A digital SLR will not use an SLR lens in the same way as a film SLR. Furthermore, not all digital SLRs will use an SLR in the same way as one another. The only factor that remains constant is the Film SLR and that is why we refer to the 35mm Film Equivalent as the industry standard.
This article will help you understand how a Film SLR uses an SLR lens, how a digital SLR uses an SLR lens and what the differences are as they relate to the focal range (or zoom) only. Most importantly, this article will explain how those differences affect you. Please note that in this article I will refer to 35mm as it relates to the size of the film strip and film equivalent, but I will also refer to other measurements as they refer to the focal length. Do not confuse the two.
The focal length is the zoom capacity as measured in millimeters. It is (or should be) an absolute measurment that is the same on one camera to the next. This is the alternative to reffering to the zoom. A zoom on the other hand is a relative term as it relates to your camera. So if you have a lens with an 18-55mm focal length, the zoom would be 3X.
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A 35mm Film strip (as illustrated on the left) was the industry standard for many years. It has only one shape and size. It is 35mm wide. So if you were to buy Fuji Brand or Kodak brand film for your camera, it would be 35mm every time. Each cell of film can be used only once and each roll of film can only be used for as many pictures as that roll allows. Once it is done, the roll must be replaced. That is reason for its consistency in size.
A Digital SLR does not use film. It uses a sensor (as illustrated on the right) that measures light, stores it onto a memory card, and then the same sensor can be reused as many times as needed. So long as the batteries and the memory are sufficient, the photographer can continue to take pictures. The sensor can be used infinitely and can be of any size.
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The following series of images is a single shot that was edited digitally. For the sake of explination, we're going to pretend that the image was taken more than once on seperate cameras.
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The first image is placed to give you a frame of reference. This image was taken with a 35mm film camera at a focal range of 18mm.
Lets say that the next image was taken with a Nikon D80 Digital SLR. Why Nikon D80? Because it is a popular camera with a sensor that is roughly 30% smaller than a film strip, so it is a perfect candidate to prove the point.
The darkened outer border is there to illustrate what the 35mm Film camera would have seen. The inner area is meant to illustrate what the sensor sees. The lens and camera body are still designed to house a sensor/film of any size, but the sensor is not large enough to encompass the entire area. So the outer area is discarded..
The third image is the same as the previous one but it has been zoomed to fit the standard that the first image has. By comparing images 1 and 3, you realize that you see less on the third image than you do on the first image. Almost as if it had been zoomed in.
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Congratulations! now you know why the Digital SLR uses a lens differently from a film SLR. The next step is calculating your camera and lens' film equivalent. Again, we've tried to make it easier for you here. You could either learn the formula and do the math - all steps are shown below - or you can use our focal length Calculator. The calculator is programmed with the formula and some basic parameters. If your camera appears on the list in the calculator, you can just click the button for your camera and the sensor size will be inserted automatically.
NOTE: We determined the sensor sizes of each camera listed by researching this from trusted sources online. None of the information was provided by the manufacturer. This information is "unofficial"
To find the Film equivalent of your camera, you have to follow a simple algebraic formula. The following are Given
35mm refers to the Film strip. This remains fixed.
X refers to your camera’s sensor size which can be any given size. Please check with your owner's manual or online forums to find this data.
Y represents your current focal length (zoom)
Z represents the focal length you will attain at a 35mm film equivalent.
(35mm ÷ X ) • Y = Z
So to plug in an example, the Nikon D80 sensor size is 23.6mm. And let’s pretend I have an 18-70mm lens and I want to see what my widest angle is at a film equivalent. Plugging it in is very simple.
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