P&S Focal Lengths

[wingspar]

I’ve not used P&S cameras much. Only at work, and not at all since I retired in 2005, but I just picked up a used G9. Won’t actually have it in my hands for a few days. When ever I look at the specs for P&S camera focal lengths, it is always written like 7.4mm - 44.4mm (35mm - 210mm equivalent). Having used SLR’s since the late 1960's, this is confusing, and if one looks at the exif data for a for a G9, you would see something like 12.73mm as the focal length, which means absolutely nothing to me.

I just noticed the aperture for this particular photo is f/3.19. Certainly won’t see an aperture like that on an SLR.

Why do they do it this way? There must be some reason, but it’s beyond me. Color me bewildered. ???

[red hot sheep]

The sensors are very very small. So think about what would happen if you put a 24-70mm lens in front of it - the sensor would only capture a very small portion of the imaging circle, giving you the field of view of a much higher focal length.

So to give a comparable field of view to standard lenses on DSLRS they need a very wide focal length.

[rpolitsr]

Long time ago, we started to deal with focal length usually related to the 35mm film negative size (36mm x 24 mm) and with some practice we got a clear idea of what means 50mm or 400mm in terms of coverage or angle of view.

With the P&S and their tiny sensors values like 7.4mm for wide angel or 44.4mm for tele became meaningless. :confused:

Although the relationship from the actual focal length to the 35mm equivalent (usually called crop factor (http://en.wikipedia.org/wiki/Crop_factor)) is enough to calculate the corresponding focal lengths, and just easy arithmetic is involved, it is better to have a graphic scale, just like my Pro 1 has in the zoom lens.:cool:

For the G9 the values are:
Wide angle 7.4mm equivalent to 35mm
Tele 44.4mm equivalent to 210mm
Zoom factor = 6 => 7.4 x 6 = 44.4 or 35 x 6 = 210
Crop factor (approx.) = 4.73 => 7.4mm x 4.73 = 35mm or 44.4mm x 4.73 = 210mm

Drawing those values we have an easy to read graphic scale:

325027

I hope it will be of some help.

[watchtherocks]

Yeah wingspar, like everyone's saying, just look at the equiavalent specs if you wanna know how wide or long your field of view is going to be.
As for the aperture settings, I'm not to sure. I know P&S's also have weird ISO settings like 80.

[wingspar]

Thanks for the replies. With the capabilities of software these days, it seems like the conversion could be done in camera, and show up as a normal lens would in the exif data. Would make things less confusing.

This is copy pasted directly from the description of the G10 at B&H. It boasts a 28-140mm lens. No mention of the smaller sizes.

Canon PowerShot G10 Digital Camera (Black)

* 14.7-Mp 1/1.7" CCD Sensor
* 5x Wide Angle Zoom Lens (~28-140mm)
* Optical Image Stabilizer
* 3.0" LCD Display
* Face Detection Software
* RAW File Mode
* Manual Exposure Control
* E-TTL Flash Hot Shoe

When you get down to the specs, it says this.

6.1-30.5mm (28mm-140mm equivalent in 35mm terms)

I guess this is how it is, and will continue to be. Just seems unnecessary and confusing.

rafael... Thanks for the graphic. I'll put it to good use.

[gcogger]

Thanks for the replies. With the capabilities of software these days, it seems like the conversion could be done in camera, and show up as a normal lens would in the exif data. Would make things less confusing.

It would also make things wrong. The focal length is 6.1-30.5mm, which is a precisely defined measurement, so to put anything else into the EXIF would be incorrect. What you are suggesting would need another field in the EXIF info, for the '35mm equivalent (in angle of view terms)' focal length - in fact I think I've seen this done on an older camera I owned.

[wingspar]

It would also make things wrong. The focal length is 6.1-30.5mm, which is a precisely defined measurement, so to put anything else into the EXIF would be incorrect. What you are suggesting would need another field in the EXIF info, for the '35mm equivalent (in angle of view terms)' focal length - in fact I think I've seen this done on an older camera I owned.

I’m sure it could be done. I disagree that it would be wrong. However, I didn’t start this thread to whine. I started it to learn something, which I have.

[MarKap77]

Wingspar,

As to the weird looking aperture value of f/3.19, it should make perfect sense to someone who comes from film SLR cameras. The aperture value is always shown as a fraction of the focal length of the lens. On your G9, at the wide angle end of 7.4 mm and aperture of f/3.19 would mean that the absolute diameter of the aperture was 2.32 mm. On these cameras, the aperture can be set to an infinite number of setting between the minimum and the maximum. f/3.19 was just deemed by the camera's internal algorithm to be the correct aperture for the exposure.

Cheers

[wingspar]

As to the weird looking aperture value of f/3.19, it should make perfect sense to someone who comes from film SLR cameras. The aperture value is always shown as a fraction of the focal length of the lens. On your G9, at the wide angle end of 7.4 mm and aperture of f/3.19 would mean that the absolute diameter of the aperture was 2.32 mm. On these cameras, the aperture can be set to an infinite number of setting between the minimum and the maximum. f/3.19 was just deemed by the camera's internal algorithm to be the correct aperture for the exposure.

I’ve never seen two places beyond the decimal point in apertures before, except with P&S cameras. No SLR I’ve used has ever done that. Not that I’ve noticed.

The point of posting this question was to try an understand what seems like weird numbers for focal length and f/stops in P&S cameras. I used P&S cameras at work, but have not used one since I retired in 2005. I just made a deal on a G9. I don’t actually have the camera yet. These numbers certainly won’t keep me from using the camera, but it’s nice to just understand why things are this way with P&S cameras.

In reading up on the G9, I see a ton of other stuff that doesn’t make much sense, but I’m going to wait till I get the camera, use it, and see if I can figure out the other stuff thru use and the manual before I post any more questions, but I do like the interactions with others and the ability to learn stuff here. It’s more fun that reading a manual. :)