Focal Length Explanation

[olgap]

I am looking for a chart/explanation of focal length. If you know of a good one, could you give me the URL?

Thanks.

[Mr Guy]

I'm sorry, that's a bit broad. What are you trying to find out?

Focal length is the distance the light travels after hitting a lens to the point where the light is measured (ie, the film, your eye, the sensor). It is determined by the optical design of a specific lens. In general, the shorter the focal length the less magnification takes place and the wider the field of view, so people refer to small focal lengths as "wide angle" (around 0mm to 20mm) and long focal lengths as "telephoto" (roughly 70mm and up). The range of focal lengths that magnify an image so the viewfinder image appears similar to what you'd see with the naked eye are called "normal" or "standard" lenses. (right around mid 20s to 70mm). 50mm on a 35mm film camera had almost exactly the same view through the view finder as the naked eye, so 50mm came to be called a normal lens, but the definition has broadened somewhat. For a digital APS-C camera, a true "normal" lens is right around 30mm.

[olgap]

Thanks Mr. Guy,

I realize my question was vague, but I am going to upgrade from a fixed lens (Fuji 5200) to a DSLR (probably Nikon D90) and the cost of lenses is daunting. I like to do extreme closeups and landscapes, however, considering cost, I want a camera that will do everything else as well, so understanding lenses is critical. The D90 plus a short telephoto (a lens for which I have been told I will have to purchase a close-up attachment for macro) plus one mid-range lens is about C$2,400. at Henry's.

I was hoping for a chart that would show how different focal lengths relate to subjects, shown in meters/cm. While wanting to understand the subject, perhaps I should just have asked which lens for the D90 would be best for extreme closeups (flowers, insects, mosses, lichens, fungi) and which would be best for 'normal' shooting. From your explanation, I gather should I should buy the widest angle lens I can afford for closeups.

I did Google "focal length explanation" with over 800 hits and decided to check with forum members before starting to wade through all the available information.

[Mr Guy]

Ah! Now that's a very different question altogether!

Fixed lenses don't really relate to DSLR focal lengths in an easy to compare way. For taking extreme close ups, you'll be happiest with a "macro" lens, which isn't directly a product of the focal length like you may think it is. "Macro" in camera terms means that the size of the image on the sensor of the camera is the same as the size of the subject in real life. So when you think about it, that means you can blow them up HUGE when you view or print the image. There are ALSO some lenses that call themselves macro lenses that aren't really one to one size wise. They are typically 1:2 (or half as large on sensor) or 1:3 (a third as large). They can give decent results, but for those truly mindblowing close ups, 1:1 is probably worth the money.

I'm not an expert on macro lens, but I do know that Macro has more to do with their optical properties and their minimum focus distance than it does their focal length. I know they also come in all different focal lengths. I'm GUESSING that that the reason for that is that different lenses allow you to get your macro shot at different distances from the subject.

If extreme close ups are your thing, then what you want is a macro lens in general. What you do with the rest of your time more or less determines what kind of macro you want.

What you want as an every day walk around lens is an entirely different question. It's a hard one to tackle for someone going from a super zoom to a DSLR, because you're used to having one camera and lens that can solve pretty much all your problems. With a DSLR, you get much more power and much better results, but it comes at the cost of less flexibility. There are several different opinions on what makes a good "every day" kind of lens.

1) Maximum flexibility. There are super zooms that give you the most flexibility, but they can be heavy, and aren't necessarily the best quality. This would be something like a Nikkor 18-200mm.

2) Best bang for the buck. It's becoming very popular to learn what kind of lenses you really want by deliberately limiting yourself to a single focal length until you can understand what that lens and camera can do inside and out. A very popular way of doing this is to either use the kit lens that generally comes bundled with a camera (usually a 18-55mm or a 50-200mm) exclusively until you find out what you can't do with it but you really want to (for example, your macro shots).
Another very popular way to do that is to buy a single focal length (called a "prime" as opposed to a "zoom") lens that's higher quality than an inexpensive zoom lens would be and exclusively using it. Again, the idea is to force yourself to learn by discovering what your personal limitations are. The most popular length is 50mm, mostly because that USED to be the normal lens (as explained above) and because Canon and Nikon both offer very inexpensive reasonably good quality 50mm (nicknamed "nifty fifty") lenses. Pentax, on the other hand, offers compelling reasons to go closer to a real normal lens on a APS-C camera by offering a variety of extremely good primes like a 40mm pancake lens (it's really, really thin).

[inkista]

Originally Posted by olgap

I was hoping for a chart that would show how different focal lengths relate to subjects, shown in meters/cm.

Don't know of anything that precise, but for vague generalties , there's a little Canon focal length demonstration that's a good ballpark guesstimate.

Another good source for basic lens information/descriptions, and what all the numbers and acronyms mean is this lens primer.

While wanting to understand the subject, perhaps I should just have asked which lens for the D90 would be best for extreme closeups (flowers, insects, mosses, lichens, fungi) and which would be best for 'normal' shooting. From your explanation, I gather should I should buy the widest angle lens I can afford for closeups.

Um... no. As Mr. Guy explained, you probably want a macro lens, which is optimized for close-focus work (I'd go into a tedious description of floating elements, but I don't want your head to explode ). A wider lens will let you get closer, but the perspective may make it look farther away and there can be distortion and vignetting (light falloff in the corners). With macro lenses, you may even want a longer focal length so you'll have more working distance.

Where you might want a wider lens is if you're planning on adding further magnification by reversing a lens in front of your macro lens. Magnification when reversing lens x on lens y will be y/x. So, if y is 100mm, and x is 50mm, you'll get 2x magnification; if x is 35mm, you'll get almost 3x. I think. It's been a while since I looked that stuff up.

As for general purpose shooting, most folks prefer a walkaround zoom, in the wide-to-normal range. The 18-55 kit lens that ships with my dSLRs is limited, but low-cost, and a good example of a walkaround zoom. But it won't come close to doing "everything else."

[olgap]

Started an evening course last night at our local college - Basic Digital Photography. With that and the information you both gave me, things should become clear. Thank you for taking time to answer.

[olgap]

What is the difference, quality-wise, between using a macro lens and using a macro adapter ring with a reversed lens?

Enjoying the Basic Digital Photography Course and now looking at a Nikon D5000.