Pentax digital spotmeter: differences with Minolta Spotmeter F

[jhorvat]

Well, it seems the seller has some problems in measuring the current consumption of the second Pentax spotmeter, so I doubt I will get that information.

In the meantime, I found something interesting on web:

1) The silver oxide batteries produced by different manufacturers have different discharge characteristics (see attached file). The Pentax spotmeter (as measured on mine) uses about 20 mA of current. With that drain, the battery voltage for some silver oxide batteries can drop to 5.6V within 1 hour of continuous use. Similar happens with 2.5mA of continuous use, just later. The voltage then drops very slowly to 5.2V. The voltage on batteries I use dropped to 5.7V in less than 1 hour of (intermittent) use.

However, another manufacturer lists data according to which the batteries keep their voltage above 6 V until discharged (last figure in the attached file, data given for 2 mA of continuous use). This makes me conclude that it is best to buy the batteries made by a manufacturer that specializes in photo equipment (or the one that publicly releases the data on their batteries).
This said, I didn't notice any difference in the reading of my Pentax spotmeter as the battery voltage was dropping to the current 5.7V, as compared to the reading of Minolta F.

2.) There are difference between the Pentax and Minolta spotmeters that should obviously result in different current consumption. Minolta used Liquid Crystal Display (LCD), while Pentax uses a display that contains elements reminiscent of Light Emitting Diodes (LED). The LCD-type displays consume much less power (no light is emitted by LCD's) than the LED-based ones. This also means that the Pentax spotmeter does not need an additional light to illuminate its display when used in low-light conditions. Minolta, on the other hand, does require a small light bulb in these conditions: with light turned on, Minolta consumes about 10mA of current.

I think this settles it all, I should have realized this long time ago: the current drain I measure on my spotmeter must be its normal consumption. Additionally, the difference in the readings between Pentax and Minolta spotmeters of 1 EV seems to be due to a different calibration standard that was used years ago for some of Pentax spotmeters (as discussed on some of the threads on this site). My solution is to rate the film 1 EI faster.

[dmmartindale]

However, this all begs another question. There was plenty of talk in the forums on the need for a battery with very flat discharge curve (i.e. constant voltage) for Pentax spotmetrs.

How much does the reading change with battery voltage? Pentax specifies an alkaline battery (537) as well as silver oxide (544), so they must believe that both provide sufficient accuracy within the range of voltages high enough for the meter to avoid shutting itself down. The silver oxide battery will provide more readings before it reaches the shutdown point (it has a higher capacity to begin with, and keeps the voltage higher longer than an alkaline) but it will eventually drop to the cutoff voltage too.

I am using Black Diamond brand silver oxide cell and voltage dropped quickly from about 6.3 V to 5.7 V. It recovered overnight to 5.85 V, to drop again to 5.61 V after use. The temperatures are between 10 and 15 C. I wonder how low it will go after a few months' use. So far, I don't see any difference in reading due to the voltage drop, as compared to Spotmeter F (i.e. the difference remains at 1 stop).

Just a side note: that Black Diamond battery is suspect. Black Diamond is a sports equipment manufacturer, not a battery manufacturer. They sell that 6 V battery for use in a tiny headlamp that they sell (I have one). The battery is labelled "silver oxide" on the package, but it was amazingly inexpensive for silver oxide. I took one of them apart (you can just peel off the outer metal sleeve with a pair of pliers) and found four LR44 (alkaline) cells inside, not four SR44. Many people do not know the difference between LR44 (A76, L1154) and SR44 (357), since both will fit the same equipment. With 4LR44 and 4SR44 (what the Pentax takes), you can't even see the markings on the individual cells, so you can't check for yourself.

The Pentax should still work with the Black Diamond battery, but it probably is alkaline, not silver oxide.

Comparing the Pentax against your Minolta is a good idea. As long as the difference between them is constant, it is unlikely that the calibration of either meter has drifted.

- Dave

PS: Yes, I know this thread is nearly a decade old. But the information is still useful, and still accessible to people (like myself) who only found it now. I don't expect the person who posted the original question to check back and read my comment; but I expect it may someday be useful to someone else - perhaps a decade from now.

[CedarMesaPhoto]

I got a Spotmeter M to use as a smaller field meter in addition to my Pentax. They read exactly the same. I hate all that calculator stuff on the Minolta, though. I printed the numbers from the Pentax dial on a mailing label and stuck it on the Minolta. Now I just read direct EV at ASA 100 and my chart gives me the numbers corrected for myself film speed I use. Makes it as simple as the Pentax.

Sent from my XT1080 using Tapatalk

[CedarMesaPhoto]

I like the Minolta because it fits in my TLR bag where the analog Pentax does not. I couldn't believe all that dreadful electronic calculator stuff when I got it, so quickly figured out how to make it be simple to use in the field with my EV sticker.

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[Drew Wiley]

The condition of the battery won't affect the reading per se, but whether it reads at all. When a battery starts getting weak, sometimes the LED will get dim and
give you a clue. But when the battery is just too cold or outright expired, you don't get any reading at all. The electronics in the meter assure that if there is a
reading, it will be correct regardless. Meters can drift over time and need recalibration; but that is a different issue and incremental. Alkaline versus silver oxide
is irrelevant to this discussion. Buy good batteries, not private-label ones, because they're generally cheapos. I hesitate to say anything about Black Diamond
because of personal connections that have nothing to do with equipment per se, except that sometimes a freebie test sample of some tent or gadget might be along for the trip too.

[dmmartindale]

I got a Spotmeter M to use as a smaller field meter in addition to my Pentax. They read exactly the same. I hate all that calculator stuff on the Minolta, though. I printed the numbers from the Pentax dial on a mailing label and stuck it on the Minolta. Now I just read direct EV at ASA 100 and my chart gives me the numbers corrected for myself film speed I use. Makes it as simple as the Pentax.

Yes, I like the simpler calculator on the Pentax. The "Highlight" reading button on the Minolta just subtracts a fixed offset (2 1/3 stops?) from the reading, on the assumption that a highlight is that much brighter than mid-grey. Setting the Pentax meter reading opposite IRE 10 on the TV scale does exactly the same thing. The Minolta "Shadow" reading button just adds a (different) fixed offset (2 2/3 stops?) to the EV - exactly the same result as setting the Pentax reading opposite IRE 1 on the TV scale. If you read both the shadows and highlights, you can see where the two readings fall simultaneously on the Pentax's calculator, and if the range is too large, you can decide which one to favour.

With the Minolta, you have to do some of the comparison math in your head, and you can have it display either EV or F/#, but you can't see both simultaneously. The Pentax lets you see the range of EVs in the image, and the set of shutter speed/f-number combinations, all at once in a natural way. This is one place where the simple mechanical slide rule (Pentax) still beats the electronic calculator (Minolta).

- Dave

[Drew Wiley]

I've owned a Minolta too. It was a little more compact than the Pentax, but read identically over the full range. I think the dial on the Pentax is more intuitive to read, though at my age I often need reading glasses or a magnifying glass. And I've heard that Pentax meters are more solidly built internally.