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How-to's Lens Reviews

FAQs - What Lens

J. Ramon Palacios (jrp)

Keywords: basics, focal_length, depth_of_field, dof, hyperfocal_distance, faq

Show pages (3 Pages)

I remember one sunny afternoon -about fifty years ago- in downtown Mexico City, glued to the window of Foto Rudiger, looking in awe at hundreds of lenses in display. Why are there so many lenses? I asked my father. His answer: "Because lenses are designed for specific purposes, subjects, situations and needs, including budgets."

A few MF (manual focus) Nikkor lenses

That afternoon -around several cups of Brazilian coffee- I also learned that all laws have as reason to exist the promotion of a good and/or the avoidance of an evil, and that physics laws of optics are not different. And so, lenses designers look at ways to apply those laws better and better, to promote greater magnification with amazing detail or wider splendid vistas, avoiding distortions and other aberrations as much as glass technology allows at the time.

I also learned that afternoon that the selection process needs not to be a puzzle; once your own set of variables is clear in your mind, choices pop up at you by themselves. That simple. To tackle this subject comprehensibly, although seemingly an enormous task, it is not.

The reasons for the initial overwhelming are, mainly: the wide variety of both lenses offerings and the variety of needs of each individual photographer (the set of variables):

The primary interest (sports, fashion, portraits, landscapes, wildlife, architecture, weddings, photojournalism, macro, travel, etc.)

Which comes immediately into mind, especially if one selects more than one interest; and the seriousness of the photographer, really meaning how much can that budget be stretched.

What camera is in use and with what format (DX or FX); what others are being considered.

What other lenses may already be in the bag.

Film or digital; is there a transition from one media into the other in the near future, and so on.

Let's look first at the basic characteristics of lenses to later match them to our own set of variables.

Lenses can be classified according to focal length, its variability, focusing method, and its relative speed as follows:


By FOCAL LENGTH: Fish-eye; wide angle; normal; short, medium and long telephoto

 Lens Type

Focal length 35mm or FX
Angle of View*
Typical use Example
 Fisheye 6 to 16mm 220° - 180° Architecture 8mm f/2.8
 Wideangle 17 to 35mm 114° - 62° Landscape 24mm f/2.8
 Normal 45 to 55mm 51° - 53° General 50mm f/1.8
 Short telephoto 85 to 135mm 28° - 18° Portraits 85mm f/1.4
 Medium telephoto 180 to 300mm 13° - 8° Sports 300mm f/2.8
 Long telephoto 400 to 1000mm 6° - 2.5° Wildlife 500mm f/4

Zoom lenses have now crossed the typical categories above, in an effort to widen the range of applications a single lens can be used for.


  • With fixed focal length or "prime" lenses
  • With variable focal length or "zoom" lenses
    • Zoom lenses with variable "speed" or aperture
    • Zoom lenses with constant maximum "speed" or aperture


  • Manual focusing or "MF" lenses (Rotating a ring in the lens barrel)
  • Auto focusing or "AF" lenses (Depressing the shutter button on an automatic camera)

By "SPEED" or "LUMINOSITY" in terms of how much light they allow into the film or digital sensor at its widest aperture

  • FAST. With a small f/stop number, wide aperture, (f/1.4, f/2.8), called "fast" in reference to the relatively fast shutter speeds you can use with these lenses.
  • SLOW. With a not so small f/stop number, not so wide aperture, (f/4, f/5.6), called "slow" because you cannot use as fast shutter speeds as with "fast" lenses.



Focal length is the distance between the lens node of emission to the plane at which objects at infinity are brought into focus to form a sharp image; i.e. at the film or sensor plane in a camera.

The focal length determines the size of the subject image on film when the lens is focused at infinity.

Such image size is directly proportional to the focal length. A 100mm lens will render an image size twice as large as that from a 50mm, a 200mm a four times larger one, etc. This also tells us that with focal length the magnification of the subject changes for a fixed lens to object distance, but not the perspective. The perspective changes only when you change the lens to subject distance.

The angle of view (AOV, sometimes referred to also as Field of View) is the angle covered by a lens of given focal length at infinity. Typically it is measured diagonally and for a given film or digital sensor format. In a rectilinear lens, it is possible to approximate it with the formula AOV = 2 arctangent [x / (2 f)], where "x" is the diagonal of the film (43.2666mm for 35mm format) and "f" the focal length in mm1.

In practice, with complex optical formulas employed in modern lenses, the AOV has to be measured with specialized instrumentation. But the important thing is that the angle of view is inversely proportional to the focal length, so the shorter this is, the wider the AOV and viceversa. Also, to increase the focal length to twice, reduces the AOV in half.

