yigitaltay Posted October 5, 2016 Share #1 Posted October 5, 2016 Advertisement (gone after registration) I have a very limited knowledge on the grain focusers. I know that they have a mirror and you focus on the grains instead of the image. It is totally understandable. But the part I couldn't get is that you are not doing the focusing on the paper level (mirror of focuser always stands higher above the paper). So how that really works? Quote Link to post Share on other sites More sharing options...
Advertisement Posted October 5, 2016 Posted October 5, 2016 Hi yigitaltay, Take a look here How Does a Grain Focuser Work?. I'm sure you'll find what you were looking for!
yigitaltay Posted October 5, 2016 Author Share #2 Posted October 5, 2016 I am using this kind of focuser btw. Welcome, dear visitor! As registered member you'd see an image here… Simply register for free here – We are always happy to welcome new members! Quote Link to post Share on other sites Simply register for free here – We are always happy to welcome new members! ' data-webShareUrl='https://www.l-camera-forum.com/topic/265253-how-does-a-grain-focuser-work/?do=findComment&comment=3123919'>More sharing options...
erl Posted October 5, 2016 Share #3 Posted October 5, 2016 My understanding is that the mirror creates a virtual image for you to focus on EXACTLY the same distance that the mirror is above the paper. Hope this makes sense for you. Quote Link to post Share on other sites More sharing options...
yigitaltay Posted October 5, 2016 Author Share #4 Posted October 5, 2016 My understanding is that the mirror creates a virtual image for you to focus on EXACTLY the same distance that the mirror is above the paper. Hope this makes sense for you. I thought of that too but the virtual image that is created by the mirror should have the same distance between the mirror and the film (not between the mirror and paper). So it still puzzles me. Quote Link to post Share on other sites More sharing options...
yigitaltay Posted October 5, 2016 Author Share #5 Posted October 5, 2016 Oh but wait. I always assumed that they are using a plain mirror. If the mirror in the focuser is convex or concave you can adjust the focal distance in a way that it is matches with the distance to the base-plate. I think that solves my problem Quote Link to post Share on other sites More sharing options...
pop Posted October 5, 2016 Share #6 Posted October 5, 2016 I would think that the virtual image was created by the loupe in the focusing aid. The mirror merely reflects the real image that is projected by the enlargers lens. 3 Quote Link to post Share on other sites More sharing options...
yigitaltay Posted October 5, 2016 Author Share #7 Posted October 5, 2016 Advertisement (gone after registration) I would think that the virtual image was created by the loupe in the focusing aid. The mirror merely reflects the real image that is projected by the enlargers lens. A loupe does not create a virtual image but a mirror does by law (scientific one). Quote Link to post Share on other sites More sharing options...
pop Posted October 5, 2016 Share #8 Posted October 5, 2016 A loupe does not create a virtual image but a mirror does by law (scientific one). There seems to be some kind of misunderstanding. When looking through a loupe, you will see a virtual image, typically one that is larger than the object or the real image you are looking at. A flat mirror does not make any images at all, neither virtual nor real ones, albeit it will reflect both kinds. The lens of the enlarger, OTOH, will project a real image onto the paper under the enlarger. 1 Quote Link to post Share on other sites More sharing options...
mjh Posted October 5, 2016 Share #9 Posted October 5, 2016 With a grain focuser you have a folded light path that must be exactly as long as the light path between lens and paper. Just draw a sketch showing the light path with and without the focuser and you will see why the mirror has to occupy the position it does. 1 Quote Link to post Share on other sites More sharing options...
honcho Posted October 5, 2016 Share #10 Posted October 5, 2016 (edited) Oh but wait. I always assumed that they are using a plain mirror. If the mirror in the focuser is convex or concave you can adjust the focal distance in a way that it is matches with the distance to the base-plate. I think that solves my problem You seem to be creating a problem for yourself where no problem exists. You have a Paterson grain focuser. Slacken-off the eyepiece locking screw below the eyepiece. Adjust the eyepiece up or down until the piece of wire across inside the focuser tube is in sharp focus and tighten the locking screw. The grain will then snap in and out of focus as you adjust your enlarger focus mechanism - as long as you keep your head out of the light path. Edited October 5, 2016 by honcho Quote Link to post Share on other sites More sharing options...
erl Posted October 5, 2016 Share #11 Posted October 5, 2016 With a grain focuser you have a folded light path that must be exactly as long as the light path between lens and paper. Just draw a sketch showing the light path with and without the focuser and you will see why the mirror has to occupy the position it does. Ah, yes mjh is correct. That is what I was trying to visualize. Simplicity and accuracy. Thank you mjh. Quote Link to post Share on other sites More sharing options...
wlaidlaw Posted October 6, 2016 Share #12 Posted October 6, 2016 To understand real and virtual images this: makes it all evident. Wilson Quote Link to post Share on other sites More sharing options...
Jeff S Posted October 6, 2016 Share #13 Posted October 6, 2016 Some focuser instructions indicate the need to place a piece of paper under the focuser, the same thickness as the one used, for best accuracy... http://www.largeformatphotography.info/forum/showthread.php?76092-Looking-for-Peak-Micromega-Model-1-Manual&p=726659&viewfull=1#post726659 Excerpt from Peak Model 1 operating instructions... FOCUSING Set up the enlarger for printing; size andcompose the print; throw the enlargerinto approximate focus, and with themirror cover open, place the Micromegaon the easel. For highest accuracy, keep a scrap of enlarging paper under theMicromega to position the base of the instrument at the exact paper plane. Jeff 2 Quote Link to post Share on other sites More sharing options...
pico Posted October 6, 2016 Share #14 Posted October 6, 2016 (edited) What Jeff wrote. Do that. I use the Micromega (long mirror). It's great. (In fact, I just got a brand new mirror for it.) Edited October 6, 2016 by pico 1 Quote Link to post Share on other sites More sharing options...
Jeff S Posted October 6, 2016 Share #15 Posted October 6, 2016 (edited) I always adhered a piece of my favorite paper to the bottom of the focuser....no fuss necessary. Jeff Edited October 6, 2016 by Jeff S Quote Link to post Share on other sites More sharing options...
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