Reciprocity Correctiongreenspun.com : LUSENET : Large format photography : One Thread
I have been enjoying doing still lifes on a table top with misc. subjects and was wondering about staying with a very small aperature. For instance; If I am at f/64 and I find I need a 1 sec. exposure the film I am using recomends adj. to 2 secs. for reciprocity. I am also doing 1:1 so I would have to allow for belows compensation which would be two full stops and that would put me at f/32. The question is how do I keep at f/64? I noticed that the reciprocity factor doesn't follow a doubling sequence as do asa/fstops/shutter speeds. I hope I have explained this correctly and someone could help. Thank you in advance!
-- Michael Pry (firstname.lastname@example.org), April 20, 2002
You can extend the exposure time. So if your original exposure is 4 seconds at F 64 and you need two additional stops, then expose at f64 for 16 seconds. That is the good news; the bad news is that you may have to reduce development to keep the contrast in your negative or slide from accumulating too much. Still, there are tables available indicating the amount of development reduction vs. exposure increase. More bad news: If you are using any sort of color film you will need to factor in color shifts.
Any bright news? Yes, you might consider using some aort of studio flash. Close up on near maximum power you could get your f64 and use multiple pops of the flash where you needed to. Hope this helps and doen't confuse.
-- bob moulton (email@example.com), April 20, 2002.
One more bit of bad news. Shooting at f64 at 1:1 puts you at f128 effective aperture - if you're using anything smaller than 8x10 sharpness is going to be severly degraded.
-- Wayne DeWitt (firstname.lastname@example.org), April 20, 2002.
You may want to try some Fuji films. According to the spec. sheets on Fuji's web site, Provia 100F and Acros both require no reciprocity correction for exposures up to about two minutes.
-- Matthew Runde (email@example.com), April 20, 2002.
There are two things going on here. First, the effective aperture at a given bellow extension as regards diffraction. To use your example: f64 at a 1:1 magnification results in an effective f128 due to the 2x magnification. This increases the degradation of the image from diffraction and, as mentioned above, might be a sharpness consideration. I try not to use smaller than f32-45 for tabletop work, and usually make only small prints (where the diffraction degredation is less noticeable). That said, it is better, in my opinion, to have the depth of field one desires than less diffraction.
Now on to your original concern, the reciprocity correction. This is not as complicated as you think. Just calculate the exposure in the proper order. 1. read the meter and arrive at an exposure time. 2. measure the bellows extension and apply the factor to arrive at a new, longer exposure time. (There are tables for different focal- lenght lenses for this, or you can apply the formula) 3. apply the reciprocity correction, if needed, last (from a table as well).
If I understand your question correctly, you need to get your hands on a suitable table for reciprocity adjustment (and maybe bellows extension factors) for the film you are using to be able to calculate the proper exposure times for your longer exposures so you can maintain the aperture you need for depth of field. There are tables available from film manufacturers for reciprocity adjustments. I don't know which film you use, but for conventional films (i.e. non-T- grain) Kodak's tables are very good, and include development alterations as well. I have not used Ilford's tables, but I would imagine that they are equally good. As far as I know, Ilford does not indicate development adjustments for their films on their reciprocity tables (the film probably does need some adjustment, however, in which case you could use the Kodak tables as a starting point). T-max films do not need development adjustments according to Kodak. The tables can be found at the respective manufacturers websites.
Hope this helps,
-- Doremus Scudder (ScudderLandreth@compuserve.com), April 21, 2002.
"I noticed that the reciprocity factor doesn't follow a doubling sequence as do asa/fstops/shutter speeds. "
FYI - that is why it is called "reciprosity FAILURE" -
The relationship between exposure settings (time/aperture) is "reciprocal" - meaning a step by-step reduction in one/reciprocating increase in the other (or vise-versa) produces the same amount of light for exposure.
Go too far - and the relationship breaks down, and all bets are off - hence, there is a failure in the reciprocal relationship.
-- Matt O. (firstname.lastname@example.org), April 23, 2002.