Equivalent lectures

I hope, my stem readers, that you have practiced how to read the expose meters on your cameras. If you still have some doubts, please be confident enough to send me a comment.

When you master how to read the values, now you are prepared to make equivalent lectures. This means, that you are able to change shutter speed or ƒ numbers to accommodate your needs or the end result you wish, without changing the quantity of light that is going to get into the film. Maybe by now, you are going to be able to have an idea if you know what each light step means.

This is going to be a little bit more practical than before. First of all, lets imagine that a friend of us has a hand expose meter, and he (or she) measures the light on his face, and the displays says: s 1/250 @ ƒ 4.0. In this case we want to blow away the background, in other words, make it unfocused. How do we do that? Easy one! We just open the lens aperture, in this case we are going all the way to ƒ 2.0. If we write an scale:

ƒ numbers   1.4   2.0   2.8   4.0   5.6   8.0   11   16   22

Speed   15   30   60   125   250   500   1000   2000

Note: I did not write the 1/ of the speed

Each number is a step, in which I am going to let pass the double or the half of light, so if we are in ƒ 4.0 and we want to go to ƒ 2.0, how many steps are we moving? Just two steps, but in this moment we let enter four times the amount of light to make a perfect exposure, so what do we do now? We have speed in the other hand and as we are not worried about its effect, we are going to move it the same number of steps until it compensates the light we let pass with the aperture, this means we are going to move the speed to 1/1000, as it let passes four times less light than before, and so our new lecture is going to be 1/1000 @ ƒ 2.0 and we assure a shallow depth of field as we wanted to do the portrait of our friend.

This also applies to the case in which you are more concerned about the speed, for example not letting it go below 1/30. For example, we are in a place where the Sun is about to set, the shadows are very long and dark, but we found that some places where the lights still passes trough is interesting, we make a lecture with our camera, and it says: 1/8 @ ƒ 8.0, and also we do not bring any tripod, and there is not a table at sight. 

ƒ numbers   1.4   2.0   2.8   4.0   5.6   8.0   11   16   22

Speed   8  15   30   60   125   250   500   1000   2000

Note: I did not write the 1/ of the speed

We want to be safe about shutter speed, so we want to shot at 1/60, so from 1/8 to 1/60 are three steps, in which we will not let pass much light at all, like about 8 times, so what do we do next, is compensate with the ƒ number, the same number of steps, so we let pass more light to it, from ƒ 8.0 we head to ƒ 2.8, and this way we end letting pass the same amount of light as the original reading.

The next pair of images exemplifies the Equivalent lectures.

1" @ ƒ 8.0

1/15 @ ƒ 2.0

Contrast seems a little bit different on each… but as you can observe, both have similar amounts of light on them, the only difference is how I wanted to move DoF, so only the little turtle was in focus. As you can notice both values moved five steps, if I just moved one without compensating the speed that would be blank image.