Relationship between film speed and aperture

If I double the shutter speed on my camera (1/250 to 1/125), I can go one stop down in aperture (f4 to f2.8) and get the same exposure. Everything balances out nicely.

My question: If I increase me film speed, for instance ISO 200 to ISO 400, is there the same straightforward relationship? Essentially, does doubling the film speed get me one stop in aperture?


That property is called reciprocity, when you change two exposure factors in opposite directions and get the same exposure. As you see there are three primary factors that all have to be in agreement. It works within normal limits but fails with some films at extremely long or short shutter speeds but that is almost never a concern in general photography.

Keep in mind that while changing shutter speed and aperture combinations can get you the same exposure they won’t take the same picture as depth of field and sometimes motion blur are affected. Film speed also effects grain with faster films giving coarser images.

Confirmed. Note the extra grain in faster film speed, not so much a problem these days, up to ISO 400

Thanks all. (Especially Q.E.D. for the interesting site.)

What do the different settings mean? I’ve never had a camera with all those options, but I’ve heard them all over the place.

What is film speed? an F stop? (exposure speed I understand)

Nevermind. I skimmed over QED’s link and it answered my questions.

To expand on Padeye’s point:

Reciprocity failure can occur with shutter speeds of over a second or more, which result in underexposed images if the exposure reading is determined normally. As a general rule, it’s a good idea to add a ½-stop for exposures of 4-8 seconds, and a full stop for over eight seconds, but experimentation with your own camera and particular film is obviously the best guide.

For exposures of minutes or hours, don’t bother compensating.

Many films also show a subtle color shift with long exposures (over a minute), which can be corrected with filters. Film manufacturers have technical data about their films which can assist with this.

I’ve condensed this from Photography Outdoors, © 1995, by M. Gardner & A Wolfe. ISBN
0-89886-430-5. It’s a great little guide which you can tuck in your backpack when you’re travelling with a camera.


No it doesn’t.

First of all, when you say “double the shutter speed” I assume you mean double the amount of time the shutter remains open. That is what happens when you switch from 1/250 sec. to 1/125 sec. Since 1/125 sec. is twice as long a time period as 1/250 sec., the shutter remains open for twice the amount of time, thus allowing twice the amount of light to reach the film (or CCD, or CMOS, or what have you).

Switching from f/4 to f/2.8 is stopping up not down. f/2.8 represents an aperture with a diameter twice the size of that which is represented by f/4, and therefore allows twice the amount of light to reach the film (or CCD, or blah blah blah).

So, in your example, what you’ve done is created a larger opening for the light to travel through (thus, more light gets through) and allowed the light to travel through that opening for a longer period of time (thus, even more light gets through). So things don’t balance out nicely. You’ve just over exposed that one in a million shot of Elvis sipping tea with two aliens in a Paris bistro by two stops or 4 times too much light!

You and I both know what you meant though. You can half the amount of light with one function and double it with the other and things will balance out nicely. In other words, a setting of 1/250 @ f/4 will yield the same exposure as a setting of 1/125 @ f/5.6.



Both of these alterations double the amount of light hitting the film, so together they actually increase the amount of light by four.

I think what you meant to say was “If i double the shutter speed on my camera (1/250 to 1/125), i can go one stop down in aperture (f2.8 to f4) and get the same exposure.”

The term “stop down” is somewhat counter-intuitive if you look at the numbers, because as you stop the lens down (i.e. make the aperture smaller), the numbers actually get bigger (2.8, 4, 5.6, etc.)


And now i’ve posted i see that photog beat me to the punch.


Stop picking on me! I knew I’d screw up one of those damn directions somewhere in that post. :slight_smile:

I should have caught that.

The reason the numbers are “backwards” to what seems right is because aperture number is the focal length relative to the absolute diameter of the aperture, focal length divided by aperture actually. The measured size of f2.8 depends on the focal length of the lens but in all cases will give the same exposure with all other factors being equal.

Because it’s a measure of diameter and *area is what determines how much light gets in is why the scale isn’t by a facor of two. The factor is the square root of two but 1.4 is more than precise enough for this purpose. The normal scale of “whole” stops usually goes f1.4 - 2 - 2.8 - 4 - 5.6 - 8 - 11 - 16 - 32 - 45. As you can see there is a bit of fudging but those are the traditional numbers everyone is familar with and adding an additional deciimal place would be useless.

Aperture also controls depth of field. A large aperture (small number) gives a shallow depth of focus with the foreground and background quickly blurring while a small aperture (large number) puts more of the scene in focus.