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Thread: Clarification of Near-Far F-Stop Procedure

  1. #1
    dperez's Avatar
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    Clarification of Near-Far F-Stop Procedure

    Hello guys and gals,

    I own an Arca Swiss F-Line Basic.

    My question is as follows:

    I was reading Quang-Tuan Luong's article regarding determining the proper f-stop and while for the most part the article made sense and was written quite clearly, I find that I do need some clarification regarding the process.

    When measuring the position on the near and far focus spread where do I measure from? Or does that even matter as long as I am consistent? (ex: the gap between the front and rear function carriers measured from the base, or should I measure the distance between the inside of each format frame taken at a point from the center of each format frame?) Iím just not clear about this and how one takes movements into account.

    I have attached an image to help illustrate what Iím asking here.

    Thanks in advance for your help.

    -Daniel

  2. #2
    Richard M. Coda
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    Re: Clarification of Near-Far F-Stop Procedure

    The way I do it on an Arca (as taught by Rod Klukas) is...

    using your back rail's ruler, focus near and remember what position the back of the function block ends up on (say 32mm). Then focus far and see where the back of the block ends up and remember (say 26mm). That distance is 6mm. For 4x5, multiply by 5 (for 8x10 by 2.5) and that is your minimum f-stop for everything to be in focus.

    Then bring your focus back by HALF, in this case 3mm (to 29mm). Check your focus again with the min. f-stop and adjust if necessary.

    That said, this does not always work, especially with "canyons" or voids that dissect a scene. In that case, find the middle, focus, stop down, adjust as necessary, and expose.
    Last edited by Richard M. Coda; 8-Jan-2009 at 13:39. Reason: forgot something...
    Photographs by Richard M. Coda
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    Primordial: 2010 - Photographs of the Arizona Monsoon
    "Speak softly and carry an 8x10"
    "I shoot a HYBRID - Arca/Canham 11x14"

  3. #3

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    Re: Clarification of Near-Far F-Stop Procedure

    If you focus on near then far by only moving the front standard, wouldn't all of the distances (changes) indicated by your arrows be the same?

    Obtaining an accurate measurement between the top of the front and rear carriers would be difficult.

    Beautiful camera!

  4. #4
    dperez's Avatar
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    Re: Clarification of Near-Far F-Stop Procedure

    Quote Originally Posted by Eric James View Post
    If you focus on near then far by only moving the front standard, wouldn't all of the distances (changes) indicated by your arrows be the same?

    Obtaining an accurate measurement between the top of the front and rear carriers would be difficult.

    Beautiful camera!
    I was thinking the same thing, but then I wasn't sure how that would work in conjunction with movements and so forth.

    I should have indicated that the camera pictured is not mine.

    Thanks for the help guys, I appreciate the clarification.

    -DP

  5. #5
    Downstairs
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    Re: Clarification of Near-Far F-Stop Procedure

    I've always held the belief that, on the camera rack, half way between nearest and furthest focus corresponds to the mythical third of the focus spread on the ground.
    I stick some tape on the rack, focus, and make two marks with a pen, then set the focus on a third mark in the middle. Front focus or rear focus works the same (strangely enough). I can judge by experience what stop is needed to cover the spread. I did a test years back with layed-out newspapers and seeing was believing.

  6. #6
    Richard M. Coda
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    Re: Clarification of Near-Far F-Stop Procedure

    Further clarification on MY post... you should always focus using the rear standard. I thought that was understood. You use the movement of the back standard to measure from.
    Photographs by Richard M. Coda
    my blog
    Primordial: 2010 - Photographs of the Arizona Monsoon
    "Speak softly and carry an 8x10"
    "I shoot a HYBRID - Arca/Canham 11x14"

  7. #7

    Re: Clarification of Near-Far F-Stop Procedure

    A number of people mention adding a ruler to the camera bed in this thread. It is easy to start with. The focus spread in mm and the corresponding f-stop is also on a chart in the thread.

    http://www.largeformatphotography.in...ad.php?t=44307

    All I have to do is put the scale back on my "push tab" DOF knob that is mentioned and it's finished. All I will have to do to find the optimal DOF is; focus on a distant spot, push the tab to set the DOF knob to zero (without coming out from under the dark cloth), focus on a near spot, come out from under the dark cloth, split the focus spread in half on the knob and the optimal f-stop is listed.

