Biber said:
Jun Yan: I've never used an MR-4 but my guess is that it works fine handheld too. The biggest problem will probably be the batteries, unless it has been modified in someway it needs mercury batteries that are quite hard to find. There are a few threads about replacements for those batteries if you search the forum.
The non-availability of mercury batteries is much less of a problem than it might first appear. There are a number of practical alternatives:
1) Many lightmeters and cameras can be recalibrated to use silver oxide batteries. I believe that this is the case with the MR-4 but I suggest that you contact Quality Light Metric via eMail to see if they can recalibrate it for you. If the camera is in for a CLA, calibration for a siver oxide cell is usually done at no additional cost. The S625PX silver oxide battery is the same size as the mercury cells and can be used directly or use the very common SR44 cells with an O-ring. The SR44 cells go for $5/pack of three where I live.
Quality light metric can repair most lightmeters. The cost runs about US$65 to restore a dead Weston III, IV, or V or a Weston Ranger (my favorite). A calibration is part of the service so you wind up with a meter that is dead on.
2) The Wein-type Zinc-air cells can be used as direct substitutes for the mercury cells with no recalibration. These sell for about US$6 for a cell in a case the same size as the mercury cells. A cheaper alternative is to use Zinc-air hearing aid batteries with an O-ring or washer to make up for the difference in diameter between these cells and the mercury batteries. This is a cheaper alternative with the hearing aid batteries selling for about a dollar apiece in packs for ten.
The Zinc-air cells are a bit less convenient as they don't last as long as the mercury cells which can make the use of hearing aid batteries attractive.
3) As has been mentioned, Gossen has an adapter available which permits use of silver oxide cells in a Lunasix of LinaPro meter with no modifications. This is a bit expensive, selling for about US$30 but seems to work quite well. If you can get a good price on a Lunasix, this might be cost effective route to go.
4) There are also adapters which drop the 1.55 volts of a silver oxide cell down to the 1.35 volts of the mercury cells. In principle, one puts a silver oxide cell into this adapter and just drops it into the battery compartment; these sell for about US$30.
The CRIS adapter is of this type; the problem is that these do not always work properly in all cameras and meters, notably in the Leicaflex SL and the Nikon Photomic prisms. There is a fellow in Holland who also offers his own adapter which evidently will work where a CRIS adapter does not.