How to calculate one ev step of exposure

Question

I'm curious how I would calculate one ev step of exposure. For example, what's the difference between f/1.8 and f/1.4? Thanks in advance.

Answer 1

The F-stop is a measurement of the amount of the lens iris is open. It is given in the formula N=f/D Where N=f-stop  f= focal lenght and D=lens diameter.

So, the larger the f-stop, the smaller the lens opening, meaning, that less light is entering the lens, which results in a darker image. So, f/1.8 will be darker than f/1.4.

The F-stop measurement also has to do with the Depth of Field (DOF), which is how much of the frame is in focus. A low f-stop will give you a narrow DOF and a high f-stop will give you a deeper DOF. For example, in a photo at f/20, the most likely, the entire frame will be in focus. At f/2, the foreground may be in focus, but the background is out of focus, or vise versa, of course. Focal length (the amount you zoom in) also plays a big role in DOF.

In a short recap, the bigger the f-stop, the darker the image, but the deeper the DOF.

Hope that helps.

Answer 2

According to wikipedia, each increment of 1 EV corresponds to a change of 1 stop in exposure. I know this is an accurate definition since I use -1 EV to do an in-camera double exposure, -1.5 EV for a triple exposure and -2 EV for four exposures on one frame.

Using EV is one way to bracket your exposures. Suppose you're shooting at f11 and 1/125 sec. If you don't want to switch to Manual Mode* but want to see the effects of 1 stop underexposure and 1 stop overexposure you'd use -1 EV for 1 stop underexposure and +1 EV for 1 stop overexposure. In effect, you're making one exposure at f11 and 1/125 sec., one exposure at f8 and 1/125 sec. amd one exposure at f16 and 1/125 sec. Or you'd be making one exposure at f11 and 1/125 sec., one exposure at f11 and 1/250 sec. and one exposure at f11 and 1/60 sec.

* If you're shooting in Aperture Preferred any change in your aperture will result in a change in the shutter speed selected by the camera. In Shutter Preferred any change in the shutter will result in the aperture selected by the camera. So if you want to bracket your exposures you either have to be in Manual Mode or use the EV settings.

Answer 3

That is one third of a stop ... to calculate?  I have really never had to do that ... in over 30 years ... the handheld meters show were exactly between f/2.8 and f/4 you need to set the aperture ... we just call it one third down or up from the marked f/stop.

There are charts out there someplace I would think, but after while you get so used to shooting the camera, you get to where that all becomes intuitive and only looking at the EXIF data on the image file will tell you the exact exposure settings

Answer 4

One EV is a doubling or halving of exposure, i.e one F-stop or a doubling or halving of shutter speed.

F-numbers are related to the diameter of the lens aperture. F/1.8 means the lens focal length divided by 1.8. The exposure effect is proportional to the AREA of the aperture, which varies as the square of the aperture diameter.

[Donning professor cap]

You need to use logarithms to accurately calculate EV.

When comparing F-numbers f1 and f2

EV = log (f1/f2)^2 / log 2.0

or EV = (2 log (f1/f2) ) / 0.301

The logarithms are base 10 logs.

For shutter speeds, EV is directly proportional., so you don't need the squaring, thus for shutter speeds S1 and S2

EV = (log (S1/S2) )/ 0.301

An increase in EV indicates more exposure.

In your example,

EV = (2 log(1.8/1.4)) / 0.301 = 0.725 EV

Answer 5

All other factors being equal, that just gives a difference of 2/10ths of an f stop and will not be all that noticeable. If you are using a digital camera you should have an ev slider in the menu that will let you do those calculations, guessing that you don't want to use the auto mode.

Answer 6

Between 1.4 and 1.8 is 2/3 of a stop.

Each LV value is twice the light of the previous value.

So if you light is at LV 15 and you are shooting 100 ISO film, you'd have your f-stop at f/16 and your shutter at 1/125. If the sun went behind a cloud and the LV dropped to 14,  it would mean the light got halved. To compensate, you would have double your exposure.

To do this you would either change your shutter to 1/60 or your aperature to f/11. Either action would double the exposure.

Built-in meters don't tell you directly what the LV is. It'll just tell you whether you are above or below the correct value.

For more on the value of the stop, see the link