Ah I was thinking of depth of field, sorry, yes, do the test at infinity focus. Note to self: never offer advice before the morning coffee break.
Anyway a wide open lens should be fast enough for a good test.
REfract , not defract, (rab knows, but R is really close to D on the keyboard) between the non-ground side facing lens and the GG imaging surface.
The thickness of the glass blank between the lens and the groung glass image shifts the focal point further (longer) by the an amount = glass thickness/glass index of refraction.
My method of checking is to palce a scrap piece of film into a film holder and place it in the camera. With a lens board but no lens fitted, measure the distance from the emulsion of the film to the front of the lens board using a steel rule.
Then remove the film holder and take the same measurement - this time it's from the inside face of the ground glass to the front of the lens board. They should be the same.
If you're worried about scratching the ground glass, put a few strips of tape over the end of the rule (before you take the first measurement).
Yes but there is no refraction in the center where the light rays are perpendicular and there is increasing refraction towards the edges where the light rays are at an increasing angle to the GG. So if you wanted to critical focus a wall, then if the center was infocus the corners wouldn't be and visa versa.
Originally Posted by Murray@uptowngallery
(Murray knows this but I just want to make it clear for the OP so he goes with the ground side facing the lens)
Yes its just like depth depth of field except it happens either side of the film plane. If you imagine an X where the left side of X is the light rays from a point in the subject. The center of the X is where the film should be and the right side is where the light rays would project behind the film.
Originally Posted by Ray Heath
Depth of focus is the distance from the left edge of X to right side of the X where the height of the X is the maximum acceptable circle of confusion (coc) or airy disc as some like to call it. If the film is shifted left or right in relation to where the Ground glass surface is then it shifts the focus plane of the subject which is also shifting the near and far depth of field points.
I'm not good on the maths for this but getting your GG surface distance correct removes any doubt about it being the cause of focus problems.
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this is all very interestingt guys, but does it matter in everyday use
can it be accurately measured?
Steve suggests a piece of tape over the end of the ruler, wouldn't that throw off the measurement if hundredths of a mm is the kind of effect you are allowing for?
actually, it's not even possible to measure to that accuracy with a steel rule
so, if the film plane and gg plane were different by up to say 3mm, would it matter, would it be noticable on the resulting print?
what about the effect of the film bowing out in it's holder?
Yes it can be measured with a simple depth micrometer. 3mm is a very large amount. If you imagine a 35mm slr which has film under tension with a pressure plate, the margin of error in film plane placement is minimal. The same is true for medium format cameras. You will often hear large format photographers tell you the jump in quality from 35mm to medium format is much bigger than the jump from medium format to large format. Yet the 35m format size is 864mm², 6x6 is 3136mm² and 4x5 is 10800mm² or thereabouts. So 6x6 is approx 3.6 times the area of 35mm and 4x5 is approx 3.4 times the area of 6x6. The increase in size to 4x5 is similar to the increase in size to 6x6 from 35mm so why is the jump to medium format bigger than jump to 4x5?
Originally Posted by Ray Heath
The answer is because obtaining film flatness and correct film plane registration on 4x5 or larger cameras is very difficult to achieve and if you don't achieve it, then you are throwing away the benfit of 4x5.
Applying tilts and swings makes it even more difficult and requires extreme precise focussing and film plane registration otherwise you are better off with medium format or larger than 4x5.
So to answer your question, 3mm is a massive amount, especially if you have for example, applied some tilt to obatain near to far sharpness. If the film plane is out by 3mm, what you thought was going to be sharp near to far will all be unsharp. View camera movements also narrow the depth of field so there is less margin for error. How much error is acceptable? You aim for zero error. If you don't aim for zero then you are wasting your time with largeformat.
Sorry, I'm not good at explaining what I know, and often not good at knowing what I try to explain. The cone of light at f/16 is 16x longer than the diameter of the cone. 1/16 of 2mm is .125mm.
Originally Posted by Jim Jones
Nothing is more dangerous than Americans trying to think in the metric system... :-> Wasn't a Mars lander lost because of a wrongly applied conversion?? Yes, 3mm is an awful distance for wrongly put gg. Definitely not a correct way to put the gg as you found it. The correct registration is always tricky and must be solved with the camera construction in mind. Which probably explains why you get different advice (some of it very good).
Steve's method works accurately, at least, as accurately as your eyes allow you to measure, and the tape won't matter if you have it on the ruler for both measurements. I'll add the following caveats:
Use a long ruler, and have the other end resting on a reference point in front of the camera. A slight change in angle will through off your measurements.
It doesn't matter what the distance from film plam/gg to lensboard is, since all you're checking is that the distance matches exactly. When I check my cameras, I usually adjust the extension to get an even reading, like 10 inches.
And yes, the ground side should always face the film, as it is very susceptible to temporary marks (like oily fingerprints) and permanent marks (like scratches from loupes and any other contact with hard objects). Having the ground surface on the outside would insure a damaged surface that just gets worse and worse.
Oh, and if the image is upside down, take the ground glass off and rotate it 180 degrees...