Lens Length vs. Magnification

[leonarda]

Can anyone tell me how to calculate the effective optical magnification for a given telephoto lens length? For example, if a 50mm lens is considered the normal baseline, does a 300mm lens magnify the image 6 times?

When I read about point-and-shoot cameras, the description usually includes a statement about the camera's lens such as "3X optical zoom." Is this just a statement about the zoom range of the camera, equivalent to saying that an SLR lens is a 50mm-150mm zoom lens?

This question has come up because I recently bought a D300 kit with the standard 18mm-55mm lens. When my wife looked through the camera and zoomed the lens, she complained that the zoom wasn't as powerful as the zoom on my daughter's Canon S400 point-and-shoot, which is around 3X I think. I intend to purchase a zoom telephoto lens for the D300 (looking at the 28-135 or 70-200 f4) and I'd like to calculate how these lenses would compare in their zoom effects to that of point-and-shoot cameras' zooms.

Thanks in advance for your help.

[hmhm]

The "3x" in "3x optical zoom" is just the ratio between the longest and shortest focal lengths the zoom offers, so the 18-55 is a 3x zoom. This doesn't tell you how long or short the lens goes, the 100-400L lens is a 4x zoom, but if there was such a thing as a 10mm-50mm lens, that would be a 5x zoom. Ultimately, you just have to look at the short and long ends of the range to see what the lens offers.

Most "point and shoot" cameras give their focal length ranges in "35mm equivalents". In other words, the lens may be 7mm-21mm, but with a small sensor they might give the same "angle of view" as a 35-105mm lens would on a 35mm film camera (or 35mm full-frame digital SLR). You could think of a typical point and shoot as having a "5x crop factor", just as your 300D is sometimes said to have a "1.6x crop factor".

To calculate "35mm equivalent" focal length for your 300D, just take the _real_ focal length (e.g. 18-55) and multiply by 1.6. If you do that, you come up with something like 29mm-88mm, and the S400 specs say 35mm equivalent is "35mm-108mm", so your wife is right.

Then again, if you crop your 300D image a little bit, you can get a 4MP image that corresponds to a focal length slightly longer than the S400's, in case you need a "defense" for this issue. :)

And, of course, it's really not all that hard to find a lens for your 300D that will beat the S400 in terms of reach...
-harry

[yb98]

I have already ask the same question :wink:
Look here :

http://photography-on-the.net/forum/showthread.php?t=29133&highlight=

Yacine.

[MarkH]

The 3x zoom will have 3x the focal length at max zoom compared to min zoom. Unfortunately this tells us little about what the zoom range is (commonly 35-105, but not all are the same).

The marketing for the mass market often involves simple but impressice terms like 3x or 4x zoom. A much more accurate description would be something like 35-105mm or 35-140 or 38-114 or whatever.

On the 300D the 18-55 lens gives the equivalent field of view of a 29-88mm lens. One of the best features of an SLR camera like the 300D is that you can change lenses, so if you want a zoom with much more telephoto then you can get one and fit it to the camera.

I mainly use a Canon 28-135 IS Lens, equivalent to a 45-216mm (when compared to a 35mm film camera). This is an excellent lens with a very useful focal length and an Image Stabiliser which allow a steady shot hand held at slower speeds than normal. The 18-55 is a good lens to keep for a cheap wide angle lens.

[leonarda]

Thanks for the information. Between the explanations given here and those previously given to yb98's posting, I understand the zoom versus magnification questions.

Arnie