Latitude of Reversal film compared to Colour Negative

I had awful problems scanning Ektar. I finally gave up on negative color film and went back to chromes to scan. First, you know immediately if you have good exposure. Plus there's no mask or weird reversal colors. It's just simpler scanning chromes.

DREW WILEY said:

Yes, indeed - I see many many Ektar images posted, and indeed tweaked despite significant exposure and filtrations errors - "I can correct anything in Photoshop " - and indeed looking awful ! This film is capable of much cleaner results if it is properly understood. And when output tweaking is needed, masking skills can do it in a darkroom for sake of RA4 printing just as well as PS qualitatively, maybe better - I didn't say faster. But once it's scanned, you have no choice but digital printing too, which I find rather disappointing compared to real darkroom prints, especially when the heavy artillery of large format film is involved.
Now ... the very curves you've posted have decent linearity only over a six stop range, or perhaps 7 stops fudged a bit, which is right around what actual experience with Ektar tells me too. But what you need to do is go to the tech sheet charts which show how the actual spectral sensitivity overlaps - where those three little dye mountains overlap down on the alluvial plain and create mud rather than pure Pyrenees tri-colored water. Once mud is mixed, it's tricky to unmix it.

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Truth is that if the scan is halfway decent & not mixing everything to midtone with semi-intelligent autocorrection of 'skintone' or anything similar, you'll very quickly see the same crossover as in optical RA4. Sure you can fix it to a somewhat believable extent via colour replacement/ reconstruction etc (and all that LUT creator is doing is automating it), but that's a hell of a lot of effort for what you and I and anyone who's actually used a decent amount of narrow latitude film knows is really mostly easily resolved in camera by the most baseline usage of an incident meter or spotmeter with a highlight index. It's the blue/ green curves' divergence at greater exposures on Ektar that seems to be part of the problem - they track more closely for a bit longer on Portra 160. Ektar definitely delivers 7 stops, but the margin of exposure error is not high.

Proper filtration at the time of the shot is by far the easiest way to deal with the Ektar cyan issue. It's a crossover problem at both ends - both highlights and deep shadows. I won't go into the details right now. Have explained it many times before on various threads.

Within the last year, I decided to "go back" to film-- but I'd never been that serious about it before, my last film camera being a Kodak Instamatic with 126 film (and flash-cubes). About the time I had money to get into photography, digital was becoming The Thing, so I went that way.

The single most annoying advice I've been given is "Don't trust box speed! You should over-expose!!"

My experience so far with Arista EDU 400, TMax 400, Fuji 400H and Portra 400 (yeah, I know...), has been that box speed, when properly metered produces fantastic results. Both 400H and Portra 400 seem to start getting squirrely around +2.

Overexposed TMax, while usable, is not my friend, and frankly, I wasn't that happy with Arista shot at 200, either.

I was able to pull some of my over-exposed 400H (+2) shots back to reality by doing some channel tweaking in the CMYK space (primarily Cyan), but not all of them.

grat said:

Within the last year, I decided to "go back" to film-- but I'd never been that serious about it before, my last film camera being a Kodak Instamatic with 126 film (and flash-cubes). About the time I had money to get into photography, digital was becoming The Thing, so I went that way.

The single most annoying advice I've been given is "Don't trust box speed! You should over-expose!!"

My experience so far with Arista EDU 400, TMax 400, Fuji 400H and Portra 400 (yeah, I know...), has been that box speed, when properly metered produces fantastic results. Both 400H and Portra 400 seem to start getting squirrely around +2.

Overexposed TMax, while usable, is not my friend, and frankly, I wasn't that happy with Arista shot at 200, either.

I was able to pull some of my over-exposed 400H (+2) shots back to reality by doing some channel tweaking in the CMYK space (primarily Cyan), but not all of them.

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You aren't wrong, but you need to understand that those who suggest adding a stop of exposure are most likely basing their decisions on meter readings that concentrate on shadows and shadow details.
And that approach is an excellent approach for those who do their own darkroom work, including either printing in the darkroom, or post processing with darkroom printing in their background.
ISO/box speed numbers are based on criteria that reflect "best print" data, and that data puts more weight on mid-tones and highlights, as they reveal themselves in straight prints. Most people, when they look at prints, respond much more to how the mid-tones and highlights appear, than how shadows appear.

