Finding a Densitometer

Since posting I have looked in some of my books to see if there is another way of testing. I found in AA "The Negative" that; Value 1 should have a density of 0.10, Value V a density of 0.70 etc. This is clearly a difference of 0.60 He then goes on to say that a spot meter can be used and that each 1/3 stop exposure interval on the meter will correspond to a density increment of 0.10 From that I would have thought 1 stop would then equal a density increase of 0.30. How then is it that he states the value difference between V1 and V5 is a density difference of 0.60, which he also says above in , blue is 2 stops, when we have learnt it is a difference of 4 stops?

Confused? I certainly am.

Since posting I have looked in some of my books to see if there is another way of testing. I found in AA "The Negative" that; Value 1 should have a density of 0.10, Value V a density of 0.70 etc. This is clearly a difference of 0.60 He then goes on to say that a spot meter can be used and that each 1/3 stop exposure interval on the meter will correspond to a density increment of 0.10 From that I would have thought 1 stop would then equal a density increase of 0.30. How then is it that he states the value difference between V1 and V5 is a density difference of 0.60, which he also says above in , blue is 2 stops, when we have learnt it is a difference of 4 stops?

Confused? I certainly am.

The difference in the scene of 4 stops results in roughly 2 stops of density on the negative.

I have a densitometer that I got from Goodwill, but haven't got working

(or tried very hard). I think it needs a new mirror which reflects the light

though the hole, and then the negative. Well, it cost $10, so I decided to buy

it and figure out later what to do with it.

I did learn some about how it works, though. The current from a photomultiplier

tube is proportional to light intensity and exponential in applied voltage.

So, there is a circuit to run it at constant current, so voltage is logarithm

of the intensity!

Anyway, you might look at the used market. Much darkroom (or not)

photography equipment is being sold off, and you might find one for

a low price.

The difference in the scene of 4 stops results in roughly 2 stops of density on the negative.

If you look at the characteristic curves you will see this depends on the development but even at highest contrast development the slope is still less than 1.

C41 negatives, including XP2, usually have gamma close to 0.5.

Black and white might be 0.6 or 0.7, though push processing can

get close to or over 1.0.

Well, the data sheets have both the characteristic curve, which usually are

curves without the long straight section that you hope for.

Then a contrast index, which is the slope of a line between two specific

points on the curve. They show both of them.

Since posting I have looked in some of my books to see if there is another way of testing. I found in AA "The Negative" that; Value 1 should have a density of 0.10, Value V a density of 0.70 etc. This is clearly a difference of 0.60 He then goes on to say that a spot meter can be used and that each 1/3 stop exposure interval on the meter will correspond to a density increment of 0.10 From that I would have thought 1 stop would then equal a density increase of 0.30. How then is it that he states the value difference between V1 and V5 is a density difference of 0.60, which he also says above in , blue is 2 stops, when we have learnt it is a difference of 4 stops?

Confused? I certainly am.

Density is a measurement of the blackening.

We measure the blackening of film which is a result of exposure and subsequent developing, using a unit called “density”. The root of this value is a delta of 2X. In other worlds, a change in exposure of 1 f-stop is equal to a doubling of halfling of density. By tradition the density unit is logarithmic. A 2X change in density equals 2^0.3 (2 elevated to the 0.3 power). In ordinary math notation this is 0.30 units of density. It goes like this: 1/6 f-stop = 0.05 units of density – 1/3 f-stop = 0.10 – ½ f-stop = 0.15 – 2/3 f-stop = 0.20 – 1 f-stop = .30 – 1 1/3 f-stop = 0.40 – 1 ½ f-stop = 0.45 – 1 2/3 f-stop = 0.50 – 2 f-stop = 0.60 – 3-f-stop = 0.90 – 4 f-stop = 1.20.

This method dates back to the turn of the 20th century. Photo scientist used log notation because before the calculator, they used the slide rule. By the way, a gray card with 18% reflectivity reads 0.75. This is zone V, the calibration point for exposure.

I think my favorite named law that I learned about in college is Beer's law.

(How can it not be fun if it is named after beer!)

That is, that transmittance is exponential in absorbance,

and absorbance is proportional to path length, and to

concentration of attenuating material.

(It seems that it was discovered in wine, and not beer.)

If one filter lets through 10% of the light, two such filters will

let through 1% and three 0.1%. So using a quantity that is

logarithmic in transmittance makes sense.

By the way, SPF on sunscreen is not exponential. The actual

amount of UV absorbing chemical is log(SPF value). Think about

that when buying it. Usually, though, pricing is close enough to

log(SPF), as maybe it should be.

There was a lot of discussion, some years ago, about using scanners as densitometers. There are too many caveats to go into here, but it's certainly feasible. Search for 'scanner as densitometer' or something similar.

Looking on the web, I see they are from a bit under £1000 up to £6000. Does anyone know of a type / sales outlet that makes something at a more reasonable price point or

You're obviously looking in the wrong place.

Small handheld pencil-type densitometers were used for analysing medical X-rays and personal radiation-monitor badges. There should be piles of those things lying around unused these days.

Search for a Sakura PDA-81, or a Wogan Radix. If you're lucky the seller won't have a clue what they've got.

