Ilford FP4+ Characteristic Curve

For those that are interested, here's what the characteristic curve looks like for Ilford FP4+ in replenished XTOL.

It was generated with what tools I have available to me, and while not scientific by most measures, gives a reasonable approximation of what to expect if shot and developed using a similar development setup as me.

The development setup:
Replenished XTOL, developed in a JOBO with constant agitation at 24C for 7:00 even. The XTOL is in a 2.5 liter bottle and is replenished with 70ml of new for every 80 square inches of film run through it.



The green line is the zone system contrast curve, the red line is ISO standard contrast curve, the yellow line is what FP4+ is doing. The vertical axis on the left is film density in log units, 0.000 is film base plus fog. The horizontal axis along the bottom is relative exposure of an 18% grey card in full stop increments with the card filling the entire frame and the lens at infinity focus. The zero point on the horizontal axis is the exposure index for a correctly exposed 18% grey card, so horizontally to the right is the grey card given more exposure, and horizontally to the left is the grey card given less exposure, in full stop increments. -4EV on the left side of the horizontal exposure axis is placed 0.1 log density units above film base plus fog and is the speed point for the red and green lines.

I can very reliably control the amount of light hitting the grey card to within a 1/10 of stop up and down the exposure scale. It's not without error, but it's consistently very close.

So, looking at it, I'd say that with Replenished XTOL at 24C in a JOBO, Ilford FP4+ is at least an ISO 125 film. At EI 125, -4EV down has a density unit of 0.128, with the 0EV point hitting 0.756. If development time was added to bring 0EV up to ISO contrast (the red line), you could probably expose it at EI 160 to bring down the -4EV spot, though more development time would mean the highlights from +1EV and up would have more than ISO contrast.

Enjoy.

Adrian,

This is helpful as I've recently started using XTOL replenished again, and there have been some comments online about film suffering speed loss with this system, which your graph would seem to contradict.

Tom

Hello,

from my numerous tests with both FP4+ and its "brother" Delta 100 I can say that both films generally react in a very good way to "influencing / shaping" their characteristic curve by different developers.
They are quite flexible in that regard, which is very good. Because you can get the tonality you want for certain subjects by using different developers.
For example in DD-X both films deliver a very straight, linear and almost identical characteristic curve.
But if you don't want such a perfect linear characteristic curve, but instead want a cc with a compensating effect in the highlights, just use a (semi) compensating developer (e.g. FX 39II, ID-11 1+2, Rodinal 1+74 etc.) and you will get exactly that: compensating effect in the highlights with less density in zones XIII - X and a curve flattening out in the highlight zones.

Best regards,
Henning

Tom Kershaw said:

Adrian,

This is helpful as I've recently started using XTOL replenished again, and there have been some comments online about film suffering speed loss with this system, which your graph would seem to contradict.

Tom

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Usually when you see comments like that it’s because if you run it for the same time as stock, it under develops, either that or they’re just repeating the Internet echo chamber and don’t actually know because they haven’t actually tried it or measured it.

I think your graphing method is pretty cool. I would move ISO reference though. I also would move Zone reference to align Zone I with the same 0.1 speed point. The metered point is not expected to coincide with Zone V. I'd expect ISO reference to be shifted 2/3 stop from Zone reference. In your graph I see ISO one full stop to the left, and I see your Zone Reference is one-third stop too far to the right. The net difference is 2/3 stop.

By definition ISO locks on the horizontal axis to the point where the curve meets 0.1 above base+fog. It’s only tentative though. If you continue with different development times and superimpose the different curves... the one which best fits ISO contrast would give you your film speed and your ISO reference for the whole graph. I would expect that one curve to be really close (within roughly a third stop) to rated film speed given your reasonably good lab technique.

When it comes to the Zone Reference for the whole graph (as you work on your N developing times)... I would expect you to find N pretty close in time to the time you already have. (Your ISO time might be a minute longer I didn’t check to be sure but ISO needs more time in developer, Zone System N is about right).

Bill Burk said:

I think your graphing method is pretty cool.

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The power of google spreadsheets. I have no problems with sharing my workbook with anybody who wants access to it. All I ask is that if you want to change something, either make a copy of a tab and do it there, or ask me to change it. PM me with an email address if you want to see the workbook.

Bill Burk said:

I would move ISO reference though. I also would move Zone reference to align Zone I with the same 0.1 speed point. The metered point is not expected to coincide with Zone V. I'd expect ISO reference to be shifted 2/3 stop from Zone reference. In your graph I see ISO one full stop to the left, and I see your Zone Reference is one-third stop too far to the right. The net difference is 2/3 stop.

