Confirming Film Development Time using an 0.30 Neutral Density Filter

Bill Burk · Jul 7, 2025

ags2mikon said:
@Bill Burk, are you reading the density through the .30 gel on your densitometer? If so could you just shoot a grey card?

In this context I am only using the gelatin filter for viewing.

Bill Burk · Jul 7, 2025

brbo said:
Thanks @koraks! It was just to confirm that I understood the principle. For BW I'm basically "read the development times from the box/MDC and you'll always be good" kinda guy...

But I can see that this method can easily be applied to scanning with a lot more precision than in 100% analogue workflow visual inspection so @Bill Burk (or someone else) might expand on this article.

This is a “go”, “no-go”, “under”, “over” check.

0.5 gradient is an arbitrary aim that I just thought would be easy for people to wrap their head around.

Bill Burk · Jul 7, 2025

runswithsizzers said:
I found a couple sellers on eBay (US), but I did not see that many Kodak ND 0.30 listed right now.

I ended up buying two, so if it works out I may have an extra for sale. I wish I thought to check for other brands, as I thought the Kodak ones are rather expensive.

Good you got some. I'm thinking they can be cut to share, because you really only need enough to cover the negative. I picked up a 100mm square and could get four out.

The Kodak ones have good optical qualities and also good reliable density.

xkaes · Jul 7, 2025

koraks said:
That's what I noticed too, but I assumed that 0.5 would be close enough to 0.6-0.65 which would be a more common CI to aim for. And since this method based on a visual assessment would only be useful for spotting gross deviations, it wouldn't matter too much that the 0.5g-bar would be a little on the low side. The idea here is probably to see you're not hitting way too low or way too high, but somewhere in the ballpark.

I'm surprised it took so long to get to this point. I use a C.I. for my gear, processing, taste, preferences, and .5 isn't it. But my base is a diffusion head. Other people will have other standards -- or they ought to. Finding an ND filter with your exact C.I. -- if you can find it -- it one way, but as mentioned by others, I'd modify the approach anyway -- for example, with a grey card and a meter of some sort.

dcy · Jul 7, 2025

brbo said:
Thanks @koraks! It was just to confirm that I understood the principle. For BW I'm basically "read the development times from the box/MDC and you'll always be good" kinda guy...

Yeah. But some of us like to experiment with weird developers

At the risk of drawing an exasperated sigh from some forum members, I'll admit that I'm going to keep using PC-TEA often to develop film. I just find it convenient and it seems to give results similar to what I get when I use D-76 or D-23.

Bill Burk · Jul 7, 2025

xkaes said:
I'm surprised it took so long to get to this point.

I’ve been shooting two shots, two f/stops apart for a while with the idea that it will give enough information for process control.

But until recently I couldn’t figure out how to make the results meaningful.

I had to be embarrassed by saying something stupid and wrong to make me sit down and come up with a real plan. And this idea was a doozy.

Once I made up my mind, I had to buy and wait for the 0.30 ND filter, and then I had to develop some film to 0.5 (I had a good clue what that would take since I have been skirting 0.62 for a long time, I knew I just had to cut back the time).

Anyway it’s been two hundred years coming.

An easy way to judge development time that takes two test shots that don’t even waste film because they’re basically just brackets that would both be good anyway.

I’m going to look through some of my old negatives for test pairs that might be on underdeveloped or overdeveloped rolls.

Next, we’re supposed to download the photo and cut and paste from one side to the other in Photoshop. Or edit and use the sliders to dial up the contrast until you can see it.

Even if the difference is slight, we should all be able to conclusively say which one is darker and which one is lighter.

You saw my photos of a perfect test pair. That’s boring because they ARE the same.

Wait until we see an imbalanced pair, the test will make more sense.

It’s the edited cellphone shots that will help us all see the difference.

dcy · Jul 7, 2025

Bill Burk said:
0.5 gradient is an arbitrary aim that I just thought would be easy for people to wrap their head around.

Let me see if I follow the math:

Let EV = number of stops of overexposure.

2^EV = how much more light the film receives.

γ = target slope in the log-log plot; should be 0.65 but you're simplifying it to 0.5 to make it easier to test with an ND2 filter.

(2^EV)^(-γ) = change in transmitted light intensity through the negative.

Your test sets EV = +2 and aims to check γ = 0.5:

(2^2)^(-0.5) = 1/sqrt(2^2) = 1/2

So the overexposed film should allow 1/2 as much light through, which we can verify with an ND2 filter.

So far so good?

