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  #41  
Old 10th March 2020, 02:02 PM
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Does this mean that when using a blue/green light enlarger, one would have different focus points using either blue or green or both? And if so wouldn't the print using both blue and green light have a sharp/soft focus effect?
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  #42  
Old 10th March 2020, 02:17 PM
John King John King is offline
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I suppose so yes - in theory. But in practise any discrepancy should be covered by the depth of field. However, starting of with the lens fully focussed at full aperture it will be sharp whatever the 'F' number is.

When I use my Rodagon APO lens to print B&W, I can see a colour fringe on an edge where there is good contrast If the lens is not fully focussed correctly it is red one way but by adjusting the focus to correct this, the fringe turns green. When it is spot on the fringes disappear completely. With this you can see a definite softening of the grain. With a colour negative there are no fringes at all.

If I do the same with my non APO Rodagon 80/5.6 (Quite old) the fringes never appear at all.
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  #43  
Old 10th March 2020, 03:13 PM
Alan Clark Alan Clark is offline
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I think we need to be clear about what Gene Nocon was actually saying, because he has been somewhat mis-quoted by John in this thread, and in the other one.
If you haven't read Gene Nocon's book and have only read these posts, then you might believe that Gene Nocon said that to focus accurately you need a blue filter on the grain magnifer - full stop. But he didn't actually say this.
To paraphrase him what he did say was that photographic paper is mostly sensitive to blue light. So if you use an enlarger with a cold light head light, which is a blue light, then you can focus accurately without needing to use a blue filter on the focus magnifier., because you are seeing, in effect, what the paper sees. He said the problems start when you use an enlarger with a tungsten or normal incandescent bulb, as these have a warmer colour. He actually says that this may not be a problem if you are using a normal negative as this will have a bluish cast which may counteract the warmth of the light. But these bulbs cause problems when used with chromogenic film because this film has a warm cast. This is when he found (he claims) that you need a blue filter on the grain focusser. Without it you are looking at light that is too red and will throw the focus.
John has said that Gene Nocon discovered the need for a blue filter when he was using grainy film. But he wasn't; it was fine grain chromogenic film. This was what was used to print the two comparison photos that show the model's armpit.
The other reason for the confusion is that Gene Nocon doesn't actually write very clearly about any of this. You have to kind of guess what he is trying to say. But to be fair to him he didn't make a big song and dance about the need for a blue filter when focussing. He covers the entire subject in a couple of pages. My interpretation of what he said is that you only really need a blue filter when using chromogenic film in an enlarger with a tungsten or normal incandescent bulb. Was he right? Well, I don't know because I have never used chromogenic film. But when I tried the blue filter with normal film (i.e. FP4 and HP5), it made no difference.

Alan
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  #44  
Old 10th March 2020, 06:11 PM
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Perhaps complicating the chromogenic film thing. Kodak chromogenic B&W film had quite a warm cast but Ilford's XP-2 is not the same.
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  #45  
Old 15th March 2020, 09:24 PM
Nat Polton Nat Polton is offline
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I have just found the blue filter that came with my Peak original model.
Adjusted the eyepiece, an old Ilford HP4 negative, Durst enlarger with 12v 100w lamp in.
The biggest difference I can see is the image is a lot more contrasty with the blue filter on. Without the filter it is difficult to tell what you are looking at. Filter on and the picture becomes contrasty and easier to see the detail of the image itself.
So I think it is to see the image clearer, but there does not seem to be any change in the grain visibility.

I shall have a bit more of a play around later in the week when I have some spare time.
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  #46  
Old 17th March 2020, 09:30 AM
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I use a Peak and have tried the blue filter a few times but to my eye I can't see any advantage. But I have heard some Peak usrers praise it's use.

Neil.
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  #47  
Old 17th March 2020, 10:57 PM
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If you use a colour enlarger 20m + 60c = blue 80a filter
17m + 53c = blue 80b try it to see if it helps you to focus at full aperture.
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  #48  
Old 18th March 2020, 12:56 PM
Terry S Terry S is offline
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Now that's a way of doing it, that I never thought of Norm.

It'll be interesting to hear peoples views if they try it.

Terry S
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