Stouffer's 4x5 step wedge for exposing negative?

[jvuokko]

Hi,

this is a bit dumb question, but how do you manage to put 4x5 step tablet with a film on the film holder (sandwitch -style)? This is the most common style that I have seen and is taught by books (except BTZS).

But I cannot put it on my holders, the step tablet itself will fit fine like does film. But film plus step tablet. No way, the holders are just too tight!

Earlier I used small step tablet, but full 4x5 size is a lot better.... If I manage to put it in the holder.

What are other options? Use of darkroom and enlarger, but as I use also orthopan and full orthochromatic films, the color of enlarger light source can affect results too much.
Also by exposing wedge thru camera, all camera inducted problems come to taken account like in real life.

How about just putting wedge against transparent glass fit it on the negative holder, illuminate it evenly from back and take photograph from it?
Ofcourse, the bellows extension factor must taken in to account.


What's your method?

Thanks,
Jukka

[salihonba]

make a slide with Laminating sleeve, exactly same size with the slide of holder, this lami- slide holds wedges in the center, use strong black tape to block the transparency area outside the wedge, and to enforce the slide. So when you want to test film, pull out the normal slide, and insert the lami-slide in, wedge in slide will just cover the film, then expose.

Since wedge is not tightly stick with film, image you get will be a little fuzzy, but that is OK for densitometer, and you can ignore density that laminating sleeve bringing in, it can be treated as film base and zeroing.

[Len Middleton]

Jukka,

The only 4x5 step wedge that I know that Stouffer makes is their project wedge (e.g. TP4x5-21). I believe this intended for use in your enlarger for paper testing (hence available in different formats), and that is what I purchased mine for.

You already know about the smaller step wedges, but with film holders designed to hold your film flat you will not / should not have any additional space, otherwise your film will flop around and could be out of focus or with uneven focus.

An alternate might be to tape the 4x5 step wedge directly onto the outside of your film holder in front of the dark slide and gently put that into your camera making sure it does not come off. You might be able to remove the lens to inspect it to ensure it remains in place. The edges of the steps may not be hard, but then that is not what you need.

If the back of your camera is removeable you might be able to tape the 4x5 step wedge directly onto the film holder with the holder in place on the back.

Hope that helps,

Len

[salihonba]

Stouffer got 31 steps wedge in 4x5 size, just 1/3 stop for each step, very easy to use when do the film test.

[nolindan]

Use of darkroom and enlarger, but as I use also orthopan and full orthochromatic films, the color of enlarger light source can affect results

Another option is to use a color correction filter to balance the enlarger's light to daylight. An 80A should work close enough. It will also give you 2 stops of light loss which will help slow the exposure times.

Since the step tablet is being contact printed in the camera the camera flare won't be incorporated into the results. In any case, flare is a very tricky proposition - very different if you have a black spot on a white field or a white spot on a black field.

I have had success in 35mm mounting a 35mm frame-sized tablet in a slide mount and photographing it with a slide duplicator. You might try something similar by taping the step tablet to the window with some translucent backing material and taking a picture of it.

A problem with using daylight is that it can be variable and it is hard to get as consistent exposures as you would like for film testing. An enlarger is a much more constant source.

[salihonba]

film test with wedge sleeve slide is quiet easy, prepare your film holder as usual, normally I will prepare for 12 films, that is six holder.

then use your camera/lens combination, focus to infinity, and then place the camera/lens close to a window with semi transparent glass, make sure that sun light on the glass is even and bright.

lens toward the window. meter the semi-trans glass, put it on Zone X, then place your film holder, pull out slide out, insert lami-slide in, expose, pull out lami-slide, insert normal slide, done! next film!

within 5 min, I can expose 12 films, all in same expose condition, all you have to do is change the film developing time, and dilution of developer.

to me, I will make 12 films into 3 group, with 3 dilution, and 4 developing time (4,6,8,10), you will have 3 curve, enough for n-2~n+2

[salihonba]

well, if your window is all transparent, prepare an acrylic plate which is semi white, about 20x20cm, put it on the window.

when you place lens toward window (or plate), make your lens as close as possible, better front end touch the window (plate), or you can use a lens hood to make it closer.

for the sun light, film test should be at a sunny day, and about 11am~2pm, when there is no cloud, and the sunlight is steady bright at least 10min, enough for your test!!

good luck to you!

