"Focusable" focusing loupes for view cameras?greenspun.com : LUSENET : Large format photography : One Thread
I read some old threads on this site which brought up the correct way to focus an image on the ground glass: The general idea was to make sure that you are viewing the image that's being formed by the lens on the frosted side of the ground glass (and not any other plane).
Two suggestions to accomplish this were to either use a simple hand held magnifying glass (like those used by stamp collectors)to get a magnified view of the image formed on the frosted side of the ground glass, or to use a "focusable" focusing loupe. With such a loupe, you "calibrate" the focus by removing the camera lens from the front of the view camera so that the ground galss is bathed in unfocused light, then adjust the loupe's focus by viewing the ground glass, focusing until the grains on the frosted side of the ground glass were sharpest.
Now the equipment question is: can anybody out there then recommend a focusable focusing loupe compatible with view camera use? I would be particularly interested to know if the Wista 7x loupe, which is long and can be used through a folding hood, is focusable... I do realize that many loupe have "diopter correction", but I'm pretty sure this relates to calibrating for variations in eyes, and not to adjusting where the actual plane of focus is. Thanks for your help in advance. Andre
-- Andre Noble (firstname.lastname@example.org), November 01, 2000
I have used many types of loupes and the best one I have come across (surprisingly) is a 4x Hoya focusable loupe. It comes with a frosted collar and an opaque collar with a case. The best part about it is the absolute edge to edge sharpness! Put it up against a Schneider and see the difference... I did and was surprised and top it off, It cost $60. USD.
-- Scott Walton (email@example.com), November 01, 2000.
Here's another, I wear glasses so I use a jeweler's loupe on my glass frame. stays out of the way and is handy
-- carson (firstname.lastname@example.org), November 01, 2000.
I use a focusable chimney finder (5x, Nikkor glass) from a Bronica S2A. It has a little curvature of field, but allows for quite precise focus in the center and the base is square, so it fits into the corners.
-- David Goldfarb (email@example.com), November 01, 2000.
I have tried several loupes on 4X5 and find that 7 times magnification is perfect on my Linhof GG+Fresnel. I use a Horseman which provides a bit greater eye-GG distance than jeweler's loupes which means you will not have your nose against the Fresnel. For that matter there is also a Horseman, 6" long with 6X magnification but I do not find it as sharp and it is heavier. Greater than 7X I find counterproductive: the image becomes more diffused because of the grain in the GG. Less than 7X I find insufficient for best focus. I use 3.75X prescription lenses which allow me to see the whole frame from a very short distance. With these, images in the GG may appear extremely sharp, until I use the loupe, that is, for the loupe always shows that focus was less than optimal using the prescription lenses. My Linhof GG is inscribed with a 6X7 frame on the GG side. By focusing on the frame until it is sharpest you get the loupe on the right plane. If your GG is not inscribed, focus on a sharp black object by moving the lens forward and backward until the image is best, then view through the loupe addjusting it to best contrast. By iteration you will optimize the loupe's correct position relative to the GG.
-- Julio Fernandez (firstname.lastname@example.org), November 01, 2000.
I have experimented extensively in this area and can offer some assitance. All new modern loupes have more than sufficeint Depth of Field to focus on whatever lies in their path! If the image is formed on the grainy side of the gg, than that is exactly what you will see looking through the loupe, because that is the only source of light and also it is well within the DOF of the loupe. As far as I can discover, this concept of focussing on the grainy side of the gg was a throwback to yesteryear when loupes had very tiny DOF ranges and needed a method to move the DOF area to encapsulate the grainy side of the gg. Today the focussing components, when they do exist, are used primarily for eye corrections. If you want to experiment place a chrome under some layers of glass on a light box, this will simulate the thickness of the gg, you will see the image in perfect focus. If your loupe can not do this, than obviously you should adjust the foucus accordingly.
As for magnification.... I once read that no gg can capture an image with sufficeint detail to benefit from high magnification. A 4x loupe is plenty of power considering a 10x loupe would offer you the same foucus position...I have both and learned through experiments thats just the way it is.... the grainy side of a gg is there to provide a plane which is exactly equal to where the film plane lies...not for producing an image that can be enlarged sufficeintly 10x.
The best field loupe I have seen are the Toyos, they have rubber coated end to ensure the gg from being scratched and have just enough power, I think their new one is like 4x, while the older model was 3.6x. They have no focus capability. They are also reasonably priced. Better loupes are made for light box examination of chromes.
-- Bill Glickman (email@example.com), November 02, 2000.
Bill, thanks for a reply that didn't "skip to the end of the movie". You succinctly got to the heart of the original question, which was basically: How can one be sure he's focusing on the same plane as that of the sheet film?
(And I would also like to thank other commenters as well for useful information.) Andre
-- Andre Noble (firstname.lastname@example.org), November 03, 2000.
Bill: I suspect that your statements about depth of field need some thinking. Depth of field as far as I know depends on magnification and if that is so, there is not optical design trick that can give a loupe with a fixed magnification greater depth of field. Lower magnification loupes may indeed have greater depth of field and would account for not seeing a difference between fresnel focusing and GG focusing with 4X loupes. I think that some in the LF forum have more in-depth knowlege of optics than I and would like to hear from them.
-- Julio A. Fernandez (email@example.com), November 03, 2000.
Julio, I agree with you comment about DOF being an factor of magnification. However, my point was this, most all loupes when placed on the gg, have sufficeint DOF to reach the grainy side of the gg. As I mentioned above, simply put a chrome on a light box, move the loupe up and away from the light box...notice how far you can move it upwards before the chrome becomes soft, measure this distance. Now measure the thickness of the gg. As long as the gg measurement is less than the DOF measurement you acheived by raising the loupe off the lightbox, then your loupe requires no adjustments and will easily focus on the grainy side of the gg.
When a LF guru once convinced me that this was not the case, I and 3 other LF users all ran the same experiment, and all had the exact same results. DOF was always sufficeint to reach the grainy side of the gg. The test is simple and should convince anyone of what their loupe / gg is capable of. It's always better to test yourself to feel confdident.
Your point is well taken though, as magnficaiton increases, DOF becomes more shallow, and possibly with a 10x loupe an adustment may be required. However, in my opinon 10x loupes are over kill anyway. The 10x sucking loupe I own is designed to be mounted on the gg, so it has never suffered this DOF focus problem. (I no longer use it after discovering it offers no better critical focus than a 4x loupe for the reasons mentioned in my previous post) It is possible that a high magnification loupe (8x or greater) designed for a lightbox might suffer this problem on a gg. I never researched that scenario.... so if anyone wants to use high magnification loupes on gg, they should run this lightbox experiment to be sure ... hope this clears things up...
-- Bill Glickman (firstname.lastname@example.org), November 04, 2000.
Bill: thanks for clarifying. Now that I see what you meant I agree with you. I now see that what you called depth of field was really focusing range, which in some loupes like the Leitz is quite shallow and for glass wearers can be problematic. Must confess I took that for granted but as you point out, it is very important. Thanks.
-- Julio Fernandez (email@example.com), November 04, 2000.