Photography #102: Primer on Lenses


“Look and think before opening the shutter. The heart and mind are the true lens of the camera.”

Yousuf Karsh


I have put a few section as quotes. These identify sections for people with advanced interest in the matter and can be skipped by others.


The “Glass”

In photography, a lens is optical device that converges a beam of light, producing an image of an object on the opposite side of the lens, projected onto the film or sensor plane. I know, I know, this is supposed to be a beginner’s class, but bear with me. The following may be boring math, but it will make you understand how lenses work and thus helps you control you camera.

The basic optical formula for a lens is:

1/S1 + 1/S2 = 1/f

where S1 is the distance from the lens to the object, S2 is the distance from the lens to the image, and f is the focal length. The reality of camera lenses is more complex, this is a generalization.

It is often said that 50mm is the “normal lens” for a 35mm film camera, making objects appears as we see them through out eyes, so let’s work with that as an example (below the 40-50mm range we have “wide angles”, beyond that “telephotos”).

When the object is at infinity, 1/S1 is 0, so S2 is equal for the focal length of the lens. The lens must this be 5 cm long, or rather the distance to the film or sensor must be 5 cm (keep in mind that the thickness of camera body counts as well). The closer the object gets, the longer the lens has to be to focus the image on the film/sensor, e.g. if the distance is 1 m (1,000 mm), S2 needs to be about 5.2 cm, and at a distance of 25 cm it would have to be ~ 6.25 cm… you see where this is going. That is what most lenses do not focus much closer than a foot or so.

Advanced: It also means that if I can extend the lens further, I can focus much closer. This is when bellows come into play. However it is apparent that once S1 approaches 50mm, S2 becomes larger, eventually infinite, then negative, which means that it now creates a “reverse” image on the same side as the real object, which obviously is not very helpful for the photographer. Another interesting aspect of this is the magnification factor, which becomes important in Macro-photography. When the image is at a distance twice the focal length, the factor 1, meaning the image is the same size as the project. This is the limit of most macro lenses.

In reality, images projected by a single lens suffer from various aberrations, and photographic lenses are designed so these are corrected to the largest extend. They are actually quite amazing, considering that many now have internal focus (they don’t change length), corrects for distortions, color shots etc etc. Many modern zooms have > 10 elements, with exotic glass and coatings, including the use of radioactive materials (up to the 1970’s apparently).

Advanced: Among others, spherical and chromatic aberrations are the most noticeable. Spherical distortion usually is “pincushion”, where lines converge to the center of the image – imagine it being “pinched” in the middle , or “barrel distortion”, which looks like someone bent the image by stretching it in the middle. Often these distortions are not very noticeable, unless you photograph a lot of brick walls.

Chromatic aberration (“CA”) can be distracting if it is severe. Lenses do not focus light of the all wavelengths quite the same. Imaging an image being made up of a several layers of color. In longitudinal CA (LoCA) the purple layer (the one with the shortest wavelength) is slightly defocused leading to the dreaded “purple fringes” around high contrast areas. This is hard to correct but usually “stopping down” the aperture (see below) improves it. In lateral or transverse CA, just picture the images being of slightly different size and shape. This is easier but not easy to correct, as it depends on focal length and lens type etc etc . Many digital cameras do so automatically.

The “Hole”

A second feature of the lens you have to know is aperture. The aperture tells you how much light gets through the lens, more specifically it is related to the size of the hole the light passes. It is actually a calculated number:

N = f/D

where N is the aperture, f is the focal length and D is the diameter of the entrance pupil (for practical purposes, the diameter of the glass part of the lens). As may not be quite apparent, the amount of light changes with the area of the “hole”, and thus is proportional to D². Thus when the diameter is cut in half, the amount of light is ¼. From this we can see that if we want to find next aperture that lets half the light through (which is the convention when we talk about “stops” in photography), we need to change the diameter by √2 (which is 1.41421356…. let’s not split hairs though). This is the reason why the “stops” on the aperture scale are such odd numbers. Here they are (in bold), with common “half” and “third stops” stops commonly found on lenses included:

1, 1.1, 1.2, 1.4, 1.7, 2, 2.5, 2.8, 3.5, 4, 4.8, 5.6, 6.3, 8, 11, 16, 22, 32….. 45, 64, 90, 128

There is very few lenses have a 0.95 aperture (the current Leica Noctilux at $10,500 probably being the only decent one), which I believe is a great thing, but is just 1/6th stop under f1 and mainly gives bragging rights.