[D. Bryant]

Hi,

this is a bit dumb question, but how do you manage to put 4x5 step tablet with a film on the film holder (sandwitch -style)? This is the most common style that I have seen and is taught by books (except BTZS).

But I cannot put it on my holders, the step tablet itself will fit fine like does film. But film plus step tablet. No way, the holders are just too tight!

Earlier I used small step tablet, but full 4x5 size is a lot better.... If I manage to put it in the holder.

What are other options? Use of darkroom and enlarger, but as I use also orthopan and full orthochromatic films, the color of enlarger light source can affect results too much.
Also by exposing wedge thru camera, all camera inducted problems come to taken account like in real life.

How about just putting wedge against transparent glass fit it on the negative holder, illuminate it evenly from back and take photograph from it?
Ofcourse, the bellows extension factor must taken in to account.


What's your method?

Thanks,
Jukka

Save yourself a lot of work and use the BTZS method.

Don Bryant

[jvuokko]

Thank you for the tips.
Now I have some good starting points :)

Enlarger with 80A sounds good also. I am not sure whether I will settle up with enlarger or continue use of camera (like I have done with narrow step strip, T2110) with LF film tests.

Enlarger method has obvious advantages because I allows easy exposure of roll films without using whole roll per test. As I use MF and 6x9 backs among with LF gears, I have to use enlarger anyway.

[ic-racer]

Personally I'd use the smaller step wedge, to concentrate the test toward the center of the lens field.

Inexpensive sensitometers are not hard to find, having one is much handier than using an enlarger. Realize that all they are is just a controlled light source and you supply the step wedge you want to use.

[mandoman7]

Its desirable IMO with calibration tests to replicate actual shooting conditions to eliminate variables. With bellows extensions, filters, incandescent light, etc., there are some variables that are getting introduced that could effect results.

After testing for awhile, I found that I was really only using zone I, V, and VIII readings to determine the spread that my dev time was giving. So my tests evolved to a method based on Hutching's described in the back of his pyro book. Using a board with a strong light at one end that graduates through a full 10 zones, you can mark the points where my meter reads I, II, and VIII, The advantage is that I'm using my regular spot meter and a normal lens with a reliable shutter speed.

[jvuokko]

mandoman7, thanks! That is new method to me. Never heard before.
I have used combination of three cards with Q&D roll film calibration. One is graycard, one is almost black and one is white at VIII, all with black velvet background. Photographed on single negative with normal lenses etc.

ic_racer, sensitometer would be really nice. There's lot of them usually at ebay, but problem is that they need 110v/60Hz current and where I live, it's 220v/50Hz.
If they're not sensitive for current's frequency then transformers are easy to found and relative cheap, but if frequency should also converted from 50Hz to 60Hz, then it is a problem.
(same problem prevents me using GraLab's timers etc).

[salihonba]

buy a ZoneMaster enlarging exposure meter, it also can be used as a densitometer under enlarger, it uses AAA battery, no current problem.

[sanking]

Hi,

this is a bit dumb question, but how do you manage to put 4x5 step tablet with a film on the film holder (sandwitch -style)? This is the most common style that I have seen and is taught by books (except BTZS).


What's your method?

Thanks,
Jukka


I adjust the height of the enlarger to illuminate an area about 8X10" in size and contact print the step wedge to a sheet of film in the middle of this area. I use a small contact printing frame for this, but you could, 1) just place a piece of glass over the sandwich, or 2) place the step wedge over the film in a film holder. .

For BTZS testing I now use a sensitometer but when I did this type of testing with an enlarger I used a Metrolux timer (which measures by light integration) to ensure accurate exposure times.

If you use an enlarger with a tungsten type light I would suggest adding an 80A filter to balance for daylight.

Sandy King

[nolindan]

Thinking about the light color ... it shouldn't make any difference in step tablet testing.

All the grains in the film have the same spectral sensitivity - the contrast of the film will not (or should not) change with the color of light.

Of course the relationship of an exposure meter reading to the resulting film density will change with light color - but if using an enlarger that's not much of an issue as the light used for the exposure is arbitrary - i.e. whatever it is that works.

The results of the step tablet test with an enlarger will be the HD curve of the film. The light meter EI setting won't be determined by this test unless you have a film-plane light meter like a Sinar, in which case the 80A filter would be a help in finding the right EI setting for photographs taken with natural light.

It is possible to find the relation between an enlarging meter reading and f-stop timer setting - the Darkroom Automation meter comes to mind, for some reason - and a corresponding light meter EV reading/aperature/ASA/shutter speed. However, unless a lot of film testing is planned this may be too much effort.