1Lens focal length and field of view, calculator by Bojidar Dimitrov et al

Lens Aperture

To obtain an exposure, a lens must be controlled through its aperture, just like the iris of the human eye. "In bright-light situations, the iris is dilated to reduce the size of the pupil and limit the amount of light which enters the eye; and in dim-light situations, the iris adjusts its size so as to maximize the size of the pupil and increase the amount of light which enters the eye"2

human eye
The anatomy of the eye2


The lens aperture controls the amount of light it transmits to the film or sensor plane, by adjusting the size of the opening in the lens diaphragm. The relative size of such aperture openings is represented by f-numbers, called apertures or f-stops.

The actual diameter of the aperture can be calculated by dividing the focal length over the f-stop value. The apertures are shown below for a 50mm f/1.4 Nikkor lens, as well as the actual diameters of each one.

50mm/16 = 3.13mm

50mm/11 = 4.42mm

50mm/8 = 6.25mm

50mm/5.6 = 8.84mm

50mm/4 = 12.5mm


50mm/2 = 25mm


50mm/1.4 = 35.4mm

The f-stop numbers follow a rounding figure convention for the sake of engraving aperture rings, now useful in LCD displays. Since each full f-stop number was established to progressively allow for twice as much light transmission as the next larger one, true f-stop values are multiples of the square root of 2 or 1.4142 as follows:

f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32
true 1.4142 2.0000 2.8284 4.0000 5.6568 8.0000 11.3137 16.0000 22.6274 32.0000

To illustrate what the apertures really mean for light transmission, lets imagine a situation where 100% of light would be transmitted at f/4, twice as much light will be transmitted one f-stop under or f/2.8, half of that light will be transmitted one stop above or f/5.6, and so on as shown in the table below:

f/stop f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32
% 800% 400% 200% 100% 50% 25% 12.5% 6.25% 3.125% 1.5625%



F-Stop and Shutter Speed

How long we let such transmitted light hit the film or sensor is the second control variable for exposure: shutter speed. The following table shows equivalent exposures, as combinations of f-stop values and shutter speeds in fractions of a second, considering a sensitivity of ISO 100 under bright light conditions in open shade:

f/stop f/1.4 f/2 f/2.8 f/4 f/5.6 f/8 f/11 f/16 f/22 f/32
shutter 1/8000 1/4000 1/2000 1/1000 1/500 1/250 1/125 1/60 1/30 1/15

What is important of the exercise above is that: for a given exposure, each one more full stop "down" (smaller in diameter) requires twice longer the shutter speed of the previous one. For each one more full stop "up" (bigger in diameter) to achieve the same exposure requires half longer the shutter speed than the stop before.

You can now select an equivalent shutter speed-aperture combination to suit your needs. If the subject is moving you will need a combination with a high shutter speed. If you want maximum depth of field, you may choose a very small aperture (higher f-stop number) with a good tripod to allow for slow shutter speeds.

A typical lens usually performs better wehen at f/8 and f/16. High-end pro glass performs equally well wide open at its maximum aperture than when stopped down.

F-Stop, Size of Glass and Cost

The wider the aperture of a lens is, the faster it can shoot when wide open (at its smallest f/stop number), the larger the glass, the higher its materials and manufacturing costs and therefore its price. For available dim light photography a "fast" lens is required.

How wide is widest possible depends on the focal length and the complexities and feasibility of manufacturing such lens. A 50mm f/1.8 lens has an effective diaphragm aperture of 50mm/1.8 = 27.8 mm (1.1") of diameter; a more manageable size than that of a 600mm f/4 that has a diameter of 150 mm (5.9"). This lens, to be a f/1.8 would need an effective diaphragm aperture of 600/1.8 = 333 mm (12.1"), a very wealthy person to order it and one strong pack mule or two to carry it.

A lens is known as a "pro" lens when it is "fast", has been corrected for all kinds of possible aberrations and it is built to last a lifetime, provided we don't drop it.

2 The Physics classroom. Lesson 6. The Eye

Focal Length, F-Stop and Depth of Field

Depth of field (DOF) is the distance between the farthest and nearest points which are in focus. This can also be identified as the zone of acceptable sharpness in front of and behind the subject which the lens is focused on. DOF varies according to several factors such as lens focal length, aperture, shooting distance, etc.

