The Local version of ƒ/Calc allows you to edit the lens table used by the Lens Advisor.
When ƒ/Calc Local starts up, it checks for a saved local copy of the lens table. If it doesn’t find it, it downloads a copy from the ƒ/Calc web site and saves it to your computer’s hard disk. From that point on, the only time ƒ/Calc downloads new lens information from the web site is when you request it (see reverting changes below) or when a new version of ƒ/Calc sees that your local copy is untouched. Once you change your local copy of the lens table, ƒ/Calc will never overwrite it with the default lens table unless you ask it to.
To make any change to the lens table, you will need to have a lens maker’s name selected in the Lens Advisor. That done, you will see the lens editing buttons in the upper right corner of the Lens Advisor panel. At first, it only shows the Add and Revert buttons. If you select a lens as well as a maker, you will then see the Edit and Delete buttons as well, as they operate on the selected lens.
To create a new lens, click the Add button in the upper right corner of the Lens Advisor. This will open a window that will let you enter information about the lens. Many of the fields have default values, but there isn’t enough information at the start to immediately save the lens, so the Save button will be disabled. At minimum, you need to give the lens a name, a focal length, and a minimum focus distance[1] that’s at least as long as the focal length.
You should be able to find the minimum focusing distance in the specs for the lens. If you can’t find this information and it’s billed as a macro lens, 4 or 5 times the focal length is a fair guess. Otherwise, make it 10 times the focal length. So for example, something in the 200 to 500 mm range is typical for a 50 mm lens.
You can get a better answer by measuring it, which isn’t hard. Stand near a flat, textured surface; an unpainted brick wall is excellent for this. Get close enough to the wall so that if you turn to face it, the front of the lens is only a few focal lengths away from the wall. Then turn in place so the camera is pointing down the wall at a fairly shallow angle, and try to focus.[2] Keep doing that, turning more and more towards the wall until you can no longer focus. If you find that you can still focus when pointing the camera straight at the wall, you’re not close enough yet, so move closer to it and try the experiment again. To be conservative, measure from the front of the camera body to the closest point on the wall where you were able to focus, not from the front lens element. See subject distance for more on this topic.
If you are entering information for a zoom lens, check the Zoom box, which will add a second focal length field, split the “AoV terms” slider so you can give a range, and add two more aperture fields.
The need to have four aperture fields requires a bit of explanation. Unlike fixed-focus (“prime”) lenses, most zoom lenses have variable maximum apertures, and many of those also have variable minimum apertures as well. That is, as you change the focal length, the minimum and maximum aperture value you can use may also change. A complete specification list for the lens will have this information, but beware that a lot of marketing information on lenses omits this, especially on the minimum aperture end. If the aperture isn’t variable, just enter the same value in both fields. For instance, the values 2.8, 2.8, 22, and 29 would specify a zoom lens with a fixed maximum aperture across the entire zoom range, but which has a minimum aperture that can get a little smaller as the focal length increases. A more typical zoom lens might use the values 3.3, 4.5, 22, 32: both the maximum and minimum apertures vary by about one stop.
All of the other items in the Add Lens window are optional. They let you set the lens’ features, used by the Lens Advisor to help you sort through a big lens table to find the one lens you want. See the glossary for definitions of the terms used here.
To change an existing lens, select it in the Lens Advisor. This will enable the Edit button in the upper right corner of the panel if it wasn’t already showing. When you click it, you get the same window as used for adding a lens, filled with the lens’ current information. If you click the Save button to close the window, ƒ/Calc immediately applies your changes to the local copy of the lens table.
To remove an existing lens, select it in the Lens Advisor. This will enable the Delete button in the upper right corner of the panel if it wasn’t already showing. When you click it, ƒ/Calc immediately removes the lens from the lens table.
To make ƒ/Calc re-download the default lens table, click the Revert button in the upper right corner of the Lens Advisor Window. ƒ/Calc will ask if you really want to replace your existing lens table. If you accept, it deletes the local copy and goes out to the ƒ/Calc web site and pulls down a fresh copy for you. It can take several seconds for this to happen, so be patient.
If you just want to get back to the stock lens table, you can use the Revert feature.
When you make any change to the lens table, ƒ/Calc throws the
old lens table file into the operating system’s Trash (Mac OS
X) or Recycle Bin (Windows). If you open that in a text editor, you
will find instructions at the top of the file that tell you how to
restore it. The specifics depend on details about your particular
computer, so I can’t give much more detail here. ƒ/Calc
figures these details out right as it’s saving the
lenses.xml file and writes these instructions
into the file.