Picture dims dramatically

Depends on the model. Have you checked the backlight, (LED in this set perhaps).

The TV's main culprits to be causing this problem are "dynamic contrast" which maybe darkening the backl light in dark scenes an example a night time or a dark scene in a movie. To make the blacks be deeper (as LCD/LED TV are rubbish at blacks) or brighten it on bright scenes to make the light be lighter and once again to make the blacks be deeper so the images pop out. Also "eco lighting" this will dim or brighten the image based on the amount of light in a room in order to be saving energy. Ether way the easy fix is to turn both of these features off. You will get rid of the problem and enjoy more subtler shades of colour that would otherwise be bleached (brightend) or crushed(darkend) out..

Melissa check you TV's picture settings, cinema, sports, movie, store, etc.and try a different display setting.
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re place lcd panel that is dead pixel

If you have the same problem with another input such as a DVD player, this TV might need attention by a qualified service technician.

A very dim picture in this case may be damage to the power supply of the set---If a LCD set it may use back lights of some type--if not on or at full brightness the picture will be very dark.

If I was looking at this set I would open and check to see if voltage for the back lights is correct first.

Time for a new lamp. they go into a dim mode when they die.

I w having this same issue when watching a Blu-Ray disc. I found it to be dependent upon what picture mode the TV was set to. By this I mean certain picture modes would cause the TV to dim during dark scenes of a movie. Changing the picture mode helped a lot.

There should be a setting in the menu of the TV to turn off the auto dim feature (used for adjusting the brightness of the picture depending on ambient room lighting) I don't have a Sony LCD here with me to walk you through the menu settings, but I am certain it is there somewhere. (Chased this issue for the wife who had the same displeasure you're having)