that is, if the light waves are parallel, as they are when coming from a great distance, there is no effect. If the light is convergent or divergent the sheet will introduce spherical aberration and chromatic aberration. The former is because light will be bent depending on its original angle of incidence. The second is really due to the same thing except that the index of refraction of glass is not constant with wavelength. Glass bends, or deviates to use a more correct term, light more for blue than for red. The amount of this difference is called dispersion. In lenses this is compensated by using combinations of positive and negative lenses of different kinds of glass. The above is the reason that filters can have a degrading effect on a lens. When a filter is used between a lens and a distant object the effect is insignificant. When used where the light is angled (vergent), as on the back of a lens in normal photography, or on either side for macro/micro photography, the effect can be significant. That's why thin gelatin filters are used where the best quality imaging is done. Gelatin has a low index of refraction, so its effect is small, plus gelatin filters are very thin compared to glass. In an enlarger, where the glass is close to the negative or transparcency, it has little effect on the light or on the quality of the reproduction. If it were next to the lens, it would have such an effect because the relative angles of the light passing through it would be much greater. -- --- Richard Knoppow Los Angeles, CA, USA |
#19
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The difference in enlarging lenses
jjs,
Please be more careful with your snipping. You have made it look like I made the statement below, which I did not. Thanks, The real HypoBob ---------------------------------- jjs wrote: HypoBob wrote: Glass between the negative and the lens affects the focus slightly, depending on the thickness of the glass. It does that by refraction, of course, which means that the effect varies with thickness of the glass as well as color of the light. [...] That reads like impressionistic keyboard engineering. I've never seen it IRL. |
#20
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The difference in enlarging lenses
Bob Salomon wrote: In article , Patrick Gainer wrote: HypoBob wrote: jjs wrote: In article , Bob Salomon wrote: [... ] In a glass negative carrier as the lenses are designed to be used with? Bob, are you saying that the glass carrier is part of the optical formula? Does a glass carrier do more than simply keep the negative flat? Tell me it does so that I have some rationalization for the dust I put up with. John, Search this ng's archives for articles by Michael Gudzinowicz about enlarging lenses. He gives the optical formulas for depth of field at the negative stage. Doing the computations for your situation will most likely convince you that a glass carrier and a "perfectly" aligned enlarger are necessary for the best results your system can deliver. Bob Glass between the negative and the lens affects the focus slightly, depending on the thickness of the glass. It does that by refraction, of course, which means that the effect varies with thickness of the glass as well as color of the light. Some lenses are designed to be used with a glass carrier of certain thickness. Whether any of us mortals could tell the difference is another matter. In any case, you can see how glass might affect chromatic aberration and curvature of the field for better or worse, depending on how the lens is designed. Any shift in focus would be compensated for when the enlarger is focused. It is a non-issue. -- To reply no_ HPMarketing Corp. Not true. The focus can only be compensated for one wavelength. The prism in binoculars affects the chromatic aberration of the system and is accounted for in the design. |
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