The quest for a reliable spotmeter

[stefan_dalibor1]

I'm shopping for a spotmeter (a real 1 degree meter, useable down to EV1 -

so please, no suggestions for Sekonics [345]* or Gossen Starlite).<p>

 

So far, I have tested the Gossen Ultraspot 2 and the Minolta Spotmeter F,

but am not satisfied: Both are rather sensitive for infrared light (hot

stove element +3EV, IR diodes like in TV remote controls or IRDA computer

interfaces +5-6EV!); the Ultraspot is non-linear below EV 2.5 (checked

with ND filters), while the Minolta (though being linear within 0.3EV

over the complete sensivity range down to EV1) has internal flare problems

(i.e. is sensitive to light sources quite a bit outside the marked 1

degree circle in the viewfinder).<p>

 

Now it's clear to me that it's probably not a beginners exercise in

opto-electronics to construct and produce a precise spotmeter for

photographic purposes, but given the amount of money these devices cost

and the experience that the manufacturers have, I had expected them to

perform far better!<p>

 

Is it really necessary to buy a Zone IV modified meter in order to get

something that delivers reproducible measurements instead of approximations

and reason for guesswork (what amount of IR radiation might this subject

contain, am I measuring far enough from distinct bright spots, can I rely

on the meters results if the light level is this low)?<p>

 

What are the downsides of the Zone IV modified meters? I suppose you have

to pay for better filtering and baffling by decreased sensivity... down

to which EV value are they working reliably, and are they useable for slide

film? Or is the spectral sensivity matched to BW film in a way that makes

them less useful than an unmodified Pentax meter for shooting slides?<p>

 

Has anybody checked the Seconics 778 regarding IR sensivity, baffling,

linearity and spectral sensivity?<br>

[carl_weese]

I'm curious--how are you testing to show that the reading of the stove

element is too high? What is the basis for saying it is +3 EV?

 

<p>

 

On a second note, I've found that while the Minolta Spotmeter F is a

useful tool (it's my primary meter) it is distinctly non-linear in the

top end of its range, overestimating the brightness of, say, a white

object in full sunlight.

[walter_glover]

Stefan,

 

<p>

 

I am surprised at your report of the Minolta Spotmeter F. I had

one until recently (for the odd flash assignment) and checked it

for linearity, flare, etc. against my Zone VI modeified Pentax

Digital and it kept pace really well.

 

<p>

 

I use the Zone VI Pentax for both colour (reversal & negative) and

Black & White. Its only foible - determined by testing and

checking density with a MacBeth Densitometer - is that when

reading through a 25a (Red) filter I have to give 1 more stop

exposure.

 

<p>

 

I periodically shoot in very dim areas (black-painted rear

projection booths for home theatres is an example) and I find

that the low light accuracy is totally trustable.

 

<p>

 

My linearity test uses NDs to 3.0 (10 stops in a given light level)

and the error is one LED blob ( - 1/3 stop )

 

<p>

 

Walter Glover

[jerry2]

Stefan, quite impressed by your thorough research. I am curious also

about how you are checking these meters for IR sensitivity? Please

explain how you are doing this. Also, where do you see this being an

issue in outdoor shooting using color film?

 

<p>

 

As for spot meters... I have tested several Sekonic spot meters

and I have read the test results a few years back on the test results

of other makers...and unfortuantely I do not have better news for

you... the meters are never linear througout their entire working

range. This can easily be tested by putting the the spot meter in

the back of a view camera and stopping down one f stop at a time,

then re check redings. Your ND test is also useful.

 

<p>

 

The compromise I have made is to use spot meters in the mid

range of their working range where they are reasonably accurate and

linear. However, when very low light levels are encountered, I rely

on an ambient meter well suited up to -8 EV. It is made by

Quantum. The good news is, once you are confident in your testing

methods, you can record how far the meter is "off" at different EV

levels and tape a cheat sheet to the meter! Just adjust in the field.

 

<p>

 

The other thing you need to be aware of.... not all spot

meters are calibrated to the same reflectivity value. Some are set

at 13% such as Sekonics while other are at 18%. So don't expect

apples to apples readings between different makers of spot meters.

Its not always easy tracking down this information. Also, not all

grey cards are 18%. Hard to beleive there is so many issues with

light meters, huh.... I agree...

[lars ake vinberg]

Interesting... I'm sitting here with a Sekonic 778, always assumed

that it was both accurate and linear.

 

<p>

 

Stefan, do you think that you could write a step-by-step testing

instruction? I'd be happy to go through the steps and post the

results.