FWIW: with the DA settings, a total exposure of 7.0 stops [meter reading + timer setting] works well for contacting a step tablet on to TMX-100 film. That's about 3 stops less exposure than what will produce max black on MGIV RC with a grade #2 filter - where the film is exposed without the filter. The film will just rise above B+F on step 30 of a 31-step tablet.

[jvuokko]

Salihomba, thanks for the tip!


About the using 80A. It's a good idea, because it simulates "general" daylight. Especially with orthochromatic or orthopanchromatic it could make difference. At least I would imagine so.

But... after taking reading measurements of my enlarger, with f/11 it gives EV4 for ISO 100 when lowered to 22,5cm (about 8.8 inches). So it's already a quite low. Get only about twice a size of 4x5 sheet of lighted area (about 8x10).

So if I add 80A, then I am out of light, especially because orthopanchromatic and otrhopan materials tend to be slow..

Measurement is taken with spot meter from bright white card.

Often I have seen tip to use light meter with cell pointed towards the enlarger lens (and thus the light source), but it doesn't seem reasonable. With incident meter it would be ok, but... Because the light comes from only above, it gives way lower reading than reflective readings from white card.
Perhaps reading thru step wedge's middle density towards the light would work..

But I doubt that it would give any help to my unpowered enlarger.

[salihonba]

A step wedge slide for testing.
http://lh4.ggpht.com/_gvRcWa2LHhg/SrMnQ5wdtwI/AAAAAAAAB7w/bxn2OmyiJkc/s400/P9180401.JPG
camera with 210mm lens focus on infinity and then put it close to acryic plate by the window, then treat plate as Zone X, meter and shoot.
http://lh5.ggpht.com/_gvRcWa2LHhg/SrMntJn1h_I/AAAAAAAAB74/R7_27UUOcyo/s400/P0000303.JPG

[jvuokko]

Okay, it really seems that the enlarger is not the best option (but it would allow the testing of 120 and 135 material easily).
Why I don't like enlarger?

Color temperature issues - my enlarger has an halogen light source, so I guess that it's somewhat close of 3400K or normal 'tungsten'. But despite of 100W power, it does not give enought light for BTZS's 'standarsized EV4/ISO100/f11'. Perhaps the diffusion chamber is just too much. Condenser enlarger would give more light (I have old Meopta Opemus 4 at the cellar, perhaps it would do the trick!)

Use a 80A with this candle? No way...

It takes ~0.25 sec until the maximum brightness is reached, so exposure times shorter than one second suffers badly. Is the halogen slower than normal tungsten bulb?

It is good that I actually measured the time that enlarger needs until the light is fully lit up. I have used short times often and sometimes wondered why my calculations don't match when I change aperture/time combination. I really thought that it would took less than 0.1 second to get light on..

[nolindan]

I really thought that it would took less than 0.1 second to get light on..

A standard PH212 enlarging bulb requires a 50 mSec adjustment for the turn on delay and the turn off afterglow of the bulb.

A test for determining the effective adjustment time you need to compensate for the lamp delays is described in an application note on the Darkroom Automation web site: http://www.darkroomautomation.com/support/AppNotePH212LampDelay.pdf.

Claimer: I am associated with Darkroom Automation (http://www.darkroomautomation.com/da-main.htm) and I do have a vested interest...but I'm also a satisfied customer.

[jeroldharter]

I would use the BTZS method on the baseboard of your enlarger. I made a simple jig with a right angle of film scraps taped to a piece of black matboard so that I can slide the sandwich of step wedge/film into the right angle on which I place a sheet of glass and then expose.

A couple of questions that I can't remember off the top of my head:

Is the recommended EV reading off the baseboard 4?
I have used the white light from a dichro head in the past. Now I have a cold light. Will that make any difference to future testing?

[uhner]

Yes, in Beyond the zone system Davies (1999:71) recommends using 4 EV for testing ISO 100 films with an exposure time of 0,4 seconds.

In my experience, this works quite well.

[jvuokko]

Yes, in Beyond the zone system Davies (1999:71) recommends using 4 EV for testing ISO 100 films with an exposure time of 0,4 seconds.

In my experience, this works quite well.

Now I am really wondering why I got EV 4 level with f/11 only by lowering the enlarger head quite low (it really covers a way less than 8"x10" area)?

