Reciprocity Failure

I don’t generally dwell on highly technical subjects in this blog, or rather I should say I haven’t done so for a while, but I am doing a bit of research for my current project and need to get this down somewhere where I can find it easily. I am certain I will forget most of the information I dug up on this subject within a few days so I am afraid I must bore you with it for the moment.

My current project involves exploring black & white pinhole photography. If you’ve been following this blog for any time you know I prefer color film in my pinhole cameras, usually Kodak Portra 400, and though this film is not immune to reciprocity failure I haven’t really had any problems with it, probably because my exposures rarely go longer than 20 to 25 seconds. My “go to” film for black and white photography however, is Tri-X and it is quite sensitive to reciprocity failure – thus my current quest for another black & white film, preferably with an ISO of 400.

In case you have no idea what I am talking about reciprocity failure is simply the tendency of some photographic films to become less sensitive to light the longer they are exposed to light. 

It seems odd that something specifically designed to be light sensitive should become less so simply by being exposed to it more but there it is. Like so many thing in life its just the way it is and we must learn to deal with it. Reciprocity failure is rarely a factor with exposures lasting less than a second but for longer exposures it can have significant impact. 

For instance a film with no reciprocity failure like Kodak BW400CN might require an exposure of 5 seconds in a given situation. A similar film with the same speed (ISO  400) that is subject to reciprocity failure like Kodak Tri-X or Ilford HP-5 will require an exposure of 15 to 20 seconds to properly capture the same image (this information was taken from the published data sheets for Kodak BW400CN, Tri-X and Ilford HP-5 films).  

In addition, reciprocity failure increases exponentially so that the longer the exposure the greater the failure. For example, an image requiring a 25 second exposure with the BW400CN will require an exposure of 150 seconds with Tri-X. This is a 5 times longer exposure for BW400CN but Tri-X requires an exposure 7.5 times longer for the same image. 

Since I generally prefer to shoot 400 ISO film in my pinholes I will focus on those films for now and offer the following notes:

Kodak Tri-X, 400 – If I understand the published technical publication data correctly Tri-X is fine down to about 1/10 of a second. At one second you should add one stop to your exposure. Beyond that it gets complicated and you should consult the technical data sheet (see link below) or just consider another film stock as I am doing.

http://www.kodak.com/global/en/professional/support/techPubs/f4017/f4017.pdf)

Kodak T-Max, 400 – This is an excellent film stock with good reciprocity characteristics. No adjustment is required down to 10 seconds exposure. At 10 seconds the technical data sheet recommends a 1/3 stop adjustment (why bother?) and at 100 seconds the recommended adjustment is only 1.5 stops. See link to technical data sheet below.

http://www.kodak.com/global/en/professional/support/techPubs/f4043/f4043.pdf

Kodak BW400CN, 400 – No problem! Apparently this black & white film develops in C-41, color processing just like color films and is immune to reciprocity failure for exposures up 120 seconds.  Beyond that you’re on your own to experiment and find out what works but here’s the technical data sheet below.

http://www.kodak.com/global/en/professional/support/techPubs/f4036/f4036.pdf

Ilford HP-2, 400 – This is a popular C-41 black & white film like the Kodak BW400CN but unlike that film, which has no reciprocity problems, Ilford HP-2 is similar to Tri-X in its susceptibility to reciprocity failure. See technical data sheet below.

http://www.ilfordphoto.com/Webfiles/201142711321939.pdf

Ilford HP-5, 400 – This very popular traditional black & white film is very similar to Tri-X and HP-2 in its susceptibility to reciprocity failure. See technical data sheet below.

http://www.ilfordphoto.com/Webfiles/20106281054152313.pdf

Ilford Delta 400 – Another black & white film similar to Tri-X, HP-2, and HP-5 in its susceptibility to reciprocity failure. See technical data sheet below.

http://www.ilfordphoto.com/Webfiles/2010628953322222.pdf

Fomapan 400 - Another black & white film similar to Tri-X, HP-2, HP-5, and Delta 400 in its susceptibility to reciprocity failure. See technical data sheet below.

http://www.fomausa.com/pdf/Fomapan_400.pdf

There are other 400 ISO black & white films available but I was unable to find any info on their reciprocity characteristics so I have omitted them for now. 

In conclusion, it seems that although Ilford and others have some very fine and popular emulsions if you’re worried about reciprocity failure you’re better off with Kodak. The T-Max and BW400CN both are very forgiving for exposures of up to about 2 minutes. Unfortunately the latter is only available in 35mm format but T-Max is available in medium and large formats.

I will update this information as I come up with additional data.

Oh, by the way, the image above was taken with my Olympus XA2 on Tri-X.