EI and Reciprocity.
I have a simple question, want to know if Im doing it right.
I expose my film at night at a certain EI, and do not compensate for reciprocity, lets assume that exposure is 'perfect' for that EI apart from reciprocity, lets assume that EI is faster than the box speed of the film and I am pushing.
So if my box speed is 100, my EI I chose is 400, my exposure @ 400 is 1 minute.
Then I'd use the 4 minute mark for calculating number of stops to add for reciprocity? Lets say 4 minute mark is 2 stops, and 1 minute mark is 1 stop, I'd add 2 stop and expose for 4 minutes EI 400, and not 2 minutes EI 400?
Reason I am asking, is because I pushed some Tri-X quite far to see how it would handle... one of the shots I did not compensate for reciprocity, and the mark for reciprocity based off converting the exposure time back box speed and use the difference in stops on top of my EI, puts it at a bit over 3 stops.
The mark for the time without converting down was 3 stops flat anyway in this case.. so either way would have been similar..
So would I say I underexposed the film 3 and a bit stops from my chosen EI? I still got a picture out of it, albeit grainy..
The reason I am asking is because I pushed the Tri-X in Rodinal to 6400, shot at EI 6400, one of the exposures that counted for reciprocity looks pretty good, the other I did not (exposure was 2 min, with reciprocity counting it the way I assumed, then it would be ~25 min) and has rough as guts grain (6x7cm neg), if I count the number of stops of reciprocity I did not compensate for as the number of stops I underexposed under 6400... then it puts the EI for that shot @ 80,000, which I find absolutely ridiculous figure, and ridiculous that its giving me a picture at all.
Basically, what I want to know is this:
If I fail to account for x stops in reciprocity compensation, do I count that as being x stops underexposed?
I want to work out how much underexposure in stops this shot has had, it was 2 minutes @ EI 6400 - that being the 'perfect' exposure for the scene for EI 6400 without compensating for reciprocity (fresh Tri-X 400)
I thought I understood reciprocity, but you just gave me a math migraine. Ow ow ow.
I think reciprocity correction is based on a fractional exponential function, one derived from curve-fitting actual data collected from many standard types of films. I don't remember where I've seen the formula, but hopefully someone will chime in with it.
I don't think it's based solely on "X" number of stops. The amount of correction depends on the initial, uncorrected time; the large the uncorrected time, the corrected time grows exponentially larger.
For example, someone recently posted an 8x10 pinhole camera image captured on Tri-X; their camera's f/stop was somewhere around f/300, and resulted in an exposure time of almost a minute. By comparison, I shoot paper negatives in pinhole cameras of the same f/stop ratio, which I rate at EI=12, and my exposure times are about the same as the (supposed) ISO320 film. This is because the reciprocity failure is so severe in the case of the Tri-X that the photo paper (which is very linear) ends up being faster than Tri-X and many similar films in extremely low light levels. Sorry I don't have any quantitative numbers to share, but that's all I can manage right now.
I understand that Fuji Acros is very much more linear than more traditional films.
Right. So, Athiril, if your light meter says you need 1 minute exposure, you will need, due to the length of that exposure, to add on further time to account for reciprocity failure. If you fail to add on that extra time, your frame will be underexposed accordingly.
Originally Posted by Joe VanCleave
So, if you are shooting ISO100 film at 400, then you are already underexposing by 2 stops. If you then ignore reciprocity failure, there will be further underexposure. Depending on the figures for your particular film, you may be able roughly to approximate that further underexposure as 1 extra stop, and then try to compensate in processing as if you had underexposed by a total of 3 stops.
But I would normally think of reciprocity failure in terms of a specific necessary time correction, rather than a number of stops, because reciprocity failure is really the breakdown of the usual relationship of doubling/halving that stops describe.
Last edited by Ian David; 04-28-2010 at 09:31 PM. Click to view previous post history.
Good question! Don't know the answer, but my guess is yes. I think you need to experiment yourself...
Originally Posted by Athiril
The harder I work, the luckier I get.
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And here comes the sacrificial sheet film now....
I don't have an answer, but I know I have read this; Probably in Adams' The Negative if memory serves me, which, lately, it hasn't been.
I've been thinking about this reciprocity thing lately, Fomapan being rather extreme in that quality. However I've never seen an example of "hey mate, look at that reciprocity, it's off the scale" unlike under/overexposure, or over/under development.
Can someone tell me what reciprocity failure looks like? I pretty much guess the EI (usually as time rather than aperture) when I'm hitting 2 or 3 seconds, and for my purposes--salt printing--it works OK.
Regards - Ross
Reciprocity law failure has ONLY to do with time of exposure. If you're in reciprocity law failure territory - then it means you have to compensate. If your film loses reciprocity at one second - then it's completely irrelevant what the box speed is. HOWEVER - where things get messy - is that any time you're compensating for RLF - you're ALREADY pushing your film significantly. Because the highlights will record to the film more efficiently than the shadows - your contrast goes up. It's a VERY slippery slope. If you want to TRULY compensate for reciprocity effects to a more accurate degree - you need to drop your dev time a whole bunch - decrease more the longer your exposure is. But this, in turn, requires dropping your effective film speed again! So what was a five second exposure becomes, say THIRTY seconds just for reciprocity law failure exposure compensation but to get the same kind of contrast you'd get at 1/8 sec, you may need to drop your development by, say 25% - which would require you to again compensate the exposure to something like two minutes... which again would require further development compensation, albeit less. All you can do is target a development time/exposure time pair that you know works, and stop down to the aperture that will give you a good result. With a hypothetical film - that means dropping the box speed from, say, 125 down to something like 20 or even less. I know it's kind of confusing - but you'll get the hang of it...!
Hmmm... so by exposing at 2 minutes... I would have to look at what reciprocity would correct to 2 minutes for Tri-X in that setting/amount of light, count that number of stops.. and find that as the effective EI.. etc.. maybe...
I just want to work out how many extra stops it was under exposed not including my pushing it to 6400, in any case, its phenomenal that it gives a picture... my 6400 corrected for reciprocity is very fine grain compared to the uncorrected one.
EG, if I were to test if I were to get a picture from EI 80,000 to prove that you use the stops difference from uncorrected towards corrected time as the amount that its underexposed, I'd need to shoot EI 80,000 in a non-reciprocity scenario... in Sunny-16 with like 6 stops of ND (which I have) and 1/400th and f/32 should do it.. by 1/400th EI 80000 converted to shutter speed in EI 400 should be -just- away from reciprocity... I'll test that at mid day tomorrow
Even though I shot 6x7, it was Tri-X @ 6400 pushed in Rodinal.. Im surprised at the grain.. and the further underexposed (how ever many stops it works out to be) to come out at all
I'll read the articles and see if i can get a copy of The Negative later guys, Im going out to watching cartoons at my friends house