Reciprocity Failure

[ckiang]

Originally posted by sriram
The exposure adjustment you need to make for different films is available on the websites ( www.kodak.com etc ). There's no hard and fast rule of 10 sec for Kodak or 16 sec for Fuji films. It varies on a per film basis. For example, Velvia needs compensation after 4sec, but Provia 100F can handle 120 seconds without failure.

Sorry that's not right. It is a reciprocity failure issue. Velvia's spec sheet states that you need to use magenta filtration when shooting longer than 4 sec. I read that it's because of the uneven response of the different colour layers beyond 4sec.

Eh? I never knew that, I always associated colour casts with improper colour balance. ;p

Regards
CK

 

[jimtong]

Originally posted by Ian
This is a slightly more technical explaination than CK's.....

First off you need to understand what teh Reciprocity Law actually is.

The law was first devised by Bunsen (of Bunsen Burner fame) and Roscoe in 1862 and states that for any photosensitive material the photochemical effects are directly proportional to the incident light energy. That is the product of a given light level (illuminance) and exposure duration as expressed in the formula H = Et where E = illuminance and t = time. H = product

In photographic terms as long as H is kept constant the values of E and t can be changed by manipulating the aperture and shutter of the camera (as per CK's post).

However, someone (Abney I think it was) demonstrated that the photographic effect depends on the acutal values of H and t and not solely on their product. This failure of the law of reciprocity is caused because the effect of exposure on a photograhic material depends on the rate at which energy is supplied.

Reciprocity failure is a fact of life with all photographic emulsions, and it occurs not only with low lighting levels but also with extremely high illuminence levels (bright light).

In the real world the high light levels aren't often encountered, unless you are photographing high powered laser pulses, atomic bomb detonations or the sun with very large telescopes. However, the low level reciprocity failure is often encountered and depending on film the time for reciprocity law failure can be as little as 1 second or less. With low illuminence level (eg at night) failures a loss of contrast and colour shifts are the two most common indicators of reciprocity failure. However the good news is most films have published compensation charts that allow the photographer to compensate for reciprocity failure.

:what: CHIM!!!!:what:

But I saw some picture with reciprocity failure produced very nice effect. The color tone it produce can be very beautiful.

 

[mervlam]

Originally posted by jimtong


:what: CHIM!!!!:what:

But I saw some picture with reciprocity failure produced very nice effect. The color tone it produce can be very beautiful.

huh? what r u talking about? :dunno:

 

[Midnight]

Originally posted by Zoomer
So, from what I gather, digital is not at all affected, right?

I think you're right there. Even if the response curve of digital sensors is not linear with respect to incident light level, since everything's electronic, the programmers can simply calibrate the available exposure settings so that they give the correct result as far as the end-user is concerned.

 

[snappez]

HeHe...I'm kinda new to these photographics jargon and concept. After reading thisthread about reciprocity failure, I gathered that the failure will happened only to colour slides when shooting under poor light condition and not normal B/W films. Is this understanding correct?

 

[ckiang]

Originally posted by snappez
HeHe...I'm kinda new to these photographics jargon and concept. After reading thisthread about reciprocity failure, I gathered that the failure will happened only to colour slides when shooting under poor light condition and not normal B/W films. Is this understanding correct?

Nope, can happen to all films with varying degrees. I originally understand that reciprocity affects effective film speed during very long exposures, but apparently affects colour too, as sririam brought out.

Regards
CK

 

[snappez]

Hmm.... I see. Thanks.