Sony LCL90AM Focal length, Maximum magnification ratio, Technical definition of focal length

Focal length, angle of view and perspective

Macro photography

Focal plane (image sensor plane)

Angle of view

(measured diagonally)

Focal length

Focal length

Focal length, or focal length range in the case of zooms, will usually be the foremost consideration when choosing a lens for a specific photograph or type of photography.The focal length of a lens determines two characteristics that are very important to photographers: magnification and angle of view.

Longer focal lengths correspond to higher magnification, and vice-versa. Wide-angle lenses with short focal lengths have low magnification, which means you have to get physically close to an average-size subject to fill the frame. But that also means you can fit large subjects in the frame without having to shoot from a distance.Telephoto lenses with long focal lengths have high magnification, so you can fill the frame with subjects that are further away from the camera.

Maximum magnification ratio

As mentioned on the previous page, the magnification of any lens is determined by its focal length. For macro photography we are also concerned with how close we can get to our subject.These two factors, focal length and minimum focusing distance, determine the lens’s maximum magnification ratio, sometimes referred to as “reproduction ratio.”The closer you can get to your subject with a lens of a given focal length, the higher the magnification ratio you’ll achieve.

The classic definition of a macro lens is one that has a maximum magnification ratio of at least 1:1, or “1x” in lens specifications.This means that a subject can be reproduced at full size on the camera’s image sensor: a 10mm object can be projected onto the sensor as a 10mm image when the lens is sufficiently close to the subject. A maximum magnification ratio of 1:2 or “0.5x” would

Secondary principal point of lens

Focal length vs. angle of view				A technical definition of focal length
				The focal length of a lens is defined as the distance from its secondary principal
	With 35mm full-frame image sensor	With APS-C type image sensor
				point to its rear focal point when focus is set to infinity. The secondary principal
				point is one of six “cardinal points” that are used as points of reference in an
				optical lens (front and rear focal points, primary and secondary nodal points
			Wide	and primary and secondary principal points). There’s no predefined location
				for the secondary principal point in a compound lens—it could be somewhere
	16 mm	16 mm		inside the lens barrel or at some point outside the barrel, depending on the
	Fisheye	Fisheye		design of the lens—so there’s no easy way to accurately measure the focal
				length of a lens yourself.
	16 mm	16 mm		Focal length and angle of view
		(24 mm)
				“Angle of view” describes how much of the scene in front of the camera will
				be captured by the camera’s sensor. In slightly more technical terms, it is the
	18 mm	18 mm		angular extent of the scene captured on the sensor, measured diagonally.
				It is important to remember that angle of view is entirely determined by both
		(27 mm)		the focal length of the lens and the format of the camera’s sensor, so the
				angle of view you get from any given lens will be different on 35mm full frame
				and APS-C format cameras. Different lenses of equal focal length will always
	24 mm	24 mm		have the same angle of view when used with the same-size sensor.
		(36 mm)		The “Focal length vs. angle of view” comparison to the left illustrates this
			Mid-
				relationship for both 35mm full frame and APS-C format cameras.
	35 mm	35 mm	range	Perspective
				With long focal lengths, foreground and background objects will often appear
				to be closer together in the final image.This effect is sometimes called “telephoto
	70 mm	70 mm		compression,” although it is not actually caused by the lens itself.What really
				happens is that when using a telephoto lens, you will need to be further away
		(105 mm)
				from your subjects.As such, the distance of the subject from the background
				relative to the subject’s distance from the camera lens becomes smaller and
				smaller the further away the photographer stands. From that perspective
	100 mm	100 mm		they actually are closer together! Another way of saying this is that since both
				the foreground and background objects are at a considerable distance
		(150 mm)
				from the camera, their relative sizes in the final image will be closer to reality.
				When shooting with a wide-angle lens you normally need to get close to the
				foreground subject so that it is sufficiently large in the frame, which is why
	135 mm	135 mm		more distant objects look comparatively smaller.The difference in apparent
				perspective is actually a result of how far you are from your subject.
		(205.5 mm)

mean that the maximum size that an image of the same 10mm object could be projected onto the sensor would be 5mm, or just half its true size.

0.35x

Other macro lens characteristics you should know about

Macro lenses are specifically designed to deliver optimum optical performance at very short focusing distances, and will usually be sharpest at close range, but that doesn’t mean that you can only use them for macro photography. Many macro lenses are also capable of excellent performance when shooting normal subjects at normal distances as well.

Another important characteristic of macro lenses used at short range is that they have very narrow depth of field.That means they have to be focused very carefully to get the desired details in perfect focus.A tripod can make focusing easier in some situations.You might have to stop the aperture down quite a bit to achieve sufficient depth of field with some subjects. But shallow depth of field can be an advantage, emphasizing the essential in-focus detail while defocusing and de-emphasizing distracting background.

1.0x

30mm Macro lens (SAL30M28)

Working distance (approx. 2 cm/0.8 in. at 1x magnification)

Minimum focusing distance (approx. 13 cm/5.1 in. at 1x magnification)

Image sensor plane 100mm Macro lens (SAL100M28)

Working distance (approx. 16 cm/6.3 in. at 1x magnification)

Minimum focusing distance (approx. 35 cm/13.8 in. at 1x magnification)

Image sensor plane

Minimum focus and working distance

250 mm	250 mm
	(375 mm)
400 mm	400 mm
	(600 mm)

*Focal length in ( ): equivalent focal length when mounted on interchangeable-lens digital cameras with 35mm full-frame sensors.

Telephoto

24mm focal length,*	300mm focal length,*
84° angle of view	8° angle of view
* 35mm format equivalent

The “minimum focusing distance” lens specification can be confusing. Minimum focusing distance is measured from the subject to the rear focal point of the lens, which is at the image sensor plane in the camera body.

The term “working distance” is used to describe the distance between the subject and the front element of the lens.

If a lens is specified as having an 0.2 meter (20 centimeter) minimum focusing distance, for example, depending on the thickness of the camera body and the length of the lens, you might only have a few

centimeters of working distance when focused at the minimum focusing distance in order to take a 1:1 macro shot. Being that close to your subject can make lighting difficult (special macro flashes and ring lights are available to overcome this type of lighting problem), focusing can be difficult if the subject or camera moves even slightly, and you’re likely to scare away living subjects at such close distances. If any of those problems occur, you need to choose a macro lens that has a longer focal length for more working distance.

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