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Hey guys! Was searching around on this site and found the ultra-cool Quickdisc.... Really want to try this out but the ruler is a bit small for 11x14. (Or I can do some pretty incredible enlargements....either way) Has anyone developed a longer measuring device that they could share? I sent Philipp Salzgeber (creator of the disc) an email but haven't heard back yet.
Thanks!
--A
Alec,
The QD is scaleable, as it works on the principal of reproduction ratio on the gg, so you can scale up the disc and the measuring device to produce the same result.
If you are talking about very large reproduction ratios, then you can simply make a longer 'yardstick', but you'll need to work out the ratios. However, that is simple enough to do. If I recall correctly, the formulas are in the documentation for the QD.
---Michael
Out of curiosity, the quickdisc wouldn't be accurate for telephoto lenses, would it? SInce the QD relates focus distance to bellows extension, it has to assume normally designed lenses...right? I don't know enough about tiehr subject to say for sure, but I'm curious :D
If you are not averse to doing some calculations using a scientific calculator, you can make your own 'Quickdisc'. The basic principle is to estimate the magnification in the plane of exact focus by taking a circle of known diameter and measuring its image on the gg. If the divide the latter by the former, you get the magnification. So suppose you are doing 8 x 10. You could make your disc to be 150 mm in diameter. If you measured 75 mm on the gg, for the image diameter, you would know the magnification is 75/150 = 0.5. I've given a couple of tables below which you can use to determine the change in number of stops based on the magnification, and you can skip to there if you want.
Once you know the magnification, you get the crucial multiplier for exposure calculations by adding one to it. So, in the above example, the multiplier would be 1 + 0.5 = 1.5. The best way to use this number is to reduce it to a number of stops or fractions of a stop and then to adjust the exposure appropriately, either by opening up by that number of stops or by increaqsing the exposure time appropriately. The relation between the number of stops and the multiplier is given by the formula
2 x log(multiplier)/log(2)
This can be found by using a scientific calculator. (Note that every computer operating system interface gives you a scientific calculator as an accessory.) But most people wil find it most convenient to prepare a table and carry it with them. Here is such a table. The first number in each pair in the list is the magnification and the second is the number of f-stops change. It goes up in increments of 0.1 for the magnification from 0 (at infinity) to 1 (for scale 1:1).
0, 0; 0.1, 0.28; 0.2, 0.53; 0.3, 0.76; 0.4, 0.97; 0.5, 1.17; 0.6, 1.36; 0.7, 1.53; 0.8, 1.70; 0.9, 1.85; 1, 2.
So in the above example, with a magnification of 0.5, you would have to open up 1.17 stops, which you could reasonbly either round up to 1 1/3 stops or round down to 1 stop. In the second case, that would mean you could instead double the exposure. (Generally, each stop change corresponds to doubling the exposure or moving to the next slowest speed.) Of course, for intermeidate values of the magnification, you would interpolate, probably rounding up to the nearest third of a stop.
It is easy enough to create a scale to use directly on the ground glass to read the magnification. Just measure off distances. Thus, if the disc is 150 mm in diameter, a magnification of 0.1 corresponds to 15 mm, of 0.2 to 30 mm, etc. To make it even faster, you can put the number of stops corresponding to that magnification, given in the above table, on the scale instead and read off directly the number of stops change. For that purpose, it might be useful to have a table for reverse lookup, which gives the number of stops in increments of 1/3 stop and the corresponding magnification. Here is such a table
1/3, 0.12; 2/3, 0.26; 1, 0.41; 1 1/3, 0.59; 1 2/3, 0.78; 2, 1.
To encode your scale, just take the disc diameter, multiply it by the second number in the list and put the stop increment at that distance. Here are the stops and distances for a 150 mm disc.
1/3, 18 mm; 2/3, 39 mm; 1, 62 mm; 1 1/3, 89 mm; 1 2/3, 117 mm; 2, 150 mm.
Just measure out those distances on a linear scale and put the appropriate stops. When you put the scale on the gg, with the zero end at one end of a largest diameter, the other end will fall between two stop marks and just round up or down as you feel appropriate.