DOF is inversely proportional to focal length, i.e. the shorter the focal length (f), the larger the DOF. So, in general terms, wideangle lenses -a 28mm for example- have larger depth of field than telephoto lenses -a 105mm for example- which are therefore said to have a shallow depth of field. So, if you want a shallow DOF in a portrait, it is best to use a telephoto lens.

DOF is directly proportional to the f-stop number, i.e. the smaller the aperture (bigger f-stop number), the larger the DOF. Images taken at f/11 will have more depth of field than those made at f/4, for example. Therefore, if you want a shallow DOF in a portrait, it is best to use a telephoto lens at a wide (small) aperture, like in the splendid candid by Nikonian Luc Van Nieuwenhove (LucVN) shown at right, made with a 200mm f/2 at f/2.

Below a table with sample values to illustrate the two points above.

Lens focused at
 20 ft / 6.1m
f/stop Near focus limit Far focus limit DOF
 24mm f/8 6.43ft / 1.96m Infinity Infinite
 24mm f/2.8 11.5ft / 3.5m 78.6ft / 23.9m 67.1ft / 20.5m
 50mm f/8 10.1ft / 3.1m 1,989ft / 606.3m 1978.9ft / 603.2m
 50mm f/4 16.1ft / 4.6m 26.4ft / 8.05m 10.3ft / 3.1m
 135mm f/8 18.8ft / 5.7m 21.4ft / 6.5m  2.6ft / 0.8m
 135mm f/4 19.4ft / 5.9m 20.7ft / 6.3m  1.3ft / 0.4m
 200mm f/8 19.4ft / 5.9m 20.7ft / 6.3m 1.3ft / 0.4m
 200mm f/4 19.7ft / 6.0m 20.4ft / 6.2m  0.7ft / 0.2m
 300mm f/8 19.7ft / 6.00m 20.4ft / 6.22m  0.7ft / 0.21m 
 300mm f/2.8 19.9ft / 6.07m 20.1ft / 6.13m 0.2ft / 0.06m

You can find at the following link a practical illustration for Understanding Depth of Field, Aperture & Shutter Speed Relationships.

Depth of Field and Hyperfocal Distance

In simple terms, the hyperfocal distance is the intrinsic characteristic distance of a lens where DOF is maximized when the lens is focused at it; then the near focus limit will be half the hyperfocal distance and the far focus limit will be infinity. For more on the subject see:DOF & Hyperfocal Distance, samples, tables, calculator and formulas.

(6 Votes )
Show pages (3 Pages)

Originally written on May 30, 2010

Last updated on December 31, 2020

J. Ramon Palacios J. Ramon Palacios (jrp)

JRP is one of the co-founders, has in-depth knowledge in various areas. Awarded for his contributions for the Resources

San Pedro Garza García, Mexico
Admin, 46125 posts


Adam Carnol (adamsc) on September 23, 2019

This is awesome I really appreciate your post. It helps me out about my particular lens. This is my thought that lenses are something that aren't worth skimping on.

Bo Stahlbrandt (bgs) on December 21, 2017

One of the two c-founders, expert in several areas Awarded for his valuable Nikon product reviews at the Resources

@Bill - post any of your questions in (1) New to photography forum (or if you are not new to photography) post your questions in (2) Manual Focus forum. Make sure to release the all CAPS when asking :-)

Bill Schneiner (wineguru1) on December 19, 2017


J. Ramon Palacios (jrp) on November 18, 2015

JRP is one of the co-founders, has in-depth knowledge in various areas. Awarded for his contributions for the Resources

Camera lens F Stops use an apparently odd set of numbers. The problem is that the amount of light entering the lens is governed by the area of the aperture; a squared function, not a linear one. If the F Stops increased in a 2 fold linear form (1,2,4,8...) then the exposure would not increase two fold but as a square of this, which is 4 fold. Opening the aperture by one F Stop would therefore quadruple the exposure as 2² = 4. So a set of numbers is needed, that when squared comes to 2, to give the double exposure required. In other words, numbers need to increase by √2, as √2² = 2, to give twice the area.

User on October 15, 2015

Hi, you wrote:"true f-stop values are multiples of the square root of 2" I don't think so.... It is SQRT(2)^n; n from Z: ..., 0.5 , 0.707 , 1.0 , 1.414 , 2 , 2.828 , 4 ,...... (...,0.5,0.7,1,1.4,2,2.8,4,... sounds familiar) (whole number) Multiples of the square root of 2 would be: ..., -1.414 , 0 , 1.414 , 2.828 , 4.242 , .... (...,-1.4,0,1.4,2.8,4.2,... sounds strange - and it is!)