Advanced: According to the above math, lenses like the 50 mm f0.95 have to have a diameter of at least 5.2 cm, and other “fast primes” are similar, e.g. a 85 mm f1.4 needs 6.1 cm, the superb 200 mm f2 must be 10 cm. But there is no 400mm f2, which would be ~ 20 cm across. The 400mm f2.8 lenses are huge. There is a 200-500 f2.8 zoom produced by Sigma, costing $26,000 and weighing 35 lbs, its outer diameter is almost 24 cm. No other 500mm lens with such a wide aperture exists to my knowledge.

As you probably would have guessed, wide aperture, so called “fast” lenses command premium prices and are generally considered “pro gear.” On the other end of the spectrum, you have “slow lenses,” mostly zooms, which often have a variable aperture, meaning at 18mm focal length they may be f3.5, but at 200mm they go to f5.6 or even f6.3.

But why would you want such a wide aperture?

There are two main reasons. For one, more light means shorter exposure times. It may make the difference between a sharp picture and a blurred one. But almost more important is the creative side. When you have a large aperture, the depth of field (DOF), meaning the range of distance where objects appear to be in focus, is very shallow. As an example, using a 85 mm lens to take a portrait at 5 ft distance, at an aperture of f1.4, the DOF is less than 2 inches, that means if you focus on the nose, the eyes may be out of focus (as half of the DOF at that distance is in front of the point of focus). Hey that never happens to me ever! Conversely, if you use f8 and focus on the eyes, you’ll likely render nose and ears sharp. This fairly simple. However if you stop down too far, say beyond f11, the picture becomes fuzzy throughout due to diffraction (if you don’t recall you high school physics, click here).

Advanced: DOF can confuse you easily. It is a somewhat flawed concept. Generally it described the area that is “acceptable sharpness” at a “normal viewing distance.” Say for a 8×10 print that would be a foot or so. If you look at pixels on your monitor, things will be different.

For any given image size, the smaller the sensor, the larger the DOF. As an example , using Nikon DX (“crop sensor”) and FX (“full frame”) cameras, a 35mm and a 50mm lens on DX and FX, respectively, give about the same angle of view. At f2.8 and 10 ft distance, the DX camera has a DOF of 2.8 ft, where as the FX camera has 2.1 ft. Note that the FX camera has DOF of 4.36 ft with a 35mm lens, whereas the DX lens has a DOF of 1.4 ft with the 50mm at the same distance and aperture, but the magnifications/angle of views are different so that comparison is irrelevant.

As a rule of thumb:
– the smaller the focal length, the larger the DOF
– the closer the object, the smaller the DOF.
– the larger the sensor, the smaller the DOF for the same angle of view.

A caveat, many lenses may have wide apertures but the quality wide open is less than stellar. Take for example the 50mm Noktor, a re-branded CCTV lens, and Leica Noctilux. The Noctilux costs 10 x more, as for sharpness, well…. visit Steve Huff’s blog for a comparison, he has shot both (I only wish I had the ‘Lux).

Advanced: One other, and major. reason to use a lens wide open if “bokeh,” the rendering the the out of focus areas in the background. This is partly a function of the aperture, which just determines how much “blurring” you get, but also has much to do with lens design. There is much discussion and argument about this. Descriptions vary, i.e. “creamy”, “harsh” etc – and it is also a matter of taste.

Autofocus (AF) or Manual focus (MF)?

If you are a beginner, I would not bother with MF lenses. Why? Because you can manually focus AF lenses if you need to and because you’ll likely use AF in most situations. Get familiar with the equipment first. I am a fan of lenses that still have aperture rings, but they are going out of fashion, too.

As you get better at it, consider taking a look at some older MF lenses. They can be very good (provided they are in good shape) and a great bargain. Sometimes an equivalent AF version does not exist – for example I have a 50mm f1.2 for my Nikons, Tilt/Shift lenses are another genre that is just MF, or they may even be better than the consumer grade AF version.