I'm not sure what you hope to gain by using a densitometer anyway. Just expose some film at box speed on an 'average' subject, and develop it according to the recommended time and temperature. You should end up with a roll of negatives that have no more than a Dmax of 2.1, with the printable highlights hovering around 1.8D above base+fog.

If what you're getting doesn't look like these reference negs, then it's time to alter your exposure or development regimen.

Obsessing about densities and 'calibration' is just a waste of time. The guys (or gals) at Kodak, Fuji and Ilford have done all that tedious stuff for you. As long as you don't go off piste and try to reinvent the wheel.... which incidentally isn't a very useful mode of transport off piste!

P.S. I just did a quick search.

There's one here for 43 quid.

From that I would have thought 1 stop would then equal a density increase of 0.30. How then is it that he states the value difference between V1 and V5 is a density difference of 0.60, which he also says above in , blue is 2 stops, when we have learnt it is a difference of 4 stops?

Confused? I certainly am.

Real life brightness values don't have a 1:1 relationship with the density of the developed film.

A typical negative has a 'gamma' of 0.55 to 0.65. So you have to multiply the exposure stop differences by about 0.6 (and then by 0.3) to get log density differences.

So 4 stops exposure difference will give 4 x 0.18 (0.6 x 0.3) = 0.72D difference in density - approximately. It's approximate because the slope of film gamma varies depending on where you measure it. That's what H&D curves tell you.

If you're not familiar with film curves showing density against log exposure (H&D curves), then a densitometer will be useless to you until you learn about them.

If you know how to use one fine, other wise you might have to download the X-rite 811 manual and spend a weekend reading it. It is not easy reading...

You're not wrong there. I read the instructions online. Phew!

I'm not sure what you hope to gain by using a densitometer anyway. Just expose some film at box speed on an 'average' subject, and develop it according to the recommended time and temperature. You should end up with a roll of negatives that have no more than a Dmax of 2.1, with the printable highlights hovering around 1.8D above base+fog.

 

If what you're getting doesn't look like these reference negs, then it's time to alter your exposure or development regimen.

And that's the problem. Without a Densitometer I cant tell what the Dmax or densities are. Will keep looking on the web though, thank you.

Without a Densitometer I cant tell what the Dmax or densities are.

Here are two simulated density wedges. One in sRGB 'space', and the other in Adobe RGB.

If you place your negative over a full-white area of your screen you should be able to judge which density square most closely matches the Dmax or highlights.

If any area of your negative is denser than any square shown above, then you need to drastically cut your development time. OTOH, if it doesn't have any density that closely approaches those squares, then probably more development is called for.

A well exposed and properly developed negative should have a base+fog (minimum density) not much more than 0.2D, and then the darkest printable highlights should be a match for the 2.0 or 2.1D squares at most.

Basically, if you can barely see a domestic light bulb through the darkest parts of your negative, it's too dense. And if you can easily read a newspaper through it, it's too thin.

What you need is an M.C.M Wedge ;):

LINK -- -- -- The M.C.M Wedge Manufactured by Ilford Limited

Seriously, to assess the density of the various bands, I put the wedge on a light box and measured the light values of the sectors using my Sekonic View Spot Meter L438, with the spot set at 4 degrees. Perhaps the same technique could be used on a negative?

The Miniature Camera Magazine Step Wedge:

If there's a difference between that sRGB and Adobe, it must be subtle. I can't see it.

In places it's no more than a one bit difference, but it gets progressively lighter in the shadows, with a cumulative 'lightening' of the shadows.

Of course, on a system with an Adobe RGB profile those density steps will look (nearly*) identical to the sRGB steps on an sRGB calibrated system.

*Nearly, because an exact match would require fractional bit differences in brightness.

using my Sekonic View Spot Meter L438

To be picky, spotmeters are only accurate at infinity and long distances. They need to be fitted with an appropriate close-up 'dioptre' to be accurate at short distances.

Here are two simulated density wedges. One in sRGB 'space', and the other in Adobe RGB.

[ATTACH=full]1378462[/ATTACH]

(snip)

 

Basically, if you can barely see a domestic light bulb through the darkest parts of your negative, it's too dense. And if you can easily read a newspaper through it, it's too thin.

I hadn't thought of a good way to say it, but I think that works.

The small exception that some scenes might have a large shadow region, big enough to read through.

But yes, normally there should be some dark areas, and some light areas.

If all are dark or all are light, you are too far off.

The small exception that some scenes might have a large shadow region, big enough to read through.

The usual problem, with random pictorial negatives, is that usually there's no single area big enough to get a true density reading from. Densitometer heads generally take an integrated reading from anything between a 3 and 6 mm diameter circle of the negative.

This tends to set one on the path of taking pictures of grey-cards, step-wedges and other pointless and boring stuff.

Leave that to the boffins at your favourite film factory - they've been paid to do it.... and a darn sight more accurately than you realistically can!

Everything you need to know about a film is available online. Just follow the frigging instructions that come with the film/developer/fixer, etc. and everything will be fine.

Also severely ignore any 'how to' processing videos on YouTube. They all seem to have been posted by idiots with zero experience, knowledge or skills.