Click to expand...

In the interest of transparency, here's the values that make the graph up. I didn't include the 6 minutes, but do have values that I can plug in up to +6 EV just like the 7:00 minute column. 6 minutes isn't enough time.



I can drop the zone 0 point off the graph if it would cause less confusion, though Ralph might feel a little clipped . For the ISO Reference, 4.3 stops up puts the log density at 0.9 above FB+F, however 4.3 stops is about a 1/3 of the way between 0EV and 1EV and I want the value at 0EV which is 4 stops up, not 4.3 stops up because I'm using full stop resolution. Soo.... I took the ISO contrast (0.8/1.3 = ~0.615) and calculated each full stop point starting at 0.1 using the 0.615 gamma. I'll be the first to admit that I'm not a math wiz, so if somebody has a more accurate ISO reference for each full stop value starting at 0.1 density, I have no problem with plugging that in.

The 0EV point is a meter reading with an incident meter placed where the grey card is, facing the camera and is illuminated with a giant 10x16 foot white wall behind the camera that has two studio strobes with umbrella reflectors hitting it with light. The grey card is about 10 feet away from the wall. It's as even lighting as it's going to get and I can exact some pretty consistent light with it. Since FP4 is available in 35mm, I generated this using my T-Stop rated lens which has a de-clicked aperture control ring on the barrel of the lens. Since it's transmission rated, I'd expect each t-stop mark on the barrel of the lens to be extremely close if not exactly on for the amount of light going through the lens and hitting the film inside the camera. The setup is not sensitometer accurate, but it's within reason and very consistent.

So, given the spreadsheet values, what would you change again?

A somewhat more twisty curve once it launches off the toe than I'm accustomed to. But I've never worked with Xtol, and see no reason for trying to now. I prefer a straighter line.

DREW WILEY said:

A somewhat more twisty curve once it launches off the toe than I'm accustomed to. But I've never worked with Xtol, and see no reason for trying to now. I prefer a straighter line.

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Yeah... Continuous agitation tends to have that effect with XTOL with a lot of emulsions. If you pull the development time in to keep the highlights within ISO contrast, the toe and shadows have a tendency to drop a little and make a bit of an S-Curve, so I generally shoot to have the toe and shadows hit at least Zone contrast and the highlights stay within ISO contrast. If I can do that, then I'm personally happy.

Thanks for the data points. I’ll graph it on paper and show what I think the references should be!

Unless it's a modest contrast scene, I always cut the speed to 50 to get the full range of the exposure up on to the straight line section. There will still be room at the top not to shoulder off unless you overdevelop or it's a very high contrast subject.

I propose this framework for the references.

Well done. Thank you for sharing. What speed did you use on your jobo?

Bill,

I suspect we're graphing the same thing, but using different terminology, and putting our dots on each curve along with a correctly exposed grey card in different places from each other along the curve. When I remove the FP4+ characteristic curve and just look at the reference data (ignoring the vertical and horizontal scales for the moment) it looks an awful lot like what you just graphed (see below)

So, a few things... Please define the vertical and horizontal scale of your graph. Also which zone system are you using for your graph?

I think it would be helpful if we came to an agreement about what log density above film base plus fog a correctly exposed grey card should be, and what zone that should be in, and we should also come to an agreement on what correctly exposed means. Does the ISO standard define what density a correctly exposed grey card should be? I don't recall that it does, as that's a zone system thing, though I might be wrong about that. It happens.

All that being said, on my chart, the horizontal axis is relative exposure. -4EV is zone 1, which puts zone 5 (per Lambrecht/Woodhouse, 2nd ed. pg. 116, figure 4) at 1.2 log units of exposure (otherwise called 0EV on my chart) above zone 1. If you'll notice, each of the little squares in my chart actually lines up with 1/3 stop increments along both the vertical axis and the horizontal axis, so I can just as easily say -4EV is 2.51 log (as an example), and then each little square is 0.1 log units up or down in exposure depending on the direction you're going. This also means that the ISO curve starting at the speed point of 0.1 on the vertical axis is also correct, because if you go horizontally over 1.3 log units, that puts you in the little square just past 0EV. If you follow that up to 0.9 above FB+F (or 0.8 above the speed point) the ISO curve line passes right through there, I just don't happen to have a dot on my line there, as it's a third of a stop over so that it's 1.2 log units above the speed point instead so that it lines up with zone 5, which is also 1.2 log exposure units above the speed point. I just simply chose to label the horizontal axis in stops of light instead of log units of light as a fair number of photogs get the stops of light thing and can then make decisions about exposing up or down. Log units make a lot of peoples eyes glaze over and many don't understand what that means in terms of actual camera exposure.