For ISO 400 grade film that would be 0,4s with f/22 - but I just can't get there.
Part of problem is surely the huge lamp delay, which I estimated to around 250ms.

After first initial test, it was obvious that I had to change time from 0,4s to 0,6s to compensate delay and as the test strip was so thin, I had to also open aperture by one step to f/16.

Ofcourse Davis also mentions in his book that 0,4s (2/5s) is a start point for proper time and if it won't give a good exposure then enlarger head must be raised or lowered. But in my case, there's no much chance for lowering enlarger head. So I have either use larger aperture (which is not good, because len's maximum aperture is only f/5.6 and I need to test slow films also, not just ISO 100 and ISO 400).


To put all this together, I can get desired EV4 for ISO 100 only by using f/8 and thus I cannot test orthochromatic films. With 80A I should use something like f/4 for ISO 100 and my lens is not capable for that. Not saying that orthochromatic films tend to be at ISO 25 range... :(

[SAShruby]

I suggest to put your enlarger as high as possible to prevent light fall-off. it could give you false readings on the edges.

[D. Bryant]

Inexpensive sensitometers are not hard to find,

If that is the case I wish you would find me one.

Don Bryant

[Kirk Keyes]

Me too!

[Kirk Keyes]

Thinking about the light color ... it shouldn't make any difference in step tablet testing.

All the grains in the film have the same spectral sensitivity - the contrast of the film will not (or should not) change with the color of light.

That's not right. The spectral sensitivity of an emulsion does vary with wavelength.

And while we are at it, the contrast of a film does increase as the wavlength increases, so red light gives more contrast than blue light.

[jvuokko]

Story continues.
Shooting against window with a camera using 'sandwitch' method on film casette would be nice but what I have tested, the light level is just too dim. Could get decent exposure times and apertures for iso 100 and slower films.
Even tried to put step wedge directly against the window and photograph it because the acrylic plate needs couple of stops more light.
No success, the light is too low.

Perhaps during summer it would be different.


So going back to the BTZS-stylish enlarger exposure. I grabbed my old condenser enlarger from the basement and to my big surprise, it's light is even weaker than my current diffuser enlarger's! Got ev 2 on f/11 @ISO100 (measured from glossy white surface).


It really seems that my only option is to use sandwith technique like salihonba's example, but using some studio flash lights instead of daylight.

Drawback: Cannot do tests for 120 materials, only 4x5.

[jeroldharter]

...
So going back to the BTZS-stylish enlarger exposure. I grabbed my old condenser enlarger from the basement and to my big surprise, it's light is even weaker than my current diffuser enlarger's! Got ev 2 on f/11 @ISO100 (measured from glossy white surface)...

Why not open the lens?

EV2 with ASA400 film should be about right by the way. If you are using some of the ASA25-50 film you will of course need a corresponding increase in light.

What sort of enlarger has such a dim light?

[salihonba]

Drawback: Cannot do tests for 120 materials, only 4x5.

Why not?
http://lh4.ggpht.com/_gvRcWa2LHhg/SrLkj7aylyI/AAAAAAAAB44/-eLXhLTDe5Q/s400/P0000764.JPG
http://lh4.ggpht.com/_gvRcWa2LHhg/SrLknMZh9yI/AAAAAAAAB5E/LjFxi9L99bA/s400/P0000768.JPG
http://lh4.ggpht.com/_gvRcWa2LHhg/SrLkyWDQe1I/AAAAAAAAB4w/6N_1W2MxOu4/s400/P0000769.JPG
http://lh4.ggpht.com/_gvRcWa2LHhg/SrLlTObh6bI/AAAAAAAAB5Q/9_RvAX5nLAA/s400/P0000773.JPG


but I think Finland's Autumn got too weak the sunlight.

[salihonba]

http://lh3.ggpht.com/_gvRcWa2LHhg/SrLlYlFmUUI/AAAAAAAAB5Y/saDACTf39lE/s400/P0000774.JPG

[jvuokko]

Salihomba, that's awesome. Never thought that!

jeroldharter, I don't open to maximum aperture in the fear of uneven light.

One thing that I haven't tried is to fit smaller diffusion chamber and try to use 2.8/50mm enlarging lens. It's two stop faster than my 6x9 lenses and perhaps smaller chamber also gives brighter light.

These enlargers are Meopta Magnifax 4 with multigrade diffuser head and condenser are cheap and lightweight Meopta Opemus 5 (only up to 6x6).
I have never before considered them slow or dim enlargers, especially Magnifax 4 gives too strong light for normal printing, I have to adjust light intensity to lower settings usually.

