52mm/67mm lens difference?

liamo

Hi,

I'm curious about the reasons for (and differences between) 52mm and 67mm lenses.

I have a Nikon D3000 with a 52mm 18-55 and a 55-200 lens. A friend has a D5000 with a 67mm 18-105 lens.

The lenses are interchangeable between the two cameras and the 18-55 and 18-105 are both 3.5-5.6 so there doesn't appear to be any aperture advantage.

I've had a bit of a Google on the subject and can't find any information. Can anyone shed any light on the subject or point me to a URL?

Thanks

Squidgy Black

Err that's just the filter size. It's the diameter of the lens cap and filters that fit on the lens, means nothing else.

bullpost

Its more glass, which means more light, which theoretically would mean faster or better quality. But from your observations , not always.

DaireQuinlan

bullpost wrote: »

Its more glass, which means more light, which theoretically would mean faster or better quality. But from your observations , not always.

There isn't any real correlation you can make between filter thread size and lens speed. I have a bunch of f/2.8 and f/1.8 and f/1.4 lenses for example, all of which take 52mm filters. I have another f/2.8 which takes 77mm filters, and a f/5.6 which takes 67mm filters. So ultimately the filter thread size isn't indicative of anything other than the thread size of the filters you can screw into the lens.

or, in short, what stetyrrell said.

liamo

Thanks for the responses which confirm my own (limited) observations that there isn't an immediately obvious correlation between lens diameter and speed.

That being the case, my question still stands. Why would the manufacturer create lenses with different diameters (and, presumably, additional costs) unless there's a good reason?

I'm going to keep looking on-line for an answer but if anyone has an explanation in the meantime, please shout.

charybdis

liamo wrote: »

Thanks for the responses which confirm my own (limited) observations that there isn't an immediately obvious correlation between lens diameter and speed.

That being the case, my question still stands. Why would the manufacturer create lenses with different diameters (and, presumably, additional costs) unless there's a good reason?

I'm going to keep looking on-line for an answer but if anyone has an explanation in the meantime, please shout.

Different design specifications require different elements and arrangements of elements within a lens. Often, making a lens to the required specifications necessitates tradeoffs in things like size, weight, filter thread diameter, etc..

It would almost certainly be more expensive to make a wide variety of lenses with uniform filter thread diameters because either all lenses would have to use the same filter thread diameter as the lens with the largest filter thread diameter, or the optical engineering would become prohibitively complex and impractical (not to mention physically impossible in certain circumstances).

pixbyjohn

http://www.howeverythingworks.org/page1.php?QNum=1525

Question 1525: Does a bigger camera lens always produce a better picture?

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Is it true that the bigger the lens on a camera, the more light goes through it and the better the photo or video? My film teacher says that while this idea is logically correct, he didn't know if it was true. Your lecture slides say the answer is yes, but my teacher still doesn't believe it. We were wondering about your source for this material. — PJ

I'll assume that by "bigger lens" you mean one that is larger in diameter and that therefore collects all the light passing through a larger surface area. While a larger-diameter lens can project a brighter image onto the image sensor or film than a smaller-diameter lens, that's not the whole story. Producing a better photo or video involves more than just brightness.

Lenses are often characterized by their f-numbers, where f-number is the ratio of effective focal length to effective lens diameter. Focal length is the distance between the lens and the real image it forms of a distant object. For example, if a particular converging lens projects a real image of the moon onto a piece of paper placed 200 millimeters (200 mm) from the lens, then that lens has a focal length of 200 mm. And if the lens is 50 mm in diameter, it has an f-number of 4 because 200 mm divided by 50 mm is 4.

Based on purely geometrical arguments, it's easy to show that lenses with equal f-numbers project images of equal brightness onto their image sensors and the smaller the f-number, the brighter the image. Whether a lens is a wide-angle or telephoto, if it has an f-number of 4, then its effective focal length is four times the effective diameter of its light gathering lens. Since telephoto lenses have long focal lengths, they need large effective diameters to obtain small f-numbers.

But notice that I referred always to "effective diameter" and "effective focal length" when defining f-number. That's because there are many modern lenses that are so complicated internally that simply dividing the lens diameter by the distance between the lens and image sensor won't tell you much. Many of these lenses have zoom features that allow them to vary their effective focal lengths over wide ranges and these lenses often discard light in order to improve image quality and avoid dramatic changes in image brightness while zooming.

You might wonder why a lens would ever choose to discard light. There are at least two reasons for doing so. First, there is the issue of image quality. The smaller the f-number of a lens, the more precise its optics must be in order to form a sharp image. Low f-number lenses are bringing together light rays from a wide range of angles and getting all of those rays to overlap perfectly on the image sensor is no small feat. Making a high-performance lens with an f-number less than 2 is a challenge and making one with an f-number of less than 1.2 is extremely difficult. There are specialized lenses with f-numbers below 1 and Canon sold a remarkable f0.95 lens in the early 1960's. The lowest f-number camera lens I have ever owned is an f1.4.

Secondly, there is the issue of depth-of-focus. The smaller the f-number, the smaller the depth of focus. Again, this is a geometry issue: a low-f-number lens is bringing together light rays from a wide range of angles and those rays only meet at one point before separating again. Since objects at different distances in front of the lens form images at different distances behind the lens, it's impossible to capture sharp images of both objects at once on a single image sensor. With a high-f-number lens, this fact isn't a problem because the light rays from a particular object are rather close together even when the object's image forms before or after the image sensor. But with a low-f-number lens, the light rays from a particular object come together acceptably only at one particular distance from the lens. If the image sensor isn't at that distance, then the object will appear all blurry. If a zoom lens didn't work to keep its f-number relatively constant while zooming from telephoto to wide angle, its f-number would decrease during that zoom and its depth-of-focus would shrink. To avoid that phenomenon, the lens strategically discards light so as to keep its f-number essentially constant during zooming.

In summary, larger diameter lenses tend to be better at producing photographic and video images, but that assumes that they are high-quality and that they can shrink their effective diameters in ways that allow them to imitate high-quality lenses of smaller diameters when necessary. But flexible characteristics always come at some cost of image quality and the very best lenses are specialized to their tasks. Zoom lenses can't be quite as good as fixed focal length lenses and a large-diameter lens imitating a small-diameter lens by throwing away some light can't be quite as good as a true small-diameter lens.

As for my sources, one of the most satisfying aspects of physics is that you don't always need sources. Most of the imaging issues I've just discussed are associated with simple geometric optics, a subject that is part of the basic toolbox of an optical physicist (which I am). You can, however, look this stuff up in any book on geometrical optics

liamo

Thanks, everyone, for the very knowledgeable responses. Its great to learn something new.