So, if I take a picture of a correctly exposed grey card with Ilford FP4+ and develop it as described before, that frame measures 0.756 log density above film base plus fog. If I then take that same grey card and same light and under expose it by 4 stops (or 1.2 log units), that log density is 0.128 above FB+F. 0.756 and 0.128 are shockingly close to zone 1 and zone 5 negative values for N development, which is what my original chart shows. If I were to increase the development time I'd be willing to bet that 1.2 log units above the speed point would also pass right though the 0.841 point on the ISO curve, and if I gave it a third of a stop exposure above that, it'd also pass right through 0.9.

Like I said, I suspect we're graphing and talking about the same thing, just using different terminology, and possibly not agreeing on where a correctly exposed grey card is supposed to be on the chart below.

foen said:

Well done. Thank you for sharing. What speed did you use on your jobo?

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This was on a Jobo duolab, it's a fixed speed, and is 60-75 RPM. I haven't measured it precisely, but at maximum capacity, it does one full rotation faster than a second passes. I have another Jobo (a newer model, not a duolab, I don't remember off the top of my head) and it produces the same thing at the faster speed. When in doubt, go faster.

Bill Burk knows his stuff. Please don’t take his suggestion as criticism. He’s suggesting that your awesome graph curves will benefit from aligning your reference point to be same convention that the industry uses. When you do that (and it’s not a difficult thing), not only will it be useful to you when/if you get deeper into photography but also more useful to the rest of us who are familiar with interpreting standard curves.

I added this thought because scrolling through, I just wasn’t intuitively “getting it” looking at the charts until I saw the adjusted graph that he made and shared. Base fog + 0.1 is at 2.51. Ok got it. Gamma is read from there. Ok got that. Now I can expose for shadows and develop for highlights.

Same info, yes.. but kinda like reading a speedometer in meters per second vs miles per hour. 28.6 m/s doesn’t mean much, but 60 mph does.

Cheers,
Jason

Thanks Jason,

Adrian, I didn’t explain my scales. The lower x-axis label is “arbitrary” and is where I marked results of calculations but the real scale is at the very top. It’s what my arbitrary scale is calibrated to and shows log meter candle seconds corresponding to film speed.

The chief differences I want to point out is that ISO meter point is 1.0 (3 1/3 stops) from the speed point while the Zone System meter point is 4 stops. This means green and red scales need to be stepped apart from each other by 2/3 stop, with the Zone System seeming to make the film look slower. You also develop the film less for Zone System so I knocked off another 1/6 stop to adjust for that.

A correctly exposed gray card with Zone System would be Zone V with an EI about 64

A correctly exposed gray card with this film at EI 125 and stopped down three and a third stops would be 0.1 when the contrast parameters are met. And that point would be at -2.2 log meter candle seconds.

I would want to put your 7 minutes film graph with its 0.1 mark at the point of -2.15 log meter candle seconds to adjust for the slightly lower development that you achieved.

Nodda Duma said:

Bill Burk knows his stuff. Please don’t take his suggestion as criticism. He’s suggesting that your awesome graph curves will benefit from aligning your reference point to be same convention that the industry uses

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I’m not questioning his knowledge. I’ve read many of his posts here on photrio. I get what he’s suggesting and don’t actually have any issues with that, except that it makes graphing it in google spreadsheets more work, so in the interest of having a relatively small table to generate the graph from, I aligned the reference points with the data points I’m actually collecting and made the horizontal exposure scale relative camera exposure in stops.

Bill Burk said:

The chief differences I want to point out is that ISO meter point is 1.0 (3 1/3 stops) from the speed point while the Zone System meter point is 4 stops. This means green and red scales need to be stepped apart from each other by 2/3 stop, with the Zone System seeming to make the film look slower. You also develop the film less for Zone System so I knocked off another 1/6 stop to adjust for that.

Click to expand...

I get that. I did it the way I did it because it’s easier in google spreadsheets. My options are to generate the ISO reference the way it is now, generate the way it is now, but with no dots, and generate it with no dots and after the graph image is generated, pull it into photoshop and manually put a dot on 1.0 for the meter point, and 1.3 for the aim point. What’s the preference?

For the horizontal scale along the bottom, converting it to arbitrary relative log units isn’t really that big of a deal, though I’d have to come up with a scale for log meter candle seconds. Is there a reference for this somewhere?