[arne_croell]

Stefan,

 

<p>

 

I both have a regular Pentax V and a Zone VI modified Pentax

digital. The latter is my regular meter. Years ago I had the chance to

try out a new Gossen Ultraspot/Spotmaster for a day and make

comparison tests with the Zone VI. Both meters agreed quite well with

each other (within 1/3rd EV), except when measuring through a dark red

filter (29) where the Gossen was 1-2EV too high. I also made an IR

diode test with a TV remote control in a dark room (pointing the

remote control directly into the meter). _Both_ meters showed a

response, but the Gossen was several EV higher, i.e. more IR

sensitive. That could be cured by using an IR filter (actually it was

a combined UV/IR filter), the type that is sold for digital video

cameras. With that filter on the meters they agreed quite well over

the whole range - the filter took out 1/3rd EV over the range but that

was the only side effect. As for flare, the modified Zone VI meter is

better than the unmodified Pentax V (comparing apples and oranges

here, but I never had an unmodified Pentax digital). However, it can

still be improved by using a lens hood (Heliopan makes a cylindrical

metal hood that fits the Pentax digital perfectly). So using a UV/IR

filter and a lens hood on an umodified meter might get you close.

[pete_andrews]

IR sensitivity is a BIG problem with most modern meters that use a

Silicon cell, not just spotmeters. The manufacturers don't pay nearly

enough attention to it.<br>I bought an Elicar spotmeter that had

exactly the same problem of excess IR sensitivity. Despite having a

cell with a supposed IR filter over it, I could still get an

appreciable reading off an IR LED, and could see a very noticeable

(>2EV) difference in readings from various black coated surfaces that

looked visually almost identical. (Epson inkjet ink appears to have a

huge reflectivity in the near-infrared, by the way.)<p>In the end, I

replaced the sensor cell with one that looked physically identical,

but that showed 1/5th of the response to an IR LED than the one

originally fitted to the meter. The response to visible

light was practically the same for both cells.<br>Whether this is a

typical spread of IR response, or whether more recent manufacturing

techniques or a different make of sensor was the reason for the

difference, I don't know.<br>Anyway, it was a worthwhile mod, IMHO,

although recalibrating the meter afterwards wasn't easy.<br>The sensor

I used was an EG&G Vactec type VTB8440B, which only cost £2 UK,

about the equivalent of $5 US.<p>If money is less of an object; I

notice that Edmund Optics Ltd have a number of filters available for

cutting IR. They sell B+W IR-cutoff filters in standard threaded

mounts, in sizes from 46 to 58mm diameter, but the cost is upwards of

$150 US.<br>They also sell 1" and 2" square visible bandpass filters,

as well as IR cutoff filters, hot mirrors, and 'tech spec' heat

absorbing glass. These are all much cheaper, in the range of $30 to

$60, and any of them would be effective at reducing the IR sensitivity

of a spotmeter. They could be fitted either internally, or in front of

the lens. However, they'd all require some compensation or

recalibration of the meter, but only by about 10 to 20%.

[WAn]

"What are the downsides of the Zone IV modified meters?"

 

<p>

 

-- the only downside I can tell is that if the brightness is below EV

2, the circle in the viewfinder is almost invisible.

 

<p>

 

 

"I suppose you have to pay for better filtering and baffling by

decreased sensivity... "

 

<p>

 

-- No. I had to pay only in terms of money.

 

<p>

 

 

"down to which EV value are they working reliably, and are they

useable for slide film?"

 

<p>

 

-- Down to 0 EV. In the letter supplied with the spotmeter (letter by

person who makes the modification) there is a statement that the

meter has been tested for linearity in the range 4-19 EV. I don't

know what tolerances he used in the testing, but the meter seems

linear even below 4 EV. (If you will test it with a film, dont forget

about Schwarzshield effect)

 

<p>

 

 

"Or is the spectral sensivity matched to BW film in a way that makes

them less useful than an unmodified Pentax meter for shooting slides?"

 

<p>

 

-- I heard the spectral sensitivity of the meter is made close to

s.sensitivity of Tri-X. Since different films (even B/W films) have

different spectral sensitivity, an orientation to a specific emulsion

can make the work with other emulsion less preise, it is obvious. But

I think it is still more precise than using the meter that is too

sensitive to IR, UV light or just to saturated colors, in spite of

that strange virtue that the latter meters are not claimed to be

oriented to a particular emulsion...

 

<p>

 

Ok, it is a dream to have a meter with a customizable spectral

sensitivity, but I'm sure the cost will be prohibitive.

