How does paper pre exposure affect exposure time

pkr1979 · Mar 31, 2025

Hi all,

This weekend I've been experimenting with Adox Lupex for the first time (after getting the paper ages ago) and I'm quite happy with it.

On some of my negatives the contrast is a bit more than whats suitable for a grade 3 paper, so I tried pre exposing the paper - but som more experimenting is needed to get where I want... if thats possible.

I do know that pre exposure reduces exposure time, but are there any rules of thumbs or anything when it comes to this? My exposure times seems to be between 1:15 and 1:45. And Adox suggest pre exposing (when needed) for between 5 to 15 seconds: https://www.fotoimpex.de/shop/images/products/media/58935_5_PDF-Datasheet.pdf

Cheers
Peter

ic-racer · Mar 31, 2025

I think it is all experimentation and trial and error. You can also try low contrast developer.

Milpool · Mar 31, 2025

I agree with ic-racer - trial and error to find an appropriate flash time for a given paper with your setup.

Typically you would first find a flash exposure (usually the maximum non-image exposure which doesn't result in any visible density) and then do your image exposure test strips/prints on top of that.

Here's an example on Ilford MG Classic developed in Dektol. You can expect a similar effect on Lupex etc. Non-image exposure reduces toe contrast and increases threshold speed.

xkaes · Mar 31, 2025

Yes, you need to run tests to figure out how much pre-exposure light is best for your set-up & goals. Did you find a testing method/approach somewhere or what? It's often -- after you have figured out your processing time, etc. -- to determine the maximum amount of light that creates no density at all.

pkr1979 · Mar 31, 2025

Thanks guys! I have no testing method/approach whatsoever. I have never done any paper pre exposing before. Is there an easy way to figure out how to find the maximum non-image exposure which doesn't result in any visible density?

koraks · Mar 31, 2025

pkr1979 said:
Is there an easy way to figure out how to find the maximum non-image exposure which doesn't result in any visible density?

Yeah, just stepped exposures just like you'd normally determine exposure time. You may want to mark the paper with a pen so you know where the strips start/end.

Keep in mind that pre-exposure flattens out the toe of the curve, which implies that the longest exposure that still produces paper white may not yield a satisfactory rendition of highlights: they tend to get very, very flat. In that case, you may have to experiment with masking or selective burning.

There's no effective theoretical approach towards determining the 'correct' pre-exposure vs. main exposure. The one thing you can assume, however, is that the shoulder of the curve will still be at the same point. So regardless of what pre-exposure you settle at, the main (imagine) exposure time needed to set the black point will not/barely be affected.

Milpool · Mar 31, 2025

pkr1979 said:
Thanks guys! I have no testing method/approach whatsoever. I have never done any paper pre exposing before. Is there an easy way to figure out how to find the maximum non-image exposure which doesn't result in any visible density?

Test strips. There's no formula. I made a little mask template to sandwich over a sheet of paper so I can do a series of increasing flash exposures on one sheet. The template has windows through which the exposures are made, which helps make it easier to distinguish any perceptible density from paper white. I find it important to inspect the test paper when dry to account for "dry-down". For this I do a short rinse after fixing and a quick dry in the microwave. Also don't assume a series of cumulative exposures = one total exposure.

Often localized flashing is better than doing the entire sheet.

Doremus Scudder · Mar 31, 2025

pkr1979 said:
Thanks guys! I have no testing method/approach whatsoever. I have never done any paper pre exposing before. Is there an easy way to figure out how to find the maximum non-image exposure which doesn't result in any visible density?

First find your threshold exposure for the paper you are using. Test your pre-flash exposure like a test strip. Find the shortest one that gives you some kind of tone other than pure paper white. You'll want your pre-flash to be less than this, usually half to three-quarters of the threshold time.

Be aware that the small pre-flash exposure has proportionally more effect in the low-density areas of your print, i.e., the highlights. Think of it this way: Each stop is a doubling of exposure. Let's call our pre-flash exposure one "unit" of exposure. If that is one second (just for simplicity's sake), and that is half the threshold exposure, then two units of exposure is the paper's speed point. Four units of exposure is one stop more, eight units = two stops; 16 units = three stops, 32 units = four stops, 64 units = five stops, 128 units = six stops, 256 units = seven stops. Six or seven stops is a typical paper range.

So, you see, that one unit of pre-flash won't make much difference, percentage wise, in the exposure for the mid-tones and shadows, but has it's greatest effect in the highlights.

If your one unit of pre-flash is added to two units of exposure from the negative, though, you'll end up with density in the highlight where none or almost none would show up without the pre-flash.

Do keep in mind that pre-flashing is the same as fogging the paper, so highlight separation will suffer a bit. If you can compare a burned in area with no pre-flash to a pre-flashed area without the burning, you'll see a definite difference. The choice to use burning or pre-flashing is highly subject-dependent.

Best,

Doremus

Alex Benjamin · Mar 31, 2025

pkr1979 said:
Thanks guys! I have no testing method/approach whatsoever. I have never done any paper pre exposing before. Is there an easy way to figure out how to find the maximum non-image exposure which doesn't result in any visible density?

