If this is your first visit, be sure to check out the FAQ by clicking the link above. You may have to register before you can post: click the register link above to proceed. To start viewing messages, select the forum that you want to visit from the selection below. |
|
|
|
Thread Tools | Display Modes |
#11
|
|||
|
|||
Focusing Scope with BLUE filter?
"PATRICK GAINER" wrote in message ... Richard Knoppow wrote: "2" wrote in message ... I picked up a focusing scope which has a deep blue filter that flips into the eyepiece. (In fact, it's in there as the default position.) What's the purpose of it? To enhance viewing contrast? Focus UV? Its supposed to prevent mis-focusing due to chromatic aberration in the enlarging lens. The idea is that printing paper is sensitive mostly to blue light and the eye to all colors. In the early days of photography, when lenses were not very good and plates sensitive nearly exclusively to blue and UV light focusing by the visible image would often result in blured pictures. A lens was said to have visible focus and _chemical_ focus, the latter being the focus for the blue light the early plates were sensitive to. Focusing using blue light is supposed to eliminate the error caused by the chromatic aberration of the lens. Even some pretty good enlarging lenses made up to the end of the 1940's can have enough chromatic to make a difference. Most modern enlarging lenses of good quality are very well corrected for chromatic and have _chemical_ and _visual_ focus which are the same. Add to this that modern variable contrast papers are sensitive to a range of colors from green to blue so that the enlarging lens has to be well corrected to get sharp images. There is another problem which can be caused by the filter. Some eyes have a significant amount of chromatic and some spectacles are also not very well corrected, so, you will have to adjust the eye lens of the grain focuser _with_ the filter in place to eliminate problems which may be caused by the eye itself. For most modern lenses the filter will make no difference. I did some research on this subject some years ago and presented the results in Photo Techniques. All human eyes have chromatic aberration. However, the idea that using a blur filter to focus for blue sensitive material does not work. It introduces two problems. First, the eye's maximum resolution is at its most sensitive wavelength, which is the yellow-green. Using any other wavelength for focusing introduced a greater random focusing error than did white light. Second, the eye's focal length varies with color. Combined with the reduced resolution, the effect was more random error with either red or blue focusing as well as a shift in focus of as much as 10 mm at the baseboard in a 10X enlargement. You can demonstrate the shift in focus of your eye with red and green color separation filters. Focus as close to a printed page as you can without a filter. Interpose a filter and see which direction you must move the object to bring it to closest sharp focus. The net result is that the least random focusing error as well as the best agreement with modern enlarging lenses will be obtained by using no filter. The random part of the error will be the more important as with proper use of a focusing crosshair the mean error will be minimized. I agree with this. The blue filter is based on a misunderstanding. -- --- Richard Knoppow Los Angeles, CA, USA |
#12
|
|||
|
|||
Focusing Scope with BLUE filter?
"PATRICK GAINER" wrote in message ... wrote: RE;Richard Knoppow wrote: Its supposed to prevent mis-focusing due to chromatic aberration in the enlarging lens. ... In the early days of photography, when lenses were not very good and plates sensitive nearly exclusively to blue and UV light, focusing by the visible image would often result in blured pictures. Focusing using blue light is supposed to eliminate the error caused by the chromatic aberration of the lens. I can't see anything there to argue with. I would state the case a little differently. The blue filter is there to assist in focusing the image on blue only sensitive materials. A Graded paper is the usual blue only sensitive target. If the lens is corrected for only the blue, as I believe is the case for some process lenses, then use of the blue filter is almost manditory. Dan I doubt that you will find a lens that is corrected for only one color. That amounts to no correction of any kind, or to perfect correction at any color as long as you expose it to only one narrow color band. If you do the experiments you will find that the blue filter gives no better focusing for blue sensitive materials than no filter. It is not the blue sensitive material that does the focusing, but the human eye. It is a demonstrable fact that the resolving power of the human eye is very poor at the wavelength of blue to which graded paper responds. It is customary to achromatize lenses for blue and green for that reason. The choice of color for achromatizition depends a lot of the age of the design and intended purpose of the lens. Very old designs were achromatized for blue and green but more modern lenses are often achromatized for blue and red. The deviation from focus beyond the two colors depends on the closeness of the match between the glasses used. Blue correction is also better in later designs than in earlier ones because glass types with a better match for anomolous dispersion became available. An example of a fairly early design corrected for blue and red is the Taylor, Taylor, and Hobson Speed Panchro lens. This was a Biotar type designed for professional motion picture cameras using panchromatic film. many earlier motion picture lenses were corrected for orthochromatic film and did not perform well at the red end of the spectrum. The Speed Panchro dates from the early 1930's and is another result of the change to sound in movies. This, for various reasons, required a change to panchromatic film. Anomolous dispersion is the result of using glass near its cut-off wavelength. At this point the dispersion begins to increase very rapidly. In most of the pass band of the glass the dispersion changes fairly regularly with wavelength but at the far blue end it can change rapidly enough so that the glass type being used to correct it can't keep up. So, the chromatic aberration becomes very great. I think this is the problem Kodak had with their early enlarging lenses (pre-WW-2). In their set up instructions for their auto-focus enlargers they recommend not focusing visually when using these older lenses because their "chemical" focus an visual focus does not co-incide. Later lenses, like the Kodak Enlarging Ektar and Enlarging Ektanon, do not have this problem. Kodak did NOT recommend focusing with a blue filter but, rather, experimentaly to find the best focus. I agree about the loss of sharpness of the eye at the far ends of the spectrum. -- --- Richard Knoppow Los Angeles, CA, USA |
|
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
'Protection Filter' Recommendations For High Quality Lens? | Burt | Digital Photography | 6 | May 10th 06 01:15 PM |
Newbie's question about UV filter | Craig | Digital Photography | 19 | April 21st 06 02:29 PM |
25/30/37/58mm Infrared 'X Ray' filter - SONY DV Cameras | yeo seng tong | Digital Photo Equipment For Sale | 0 | September 4th 04 03:59 AM |
25/30/37/58mm Infrared 'X Ray' filter - SONY DV Cameras | yeo seng tong | Digital Photo Equipment For Sale | 0 | July 4th 04 09:09 AM |