MattKing said:

You aren't wrong, but you need to understand that those who suggest adding a stop of exposure are most likely basing their decisions on meter readings that concentrate on shadows and shadow details.
And that approach is an excellent approach for those who do their own darkroom work, including either printing in the darkroom, or post processing with darkroom printing in their background.

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Understood-- and I have to admit to having never done a darkroom print in my life, although I have a decent grasp of the technology and challenges (and I keep contemplating where I could set one up in my current house).

My personal experience with scanning is that overexposing, especially for color film, will produce difficult to correct results. Still, I deliberately left myself a "get out of corner free" card-- I said "when properly metered". In my case, that includes specifically metering the shadows I want in the final image, and as a rule, trying to keep them in at least zone 2, if not 3, to borrow the zone system terms for a moment. So in most cases (7 or fewer stops of range), I should be keeping shadow detail visible, without sacrificing highlights.

Part of the reason I've posted very few photos (other than being highly self-critical) is that I've spent most of my time getting familiar with the quirks of various films and lenses, and learning how to produce what I like.

So far, accurate metering and shooting at box speed seems to produce what I consider to be "better" images.

grat said:

My personal experience with scanning is that overexposing, especially for color film, will produce difficult to correct results. Still, I deliberately left myself a "get out of corner free" card-- I said "when properly metered". In my case, that includes specifically metering the shadows I want in the final image, and as a rule, trying to keep them in at least zone 2, if not 3, to borrow the zone system terms for a moment. So in most cases (7 or fewer stops of range), I should be keeping shadow detail visible, without sacrificing highlights.

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That is a very reasonable approach, but I would suggest at least experimenting with another approach, because your approach is better if you have available to you the darkroom techniques that retrieve highlight detail. Those techniques are at least a little bit harder to mimic in a scan and post-process workflow.
Consider trying incident metering, at box speed.
Failing that, try picking a mid-tone that you wish to render at or close to a zone V and pegging your exposure to that.
That should give you very faithful to real life rendition of tones - at least the tones that surround the mid-tones.
If you base your exposure decisions on that, you are free to make intentional changes - to lighten or darken your final results, according to taste.
This example, which I have shared many times, was based on an incident meter reading, and resulted in a thin looking negative. What you see is the result of a negative scan - the darkroom print looks good as well.

DREW WILEY said:

Yes, indeed - I see many many Ektar images posted, and indeed tweaked despite significant exposure and filtrations errors - "I can correct anything in Photoshop " - and indeed looking awful !

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grat said:

My personal experience with scanning is that overexposing, especially for color film, will produce difficult to correct results.

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A big help is using Silverfast Negafix for the color inversion. Negafix has Pro color maps for each individual film, as also density for each spot is known by the scanner then is makes the Pro correction in an amazing way. It is not by chance that Frontiers and Noritsus make those amazing color inversions, real Pro inversion software is a very well adjusted thing, each have its own footprint but all are good starting points for color edition. being the CN film overexposed or not.

The case of Negafix is very interesting, this is a really well crafted Pro tool that beats many other things, in particular many Pro drum scans require a very skilled operator to get close from the Negafix level requiring a click, "influencer" Nick Carver explained it clearly in that famous video, in my opinion his statements are flawless.

MattKing said:

You aren't wrong, but you need to understand that those who suggest adding a stop of exposure are most likely basing their decisions on meter readings that concentrate on shadows and shadow details.

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IMO something can be added to that. It is not only exposing for the shadows but also developing for the highlights.

To conserve the shadow detail we may want there is no other way than exposing enough for those shadows, and if this may end in excessive densities we adjust development. This is excellent for sheet film because we may make an special developement for each individual sheet, so we carefully spot meter shadows and highlights to nail a result like it is exactly predicted before shutter release. The spot reading and the selected N+/- tells to the photographer the precise density in the negative he will get in each spot, and he even may make a good guess about the eleborated printing workflow he will require for his visualization, also before shutter release.

Usually two shots of the scene are made (Sexton explains)... but no bracketing, this is to have an exact backup, if the image is important for an artist then he wants a backup for the case (for example) the negative gets a minimal scratch while printing a series....

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We may recall that in 1959 people were overexposing by 1stop compared to 1961. In 1960 ASA standards changed and all speeds stamped on the boxes were doubled with no change in the film manufacturing. Were the 1961 negatives better or worse than the ones shot in 1959?

