You can ask ƒ/Calc what ƒ/32 plus 5 stops is, and it’ll happily inform you that it’s ƒ/181. You can put 2 mm in for the focal length in the Spot Meter calculator and get a completely ridiculous answer. Or, tell the Magnification tab you want to focus on something ⅛ inch away, and it’ll try its best to convince you that you’ve actually got a microsocope there, not a camera.
You might not care that none of that is practical with your average camera. Sometimes it’s educational to get the answers to nonsense questions.
But, if you are using ƒ/Calc to get the answer to real-world questions, it can be annoying if it gives you an answer that simply doesn’t work. Maybe you’re using the Field of View calculator to figure out if you can capture your subject with a given setup before you travel to the location. You could end up wasting a trip if the successful answer depends on having a lens that focuses closer than the lens you actually own.
This is why the Lens Advisor exists. If ƒ/Calc knows about your lens, you can pick it from the lists below the main calculator area, and it will then double-check any input values or answers against known properties for that lens. It won’t prevent you from getting an answer if it depends on a bogus setting, but it will warn you about the bogosity.
Each lens that the Lens Advisor knows about has one or more “features” that let you filter the hundreds of lenses down quickly so you can find the one you want. For instance, if you want to find the original version of the Canon 28-105 mm ƒ/3.5-4.5 zoom lens, you could select the discontinued, telephoto, and zoom features. On a Mac, you select multiple features at the same time by holding down the Command key while you click each one; on all other platforms, it’s the Ctrl key instead. When multiple features are selected, the Lens Advisor only shows lenses with all of the features; the purpose of the feature is to make the lists shorter, not broaden the search.
Some of these feature names have different interpretations depending on who you’re talking to, so they're defined in the glossary.
Companies occasionally replace an existing lens with a newer version. Sometimes the differences are so slight that when you only look at the specifications ƒ/Calc cares about, the only thing different is the name. Yet, both lenses will be in the Lens Advisor’s tables. (If you know of a discontinued lens that isn’t in the Lens Advisor, drop me a line and I’ll see about adding the missing lens.)
I’ve done it this way to avoid confusing people who know exactly which version of the lens they have. The whole purpose of the Lens Advisor is to free you from having to memorize your lens’ specifications, so consolidating similar lenses into a single entry would be a mistake.
A lens with a wide zoom range can simultaneously be considered wide, standard, and telephoto. These terms just indicate whether the lens includes the given range, not that it is limited to that range.
These definitions are given in terms of diagonal angle of view, not focal length as some people try to do. A 50 mm lens is a 50 mm lens no matter what camera you mount it on. If someone tells you that a 50 mm lens designed for a 35 mm film camera is “equivalent” to an 85 mm lens when you use it on a digital camera with a sensor smaller than 35 mm film, what they mean to say is that it has a smaller angle of view. The focal length remains the same. This isn’t just pedantry; if you lie to ƒ/Calc, it will give you wrong answers in revenge.
If a given lens works on cameras with different sensor sizes, we always make the judgement in terms of the largest sensor size the lens works with. For instance, in the case of Canon SLRs, there are a relatively few lenses in the line that only work on the 1.6× crop sensor size common to all the lower-end cameras. We categorize these few in terms of the angle of view on the digital cameras it works with. Most Canon lenses also work on 35 mm bodies, so we categorize them in terms of the lens’ angle of view with a 35 mm frame size. You can use the Lens Advisor’s digital-only feature to discriminate between these cases.
ƒ/Calc’s angles of view feature can help you to think about this issue more clearly.