    I'll post some pictures and start a thread in a couple of days when it's done and I have more time but you can look at "making a DOF knob" on the hompage to get an idea of what it's about. All I have done is made it possible to set the knob to zero without having to come out from under the darkcloth, which should save time.

    Starting with a ruler is good and you may not be inclined to make a focusing knob after all, as many people don't use them.

  8. #8

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    Re: Clarification of Near-Far F-Stop Procedure

    With respect to your question about movements and their effect on the "near-far" system - you make all of your movements first, then measure. For example, if you're using front forward tilt, get the tilt right and then do the "near-far" measurement.

    I almost always focus with the front standard and I don't use an Arca so I don't know how much of this will be useful but FWIW - I tape a length of a paper or fabric mm ruler along some part of the camera that is more or less flush with anything that moves as the front standard moves. This is usually somewhere on the top of the bed of a field camera. The length of the ruler isn't critical, I usually make it as long as possible but you mostly just want it to be long enough so that it can be used with your shortest and longest lenses. I use double-sided Scotch tape and it stays on forever.

    It may also help when counting the mms from near to far to cut out a little paper arrow-head shape and tape it somewhere on the front standard more or less flush with your mm ruler. Linhof Technikas are great because they already have a metal arrow that Linhof thinks will be used with their distance scales but for other cameras I've just made my own little arrow. Remember that the absolute numbers on the ruler don't matter (i.e. you don't have to start at 1), all you care about is the number of mms the front standard moves so the particular numbers on the ruler are irrelevant.

    This is one of those things that's simple to do and could be demonstrated in about 10 seconds but is difficult to describe in writing. Hope this is reasonably clear and helps.
    Brian Ellis
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    Re: Clarification of Near-Far F-Stop Procedure

    Quote Originally Posted by Brian Ellis View Post
    With respect to your question about movements and their effect on the "near-far" system - you make all of your movements first, then measure. For example, if you're using front forward tilt, get the tilt right and then do the "near-far" measurement.
    I have an A-S, as well, which is helpful because it has the millimeter markings on the rail. I do what Brian does and get the tilts and swings, etc., right first, then measure the spread. If your scene allows you to experiment with different tilts/swings, do so, then choose the one that gives you the narrowest spread or the effect you want.
    Mike

  10. #10

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    Re: Clarification of Near-Far F-Stop Procedure

    The distances you indicate in your thumbnail are possible measurements of bellows extension. You can get the focus spread by measuring any of those distances for near and far point and then subtracting, but that would be a crazy way to do it. Instead, tape a scale to the rail, if there isn't one there already, and then note the position of the standard you are moving to focus in relation to that scale. It doesn't matter which point you use.

    What you want to measure is the change in the position of the standard along the rail. So you have to pick some point on the standard you are moving and note its location relative to the rail. If you also do a tilt or a swing, you wouldn't want to note the position of some point on the standard relative to the rail, tilt the standard, and then move the standard along the rail. You want to measure any focus spread either before tilting or after tilting. Another possible complication is parallax error. The greater the distance from the reference point on the standard to the rail, the more the angle at which you look affects the measurment. So ideally, you should pick your reference point on the standard to be very close to where the standard attaches to the rail.



    Let me describe what I consider the proper way to use focus spread since there is a lot of confusion about this. First, consider the case where you don't intend to do a tilt or swing. In that case, you find the positions on the rail when the standard is at the near point and at the far point. The correct place to focus is generally halfway between those points. (But there are circumstances where because of the nature of the scene you may want to favor either the foreground or the background, in which case you would place the standard closer to the corresponding point.

    You then determine the f-number by some rule depending on the focus spread, which is the distance between the near point and far point in mm. If you ignore diffraction, the rule is that you divide the focus spread by 2 and then divide that result by whatever coc you find useful. A typical value for the coc for 4 x5 format would be 0.1 mm. So in that case you divide by 0.1, which is the same as multiplying by 10. This gives the simple rule: multiply the focus spread by 10 and divide the result by 2 (or vice-versa). For example, for a focus spread of 4 mm, you get 40/2 = 20. You then round up to the nearest f-stop or fraction thereof, in this case to f/22. But in using such a rule, you should keep in mind there there are inevitable errors in focusing and that lens aberrations may reduce DOF, so it would be wise to add a safety factor of up to an additional stop. In the above example, you might want to stop down from 1/2 to 1 full additional stop beyond f/22.