[jvuokko]

Actually, how should light metered?

From Davis' book I understand that incident light meter with semiphere removed should give EV 4 readings when pointed directly upwards to the lens and light?
Perhaps I missunderstood, that way I got EV 10 readings for ISO 100 when the step wedge + negative got about right exposure.

With hemisphere on it's place I got EV 2.

From glossy photograph paper at the enlarger's easel, I got EV 5 readings for ISO 100 with a spotmeter when exposure seems to produce nice scale to the negative.

The right reading should also be able to meter thru the step wedge's step 10 when a spot meter is pointed directly upward, but it's bit hard.


Acrylic plate and strong back light is easier to meter, just meter light thru acrylic plate and add 5 stops overexposure and there it is.

[ic-racer]

What are other options? Use of darkroom and enlarger, but as I use also orthopan and full orthochromatic films, the color of enlarger light source can affect results too much.


What are you trying to accomplish?

If you are comparing a single film across a number of development times or development menthods to determine gamma the enlarger light color will not mater. The exposure time and ev will probably not matter either, unless the film has very bad reciprocity characterstics and the time is greater than 1 sec or less than 1/1000.

If you intend to compare the relative speed of one film to another or set up an independent ISO testing station, then enlarger light color will have an effect, but that type of test might be better done with an in-camera test (like a zone I, 0.1 log d test).

[jvuokko]

What are you trying to accomplish?

If you are comparing a single film across a number of development times or development menthods to determine gamma the enlarger light color will not mater. The exposure time and ev will probably not matter either, unless the film has very bad reciprocity characterstics and the time is greater than 1 sec or less than 1/1000.


Normal development calibration, but the film is not sensitive to the red light. Althought I don't thing it's a big problem as long as I get curves. I can always figure out the real speed later in normal daylight test.

Regarding the enlarger problem; I found easy solution. Just took off the lens and now I have a lot of light that is even enough :)
Actually found this one from the btzs's web site's forum where Davis has wrote same tip to someone who had similar problem with too dim enlarger.

Now I am glad that I have light intensity adjustment in my enlarger's head. I can easily set the level of light low enough that it's suitable also for faster films :)

[younghoon Kil]

then use your camera/lens combination, focus to infinity, and then place the camera/lens close to a window with semi transparent glass, make sure that sun light on the glass is even and bright.

lens toward the window. meter the semi-trans glass, put it on Zone X, then place your film holder, pull out slide out, insert lami-slide in, expose, pull out lami-slide, insert normal slide, done! next film!


When I want to shoot a Stouffer's 4x5 step wedge(TP4x5-21 or TP4x5-31) regardless of time and light conditions, can I shoot it towards the LED Video Light instead of the window?

- https://www.amazon.com/gp/product/B07WD6QXMJ/

[esearing]

When I want to shoot a Stouffer's 4x5 step wedge(TP4x5-21 or TP4x5-31) regardless of time and light conditions, can I shoot it towards the LED Video Light instead of the window?

- https://www.amazon.com/gp/product/B07WD6QXMJ/

Most light sources would likely have a hot spot in the center. Use something to diffuse the light so it is evenly illuminated from center to corner. A piece of white poster board well lit will achieve same result.

[younghoon Kil]

If I can use such a High CRI LED panel, I will place an acrylic plate in front of the LED panel.
Thanks.

[bnxvs]

Wow. You spend so much time building tools for easily exposing sensitograms. In my humble opinion, it is much easier to spend time once and make a simple sensitometer. All you need for this - an arduino (Uno, Nano etc), a piece of LED strip (preferably high CRI), a i2c light sensor module (such as BH1750 or max44009), some buttons, a piece of milky white acrylic glass and a mosfet transistor key. And, of course, the stouffer's scale (T2115 for example).
You can precisely set the illumination (within 1 lux) and any, even the shortest "shutter speed" (up to nanoseconds), since the response time of the LEDs and the mosfet is very short.
Also, within certain limits, you can adjust the illumination using PWM.
Combining illumination with "shutter speeds" (more precisely - LED turn-on time) in certain proportions, you can get very accurate parameters of illumination both in units of lux per second (H) and the logH, as you wish.
p.s. To evaluate the results of density measurements with a densitometer, you can use the online script that I made for this: https://bnxvs.shinyapps.io/cc_film_plot/

[Ricardo Maydana]

