I think you have received excellent answers so far. I have just a couple of points to make about focusing using a loupe.
First, the "focusing" part of a "focusing loupe" (i.e., one with "adjustable focus") is just a distance adjustment. As Polyglot stated, if your eye cannot focus on the subject of your loupe, you have to change the loupe-to-subject distance. For those of us whose eyes don't adjust much anymore (read 50+), it can be a real convenience to have the loupe always at the right distance from the frosted side of the ground glass, and, since eyes are not all the same in this regard, clever manufacturers have built a distance adjustment into better loupes. That's all there is to it.
That said, it is often devilishly hard to see the corners of an image on the ground glass with loupes of this type, since they are (with the exception of a very few) placed at right angles to the ground glass. This difficulty is especially a problem with shorter focal-length lenses. I mostly use an 8x or 10x loupe designed for viewing 35mm negatives (like your Nikon loupe) reversed, that is, looking through the "bottom" of the loupe. This means I never touch the ground glass with the loupe, and can therefore easily adjust the angle of the loupe to the plane of the ground glass to be able to see the brightest image at the corners. This also means that I have to find the proper focus point by adjusting the loupe-to-ground-glass distance myself. I find this to be no problem.
I personally find 4x loupes to be too weak for fine focusing, although there are many who use them and prefer them to higher-power loupes. FWIW, I use a pair of 3.5 to 4-diopter reading glasses for ground glass viewing and rough focusing (that in addition to my slight near-sightedness) and do the fine focusing with an 8x-10x loupe as described above.
Also, even though we all want to have our loupes at the proper distance from the ground-glass image so that what we see is sharp, even at the wrong distance, "sharpest is sharpest." That means that even if you have your loupe at a distance that does not allow your eye to focus perfectly on the image, when you adjust focus on the camera, the image will appear more or less sharp, even with this compromised resolution. When it is its sharpest (least blurry if you prefer), then it is as sharp as it gets (other small focusing errors excluded). In practice, this means you don't even have to have a really sharp image with your loupe to find the correct camera focus.
Also, since you are new to 4x5 and the attendant focusing techniques, let me just mention that, for most of us who use movements a lot and are interested in exactly what is in or out of focus, fine focusing with the loupe is usually reserved for just a couple of key image points to place the plane of sharp focus and then check depth-of-field (using whatever method you like).
I would recommend you read the article on focusing the view camera here: http://www.largeformatphotography.in...-to-focus.html , and the one on finding optimum aperture here: http://www.largeformatphotography.info/fstop.html . These are a bit daunting and may seem overly complex at first, but are really not that difficult. The time spent mastering these techniques will save you time and wasted materials and is well-worth spending.
One last observation: I use a viewing frame, not the ground glass, to compose my images. Before I set up the camera, I know where the borders of my image will be and, since lens focal length is directly related to the distance of my viewing frame to my eye, I know approximately what lens I will be using. This often means that I don't spend a lot of time viewing and working with the image on the ground glass. I set up, make sure the borders are where I want them (if the lens is slightly wider than the critical area, I just make sure that the image is "in there somewhere") and then check the few critical focusing points with the loupe (while applying movements), stop down to the optimum f-stop and "click." In other words, if you already have composed your image and know you have your camera pointed correctly with the appropriate lens mounted, focusing with the loupe becomes just an aid to adjusting camera movements and determining the proper f-stop for the desired depth-of-field, and can be confined to the few critical points needed for these.
This got longer than I intended, but I hope it helps some.
Originally Posted by arizonafilm
You can focus the loupe in the grid marks. I have vision issues which don't really allow me to se a sharp image on the gg under any circumstances. I merely look through my loupe, rack back and forth past focus and lock when the image is the smallest..i call it the in and out..never misses..don't overthink it...EC
The grid maks are sometimes on the outside of the GG. In which case focussing on the grid marks will lead to focussing error.
Originally Posted by eclarke
To iterate, focus the loupe on the ground surface of the glass. It really is that simple.
Thanks for taking the time to explain your ideas. I got the feeling after awhile that I was either a) overthinking this or b) misinterpreted or got bad advice. With all of you folks' input, I am far more confident. I am now working each suggestion posted and will surely resolve this issue. Thanks again!
Simple procedure...that's the answer I needed! Thanks.
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