Reciprocity Failure

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darkavgr

New Member
What is Reciprocity Failure or Reciprocity Law failure ? :dunno:

Maybe the gurus here can explain....

Many thanks

ckiang

Senior Member
Originally posted by darkavgr
What is Reciprocity Failure or Reciprocity Law failure ? :dunno:

Maybe the gurus here can explain....

Many thanks
In simple terms, it's the loss of film speed when doing long exposures, and the film does not obey the law of reciprocity. (you know, the open up 1 stop of aperture, you need to 1/2 the shutter speed, etc).

So let's say you have an exposure of 1s at f/4.
At f/16, it would be 4 stops down, so exposure time would be 16s. But becoz of possible reciprocity failure, this combination might result in underexposure. To counter this, you would have to open up the aperture to say f/11 (or whatever determined by experience and experiment, depending on the film).

Regards
CK

darkavgr

New Member
Thanks ck !
understand now...

Ian

Senior Member
Originally posted by Kit
I read somewhere that this only kicks in after 30secs?
16 seconds for most Fuji emulsions
10 seconds for most Kodak emulsions.

Ian

Senior Member
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.

darkavgr

New Member
Wah...Ian..you professor of physics ...
I am impressed.

thanks...

darkavgr

M

Midnight

Guest
Now that's the difference between a pro and an amateur... :thumbsup: :thumbsup:

darkavgr

New Member
Ian & ck,

So for night shots...do I need to compensate ?
so far, I've been following the exposure values per camera.

sulhan

Moderator
Staff member
Optimally for a iso100 daylight film....

say for example for a perticular image:
shutter speed @ 1/125 and aperture f8 - (Name this part 1)

- If you move shutter one speed higher,
then the aperture needs to be larger by one stop.
(to get the same exposure as part 1)
- if you move the shutter one speed slower,than the aperture will need to be be smaller by one stop
(to get the same exposure as part 1)

When this type of compensation fails, then you will have reciprocity failure - thus image turning over or under exposed.

......

Kit

Senior Member
Originally posted by darkavgr
Ian & ck,

So for night shots...do I need to compensate ?
so far, I've been following the exposure values per camera.
I don't see the need for compensating. Hardly, if any at all where I used an exposure of more than 15 secs. With Fuji Velvia that is.

NitroTech

New Member
Originally posted by Kit

I don't see the need for compensating. Hardly, if any at all where I used an exposure of more than 15 secs. With Fuji Velvia that is.
Have you experienced this before? With Velvia i mean.... When i use it for long exposures (6 secs and above) the picture always comes out greenish. But for Provia 100 and 400 i have no such problem.

Kit

Senior Member
Originally posted by NitroTech

Have you experienced this before? With Velvia i mean.... When i use it for long exposures (6 secs and above) the picture always comes out greenish. But for Provia 100 and 400 i have no such problem.
Hmmm..... nope. Haven't noticed that on my slides.

NitroTech

New Member
Originally posted by Kit

Hmmm..... nope. Haven't noticed that on my slides.
Sheeesh.... Now another 'problem' for me to rectify....

ckiang

Senior Member
Originally posted by NitroTech

Have you experienced this before? With Velvia i mean.... When i use it for long exposures (6 secs and above) the picture always comes out greenish. But for Provia 100 and 400 i have no such problem.
That's NOT reciprocity failure, that's a colour balance issue. The slide is daylight balanced, and the greenish cast you get is probably from some artificial lighting, notably flourescent.

Regards
CK

NitroTech

New Member
Originally posted by ckiang

That's NOT reciprocity failure,

Regards
CK
Ehhhhhhhhhhhhh, I KNOW!! Sorry, my fault. I went OT!!

sriram

New Member
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.
Originally posted by ckiang
That's NOT reciprocity failure, that's a colour balance issue. The slide is daylight balanced, and the greenish cast you get is probably from some artificial lighting, notably flourescent.[/B]
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.

Zoomer

New Member
So, from what I gather, digital is not at all affected, right?

Though maybe the high powered lasers and atomic bomb detonatinos might fry the sensor. ;p

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