Sunday, August 28, 2011

What is a stop?


One of the main keys to good photography is exposure. This is also one of the big mysteries.  When I am teaching classes and we are using manual mode the first question is what should I set my camera to for the exposure?  What the person wants to know is three settings, ISO, shutter speed, and aperture.  My stock answer is “it depends”, because it really does depend. The big key is what effects are you looking for in the photo. The other answer could be, “the combination that gets you the proper exposure”. The reason for this is that there are many different combinations that will get you a proper exposure.

You can go a fast shutter and wide aperture and you stop motion (either the subject or camera shake) and get a shallow depth of field. You can go with slow shutter and narrow aperture and you get a very large depth of field, but if something moves you get blur. You can go fast shutter, narrow aperture, and a high ISO, and you get big depth of field and stop motion, but you also get digital noise in the photo. It is all about what affects you want (or compromises you are willing to make) to get the shot. So it really does depend.

As you are trying to find the right combination you will move the settings of each of these three settings in specific and related increments. The measurement for exposure is the stop. You might have heard the phrase “bring your aperture down a stop”, when people are talking about the exposure. Or when reading about vibration reduction (Nikon term – for Canon it is image stabilization – and for Sigma lenses it is optical stabilization) you might see the phrase “gives 4 stops of stabilization” (more on what that means in a minute). At the very basic level a stop difference means you are either doubling or halving the light that gets to the film or image sensor (yes it is the same for film and digital). Initially everything was pretty much measured in full stops. Technically you could have infinite adjustments for shutter and aperture, but the industry has agreed to increment things in either full stops, half stops, or third stops.

Well this probably sounds all boring and mathy. I suppose it is. But if you can at least get a good basic understanding of the relationship then it will help you pick and adjust your settings. It will also help when purchasing equipment. You will see references to stops regularly. You will also need to know the basic increments when evaluating different equipment.

So let’s start with an example or two on how this affects your actual photography shoot. Say you are out doing a shoot of some kids. You want a clear clean photo without digital noise. So you set ISO to 100 (the lower the number the less noise). You initially set your f-stop to f/8 because that is the real sweet spot of the lens (this is a whole different discussion on lens quality and construction). You adjust your shutter speed until the light meter shows a proper exposure and you end up with 1/60th of a second. Not a bad setup, except that kids are constantly moving. So you might be lucky to get one good photo out of five. The rest are blurry from motion. So you think “I need to get to at least 1/250th of a second. This is a change of two full stops. You know you don’t want to adjust ISO because you don’t want the digital noise. So you need to adjust aperture. Well if you move from f/8 to f/4 then you are dropping aperture by two full stops. You can now go to 1/250th of a second for shutter and still have a proper exposure. Tada, you have now used all that mathy stuff to get you where you need to be.

Or let’s say you are shooting with a 50mm f/1.8 at f/1.8 and the depth of field is just too short (a fast lens is not always the solution). So you increase by a stop and a third to f/2.8 to get a little more depth of field. Again, ISO is always a last resort adjustment. If you were at 1/500th of a second for shutter you would need to drop to 1/200th of a second to keep the same exposure.

To be a little honest here, I don’t have all the different numbers memorized.  I know some of them, but not all of them. I use a chart when writing. When I am “in the field” doing shoots what I simply do is to count the number of clicks I am making in the first adjustment to know how many clicks to make in the other adjustment. So if I increase shutter by two clicks then I will decrease aperture by two clicks. Then I chimp a couple shots (take a shot and look at the LCD screen and go Ooo Ooo Ooo) to see if I have what I want.

Another place that this knowledge of stops helps is with choosing new equipment. Let’s say you are trying to decide between a 55mm to 200mm zoom lens that is f/3.5 at 55mm and a 50mm f/1.8 lens. Well you are going to get two full stops more light with the prime lens. This means you could go from a shutter speed of 1/30 and move to 1/125th for a shot. This could be the difference of using flash or increasing your ISO to get a natural light shot. Don’t forget that your depth of field shortens up, but maybe that is OK.

Let’s talk again about vibration reduction. You have two lenses that you are considering. Both are the same focal length and aperture capabilities. One has vibration reduction that says you will get three stops of stabilization, and the other does not have that. Keep in mind that image stabilization only affects movement in the camera and NOT the subject moving. If you get the lens without the VR and you need to shoot at 1/500th to get a still photo, then you could get the VR lens and shoot at 1/60th and still get a clear blur free photo. This is especially important with telephoto lenses. The rule of thumb is that you always want to be at least as fast as the reciprocal of the focal length. So if you are shooting a 200mm lens you should be at least 1/200 or faster. For a 500mm it would be 1/500th or faster. So now you can decide if that extra $400 is worth it to be able to handhold the lens at 1/60th of a second. The numbers (three full stops of reduction) are not just numbers, but have actual meaning for your shooting.

Exposure compensation is also measured in stops. So when I talk about that in the next blog post what we talked about here will make a lot more sense.  One other point, Remember that ISO is the last refuge to get a faster shutter or narrower aperture. Digital noise is the bain of digital photography. In film you would get grain. Surprisingly at times grain could be a good thing. Digital noise is almost never good. So that is why digital cameras don’t have an ISO priority setting. You will almost never adjust that. That is starting to change with the high ISO capabilities of some of the newest cameras. And full frame sensor cameras are better at high ISO than crop sensor cameras. But you will want to still go as slow as possible for ISO to get your best looking shot.

Have fun shooting and take some time to practice making these adjustments back and forth to get used to them. 


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