Assuming that I got this right, we could tweak EV to get closer to the ideal γ = 0.65.

(2^(1 + 1/3))^(-0.65) = 0.548412489847313

(2^(1 + 2/3))^(-0.65) = 0.47193715634084676

Therefore, the idea overexposure is somewhere between +1 1/3 and +1 2/3. Assuming that the camera does not permit you to adjust the exposure in increments smaller than 1/3 stops, we could consider a more complex test:

1) Take one exposure at EV = +1 1/3
2) Take one exposure at EV = 0
3) Take one exposure at EV = +1 2/3

Develop the film, then put an ND2 filter on top of the middle negative. Its density should be half-way between the density of the two neighboring negatives.

Did I get this right? It's a bit more complex than your version, but if I got it right, it would be quite precise at getting γ close to 0.65.

---------------------------
EDIT: Reading @Bill Burk 's last comment, it looks like the target is γ = 0.62, not 0.65.

(2^(1 + 2/3))^(-0.62) = 0.488579984217123

So... just use the original method but with EV = +1 2/3 instead of EV = +2 ?

MattKing · Jul 7, 2025

A suggestion:
Try to avoid the terms "overexposure" or "underexposure" for this sort of analysis, because they usually imply incorrect exposure.
"Increased exposure" and "decreased exposure" are much better choices.

dcy · Jul 7, 2025

MattKing said:
A suggestion:
Try to avoid the terms "overexposure" or "underexposure" for this sort of analysis, because they usually imply incorrect exposure.
"Increased exposure" and "decreased exposure" are much better choices.

Ok.

Bill Burk · Jul 7, 2025

@dcy good.

It’s straight forward to customize the exposure difference to match any ND filter you have on hand, and then to any aim contrast you desire.

The actual ISO parameters are, once you have 0.10 above base+fog then go across 1.3 and up 0.8 (to 0.90 density +/- 0.05)

We always say 0.62 (but it’s 0.61538…)

0.55 is close to what people who do Zone System aim for as “N” normal.

dcy · Jul 7, 2025

Bill Burk said:
We always say 0.62 (but it’s 0.61538…)

(2^(1 + 2/3))^(-0.61538) = 0.491194620829931

But was my line of reasoning correct? I'm not very familiar with how these density graphs work.

Also... Why is it 0.61538... ?

koraks · Jul 8, 2025

brbo said:
For BW I'm basically "read the development times from the box/MDC and you'll always be good" kinda guy...

Same here!

dcy said:
Also... Why is it 0.61538... ?

Just to be sure - there's no particular gamma that's 'correct'. It's all rather subject to taste, like many things. How much salt should there be on boiled potatoes? Some like their negatives beefy, some like them a little thin. Some like their potatoes salty, some like them bland.

dcy · Jul 8, 2025

koraks said:
Just to be sure - there's no particular gamma that's 'correct'. It's all rather subject to taste, like many things. How much salt should there be on boiled potatoes? Some like their negatives beefy, some like them a little thin. Some like their potatoes salty, some like them bland.

Then why did Bill give such a precise number? I can't imagine that his personal preference has 5 significant figures. I assume that there must be an standard somewhere that he is referring to; probably the ISO standard where ISO speeds come from?

koraks · Jul 8, 2025

dcy said:
Then why did Bill give such a precise number?

Ask him.
I know of literally nobody who aims for a gamma of 0.61538. Some people may aim for a specific gamma somewhere in those regions. Plenty of people don't aim for any particular gamma and all and just develop the way it seems proper to them and they never even know what gamma they're working at. Some people aim for considerably lower or higher gammas depending on preference or the requirements of the process they're working with.

This is why I mentioned it; you seem to easily put a lot (too much) stock in certain details people mention. Try to take it all with a grain of salt. I know that can be difficult for some, but try; it'll make it a lot easier to make sense of the sometimes seemingly conflicting information you receive.

PS: out of the famous photographs that people like so much, you could try and deduce how many were made from negatives developed to a very specific gamma. The answer will be sobering. The major share of the photographers behind those images didn't care and/or didn't know, maybe left the whole film development to a lab who may have cared/known to a greater or lesser extent - but in the end, the gamma of the negatives played only a marginal role in the emergence of the photo as such.

Bill Burk · Jul 8, 2025

dcy said:
Also... Why is it 0.61538... ?

When you plot a characteristic curve and have a candidate for ISO, you locate the point of least exposure where the curve crosses 0.1 density above base + fog. Then you go across towards greater exposure and check the density at 1.3 log exosure across. For ISO you look for density increase 0.80 +/- 0.05.

