Reciprocity factor in paper

I've seen it mentioned a few times about reciprocity factor in paper. At what enlarging times does this come into effect and are there any noticeable effects of reciprocity even after compensation? My medium format enlargements have longish times, between 20 and 40 seconds per grade when split grade printing. On top of this there is also additional burning in time. Is the reciprocity cumulative or is it per exposure? e.g. if the longest exposure is made first does this have an effect on subsequent exposures?
Does the tonal value also get affected by which grade is exposed first? When split grade printing, if I expose for 30 seconds at grade 5 then 20 seconds at grade 0 will that give me a different tone than exposing grade 5 first then grade 0? Does the grade 5 give a flashing effect for the grade 0?

I've noticed that my 35mm prints tend to have a bit more "snap". For these I don't print above 5x7 and the times are around 12 to 20 seconds.

I'm sure that there is reciprocity failure in B&W paper, however my own experience is that unless one is doing really, really big enlargements it isn't too much of a problem.

The snappiness of the 35mm stuff could be from a few factors, minimal enlargement, higher paper grade, whiter base because the paper is not old, things like that.

Years ago, in another life I did mural enlargements, both colour and B&W. These enlargements were on paper up to 6' wide by 18' long for colour and 4' wide by 18' long for B&W.

Some of the enlarging times were measured in minutes and I mean minutes. It was not uncommon for a large print to be around 12 - 18 minutes exposure. This is mainly because the enlarger is metres away from the paper and the light loss is large.

If we were able to rack the enlarger very close, the times would come right down, sometimes we did super enlargements and then rolled the enlarger (on rails) right up to the wall for a 40x60" print from the same paper and really, there appeared to be no difference in the whites or the snappiness, in fact if there was, the client would not be impressed.

Whilst I understand that time can mean reciprocity failure my experience is that I think it's minimal for moderate enlargements and times, like yours.

With colour paper, I also have done some extremely long exposures ( 20 minutes) and also extremely short (.5 seconds comes to mind for small colour stats). Yes I'm sure it's there and measureable, but I'm not sure unless the paper is old whether it's that much of a concern.

One thing to consider is your enlarging lenses, are they the same brand/type/age. These things can and do have a serious effect on the contrast.

I know this as I sometimes swap enlarging lenses whilst keeping the same negative in the head for a different sized print that the other lens cannot do.

With my two lenses there is about ½ a grade difference in contrast between the two. These lenses are from the same manufacturer, the same year of manufacture and the same family of lenses.

Interesting question about reciprocity though, I'll look forward to hearing from more technical people regarding this.

Mick.

I've also never noticed reciprocity failure in B&W exposures of several minutes. However, Kodak's Publication No. G-1, Quality Enlarging with Kodak B/W Papers (1982), claims noticable reciprocity failure occurs with exposures of less than two minutes. It also mentions a lowering of contrast with long exposures. Safelight fog can contribute to a reduction in print quality in long exposures.

Jim Jones said:

I've also never noticed reciprocity failure in B&W exposures of several minutes. However, Kodak's Publication No. G-1, Quality Enlarging with Kodak B/W Papers (1982), claims noticable reciprocity failure occurs with exposures of less than two minutes. It also mentions a lowering of contrast with long exposures. Safelight fog can contribute to a reduction in print quality in long exposures.

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I have not tested this, but it seems reasonable to me that we would see a difference between a print made with 30 seconds of continuous light, and a print made with 60 half-second bursts.

I would expect the short-exposure version to be more contrasty, since the high values might never get enough light through the dense portions of the negative, to contribute towards print density. We could call this a kind of "clipping" of the high values in the print.

By comparison, the print made with continuous light would look "soft" - although it might reveal more of the full range of the negative.