One set of terms that you’re certain to come across when researching your next DSLR purchase are Crop Sensor and Full Frame.

35mm film is the standard by which all cameras are measured. Cameras that have sensors which replicate the 35mm film style are referred to as being “Full Frame.” Not all cameras replicate 35mm film. Most cameras have sensors smaller than 35mm. Those cameras are referred to as “Crop Sensor.” To keep you from having to buy different lenses for different style cameras, professional lenses are made to work on either style. However, they will produce different results. Crop sensor cameras produce images that seem magnified.

The illustration below shows the relationship between a crop sensor camera and a full frame camera. Your lens is round, but your camera’s sensor is rectangular. If you can remember the film days, you remember that you loaded a roll of film into the camera and then dragged it across the back of the camera. Each frame of film was a rectangle, just like your camera’s sensor.

As you can see below, the 35mm (or full frame) rectangle stretches across the entire circle, such that its corners touch the edges of the circle. It is the largest rectangle that can fit in the circle, hence the name Full Frame. The smaller rectangle represents the size of a crop sensor.

When light comes through the lens, it actually comes through the sensor opening. Imagine two separate cameras, one with a full frame sensor and one with a crop sensor, both taking the picture below, using the same exact lens and the same settings. In the diagram below you can see that the full frame sensor camera would capture much more of the image than the crop sensor camera would.

But just how much more? It depends on the camera. In the Canon world it’s 1.6. So what does that mean?

It means that if you put a 105mm lens on a crop sensor camera, the image you get would be the equivalent of a 168mm lens on a full frame camera, or 1.6 larger. This can actually be considered a feature for some people. Besides my full frame Canon 5D Mark IV, I also carry a crop sensor Canon 7D. If I’m shooting sports and need a much larger zoom lens than I have, I attach my lens to the 7D, this giving me more zoom. My 200mm lens behaves like a 320mm lens on a crop sensor camera.

The trade off however is when you are trying to shoot wide angle. Using my 17mm lens on the full frame camera allows me to actually shoot at 17mm. If I only had a crop sensor camera, that 17mm lens would be shooting at the equivalent of 27mm. I’d have to use a 10mm lens in order to shoot at 17mm.

Confusing for sure.

The bottom line is, a full frame sensor captures as much of the image as is possible for a given lens, while a crop sensor captures less of the image. It’s neither good nor bad. It just is what it is. Full Frame cameras are inherently more expensive and are considered to be “professional.” Many photographers carry a full frame and a crop sensor and use them as necessary for the shot they need to take.

You’ve seen those pictures. You know, the pictures where the subject is in focus and the background is all blurred out. The blur we all love is called bokeh (pronounced bo-ka , or bo-kay). Bokeh comes from the Japanese word boke (ボケ), which means "blur" or "haze", or boke-aji, the "blur quality." Merriam Webster’s defines it as “the blurred quality or effect seen in the out-of-focus portion of a photograph taken with a narrow depth of field.”

So that brings up another question; what is depth of field?

Depth of field is best described as the depth of that which is in focus. So what does that mean? In the image below you will see a line defined as the Focal Plane. Imagine you are focusing on your subject’s nose. Now imagine that their nose is touching a pane of glass. That pane of glass stretches from floor to ceiling and infinitely from side to side. This giant pane of glass is your focal plane. If your subject has some friends, and they all stand in a row with their noses touching the same piece of glass, then all of their noses will be in focus. Everything that is on the same plane on which you are focused, will also be in focus. Everything that is front of, and behind, the focal plane is subject to be blurred (or bokeh’d).

How far in front, and how far behind, before the blurring occurs, is your depth of field. Put another way, depth of field is the area in front of, and behind, the focal plane that remains in focus. It is the depth of the field of focus. Anything outside of the depth of field is blurred. The further the distance from the focal plane, the blurrier.

So, how do you control the depth of field, and therefore the bokeh? That is the $64,000 question, and the holy grail of photography.

There are several factors involved in getting good bokeh. The first, and most important, is using a lens with a large aperture. f2.8 is ideal. Faster lenses, with apertures of f2.0, f1.8, f1.4, and f1.2 are even better! Another factor is the distance between the subject and the background, as well as the distance from the lens to the subject. The third factor is how much of the frame is occupied with the subject. Zooming in your subject’s face will give better boken than taking a full body shot. The final factor is where you put your focus.

In the image below we can see from the red square that the camera is focused on the flower, thereby creating bokeh of the buildings behind it. Ten photographers could take this same picture and end up with ten different results. If the focus had been on the building in the back, then the building would be in focus and the flower would be bokeh. Likewise, if the focus was somewhere on the middle building, both the flower and the back building would be bokeh. The focus point establishes the focal plane, that piece of glass. Everything in front of it and behind it is subject to bokeh depending on the depth of field.

So, lets assume that my examples above were based on an aperture of f2.8. If we took the same shot at f5.6, focusing on the flower, there is a good chance that the back building would still be bokeh, however, the building on the left may be in focus because it is still inside the depth of field at that aperture. Using an aperture of f8 or higher would increase the depth of field such that everything in the picture would be in focus.

This is why lenses with low (large) apertures cost significantly more than lenses with higher apertures. You can achieve bokeh with higher apertures, but it requires much greater separation between the subject and the background so that the background falls outside of the depth of field. Lower apertures allow you to create bokeh without having to separate your subject from the background as much.

But bokeh at low apertures can be tricky. If you have a lens with a f1.2 aperture, and you zoom in to focus on your subject’s nose, there is good likelihood that your subject’s ear may be part of the bokeh, because the depth of field is so narrow. The point here is that just because your lens goes to f1.2, it doesn’t mean you should always use it. Adjust your aperture to achieve the best depth of field that keeps your subject in focus.

So, how do you know what aperture to use, or how much depth of field you’ll have at that aperture? First, I would suggest shooting in Aperture Priority mode and setting the aperture to its lowest setting. Second, there are apps that exist to help you determine how much area will be in focus. The app I like is Digital Depth of Field. It’s free. You just enter the type of camera you’re using, focal length your shooting, the aperture, and the distance to your subject. It will tell you how wide or narrow your depth of field is.

My favorite lens is the 70-200mm f2.8 zoom lens. This lens allows me to get apertures as low as f2.8 regardless of my focal length. Zoomed in at 200mm produces some stunning bokeh. Another favorite of photographers is the 85mm f1.2 lens. The bokeh at f1.2 is milky smooth and looks fantastic. I highly encourage you to try both lenses for yourself. If you have a lens rental company in your area, rent either or both of these lenses. If you are in the Houston area, I highly recommend Photo Rental Source.