Pretty good demonstration here:

pkr1979 · Mar 31, 2025

Thanks for this guys. I might get this wrong... but, if you have a very high contrast negative and wanted to reduce contrast significantly, wouldn't you then pre expose past the papers inertia in order to achieve this?

Alex Benjamin · Mar 31, 2025

pkr1979 said:
Thanks for this guys. I might get this wrong... but, if you have a very high contrast negative and wanted to reduce contrast significantly, wouldn't you then pre expose past the papers inertia in order to achieve this?

Not necessarily. It's not a question of contrast but of brightness range on the negative. Sometimes, with negs with high brightness range, detail in the higher values on the negative won't print easily without extensive burning, which you may not want as it may cause other tonal balance problems. I would use pre-flash in these cases.

Another way is to use split grade printing. This I use more with high contrast negs, or when I want full, but independent control on the highlights and the shadows.

Les McLean also has a good article on it:

Les McLean Photography

Alex Benjamin · Mar 31, 2025

Just to amend my previous post, in which I mention high brightness range, but, to be specific, it's not as much about the brightness range as it is about the higher values. Pre-flash would be when the higher values with detail on the negative cannot print on the paper without extensive burn.

Split-grade printing, on the other hand, is more related to high brightness range in the negative, or to overly contrasty ones, and offers more control on the contrast. It won't help bringing out the details in the high values like pre-flash does.

Doremus Scudder · Mar 31, 2025

The OP is using fixed-grade contact printing paper; no possibility for split-grade printing here.

Flashing will often help get some tonality and texture in the highlights when burning is not practical, but at the expense of some highlight separation. It can be really gratifying, even with the trade-off.

How much flashing is required depends on a lot of things, but if you want to have some pure paper white in the print, you can't flash past the threshold of the paper. If it's not important, then fine. Or, you can bleach back certain areas of the print to get some bright highlights if you've had to flash a lot to get the highlights to print at all.

Best,

Doremus

Alex Benjamin · Mar 31, 2025

Doremus Scudder said:
The OP is using fixed-grade contact printing paper; no possibility for split-grade printing here.

Right you are. For some reason, I did not catch that. My bad.

Milpool · Mar 31, 2025

pkr1979 said:
Thanks for this guys. I might get this wrong... but, if you have a very high contrast negative and wanted to reduce contrast significantly, wouldn't you then pre expose past the papers inertia in order to achieve this?

Yes, although localized flashing is often more useful, you can flash the entire sheet of paper to reduce overall contrast. It’s not going to be a huge effect, but it will definitely be noticeable. Note that when flashing, the reduction in contrast will be biased toward the highlights (refer to the graph I posted).

If you pre-expose (flash) the paper past the “threshold” where tone is produced, you will be decreasing both the paper’s exposure range and reflection density range, which will work against you.

koraks · Apr 1, 2025

pkr1979 said:
wouldn't you then pre expose past the papers inertia in order to achieve this?

The notion of 'inertia' is false, really. Silver halide exposure for all intents and purposes is cumulative. There is a threshold beyond which a silver grain becomes developable. If this is what you mean by 'inertia', then no, I would not recommend flashing to that level as it'll flatten out your highlights and you won't actually hit paper white anymore. This is essentially what @Milpool says above as well.

In all honesty, apart from masking, burning & dodging and local flashing, IMO the best way to approach this is to produce negatives that suit the printing process. We're kind of spoiled with VC silver gel paper and scanning & digital editing in that we've become used to being able to print well from virtually any given negative. Papers like Lupex put you back in time, effectively, when having a good negative was essential to getting a decent print.

pkr1979 · Apr 1, 2025

Thanks again fellas - this is really useful. @koraks , I agree. Its just that I have this one negative that I'd like to print despite being trouble. If its too much trouble though - I might shoot the scene again under different lighting conditions.

Klaus Mähring · Apr 1, 2025

This might not be perfect when it comes to colour balance, but it works for me. It is for colour process, but might be applicable for b/w:

If my test strips indicate an exposure time of e.g. 10s and I decided I need to preflash I do the following:
- I simply hold a ~1,5mm opaque plexi under the lens and expose for 5s = pre flash.
- then I do the normal exposure with 7s.
So half of the time for pre flash, and a bit more than half for the exposure. Pre flash timing might vary with the thickness of the plexi.

From there on you can go and vary the two times to get the desired print.

This is usually a good starting point, but NOT an exact science.

Doremus Scudder · Apr 1, 2025

Another traditional way to deal with reducing contrast on fixed grade papers was to use soft-working developers. Selectol-Soft, Ansco 120, Ilford ID-3, Kodak D-165 and a few others (as well as the Beer's variable contrast developer) were often used.

I spent a lot of time split-developing graded papers in first a soft and then a contrasty developer for different times to achieve intermediate contrast on graded papers.

One of these developer formulae might help in addition to (or even instead of) flashing.

Doremus

koraks · Apr 1, 2025

orp said:
This might not be perfect when it comes to colour balance, but it works for me. It is for colour process, but might be applicable for b/w:

I've done this for color quite a bit as well. It works quite well; as you said, there can be color balance issues, depending on the scene. But overall, for the 'average' scene, it often works fairly well indeed.

How does paper pre exposure affect exposure time

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