Hence 1.3 run 0.8 rise, rounds off to 0.62 most of the time.

You can see the 0.5 curve crosses low of ISO tolerance.

Bill Burk · Jul 8, 2025

koraks said:
Ask him.

PS: out of the famous photographs that people like so much, you could try and deduce how many were made from negatives developed to a very specific gamma. The answer will be sobering. The major share of the photographers behind those images didn't care and/or didn't know, maybe left the whole film development to a lab who may have cared/known to a greater or lesser extent - but in the end, the gamma of the negatives played only a marginal role in the emergence of the photo as such.

True we mostly don’t. But the first hundred years people were developing to 1.0 and higher. These days, mostly not. But when you see those early works, you can marvel at what they did. They controlled the light too, so that plays a part in what we don’t see as much these days but are impressed when we do.

But a lot of them got soot and chalk too, I miss that sometimes.

koraks · Jul 8, 2025

Bill Burk said:
You can see the 0.5 curve crosses low of ISO tolerance.

What your plot also neatly illustrates that in the real world, the 5 significant digits don't do much. The width and inaccuracy of a pencil-drawn curve alone renders all that numerical resolution moot. And then we're not even talking about variance in metrology, film processing etc.

Bill Burk said:
But the first hundred years people were developing to 1.0 and higher.

Well, arguably, you develop your film to match the requirements of the printing process and the aesthetics you're after. There's so many ways to do this. Which is 'best'? I couldn't mention one particular approach.

dcy · Jul 8, 2025

koraks said:
This is why I mentioned it; you seem to easily put a lot (too much) stock in certain details people mention. Try to take it all with a grain of salt. I know that can be difficult for some, but try; it'll make it a lot easier to make sense of the sometimes seemingly conflicting information you receive.

Very true. I definitely do that. But I think I'm getting better at this. I'm realizing that a lot of the apparent precision you see in discussions or YT tutorials is entirely artificial. A bit like trying to decide what color you might like by asking 10 people for their favorite color and taking the average of the RGB values.

koraks · Jul 8, 2025

I think that's a very apt comparison, in fact! And then concluding that "the best color for a bedroom wall is #FB165C". (Which turns out to be a rather oppressive shade of very bright pink that a particular friend of mine might actually like on her bedroom wall - shudder!)

Mr Bill · Jul 8, 2025

dcy said:
Very true. I definitely do that. But I think I'm getting better at this.

Hi, I've been thinking for some time that you would probably find this book interesting (and useful). Any edition would work; the later ones, I think, had sections on digital photography, but I don't think those parts were very good. (In their day, when few people were knowledgeable about digital, it was probably ok. But now? Nah.)

I would think you can get a nice copy for $8 or $10; probably well worth it for you. You'll probably find a number of things that you didn't realize you wanted to know. Fwiw the authors were all from RIT, a premier photo school of the day.

If you prefer to just ask questions on photrio (which I suspect you might) just ask away and pretend you don't have the book.

Basic Photographic Materials and Processes - Current, Ira; Compton, John; Stroebel, Leslie; Zakia, Richard D.: 9780240800264 - AbeBooks

Students and seasoned photographers have relied on Basic Photographic Materials and Processes for its comprehensive explanation of the photographic process, giving them the technical base they need to achieve the effects they want. Heavily illustrated and enhanced with study questions, this book ...

www.abebooks.com

Bill Burk · Jul 8, 2025

Mr Bill, any book including Zakia as an author is worth having!

Mr Bill · Jul 8, 2025

Bill Burk said:
Mr Bill, any book including Zakia as an author is worth having!

Yep, and John Compton was no slouch either.

xkaes · Jul 8, 2025

koraks said:
Just to be sure - there's no particular gamma that's 'correct'. It's all rather subject to taste, like many things. How much salt should there be on boiled potatoes? Some like their negatives beefy, some like them a little thin. Some like their potatoes salty, some like them bland.

I'll throw in "your gear" into this potato soup. Diffusion vs condenser? You'll want different C.I.

koraks · Jul 8, 2025

Yeah, there you go, that's also a factor. The list is long!

Bill Burk · Jul 8, 2025

Bill Burk said:
I’ve been shooting two shots, two f/stops apart for a while with the idea that it will give enough information for process control.

Darn, can't find examples where I "bracketed 2 stops" with this idea in mind. Too bad because I had some underdeveloped and overdeveloped rolls recently.

Confirming Film Development Time using an 0.30 Neutral Density Filter

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