Since 1961 images were better in general, they had more DOF or less Shake, but also images became generally worse than the 1959 in the tonality and in the shadow detail. This was extensively debated at the time... and 60 years later Zone System practitioners still have a mess when explaining if they aim Z-VI or if they rate speed to the half of what the box says, like in 1959 !! Amazing !

In 1959 meter aimed x20 the light in the speed point (Base+Fog+0.1D), while today (since 1960 change) it aims only x10 the light in the speed point.

What happens if we overepose by 1 stop ? Well... we have a safety factor, negative film usually also works fine at +1 but those shadows at -2.5 will be at -1.5. Also Image quality will be slightly better, smaller crystals will be also exposed and film will have a marginally better capability. (Slides, CMS 20 cannot be overexposed...)

If we spot meter each interesting spot in the scene we have a good prediction about what wil result in each spot, instead if the metering is averaged we don't what we are doing... for example is sun disc is included in the framing the reading changes a lot. The F5 / 6 matrix metter detects that an it is not lured, but in many other situations a brigt spot or area may end in an undesired exposure. Also shutter, aperture, error... and not all rolls are equal, film age counts. In particulars mechanic shutters in LF lenses are not exact and speeds should be checked with a shutter tester to know the actual ones...

Usually negative film reacts better to overexposure than to underexposure, so recommending some overexposure (1/2 to 1 stop) with negative film is not that weird when we don't have total control, like when we use ponderated mode with contrasty scenes.

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Some remarkable Pros, like Jose Villa "and friends", being top notch film Pros with atonishing commercial sucess, have made a living from overexposed color negative film. In the Jose Villa case, who more or less is a principal boss in the wedding sector, he usually overexposes Fuji 400H by +2 stops or beyond, not always but usually, he rates 400H at 200, and then he meters in the shadows... And we are talking about people shooting tons of film in the Pro realm and invoicing insane amounts for each job.

http://www.johnnypatience.com/jose-villa-colors/
https://josevilla.com/santa-barbara-wedding-photography-home/
https://josevilla.com/




Of course we may not want a frankly overexposed look like in the wedding shootings, but of course this is an aesthetic resource we have in the toolbox.

Well, no doubt that that CN film stands a fair overexposure abuse, do that with a DSLR ! nothing wrong at +3 or +4, but we have problems at -2, so a +1 by default will mostly benefit the average result, if we have light enough to not have shake or not having to stop to get more DOF.


http://canadianfilmlab.com/2014/04/24/film-stock-and-exposure-comparisons-kodak-portra-and-fuji/


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Of course, nothing wrong in using CN box speed in a well metered scene, and also nothing wrong in overexposing CN by 1 stop if we can't meter accurately a challenging scene.

138S said:

Of course, nothing wrong in using CN box speed in a well metered scene, and also nothing wrong in overexposing CN by 1 stop if we can't meter accurately a challenging scene.

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This is the exact advice I received when I started shooting professionally. My mentor said that a dense neg (overexposed) can always be printed down but a thin neg (underexposed) will have difficulty making a decent print. He also said the opposite was true of transparency film but with less tolerance.
As a result of this advice, I always exposed colour neg & b&w neg film by reflective meter and transparency film by incident meter and always at box speed.

foc said:

This is the exact advice I received when I started shooting professionally. My mentor said that a dense neg (overexposed) can always be printed down but a thin neg (underexposed) will have difficulty making a decent print. He also said the opposite was true of transparency film but with less tolerance.
As a result of this advice, I always exposed colour neg & b&w neg film by reflective meter and transparency film by incident meter and always at box speed.

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If you want to project (which I would recommend anyone to try and is where slide really justifies it’s existence) then you need to be pretty much bang on.

There is probably a stop or so to either side with Velvia 50 and a little more with other slide film.

But that is essentially up to interpretation and the look you want.

More than that, and you are going to have to work hard to convince yourself and any onlookers that milky gray or deep shadows is what you intended to do all the time.

It’s not a problem with modern AE cameras from the 80s and up.
But when you graduate to folders and TLRs with primitive or no light meter, you should really use that tripod and meter the scene throughly.

BUT if you have to err or chose a safe zone, then it’s better to underexposed slide a bit than to overexpose.

You will need one hell of a light to blast through the DMax of two stop underexposed Velvia 50, but surprisingly when you do, there is actually more good image information there than you’d think, with beautiful deep saturated colours.

One little discussed option is to preflash slide.
It’s tricky and somewhat of a black art with a lot of tacit experience and Gefühl, and of course some amount of luck.

