I have not been able to find this in my searching.

What lense is equivalent to 20X optical for an SLR? How do I compare lens zoom ability?


TIA

On an SLR-type camera, a 300mm telephoto lens would be 6x (300/50). A 20x telephoto lens would be 1000mm - telescope territory. A 500mm lens with a 2x teleconverter will also give you 1000mm. Are you getting the 20x from a P&S spec or somewhere else?

Telephoto and zoom are separate terms. A zoom lens is any lens with a variable focal length. A telephoto lens typically means anything longer than normal view (usually in reference to lenses 100mm and longer). You can have wide-angle zooms, normal zooms, and telephoto zooms. You can also also have wide-angle, normal, and telephoto prime lenses.

For zoom lenses, taking the higher focal length and dividing by the lower gives you the zoom ratio (not the same as zoom amount). A 70-200 lens is ~3x zoom lens, but gives about a 4x zoom equivalent at the long end. A 17-85 lens has a 5x zoom ratio, but is a 1.7x zoom on the long end. Generally the smaller your zoom range, the better your image quality will be, which is why prime lenses (fixed focal length) typically have better image quality than zooms.

Hopefully something in all that answered your question.

on the right (EF lens 101/ focal length comparison).<ul><li><a href="http://www.usa.canon.com/consumer/controller?act=ProductCatIndexAct&amp;fcategoryid=152">http://www.usa.canon.com/consumer/controller?act=ProductCatIndexAct&amp;fcategoryid=152</a</li></ul>

<ul><li><a href="http://www.usa.canon.com/app/html/EFLenses101/index.html">EF Lens 101</a></li></ul>

To add to what Tristan said...

A lot of P&amp;S camera reviews will give you a spec like "7x zoom", and somewhere near that, they will give the focal length equivalent like "300mm" (I made up that number...I have no idea what the true equivalent is). The problem is that focal length assumes you're using a standard 35mm film camera.

Things get a little complicated when you move to the digital world. While focal lengths on lenses and specs assume you're using a full frame sensor (same size as a 35mm film), the fact is, every digital sensor on a consumer dSLR (sub $2K price point) is smaller. So what happens is the image is effectively cropped. The other way to look at it is you're getting a 1.5-1.6x increase in the focal length indicated on the barrel of the lens.

So, if the P&amp;S camera specs say 7x zoom (300mm equiv), and you buy a Canon 30D dSLR, take 300 and divide by a factor of 1.6, and you'll come up with ~190mm being the focal length that would give you an equivalent field of view on that dSLR (compared to the P&amp;S).

to a focal length equivalent on a dSLR to maintain the same field of view. I don't think the numbers Tristan mentioned correlate to P&amp;S cameras.

I did pull the 20X optical from a POS camera figure

Take the camera linked below. It's widest focal length in 35mm equiv. is 38mm, and a 10X zoom provides a 380mm equivalent.

If you had another camera whose widest focal length was 28mm and they advertised a 10X zoom, that would yield approx 280mm.

So the 10X zoom designation between various point and shoot cameras doesn't allow for a direct comparison (12X is not necessarily a longer reach than 10X, it depends on the camera).<ul><li><a href="http://www.dpreview.com/reviews/olympussp500uz/">http://www.dpreview.com/reviews/olympussp500uz/</a</li></ul>

you were referencing p&amp;s to me.