These are the basics you need to know about lenses. That was painful :) But I hope it will help you choose a lens for you DSLR or EVIL.


Bottom line:

If you buy a DSLR or EVIL camera, you should think about lenses. The photograph is still made by the photographer, but not only will a good lens not limit your skill, it can also enhance you creativity. There are images you CAN get with an aperture of f1.4 that you just won’t get with f3.5. On the other hand, you may not need or want to get these kind of shots, in which case you can spend you money elsewhere.

Keep in mind: the camera body basically is a fancy (or not so fancy box) that houses the film or the sensor. It needs to be light tight and the shutter needs to be accurate. They knew ow to make this 100 years ago. Lenses have evolved tremendously, and today’s lenses are very very good. They will stay very very good 20 years from know. So the general wisdom is you should invest in good lens(es) over fancy camera bodies. Case in point: I have a 1930’s uncoated Leica Summar lens – it is basically a coke bottle bottom. It dies not matter if I put it onto my $300 Leica IIIc from the 1940’s or my Leica M9 – the image quality is not going to be great.

In terms of DSLsR, I know Nikon very well, owning many of them, but cannot really comment on Canon’s or others. Regarding the lens that comes with the camera, such as the 18-55mm with the D3100 for example: they are often quite good, but if you are demanding, you’ll soon see their limitations. I personally like Nikon because you can use almost all lenses ever made for Nikon SLR (with some limitations on the consumer bodies).

Here are a type of lenses I would recommend for a beginner. This is in no way complete. The various manufactures have a very similar line up in the same price range.

First the primes (single focal length lenses):

Wide angles: if you like a wide field of view, you should consider one. The start at 14mm and end at around 35mm (there are 14, 15, 16, 20, 21, 24, 28, 30, 35, not every manufacturer has every focal length). A good and relatively cheap staring point would be a 24mm AF-D for Nikons. A 35mm “equivalent” (i.e. 24mm on a crop sensor) is a classic photojournalist lens.

50mm f1.2/1.4/1.8: Good walk around lenses, sharp, good for low light, the 1.8’s are relatively cheap. Get a f1.4 version if you have deeper pockets. If you use DX/Crop sensor, Nikon has a nice 35mm f1.8, which would be a 50mm “equivalent.” Nicely blurs the background.

85 f1.2/1.4/1.8: usually quite expensive but if you want to get one prime portrait lens, these are the ones to get

Macro: There are various focal length available. A true Macro lens gives you 1:1 magnification (the projected image is as big as the real object), and focal length usually only is important how close you have to be to get there. With a 40mm, you’ll need to be close; with a 200mm, not so much.


18-200mm (Nikon, Sigma, Canon) 18-250 (Sigma) 18-270 (Tamron) Zoom: if you want to have one all-rounder, one you need never take of the camera, those are the ones.

70-300mm: This is a great Tele range. Nikon makes two, the VR version is the one to get. On a crop sensor, will fulfill 99% of you telephoto needs unless you shoot sports and wildlife professionally.

16-85 (Nikon) 17-50 (Tamron) 17-70 (Sigma), 17-85 (Canon) Zooms: good allrounders if you already have a longer zoom. The Tamron probably is the winner here. For full frame chose a 24-85 or 24-120mm lens.

10-20 (Sigma) 10-22 (Canon) 10-24 (Nikon) 11-16 (Tamron) 12-24 (Nikon, Tokina, Sigma): only for crop format, get such wide angle zooms if you like landscapes.

The pro-line up (the honor roll):

If you want to take it one step further, most camera makers have a line of pro lenses. They cost a fortune but are usually superb. Here is an example of a Nikon “dream photo bag.”

Primes: 24mm f1.4, 35mm f1.4, 50mm f1.4, 85mm f1.4, 200mm f2.0, 300mm f2.8

Zooms: 14-24mm f2.8, 24-70mm f2.8, 70-200mm f2.8, 200-400mm f4

That’s it folks~ Hope you enjoyed it.

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next up (coming soon): #103, What is an Exposure and How Do I Shoot It?

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