But it can get very nice and projectable results in contrasty situations (one of which is under exposing) and when pushing slide.
you need a camera with double exposure, or you need to runs the whole roll through once and flash the whole thing.

Put a tissue over the lens set at infinity, aim it at a white light source and expose three stops down from what the scene requires, as a starting point.
Less, and effect will be too subtle for you to really get a feel for it. And more will of course start to impinge on the mid tones.

It will give you a different look, but it will do magic to contrasty scenes.

foc said:

and transparency film by incident meter and always at box speed.

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I also belive that incident metering is a very suitable way for slides, it yields perfect results, personally I do it by spot metering on a grey card or with a white paper with the proper correction, which is equivalent.

Still when we have bright sky/clouds then metering the sky in spot mode allows to select the right graded ND filter to not burn the sky, of course with some experience we may also have a good guess about what graded ND would be suitable, but burning a LF slide is that painful (one remembers it for years!) that for those formats checking skt well with the spot meter brings some peace of mind.

138S said:

A big help is using Silverfast Negafix for the color inversion.

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I am using SilverFast + Negafix. You over-expose Fuji 400H, get ready for some unsatisfying color issues. Unless you like your photos slightly pastel with extra blue/green elements, in which case, go for it.

Bracket and see what works best for you.

grat said:

I am using SilverFast + Negafix. You over-expose Fuji 400H, get ready for some unsatisfying color issues. Unless you like your photos slightly pastel with extra blue/green elements, in which case, go for it.

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Grat, in my experience you have to really toast (+4 at least) the 400H to get a real pastel shift, like this exposure test shows, no shift by +3...



Sometimes, even if carefully metering, we have very differently subjects in the image, say we have something in the shadow and something under sunlight, or we have a backlighted subject with the face in the shadow and backlight bathed by direct sun rays... We may have to balance exposure to have all ok so we need to know the medium's limit.

If the scene is challenging we know that CN film stands well overexposure, so this determines the way we balance an optimal exposure.

DREW WILEY said:

It's a crossover problem at both ends - both highlights and deep shadows.

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Can't say I've had shadow issues - the toe shapes of Ektar and Portra 160 are pretty near identical - if your processing is on target etc.

grat said:

My experience... has been that box speed, when properly metered produces fantastic results. Both 400H and Portra 400 seem to start getting squirrely around +2.

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And therein lies the answer that the carelessly over-cautious (who apparently spend hours waving their meters at every object in sight, but seemingly never reading, let alone understanding what they say) don't like - it's all about the very basic skills of the correct usage of whatever meter you have. The one aspect that seems to confuse the hell out of people is understanding that shadow or highlight keying may require modifying the apparent rated speed on the meter - in quite a lot of circumstances, shadow keying can give the apparent impression of a +2 exposure. Highlight keying done correctly or incident metering, remarkably often tracks sunny 16... Funny that - almost like those scientists knew what they were doing or something... If it's taking someone more than a few seconds to make an accurate enough exposure determination in the field, I'd be genuinely concerned about their ability to tell apart their arse from their elbow. It really isn't complicated.

And I don't think that any sensible end user has really gone more than one stop over in effective EI on the wider latitude colour neg films (as opposed to Ektar & some of the other more specialist materials) - if the end result is optical printing, as the results will get out of correctable range pretty quickly. Much of the software on the minilab scanners is designed to make it as hard as possible for the profoundly incompetent to truly screw things up in a moderate size print from relatively wider latitude neg films - and certain crossover is easier to correct to something possibly 'convincing' than others. The methods of inversion take a lot more words to explain than do, but the underlying parameters are pretty simple - a gamma & colour correction, corralled into the right direction by a choice of gamut close to the intended output medium. The rest is a lot of words from people who don't want to actually use their lightmeter properly.

I'm not sure if you're actually responding to me directly, or you're merely responding to something I said that set you off.

Your post seems awfully generalized, and makes a number of slightly confusing (to me) assumptions, so I don't know if you're addressing me directly, or merely stating the general case.

I confess, my metering technique seems ridiculously simple, and I'm aware that it won't always work-- but as a rule, I meter for shadows and highlights, and generally split the difference. My humble little Soligar spot-meter only speaks EV, so I have to rely on the lookup dial(s) on the side of the meter to select an appropriate aperture and matching shutter speed. I'm willing to shift the desired EV a bit to either side for overly bright, or overly dim, scenes, but usually, I just average the high and low, and use that as my metered EV.

