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AF Working Range ... aperture

Erick Boileau , Jun 13, 2004; 01:10 p.m.

I am a bit lost in specifications

for PowerShot G5 Maximum Aperture : f/2.0 ? 8.0 (W), f/3.0 ? 8.0 (T)

is it the same than ? >>>

for EOS DIGITAL REBEL AF Working Range : EV 0.5 -18 (at ISO 100)

or is it equivalent to >>> f/0.5 ? 18 ??

thank you


Mark Ci , Jun 13, 2004; 01:29 p.m.

I have no idea what you've posted for the Powershot. Looks like meaningless gibberish.

A full specification for a lighting level includes an EV value plus an ISO. Sometimes the ISO is assumed to be ISO 100 and omitted.

Since a faster lens gathers more light, you need to include this as well as EV/ISO. So a full specification for AF sensitivity would be something like EV 1 @ISO 100 (f/1.4).

Mark Ci , Jun 13, 2004; 01:33 p.m.

Lower EV values imply less light and so greater sensitivity, by the way. But only if the ISO and f/ values are the same (which they often are: 100 and f/1.4 seem fairly standard).

Erick Boileau , Jun 13, 2004; 01:37 p.m.

then certainly for powershot G5 the meaning is EV 2 -8 (at ISO 100)

means that you can open/close from 2 to 8 for powershot G5 and for the Rebel from 0.5 to 18 .. right ?

thank you

Mark Ci , Jun 13, 2004; 02:02 p.m.

Well, looking at the G5 spec sheet, those numbers are simply the speed of the lens. At the wide setting the lens is an f/2-f/8. At the telephoto setting it's an f/3 to f/8. Which is a little weird, but whatever. Doesn't have anything to do with AF sensitivity, about which nothing is said.

NK Guy , Jun 13, 2004; 02:05 p.m.

I think you're a bit confused with the terminology, actually. The maximum aperture values for the PowerShot camera refer to the largest lens aperture available to you. The larger the lens aperture the more light the lens lets in. The built-in lens has a variable largest lens aperture which depends on the focal length setting (wide or telephoto) of the lens.

On the other hand, autofocus working range refers to the light levels at which the camera's autofocus mechanism can work. This working range is measured in EV at ISO 100, or exposure value.

The Digital Rebel does not have a built-in lens. It has interchangeable lenses, though it typically ships in a kit with an EF-S 18-55 lens. Since it hasn't got a built-in lens there's no point specifying the maximum aperture for the camera - you have to specify the maximum aperture for the lens attached to the camera.

So what are you interested in? The maximum aperture of a camera or lens? Or the ability to autofocus at low light levels? They are different things.

http://photonotes.org/ articles/beginner-faq/

Jean-Baptiste Queru , Jun 13, 2004; 03:42 p.m.

The former is a specification for the lens (the f/2.0 - f/3.0 part matches the f/3.5-4.5 part of an SLR lens specification like 28-105mm f/3.5-4.5).

The latter is actually only a partial specification, lacking an aperture specification (typically measured with a 50/1.4). It means that with a 50/1.4 the auto-focus will work in a dark street (EV 0.5 at ISO 100, i.e. LV 0.5 is a dark street) all the way to a snow landscape in the sun (EV 18 at ISO 100, i.e. LV 18, is a slightly shiny white object in sunlight). With a slower lens (like an f/5.6) or with filters the camera will need more light to auto-focus, in this case LV 4.5, which is a fairly brightly lit street.

Erick Boileau , Jun 14, 2004; 01:55 a.m.

thanks to everybody

I was just reading in canon website the specification of this 2 cameras to try to understand the (big) difference , but the 2 pages are nearly in another technical language

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