Using 35mm SLR lenses on dSLR camera bodies

Why the lens's effective focal length increases.

If you have a lens intended for use on a traditional 35mm film SLR (single lens reflex) camera, you may be able to use it on a digital SLR camera (dSLR) from the same manufacturer.

The range of lenses made by Nikon and Canon, in particular, will fit and function on most of their traditional film SLR camera bodies and also on their digital SLR camera bodies. (Some of their very old lenses may not have the newer electronic connections to enable you to use the same range of features, like auto-focus for example.)

When using such a lens on your digital camera, you will notice a difference in the angle of view, which will be reduced on a digital camera unless your digicam has a full frame image sensor. Most digital cameras don't.

Because the image sensors of digital cameras are typically smaller in size than a 35mm film frame, as illustrated by the image on the right, the lens will not record an identical image size. It will be smaller on the sensor than it is on 35mm film.

This is not the case with a full frame image sensor, which is a sensor that is the same size as a frame of 35mm film (24 mm X 36 mm). Few digital camera models have full frame sensors to record images that are identical in size to the film frame of 35mm cameras, or even larger, as a rare few are.

If your dSLR camera does not have a full frame sensor and you attach a lens from an SLR camera to it, you are increasing the effective focal length of the lens. Although the lens's actual focal length remains the same, the smaller sensor size increases the apparent size of the subject, as if you had zoomed in on it.

To aid in understanding this, think of a projector casting a picture onto a wall-mounted rectangular screen, like in a movie theater, so that the projected image just fills the screen. Then, without moving or adjusting the projector, replace the screen with a smaller one. Now the projected image is larger than the smaller screen and spills outside of it onto the wall behind the screen. In effect, the image on the screen has been cropped. That is what happens when a lens from a 35mm camera is used on a digital camera that has a sensor smaller than a 35mm film frame, as most do.

Now, if you were to make a 4" X 6" print from the image on the larger screen and also make a 4" X 6" print from the image on the smaller screen, the print from the smaller screen would appear to be closer to the subject, as if it had been photographed by a lens of a longer focal length. But you know that the projector's lens remained unchanged in any way. The effect makes it seem as if it had been changed. That is why it's called an increase in effective focal length.

THE CROP FACTOR

Digital image sensors are not all the same size. If they were, it would be easy to compare one of them with a 35mm film's frame size to come up with a ratio that would tell you exactly how much the effective focal length differs when an SLR lens is used on a dSLR.

But, the ratio can be determined for any digicam when its sensor size is known. This ratio is called the Crop Factor or the Focal Length Multiplier or FLM. It is also sometimes referred to as the Format Factor. (That is a lot of names for the same thing, don't you think? Let's stay with "Crop Factor" for our discussion here.)

When a digital camera's manufacturer provides the crop factor for one of its models, it's letting you know how much of an apparent increase you can expect in the effective focal length of any SLR lens you use on that dSLR camera. Typical crop factors are in the range 1.5 or 1.6 to 2.0.

HOW DO YOU USE THE CROP FACTOR?

As we said above, it's used to determine the effective focal length of an SLR lens that is used on a dSLR camera. How? Just multiply the actual focal length of the SLR lens by the crop factor to arrive at the effective focal length. For example, a 60mm SLR camera lens on a digicam that has a crop factor of 1.5 would be equivalent to a 90mm lens in terms of the lens's angle of view (60 X 1.5 = 90). If you are using a zoom lens, say an 80 to 200mm zoom lens, apply the crop factor in the same manner, making the lens's effective zoom focal length range from 120 to 300mm.

PROS AND CONS

What are the advantages and disadvantages to a photographer of using an SLR lens on a typical dSLR that has a smaller sensor than a 35mm film frame?

Pro - If you are shooting with a telephoto lens, its increased effective focal length provides you with what amounts to a longer telephoto lens, bringing you even closer to your subject to fill the frame.

Con - If you are using a wide angle lens, you lose width because the field of view is less. The crop factor makes a wide angle lens have a smaller angle of view.

Pro - Many lenses perform better in the center of the frame than at the edges of the lens. When such a lens designed for a 35mm SLR is used on a dSLR, the image recorded on the sensor may be sharper or may not show edge bluriness or darkness that would appear on the larger film format.

Con - The shutter speed when hand-holding the camera should be at least equal to the effective focal length, not the actual focal length, to avoid blur caused by camera shake.

DEPTH OF FIELD CONSIDERATIONS

Let us say you frame your subject so that it fills the frame using a digital camera that has a small image sensor. Then you switch to a digital camera that has a full frame sensor, using the same lens, without either you or the subject changing your positions.

Because the sensor is larger, your subject no longer fills the frame. In order to fill the frame, you must either move closer to your subject or switch to a lens with a longer focal length. When you do either of these, the depth of field becomes altered. It is shallower for large image sensors.

If you wish to maintain the same depth of field, you must use a smaller aperture setting for the camera with the larger sensor.

Of course, if you change to a smaller aperture, you must also change to a slower shutter speed to maintain proper exposure. If there is motion in or around your subject, a slower shutter speed may create unwanted blur. To adjust for this, you may need to increase your sensitivity setting, permitting you to maintain the same shutter speed at the new aperture.

DIGITAL CAMERA LENSES

Most camera and lens manufacturers design lenses with shorter focal lengths specifically for the smaller image sensors of most dSLR cameras. The circle of light that strikes the image sensor through such lenses is not large enough to fill a 35mm film frame. For this reason, these lenses will not work as satisfactorily on a full frame digital camera or on a 35mm film camera body. Vignetting (dark corners or edges) will occur in the image.

Some camera manufacturers make wide angle lenses specifically for their dSLR camera bodies that have smaller image sensors. These lenses (with some exceptions) cannot be used on their full frame dSLR camera bodies.

The lenses of point and shoot digital cameras, even though they are not removable, are smaller than the lens of an equivalent 35mm point and shoot camera. Most of the time, these lenses are labelled with their actual focal length, but sometimes you may see them labelled with a 35mm equivalent focal length, which means their actual focal length has been multiplied by the crop factor. This is done to aid the buyer in comparing the lens's field of view with that of a 35mm camera lens.