 

<p>

 

Like other posters I hope you will share your methids of testing for

linearity and IR-sensitivity ;)

 

<p>

 

Cheers

Andrey

[stefan_dalibor1]

O.K., here are my »spotmeter tests« - I don't claim them to be

of scientific quality, but think they <b>do</b> have real-world relevance...

after ruining 8 Velvia Quickloads (hopelessly under-exposed) and a probably

unique opportunity because I trusted the Ultraspot's results in a brickworks

(with a rather high level of IR radiation due to large heaps of red-glowing

bricks), I repeated Walter's tests (see

)

with the Ultraspot, and switched it for a Minolta Spotmeter F after

recalibration by Gossen didn't improve the meter's deficiencies.<br>

For all experiments, I put the meter on a tripod and repeated them in

in daylight and artificial light.

 

<ul>

<li>Low-light linearity - you need ND filters (I used two Hoya NDx4 and

a no-name NDx2):

<ol>

<li>Measure your grey card in bright light (EV 10 or similar).

</li>Under the same lighting conditions, measure the grey card through your

ND filter. Memorize the difference (e.g. EV -1).

<li>Look for a place with dim lighting, just enough to make your grey

card readout add up the low light sensivity level as spec'ed by

the meter maker <i>plus</i> the ND filter factor determined above

(e.g. EV 1.3 + EV 1 = EV 2.3).<br>

</li>

<li>Repeat the measurement from above: do you get a reliable readout (the

Ultraspot tended to show an underflow)? How much light does the

ND filter hold back now, according to the meter?

</li>

</li>

</ol>

</li>

<li>Infrared sensivity - you need a kitchen stove (and permission by your

wife to operate it until an element starts to glow red w/o having a

pot on it - anyway, keep a pot full of water at hand to cool down the

stove element if you are done with the experiments :), and a TV remote

control (the latter preferredly loaded with a fresh set of batteries):

<ol>

<li>Measure the cold stove element in dim lighting (should read something

like EV2-3).

</li>

<li>Turn on the stove, repeat measurements every minute.

</li>

<li>With my stove (30 years old Brown Boveri, element approx. 1.5kW),

both the Ultraspot and Spotmeter F started to read contiously

increasing EVs after ca. 3-4 minutes, up to EV+3 (Spotmeter F) /

EV+4.5 (Ultraspot) <b>before</b> the stove element even started

glowing (you should check this by switching off the lights regularly

and stop the experiment once the red glow become noticeable - don't

ruin your stove!).

</li>

<li>Fix the TV remote control in front of the spotmeter, so that

the 1 degree circle points to the IR diode in the remote control's

tip - if you don't want to dismantle the remote control's housing

(most have an opaque plastic cap over the IR diode), you might

have to try for a while to find out the exact location of the

diode. The distance between the diode and the spotmeter should

be chosen so that the diode fills the 1 degree circle in the

spotmeter's viewfinder.

</li>

<li>Make measurement with the remote control switched off, memorize

the spotmeter readout.

</li>

<li>Repeat measurement while operating the remote control - you'll

probably have to make several (like 5 to 10) measurements here,

because the remote controls I know send an oscillating signal.

But if measured while on, my remote control made the Ultraspot and

Spotmeter F really freak out (EV+5-7!!).<br>

If you think this has no practical relevance, try to get reliable

measurements in a room where computers communicate via IRDA interfaces

- I tried, and the Ultraspot's results where so ridiculous that even

I noticed before burning film and switched to an incident meter.

</li>

</ol>

<li>Flare - you need a piece of cardboard (grey, about 40x40cm) and a strong

light source in an otherwise evenly lit room. I measured

<ol>

<li>outside the window on an overcast day for daylight

</li>

<li>a strong electric bulb (100W, opaque glass) inside for artificial

light

</li>

</ol>

<ul>

<li>Point the spotmeter to the light source so that it fills the 1 degree

circle indicator, make measurement and memorize readout.

</li>

<li>Now put the cardboard between the spotmeter and the light source;

minimum distance to the meter should be approx. 1m (to exclude the

possibility of vignetting the meter's optical system).

</li>

<li>Cut a hole in the cardboard where the spotmeter's 1 degree circle

indicator points to - the hole should be just a bit larger than the

circle in the viewfinder (i.e. you should see the light source in the

viewfinder only through the middle of the circle indicator).

<li>Repeat measurement - do you get the same reading as the first time?

IMHO you should... for me, the ability to meter a small spot - and

<b>only</b> a small spot - is the point about using a <it>spot</it>

meter in the first place.