    If you also want to take diffraction into account, you can use one of several rules. the one described in the lfphoto web page is that due to Paul Hansma, It is based on a formula too complex to be used in the field, so most people just carry a table with them that tells the the proper f-stop based on the focus spread. In my experience, the Hansma suggestions are only valid for very large focus spreads, e.g. larger than 6 mm, at least for 4 X 5 format. For smaller focus spreads, just stopping down slightly more than a no diffraction rule suggests will be enough. But it may be worth looking both at the simple rule and the Hansma rule and stopping down somewhere in between.

    When you tilt, focus spread is used in two ways. First, when determining the tilt, your aim should be to reduce the focus spread between a near point and a far point in a desired plane of exact focus to essentially zero. If you focus on the far point and the have to move the standard further away from the lens to get the near point in focus, you need to increase the tilt. If you focus on the far point and you need to move the standard closer to the lens to get the near point in focus, you should decrease the tilt. With experience, you should find after only a few iterations of this procedure that the focus spread between near and far point is undetectable. (For swings, replace high and low by right and left.)

    Where you want to put the plane of exact focus containing the near and far points depends on the scene. For example, you may want a plane at ground level to be in focus, but that plane is also the lower limit of what you want to remain in focus. You should choose that plane so that there are several points in it to check focus. But you may not end up setting the point at which you focus at the reference plane used to determine the tilt. See below.

    After you have set the tilt angle, you can use a different focus spread to determine the f-stop. Namely, focus on the highest point above the plane of exact focus you want in focus and then on the lowest point below the plane of exact focus you want in focus. Ideally the high and low points should be equidistant on the gg on either side of where you focus. If they aren't, you should place the focus point halfway in between. In some cases, you may want to reconsider which reference plane you used as above to set the tilt angle. (As in the no-tilt situation, in some cases, you may also want to favor the high point or the low point in which case you would shift in that direction.)

    To determine the f-stop, you now use the same rule you would use in the no-tilt case, but this time use the focus spread between the high and low points. In most cases, this will lead you to stop down too far rather than not far enough, so usually it will be innocuous.

    It should be added that when determining the proper f-stop, you should also stop down to the calculated f-stop and examine the scene to see as best you can what is in focus. Most people can stop down to at least f/16 and still see reasonably well. Beyond that things begin to get too dim to see much of anything. The following rule of thumb may be useful in the case of tilts or swings (but not in the notilt/swing case.). Stop down as far as you can and still see clearly. Measure the vertical (horizontal for swings) extent in focus on the gg at some fixed distance from the lens. Find what fraction of the desired extent that is, and then multiply the f-number by the reciprocal of that ratio. For example, suppose when you stop down to f/16, you can see easily that 15 mm vertically on the gg is in focus at some fixed distance from the lens, but that you want 40 mm to be in focus. Multiply the f-number by 40/15 (reciprocal of 15/40) or 8/3. The proper f-number is 8 x 16/3 ~ 43. so you should be okay if you stop down to f/45. Keep in mind however, that there may be some vriation in what you consider in focus, so this method should be compared with the previous methods based on the focusing spread between high and low points.

    There is a corresponding rule which can be used in the no-tilt situation. Note where in the scene the limits of near and far focus are after stopping down to where you can still see. Then open up to full aperture, focus on each of these points and measure the focus spread between them. Now use in this case as a multiplier the recioprocal of the fraction this is of the full focus spread you want.

    Finally, it might be added that measuring focus spread on the rail by using a scale attached to the rail, you will not be able to measure small focus spread, e.g. 1.5 mm, very accurately. Fortunately, you would mostly want to stop down further than the rules suggest, so this may not matter. For example, using the rule I outlined above, 10 x1.5/2 = 7.5. But in such a case you probably would want to stop down to at least f/11 or f/16 in any case.

    One way around this is to place a focusing scale on the fine focusing knob. Most cameras use gearing such that a large rotation of the focusing knob is converted to a much smaller movement along the rail. In order to take advantage of this, you have to do some measurements and calculations. I described how I did this with my Toho FC-45X at

    http://www.math.northwestern.edu/~le.../dof_essay.pdf.

    See also www.largeformatphotography.info/dofknob/

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