Una diapositiva de cuña de paso para pruebas. http://lh4.ggpht.com/_gvRcWa2LHhg/SrMnQ5wdtwI/AAAAAAAAB7w/bxn2OmyiJkc/s400/P9180401. JPG cámara con lente de 210 mm enfoca en el infinito y luego la coloca cerca de la placa acrílica junto a la ventana, luego trata la placa como Zona X, mide y dispara. http://lh5.ggpht.com/_gvRcWa2LHhg/SrMntJn1h_I/AAAAAAAAB74/R7_27UUOcyo/s400/P0000303. JPG La cuña de stouffer va dengro del chasis o en la camara antes del chasis eso te permite hacer varias sin tener que hacer un sancwich con la cuña

[s389]

can we use tracing paper instead of acrylic plate?

Another solution is to use Linhof holders that are made for negative or glass plate and are built with a spring.

[acrobatic_citron]

I think you could however having tested the above method just metering I feel whatever is behind the glass needs to be an even tone i.e pointing to the sky

[Larry Gebhardt]

I haven’t tested this, but why not mount the step wedge to the translucent sheet and focus at 1:1? That would account for lens flare and bellows flare and still be easy to meter, assuming a spot meter. I’ve done btzs tests in the past and find the results are not as close to real world shooting as I would like. Flare is the most likely reason. If I were to test again I’d try for a setup that’s as close to real world conditions as possible. To get closer to normal conditions you might need to mask off the area around the step wedge since the bright area will add more bellows flare than normal.

[Bill Burk]

Regarding the idea to "mount the step wedge to the translucent sheet and focus at 1:1"...

Some people do it that way.

But the image of a step-wedge starting with 1000:1 contrast, after traveling through lens and camera will deliver maybe 100:1 contrast. Flare gives the darkest patches exposure about a stop greater exposure than the amount you want the patch to get in the test. The darkest patches are the ones that tell you the film speed. So flare ruins the speed check.

[Larry Gebhardt]

Regarding the idea to "mount the step wedge to the translucent sheet and focus at 1:1"...

Some people do it that way.

But the image of a step-wedge starting with 1000:1 contrast, after traveling through lens and camera will deliver maybe 100:1 contrast. Flare gives the darkest patches exposure about a stop greater exposure than the amount you want the patch to get in the test. The darkest patches are the ones that tell you the film speed. So flare ruins the speed check.

If it really is that big of a difference I guess it wouldn't work for the standard testing methodologies. But it does seem like a useful starting point for a new testing methodology since the flare is built into the test. Or maybe not...

[Michael R]

The problem with building flare into the test is that under real shooting conditions flare is of course variable (and not under control). It can be a good "supplemental" test to show the general effect on EI and the lower part of the curve relative to a no-flare EI test (although that is generally superfluous anyway), although obviously the flare factor will depend on the test conditions including the type/layout of the step wedge.


If it really is that big of a difference I guess it wouldn't work for the standard testing methodologies. But it does seem like a useful starting point for a new testing methodology since the flare is built into the test. Or maybe not...

[Larry Gebhardt]

I'm convinced it's not that useful of an idea given there are too many variables. Thanks for setting me straight.

[Ricardo Maydana]

excellent, if I think tracing paper is more appropriate.
I will see if the linhof is economical for the chassis.
What I am understanding from the translation of the text is that the stoufer wedge is inserted/glued into the camera plate in such a way as to make several shots without having to sandwich the plate holder.
If I'm wrong, let me know... my English is not so good...
Thanks

[Ricardo Maydana]

is very likely to happen, that
enlarging and achieving 1:1, without losing light, is complicated.
I understand, there is nothing left but trial and error, being tidy

[Greg]

For anyone wanting to work with step wedges, I suggest acquiring a copy of
photography: CONTROL & CREATIVITY
by T.L. Bollman & G.E. DeWolfe
Special Services, Rochester, New York 1972
It's a small paperback that was very popular at the Rochester institute of Technology in the 1970s.

[Drew Wiley]

Beware of excess yellowness, potentially added up from the diffusers, plus the yellowing atop most LED fixtures, plus their native spectral discontinuity, plus any distinct yellowness inherent to very old step tablets. The cumulative offset could amount to as much as that between recommended filter factors for daylight vs tungsten light. When in doubt, check the net K temp with a color temp meter. You need you test illumination balanced to that or your intended work illumination.

[Ricardo Maydana]

Gracias por el dato