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-   -   Two Focusing Problems with A1 (https://forums.steves-digicams.com/konica-minolta-26/two-focusing-problems-a1-16501/)

shene Nov 24, 2003 6:32 AM

Two Focusing Problems with A1
 
Hi A1 shooters,


I am not sure if I am the only one who encountered the following problems. The A1 focuses much faster than my Nikon Coolpix 5700, especially in low light and low contrast situations. But, ...

(1) The A1 has some problems in shooting CRT monitors and TV screens. I used full auto, flexible focus point(FFP) and continues AF to shoot TV screen and CRT monitor at 200mm, and found out that the failure rate is very high. More interesting is that the focus area jumps rather randomly and fail to focus. The 5700 has no such problem as long as it can focus.

(2) In macro mode at 200mm, all three focus modes frequently show false focus (i.e., white dot shows up but it is clear that the camera is not in focus). This is especially true when shooting at the minimum focus distance.

Does any one of you encounter the same problems? Or, is it the problem of my own A1? Any comments and input?

TIA

CK
http://www.cs.mtu.edu/~shene/DigiCam
Nikon Coolpix 950/990/995/2500/4500 User Guide

voxmagna Nov 24, 2003 7:33 AM

Re shooting TV displays: You're picking the most difficult thing to photograph. Your camera is itself a digital sampling device (including the AF which is done in time segments using processor interrupts) whilst the picture on a monitor is from a spot wanging back and forth and up and down, with the image you see produced by persistence in a phosphor and itself a light source, and a big black field blanking interval which your eye is deceived into ignoring - but which the AF will 'see'.

Why don't you try doing a half shutter press on an object at the same distance as the screen to get focus lock, then point at the monitor? Don't forget the point of focus will be a bit further back than the monitor case or glass - but you can stick something at the equivalent distance to focus on, or go down a stop for more DOF. Of course you may still get soft looking pics if your shutter is to long, as the spot is always moving and refreshing the screen! If the text on the monitor case is sharp, then this is likely the problem. However, shorten the shutter and you'll probably catch the black frame bar. VOX

browntimdc Nov 24, 2003 8:35 AM

You shouold use manual focus and the magnifier to photograph a CRT. You should also use a tripod because you need a shutter speed of 1/30 or slower and you need to aim the camera straight at the screen (focal plane parallel to the screen). Lastly, unless the CRT has a flat screen you should set the zoom fairly long (~135mm) and back away from the screen to reduce geometric distortion.

sjms Nov 24, 2003 11:03 AM

also don't forget it has both a horizontal and vertical refresh rate

mine is currently running at hoz 85.9khz
vert 85.0khz
screen res: 1280x960

http://www.pbase.com/image/23645958

roughly 100%crop

quick snapshot. i don't know looks good to me considering. note the vert phosphor lines

shot from about 12in from screen hand held 200mm eq macro in AF on single shot

oh yeah sorry i didn't clean the screen


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