So far, it's worked well. It's when I get in a hurry, and try to estimate things that my brain occasionally adds a stop instead of removing (or vice versa), or I use ISO 100, forgetting I've got ISO 400 (and there's my two stops), that things come out poorly. Obviously, the lesson here is "don't do that".

My grumble is that many people seem to grab a box of 400H, put it in their camera, and automatically rate it at 200, because "Box Speed Bad! Over-expose!". Then they make an error in metering, and complain that 400H is a terrible film.

I don't know where 138S found his exposure test samples, but the last set of samples I saw, showed color shift starting at +2 when scanned at a lab on a Noritsu.

grat said:

I'm not sure if you're actually responding to me directly, or you're merely responding to something I said that set you off.

Your post seems awfully generalized, and makes a number of slightly confusing (to me) assumptions, so I don't know if you're addressing me directly, or merely stating the general case.

I confess, my metering technique seems ridiculously simple, and I'm aware that it won't always work-- but as a rule, I meter for shadows and highlights, and generally split the difference. My humble little Soligar spot-meter only speaks EV, so I have to rely on the lookup dial(s) on the side of the meter to select an appropriate aperture and matching shutter speed. I'm willing to shift the desired EV a bit to either side for overly bright, or overly dim, scenes, but usually, I just average the high and low, and use that as my metered EV.

So far, it's worked well. It's when I get in a hurry, and try to estimate things that my brain occasionally adds a stop instead of removing (or vice versa), or I use ISO 100, forgetting I've got ISO 400 (and there's my two stops), that things come out poorly. Obviously, the lesson here is "don't do that".

My grumble is that many people seem to grab a box of 400H, put it in their camera, and automatically rate it at 200, because "Box Speed Bad! Over-expose!". Then they make an error in metering, and complain that 400H is a terrible film.

I don't know where 138S found his exposure test samples, but the last set of samples I saw, showed color shift starting at +2 when scanned at a lab on a Noritsu.

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I'm saying that you're basically doing it about right - while others seem to make a huge performance out of taking a meter reading and still get it wrong!

The trick with spotmeters is to use the IRE index for setting highlight or shadow keying - the Pentax digital meter has the index engraved (found under the zone sticker people like to deface them with) - IRE 1 for shadows with detail (better for broader latitude neg), IRE 10 for highlights with detail (better for transparency, narrower latitude neg, cinema neg - stuff where you can't burn in highlights without issues!) - depending on what material you're using. Should be obvious from the linked pic how it works - it's great with transparency etc because it'll immediately show you if shadows you want detail in will drop below 1, when holding your detailed highlight on 10 - and how much ND grad you might need to add. And for BW, it should be pretty obvious when you're going to need to pull back processing a bit (or bump it up), if when you set exposure to hold shadow details, your highlight value disappears off the end of the scale. It's pretty intuitive once you get where the indexes sit! IRE 1 is 2 2/3 stops to the left of the 'midtone' mark and IRE 10 is 2 1/3 stops to the right (though the Soligor might run the other way to the Pentax from what I can tell). As you can see however, a 'shadow key' could lead someone to claim that they're exposing +2, when they possibly aren't really - then +2 on top & you end up with what some might call +2, others +4. And NPH's relative gradient relationships of its RGB curves change with exposure, which is what throws off the colour balance as you add exposure - but depending on how well the minilab algorithm can get everything to mix to grey (allowing for subject failure - a big block of a single strong colour) & balance what it thinks are faces to about skin colour, you might get OK results further than you could in the darkroom (unless you used extended techniques - masking etc).

grat said:

My grumble is that many people seem to grab a box of 400H, put it in their camera, and automatically rate it at 200, because "Box Speed Bad! Over-expose!". Then they make an error in metering, and complain that 400H is a terrible film.

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Box speed works if the light meter has been calibrated, which it sounds like yours has not been calibrated, AND one meters without the sky in the meter view. Sun in the meter view is an almost guaranteed over expose for slide film.

Any kind of "one shoe fits all" advice is bad advice. Not all color neg films are the same. Fact. Nor are all meters the same. I use the Pentax spotmeter for all my shooting; but what counts most is being familiar with whatever metering style you prefer. But the fact that all my meters match, and are periodically recalibrated if necessary by the same outfit that does that for Hollywood cameramen means that the reading is objectively true unless there's flare involved. I use rubber hoods on these. The nice thing about these Pentax meters is that it's extremely fast and intuitive to compare highlight, midtone, and shadow values, all at a single glance. They're also particularly convenient for Zone System work in b&w photography, but don't confuse the applications like a couple of past threads have! Color is a different ballgame.

