Fixed/Auto Focus questions

I have read the C20 manual, looked up the specifications and support documents in www.hp.com and www.photosmart.com and I still confused with the fixed focus and auto-focus options of the C20. Finally, I called up the HP support at 208/376-3686 and I am not quite sure of their explanation. I hope the moderator can find the source and get to the bottom of this question.

On page 12 of the C20 manual, it says "the HP PhotoSmart C20 digital camera is fixed focus as a default.... Press the Shutter Release button part way until you see the green light in the viewfinder - this sets the focus." This raises the question of what this "fixed focus" mean here. If it means that the camera lens is set at a fixed position for all objects no matter where they are, then why do we need to press the Shutter Release part way to "set" the focus. If it means that the part way press of the Shutter Release button is "locking the focus" (as indicated in the C20 review article in http://www.imaging-resource.com/PRODS/C20/C20A.HTM), that is, locking the lens position after the camera finds the best lens position for a certain object, then what is the difference between this fixed focus mode and the auto-focus mode? Does the camera lens move in both cases to find the best lens position for the object?

The technical support person at 208/376-3686 said that the lens does not move in the fixed-focus mode. Then the phrase "this sets the focus" in page 12 of the manual is very misleading and even incorrect to me.

How does the C20 find the best focus? The C20 review article in www.imaging-resource.com (http://www.imaging-resource.com/PRODS/C20/C20A.HTM) says that is done by TTL and analyzing the contrast of the image. Is it based on the whole image or the center part (as marked in the viewfinder) of the image?

The technical specification (http://www.photosmart.com/products/cameras/cameras.html) says "Auto-focus - Normal focus from 2 feet to infinity. Macro focus from 9.5 inches to 2 feet", and the one from http://www.hp.com/cposupport/pc_photography/support_doc/bpy00083.html says "Focus and Range - Fixed Focus Range: 2 Feet to Infinity, Auto Focus Range: 8 inches to Infinity". Does the lens move in for objects from 2 feet to infinity in both fixed-focus and auto-focus modes, or just in the auto-focus mode?

I also experience that when the LCD is turned on to take a picture, the LCD image focus will go in and out before it settles in. Does it mean that turning the LCD on will automatically turn on the auto-focus?

In a nutshell, when does the C20 move the lens for focusing and for which objects, center ones or all in the viewfinder?

Here is my guess: The fixed mode is that the lens is fixed at the same position no matter where the objects are. All objects from 2 feet to infinity will be "clear" or "focus". The C20 will try to use a small aperture to obtain this long depth of focus. The "part way" press of button is just to lock the exposure. The auto-focus mode will move the lens until the whole field of view is in the best contrast. This mode will work if the objects are between 8 inches to infinity. Anything less then 8 inches will be beyond the capability of the lens and become fuzzy. The center part of the viewfinder is for exposure measurement only. Am I right?

Can the moderator please find the authoritative source and help clarify these questions. Thanks in advance.

-- Alex Wat (alex_wat@hp.com), October 26, 1998

Answers

Sorry for the long delay in answering (this and so many other questions here!) - The Oly D-400 review kept me tied in knots this last week or two.

The issue of Fixed/Auto Focus on the C20 confused us also, so we asked HP about it. (I need to check the review, maybe the answer didn't make it into the final version...)

You're right - they actually meant "fixes the exposure", not "fixes the focus" when the shutter button is depressed half-way, and the camera is operating in normal mode. I don't know what aperture they use, but this approach is common in many inexpensive cameras - they set the lens at the "hyperfocal" focusing distance for the aperture they're using, so that everything from a given point out to infinity will be within reasonable focus.

The autofocus on the HP C20 operates rather slowly (not a good choice for fast-moving sports shots!), taking about an additional second to adjust the lens, beyond whatever the autoexposure requires. The net result with autofocus enabled can be a shutter lag as long as two seconds. Turning off the autofocus decreases this to about 1 second, if memory serves. Thus, I'd recommend using autofocus anytime you're not too concerned about catching fleeting expressions, etc, and the default fixed-focus the rest of the time. (AF is necessary for close-up shots, all the time.)

Hope this helps, good job piecing the conflicting info together & arriving at a correct conclusion!

-- Dave Etchells (web@imaging-resource.com), November 02, 1998.