3

I have Samsung Galaxy Grand Quattro and I was just experimenting with the camera when I saw lines on the screen. Well this was not the first time I saw them but when I varied the ISO(from Auto to 400), the lines changed their intensity, width and speed on the screen(you can't see them in still images). Can anyone explain what's going on? The images are in the order :

ISO Auto, ISO 100, ISO 200, ISO 400.

ISO Auto

ISO 100

ISO 200

ISO 400

  • 1
    Wow! a CRT in 2015... it brings back memories from the '90s. Since you are there, why not trying also "tempest"? erikyyy.de/tempest – FarO Oct 15 '15 at 11:33
  • CRT with advanced CPU :D... – prakhar londhe Oct 15 '15 at 11:38
6

You are seeing the scanning of the screen. Try shooting an airplane propeller next! The exposure is made with a travelling slit so different parts of the image are exposed at different times and the movement causes artefacts.

Even though the shutter is not mechanical, the reset and readout has the same effect, but more complex: while a focal plane shutter is a single pair of curtains, I'm guessing here that the sensor chip has strips that are scanned in parallel. Each strip has a dark zone and a bright zone because the screen was bright or dark at the time each row was exposed. The bright bands are diagonal because the screen is painted from top to bottom, orthogonal to the direction of the sensor rows.

Different iso changes the exposure including the shutter speed.


I mentioned an airplane propeller as the iconic example of the artefacts caused by a focal-plane shutter. With the segmented electronic shutter, the same scene would show telltale signs of the segmentation. How about finding something spinning fast and posting similar shots as a comparison?

  • But it happens even when i didn't close the shutter(capture the image)? – prakhar londhe Oct 13 '15 at 15:25
  • 1
    But you're looking at your phone's screen, not through an optical viewfinder. You see a video which consists of single frames, which are still subject to the effects of shutter speed. – ths Oct 13 '15 at 16:37
  • 1
    Right, the actual shot may show different banding as the shutter speed of the live-view might not match the real exposure. – JDługosz Oct 13 '15 at 17:52
  • Your last line is very important. Changing the ISO has nothing to do with the changing lines on the screen. The different ISO just happens to make the camera use a different shutter speed, and that's what changes the lines on the screen. – JPhi1618 Oct 14 '15 at 15:37
3

This type of monitor uses a cathode ray picture tube. This is a design taken from analog TV technology. The image on the screen is glowing pixels of red, green and blue phosphorus. In the neck of the tube is an electron gun. It fires elections aimed at the inside center of the tube. The exterior of the neck of the tube is wrapped with electromagnets. These cause the path of the electrons fired by the election gun to swerve. The magnets cause the electrons to hit the tube at the upper right corner. The electromagnets force the electron hits to move from right to left tracing out a line like a like on a typewriter. The intensity of the electron hits dictate how bright the spots of phosphorus will glow. Again the hits are moving along a line right-to-left. The glowing phosphorus persists in glowing for a time. After a line is complete the electromagnets cause the electron beam to drop down to the next line. Again this line is targeted (scanned).

TV and some monitors paint every other line. That would be line 1, then line 3, then line 5, then line 7 etc. When the scan reaches the bottom it is directed to scan line 2, then line 4, then line 6, then line 8 etc. This is called interlace scanning. This was the normal in analog TV and monitor displays. The scan rate is locked on to the cycles of the AC main power grid, 60 cycles per second in North America or 50 cycles in Europe. Interlace scanning paints every odd line completing the task in 1/60 or 1/50 of a second. Then the even lines are scanned. The complete picture takes 1/30 of a second in North America and 1/25 of a second in Europe. More modern transmissions use progressive scanning; this method scans like a typewriter types. The image quality is doubled.

Keep in mind that the glowing phosphorus does persist, however when a picture of the screen is taken you are seeing different intensities depending on how many milliseconds ago that portion of the screen was painted. Your images show scan lines with different shutter speed times. Thus what is bright and what is dim is dependent on the persistence time of the phosphorous. Your image showing lines is normal for a screen of this type. You used different shutter speeds as you changed the ISO.

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.