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RobertSwe Nov 25, 2004 2:57 PM

How can you tell if a lens has high "light-sensitivity"? How do you differ bad lenses from good?

PhilR. Nov 25, 2004 8:45 PM

<<How can you tell if a lens has high "light-sensitivity"? >>

Lenses are not "light sensitive". They merely let light pass through. How much light actually gets through to the light-sensitive CCD/film varies from lens to lens.

<<How do you differ bad lenses from good?>>

By measuring the resolution of the lens in question against a known standard.

Phil R.

RobertSwe Nov 26, 2004 1:36 AM

Ok, but that was what I wondered... how can you tell if a lens will let through much light?

PhilR. Nov 26, 2004 8:27 AM

Most lenses have an aperture through which the light passes. The size of the opening varies - the more open, the more light gets through. The more light gets through, the "faster" the lens is. There is a number system to more or less indicate how fast a lens is. Most lenses have this number stamped on the front ring. The lower this number, the faster the lense.

A fast 35mm lens can have a number as low as 1.2. An average 35mm lens about 1.7. The higher the magnification (which also increases individual glass elements in the lens), the less light gets through, and the higher the number. A 35mm telephoto might range from 3.5 to 5.6.

Since I've not used a tremendous amount of digital cameras, I don't know what is the fastest lens you can get on a dc. I do know however that when one comes from 35mm, that the 2.8 max. aperture on my 12x zoom digital camera is unbelievable when the price is considered.

At equal price levels, there isn't a lot of difference in lens speed among the cameras from the major makers. If you really do need a fast lens, then stay away from the mega-zooms, unless you have the money for a good DSLR.

NHL Nov 26, 2004 9:06 AM

How bright or 'fast' a lens is determined by its F-stop. This is a ratio between the effective diameter's aperture of a lens against its focal length: ;)

JimC Nov 26, 2004 9:11 AM

With a prime (non zoom) lens, you will see one aperture listed.

With a zoom lens, you usually see two apertures listed (the maximum available aperture at wide angle, and the maximum available aperture at full zoom). Some higher quality zoom lenses can maintain a constant aperture throughout their zoom range.

Aperture is a ratio of thefocal length of the lens and thesize of the aperture iris opening.

When shopping for lenses, the aperture scale (in one stop increments) goes f/1.0, f/1.4, f/2.0, f/2.8, f/4.0, f/5.6, f/8.0, f/11, f/16, f/22... With each one stop move to a smaller aperture (represented by larger f/stop numbers), you will need shutter speeds twice as long for proper exposure.

Usually, only prime (non-zoom) lenses have very large apertures (represented by smaller f/stop numbers). A high quality zoom lens may have a constant f/2.8 aperture (which is considered bright for a zoom lens). But, the brighter the lens, the larger and heavier it's going to be.

Here is achart you can use to get anidea of the shutter speeds required for any EV and Aperture (but make sure to use your camera's metering, as lighting can vary -- this is only to give you an idea of how it works). It's based on ISO 100. So, each time double the ISO speed, you can use shutter speeds twice as fast:

Aperture also impacts Depth of Field. Thelarger the aperture (represented by smallerf/stop numbers), and the closer you are to your subject (focus distance), and the longer your focal length (amount of zoom used), the less depth of field you will have (less of the scene in focus, as you get further away from your focus point).

A lens with a larger available aperture (represented by smaller f/stop numbers) is preferred for some subjects (so that you can blur backgrounds, helping your subject stand out).

Here is an online depth of field calculator. Plug ina camera model, then change focal length, aperture and focus distance to see what impact aperture has. Note that you must use the actual (versus 35mm equivalent) focal length of the lens for computing depth of field (you'll see a link to a table with information for popular cameras).

Note that most Digital Cameras have a crop factor (the sensor is smaller than 35mm film, so the entire image circle is not used). For example, you must multiply the actual focal length of the lens by 1.5 or 1.6 on most Digital SLR models to get the 35mm equivalent focal length (same angle of view), since their sensors are smaller.

As far a lens quality, it can vary widely (even within the same manufacturer). Some lenses may be soft at one end of the zoom range, and may be soft unless the aperture is stopped down some, etc. As a general rule, the greater the focal range (wide to tele), the lower the quality - at least at some focal lengths.

Some good sources of lens ratings are here:

You'll also see some sections with detailed user reviews of lenses, and a database with more user ratings on the main page:

Another good source of lens ratings is here:

Also, keep in mind that many of the user reviews were made with the lenses mounted on 35mm cameras. So, if you have a bit of corner softness at some focal lengths/apertures with a particular lens, it may not be bad enough to be noticeable ona DSLR model (because of the crop factor).

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