Sirius Glass said:

Box speed works if the light meter has been calibrated, which it sounds like yours has not been calibrated, AND one meters without the sky in the meter view. Sun in the meter view is an almost guaranteed over expose for slide film.

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Really? Why do you think my meter needs calibration, given that I'm the one claiming box speed is the right way to go?

Further, why would you think I'd meter with the sun in the view, given that I'd have to deliberately point my 1 degree spot meter straight into the SUN?!?

Not only would I have to be an idiot, I'd probably be blind.

Sirius Glass said:

Sun in the meter view is an almost guaranteed over expose for slide film.

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Sirius, in this case it would underexpose... the meter takes much more light so it increases shutter speed or/and stops more the aperture. Same effect than when background is bright snow.

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With ponderated and average metering modes we always have that problem... bright areas in the scene may influence too much the reading, so we should frame in the sky only or in the ground only to realize who those areas will result. Matricial modes tend to solve that problem from multi-segment intelligence. A meter like the one in the F5 / F6 has no problem with that because it reads mode than 1000 RGB spots in the frame and a Neural Network interpretes the scene, with the information from DX code telling what latitude has the film to enrich the automated decision...

Still, we get the highest control by M mode and spot metering, in that way we know what exposure will result in each spot we want to check... but this is slow for many kinds of shooting.

One has to learn how his meter modes work and when one has to go manual decision because the meter is not to tell the exposure we really want.

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Another thing is if we have to use box speed or not, the majority of the automatic exposure cameras have an "exposure control dial", isn't it ?


https://www.kenrockwell.com/nikon/fe/users-guide.htm

This is not only because we may want an underexposed shot, it is also because ponderated mode fails to say a good exposure in challenging scenes where bright areas provocate the underexposure of the rest which can be the important subject in the scene...

But we may rate CN film at lower than the box... effect will depend on our taste but also a lot on what automatic metering system we use. The F5 matricial mode won't be lured by bright areas in the scene, it is intelligent enough to see what happens. Instead ponderated mode is not that smart by far, but one may prefer it because it is clear how it made the calculation so while the reading can be less trusted at least we can overide it to get exactly what we want, instead F5 matricial mode is more difficult to override because we don't know how the hell the intelligent system told the number.

The controversial nature of the box speed debate comes from that... YMMV because we meter different.

Anyway generally overexposing CN film is about taste, IMO the standard exposure aimed by meters is a bit favouring the benefits of speed (less shake, DOF) over the benefits of general quality, there is a compromise... want we better shadows or we prefer less shake and more DOF ? Do we want DOF or selective focus ?

We may want a +1/2 or +1 general overexposure... what we usually don't want is a -1 underexposure ! this IMO tells that exposure is a bit standarized to favor higher shutter speed or higher DOF. Well, this is a 1960 year debate... film box speeds were just doubled that year.

grat said:

Really? Why do you think my meter needs calibration, given that I'm the one claiming box speed is the right way to go?

Further, why would you think I'd meter with the sun in the view, given that I'd have to deliberately point my 1 degree spot meter straight into the SUN?!?

Not only would I have to be an idiot, I'd probably be blind.

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The post is for general advise for our paying public.

grat said:

calibration

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Additionally, it has to be noted that meters cannot be fully "calibrated", they can be calibrated to certain manufacturer specs but there is no norm clearly stating how meters universally should work, there two constants in the calculations that the manufacturer can vary..

It usually accepted that meters from different manufacturers may have a 1/6 to 1/4 stop missmatch ex-factory, and of course this is only for gray subjects. When we meter saturated color the difference may be way larger because spectral response of each meter is probably different.

For exampe the Pentax V spot meter has this spectral response, as each film has a particular spectral response, the metering on saturated colors may give an unsuspected result, for example metering a blue sky for velvia may no be that direct as we may think, with slides we are at risk of overexposing the sky if not knowing how our particular meter treats the sky...

The Pentax and Minolta spotmeters are calibrated identically, and have peak sensitivity at 550 nm, just like normal human vision. In certain applications there ARE fixed standards of calibration. If a completely objective industry standard weren't in mind, there wouldn't even be an IRE scale on those meters.