Hi All,
A few days ago I shot some slides of part of a TV programme. Following my normal practice, I froze the frame using the 'pause' button on the TV decoder/hard drive. I stopped down the lens aperture to somewhere around f16 and prepared for the meter to give me a reading of around half to one second. (This technique avoids problems, as over the long exposure time the odd half a field here or there doesn't impact significantly on the overall number of fields/frames captured).
For the first time, I was using a Pentax MX rather than the KX that I've used in the past. Whilst determining the correct exposure via the five LEDs, I had the strangest effect whereby I could get all five LEDs to illuminate simultaneously instead of just one or occasionally two when the exposure falls in between.
My first reaction was that the MX's metering had died, but pointing it out of the window, it behaved quite normally. My conclusion was that whereas the KX or any other analogue needle-type metering system averages out rapidly fluctuating changes in brightness (as occur with a TV picture many times a second), the MX must take regular fairly short samples of the brightness level it sees, and depending on what point of a field scan it is sampling, sends a signal to the relevant LED in the viewfinder (in the most simple terms). Thus, if it sees the space in between fields, it sees a low light level and the -2 stops LED lights, then it sees half a field and the OK LED lights, then a full field and the +2 stop LED lights, plus proportionally for the +1 and -1 stop LEDS in between, all in rapid succession, leading to the appearance of all five LEDs being illuminated simultaneously. Anyway, that was the best explanation I could come up with! It couldn't be down to changing picture content, as the frame was frozen, but I don't know whether the decoder/hard drive employs a frame store or just keeps re-reading the relevant part of the hard disc.
For anyone with an MX who wants to try this, I was photographing a CRT screen, which in the UK is displaying 25 frames/50 fields per second. Perhaps in the US with 30/60 the effect would be different?
Any other thoughts?
Steve
A few days ago I shot some slides of part of a TV programme. Following my normal practice, I froze the frame using the 'pause' button on the TV decoder/hard drive. I stopped down the lens aperture to somewhere around f16 and prepared for the meter to give me a reading of around half to one second. (This technique avoids problems, as over the long exposure time the odd half a field here or there doesn't impact significantly on the overall number of fields/frames captured).
For the first time, I was using a Pentax MX rather than the KX that I've used in the past. Whilst determining the correct exposure via the five LEDs, I had the strangest effect whereby I could get all five LEDs to illuminate simultaneously instead of just one or occasionally two when the exposure falls in between.
My first reaction was that the MX's metering had died, but pointing it out of the window, it behaved quite normally. My conclusion was that whereas the KX or any other analogue needle-type metering system averages out rapidly fluctuating changes in brightness (as occur with a TV picture many times a second), the MX must take regular fairly short samples of the brightness level it sees, and depending on what point of a field scan it is sampling, sends a signal to the relevant LED in the viewfinder (in the most simple terms). Thus, if it sees the space in between fields, it sees a low light level and the -2 stops LED lights, then it sees half a field and the OK LED lights, then a full field and the +2 stop LED lights, plus proportionally for the +1 and -1 stop LEDS in between, all in rapid succession, leading to the appearance of all five LEDs being illuminated simultaneously. Anyway, that was the best explanation I could come up with! It couldn't be down to changing picture content, as the frame was frozen, but I don't know whether the decoder/hard drive employs a frame store or just keeps re-reading the relevant part of the hard disc.
For anyone with an MX who wants to try this, I was photographing a CRT screen, which in the UK is displaying 25 frames/50 fields per second. Perhaps in the US with 30/60 the effect would be different?
Any other thoughts?
Steve
I'm glad to know why now.