1 Meter candle second = 1 lux, so if you have a lux meter it’s easy enough to take a measurement

Nodda Duma said:

1 Meter candle second = 1 lux, so if you have a lux meter it’s easy enough to take a measurement

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I do, but have never tried to get a reading off a strobe. Don’t know if that would work.

As usual, I find any rigid density value above fbf ("industry standard" or not) to be potentially misleading when making shadow placement. It's the shape of the toe I'm interested in. Otherwise, why bother plotting? I don't want a generic exposure formula; I want to understand what's really going on. Of course, it takes a whole family or suite of curves to understand even a single specific film/developer combination.

Bill Burk said:

The chief differences I want to point out is that ISO meter point is 1.0 (3 1/3 stops) from the speed point while the Zone System meter point is 4 stops. This means green and red scales need to be stepped apart from each other by 2/3 stop, with the Zone System seeming to make the film look slower. You also develop the film less for Zone System so I knocked off another 1/6 stop to adjust for that.

A correctly exposed gray card with Zone System would be Zone V with an EI about 64

A correctly exposed gray card with this film at EI 125 and stopped down three and a third stops would be 0.1 when the contrast parameters are met. And that point would be at -2.2 log meter candle seconds.

I would want to put your 7 minutes film graph with its 0.1 mark at the point of -2.15 log meter candle seconds to adjust for the slightly lower development that you achieved.

Click to expand...

Horsing around with Google Spreadsheets resulted in below. Something like that?

DREW WILEY said:

As usual, I find any rigid density value above fbf ("industry standard" or not) to be potentially misleading when making shadow placement. It's the shape of the toe I'm interested in. Otherwise, why bother plotting? I don't want a generic exposure formula; I want to understand what's really going on. Of course, it takes a whole family or suite of curves to understand even a single specific film/developer combination.

Click to expand...

That’s why I always try to make exposures down to 6 stops below middle gray. Most films have petered out by then.

Certain films have much longer straight lines than others, and this can be affected by the degree of development. TMax films will sustain a straight line at least an entire stop below FP4. Old time "straight line" films like Super XX or Bergger 200 would go down even a stop further. On the other hand, Pan F is almost entirely an S-curve, and you basically lose an entire zone of shadow separation compared to FP4. I see you're here in the Bay Area. Well, I'm hoping for some silvery light "softbox" fog in the morning; and FP4 will be fine if the wind stays calm. But out on the coast you often get that kind of softbox effect in the morning, but once the fog breaks you can easily encounter scenes in the redwoods under open sun that span twelve stops of range. FP4 just can't handle that. Of course, folks will tell you just to use compensating or minus development, which some people on this forum often incorrectly call pull development; but that is just like stomping on a Dagwood sandwich to get it thin enough to eat. Sure everything will be inside, but all scrunched up - all your delicate midtone and highlight tonality will be poorly differentiated, and the shadows will be bland. It's better to select a film more suited to a high contrast range to begin with. I prefer to think in that manner rather than place ZV at any specific point above threshold density; otherwise, you might encounter trouble at the opposite end, and shoulder off the highlights.

DREW WILEY said:

Certain films have much longer straight lines than others, and this can be affected by the degree of development. TMax films will sustain a straight line at least an entire stop below FP4. Old time "straight line" films like Super XX or Bergger 200 would go down even a stop further. On the other hand, Pan F is almost entirely an S-curve, and you basically lose an entire zone of shadow separation compared to FP4. I see you're here in the Bay Area. Well, I'm hoping for some silvery light "softbox" fog in the morning; and FP4 will be fine if the wind stays calm. But out on the coast you often get that kind of softbox effect in the morning, but once the fog breaks you can easily encounter scenes in the redwoods under open sun that span twelve stops of range. FP4 just can't handle that. Of course, folks will tell you just to use compensating or minus development, which some people on this forum often incorrectly call pull development; but that is just like stomping on a Dagwood sandwich to get it thin enough to eat. Sure everything will be inside, but all scrunched up - all your delicate midtone and highlight tonality will be poorly differentiated, and the shadows will be bland. It's better to select a film more suited to a high contrast range to begin with. I prefer to think in that manner rather than place ZV at any specific point above threshold density; otherwise, you might encounter trouble at the opposite end, and shoulder off the highlights.

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I’m in Petaluma. During warmer weather it’s not uncommon to find me and the fam out on Doran beach during the weekends. If you’re ever in Sonoma county and wanna stop by and say hi, feel free to give me a ring. I’m totally cool with that as long as I’m not already engaged in something.