</li>

<li>If you don't get the same readout through the cardboard, and are

patient, you can try to enlarge the hole incrementally until you

get the same readout as w/o cardboard.<br>

This way, I found out that the Ultraspot works really »on the

spot« (i.e. same readout within EV0.2 with a hole just large

enough for the circle indicator), while the Spotmeter F should really

be flagged as »2-3 degree meter« - the sensitive area

in the viewfinder seems to be roughly the double size (area-wise) of

the circle indicator.

</li>

</ul>

</li>

</ul>

[stefan_dalibor1]

Oops, forgot to insert URL to Walter's original tests (sorry, Walter :) in the

lenghty treatise above

<a

href="http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=006QsE">here</a>

it is.<p>

 

Stefan

[mark_sampson]

I've used the Pentax Digital/Zone VI spotmeter for ten years now.

It's quite accurate enough for any work I've ever done. Experience

shows that it's the meter to believe, if there's a question. Someone

mentioned in a previous reply about the red-filter compensation-

that's true. As I use a #25 once a year maybe this has'nt been a

problem. It's a wonderful tool.

[WAn]

Thank you Stefan.

I hope the sort of pernickety and meticulous users will finally force

the manufacturers to pay more attention to the quality of their

production, instead of resting on the laurels.

Great topic.

[anthony_harrison]

"the meter to believe" says someone, and therein lies the rub: I find

your questions frankly unrealistic, and do not understand your quest

for such an improbably high degree of metering veracity. The Gossen

has a solid reputation, I used Minolta Spot-Fs for years, and

currently I'm using the Sekonic 608, and they're all excellent meters.

Like any meter though, their readings have to be interpreted, rather

than taken literally. They are machines, so they are stupid. Sorry to

be so brutal but I think in seeking a meter that will give you totally

reliable readings under such highly specialised (arcane, even)

lighting conditions, you're on a wild goose chase.

[bill_marsh]

I've used the same UNmodified Pentax digital spot since 1983. It's a great meter, and if I had to buy a new one

for some reason, that would be my choice. I once thought about about having it modified by Zone VI, but

decided against it because, at the time, it seemed like kind of a fad (sorry, Fred, wherever you are). It might

have made the meter more accurate, I don't know.

 

<p>

 

Consistency would seem to be the key factor. Every single meter out there, even within the same type, is

different. If you pick one and calibrate your particular film-lens-EI-development-agitation-developer-etc system

to it, then it probably doesn't matter which one you get (except for the reliability and build quality thing).

 

<p>

 

FWIW,

 

<p>

 

Bill

[WAn]

Anthony, Bill,

I beg to differ.

 

<p>

 

I agree there are a lot of situations where most meters will do.

Moreover, there are situations where an experienced photographer can

guess the proper exposure without any meter at all with precision of

+/- 0.5 stop. But there are numerous real life situations where the

correct behavior of the meter is critical.

 

<p>

 

"..calibrate your particular film-lens-EI-development-agitation-

developer-etc system to it, then it probably doesn't matter which one

you get"

-- it is true for absolute value of the measurement (zero

adjustment). But the adjustment found during the calibration is

absolutely useless if the meter is not linear or if it is flare prone

or if its spectral sensitivity is drastically different from film

spectral sensitivity (including notorious IR-sensitivity) or all this

together.

 

<p>

 

Machines are stupid, but they should work as specified. If one spends

several hundreds for a meter, it is mildly speaking somewhat

exasperating if he discovers that in certain (not uncommon)

situations he has to guess and the machine instead of helping fools

him.

 

<p>

 

(See for example my funny experience with Gossen Starlite:

http://www.greenspun.com/bboard/q-and-a-fetch-msg.tcl?msg_id=007drq

Does it mean the Starlite is unusable? No, it is pretty usable in a

lot of situations!)

 

<p>

 

Solid reputation of a company � it is good. Experience of thousands

photographers in the world -- it is great. But I insist that a

machine must work according its technical specifications.

 

<p>

 

Andrey

[stefan_dalibor1]

I fully agree with Andrey - the spotmeter manufacturers either have to make their products comply with published specs, or change the specs to describe their products accurately.<br>

This would mean that the Ultraspot's sensivity has to be listed as EV3-21, and the Spotmeter F has to be described as 3 degree meter (and the viewfinder indicator circle enlarged appropriately!). For both meters, a detachable IR blocking filter (w/ description how to adapt the meter readout when the filter is applied!) should be packaged together with the meter.<br>

The way these products are delivered now make them IMHO amateur equipment... you can get some approximation of your subject's contrast range for many subjects, but you can't rely on the meter as a professional tool.<br>

Considered the $$$ that you have to pay for a spotmeter (mind you, for the official resale price of an Ultraspot you can buy a comlete 35mm or digital camera w/ lens!), this is annoying, to put it mildly.