Which Telephoto Lens and What for? - Page 2: Choosing a Lens (cont.)

Which Telephoto Lens and What for? - Page 2: Choosing a Lens (cont.)

PDF | Print | E-mail

Article Index
Which Telephoto Lens and What for?
Page 2: Choosing a Lens (cont.)
Page 3: Using Teleconverters
Page 4: Notes and References
All Pages

7. Lens type and Lens - Sensor combination

Lenses are designed for use on film and / or digital cameras. Further, some lenses work best on DX formats while others work on both DX and FX formats. Make sure to chose a type which is designed for your camera and has the correct mount for it, too.

Though some tele lenses work on both DX and FX formats the results we get when using them with different sensors are different. So far there is nothing new. One would expect the better quality sensor to produce the best results. Well, it is not as easy as that.

The level of resolution and detail you get in an image is proportional to the number of pixels used to create it. It follows that the capacity of the sensor of the camera to record detail affects the quality of the photos taken with long lenses. The question is, is it preferable to attach our longest telephoto lens on a 24.4 Mpixel full frame camera or an APS-C sized 12.4 Mpixel one? It is generally believed that, unless the full frame camera has a lot more Mpixels, the APS-C sized one should be preferred. We have tried our telephoto lenses on a Nikon D3X and a Nikon D2X.

The D3X offers 24.4 Mpixels on a sensor measuring 24x36 mm, thus those millions of pixels are spread over a surface of 864 mm2 and the pixel density is 28.240 pixels / mm2.  On the other hand, the D2x has 12.4 Mipxels spread over a surface of 370 mm2, therefore the overall pixel density is 33.513 pixels / mm2.

It is clear that the D2x offers a greater pixel density (1,18 times more pixels for the same surface). Thus, other things equal (no diffraction, very low ISO settings) the D2x offers a more detailed image for the same image surface. It would take a 29 Mpixel full frame sensor to match this.

That is not the end of it. If you attach the same lens on both cameras, it is evident that the image produced by the D2x is larger than the image of the D3x. Since with DSLRs “what you see is what you get” it is evident that the same bird, sitting at the same distance, will be larger in the frame of an APS-C sensor, i.e. it will occupy a bigger surface on it. How much larger? One and a half times in each direction (horizontally and vertically) or a total of 2.25 time larger in terms of surface.

If we now calculate the greater pixel density for the D2x sensor, we have 2.65 times more diodes recording the image than what we get in the D3X. Therefore, when you view your image on your screen, you will have 2.65 times more pixels forming the subject than you would if you used the D3x instead (with the same lens attached on both cameras).


We tried to analyze the two pictures of the heron (above) taken at the same time using the D2x which had the Nikkor 80-400mm f/4.5-5.6 VR fully extended at 400 mm (which is effectively an 600 mm lens) and the D3x with the 300 mm f/2.8 lens mounted on it, coupled to an 1.7x TC, which is effectively an 510 mm lens. What we have to compare now is the differences between these two combinations. Since we refer to 600 mm for the D2x, the difference in sensor size have been accounted for, therefore the two cameras are now regarded as full frame cameras. Thus, the D3x has a 24.4 Mpixel full frame sensor and the D2x a 29 Mpixel full frame sensor.

What the two sensors see is schematically presented in the diagram. The blue rectangle stands for the 600 mm lens and the red rectangle stands for the 510 mm lens. It is evident that the D2x will record the subject better (assuming it fits in the blue rectangle) since it will record less empty space (gray area in the diagram)). How much less space?


To identify this we need to look at the ratio between those two areas at any distance from the camera. We have a horizontal axis, in which the 510 mm will cover x510mm and the 600 mm will cover x600mm. Consequently, in the y axis, the 510 mm will cover y510mm and the 600 mm will cover y600mm. When we multiply x510mm x y510mm we will get the area covered by the 510 mm lens and the same will be true for the 600mm lens when we multiply x600mm x y600mm.

In order to make some calculations using basic trigonometry we need to get some right triangles, which are easily found if we take half from each dimension for every lens. Starting with the 600mm lens, x600mm/2 is one side of the right triangle in the horizontal axis, d is the distance from the camera and b is half the angle of view for the 600 mm lens on the horizontal axis. The distance does not matter since it is the same for both cameras. The angle of view for the 600 mm lens is 3.44o horizontally and 2.29o vertically. We will use half that values to calculate the x600mm/2, based on the right triangle which is formed there. For the 600 mm lens the horizontal dimension of the recorded area is x600mm. x600mm/2 = d x tan(b) and finally  x600mm/2=0.0300286*d. Therefore the horizontal dimension of the recorded area is two times that or: x600mm = 0.0600573*d. Similarly, working for the vertical axis, the vertical dimension of the recorded area for the 600 mm lens is y600mm=0.03997336*d. Thus, the overall surface recorded by the 600 mm lens is 0.002400692*d2.

Moving on to the 510 mm lens, the angle of view for it is 4.0o horizontally and 2.7o vertically. The horizontal dimension of the recorded area is x510mm. x510mm/2 = d*tan(a) and  x510mm/2=0.034920769*d. Therefore the horizontal dimension of the recorded area is two times that or: x510mm=0.06984153*d. Similarly, working for the vertical axis, the vertical dimension of the recorded area for the 510 mm lens is y510mm=0.04713261*d. Thus, the overall surface recorded by the 510 mm lens is 0.0032918137*d2.

A direct comparison between the two surfaces shows that the 510 mm lens will have to “cover” 1.37 times more surface than the 600 mm lens for an image standing at the same distance from the lens (any distance from the camera, this ratio is constant). Therefore the sensor of the camera with the 510 mm lens can spare 1.37 times less diodes to record the same object, since it needs to cover more surface. In reality, this means that the sensor of the camera with the 510 mm lens can devote only 100 diodes to record a subject when the same sensor with the 600mm lens will devote 137 diodes for the same subject sitting at the same distance from the camera.

Time for some (easy) multiplications: With the 510 mm lens you will get 1 Mpixel devoted to your image when you will get 1.37 Mpixels for the same image using the 600 mm lens. Going further down the road, you will get 3 Mpixels devoted to your beloved subject with the 510 mm lens and 4.1 Mpixels with the 600 mm lens. The difference starts to show.

Just to make it more interesting: If you have a 10 Mpixel sensor, which you can fill completely with your subject (all 10 Mpixels of it) using your 600 mm lens, you will only “use” 7.3 Mpixels of that sensor for the same subject if you use a 510 mm lens.Thus, a mere 90mm difference at the long end of your telephoto lens allows you to save almost 30% of your pixels. Not a bad deal after all.

It all started as a joke, while discussing the difference in image quality between the D3x and the D2x shooting the same subject at the same time. The results were quite better with the D2x which didn’t seem to make much sense (even though a TC was attached on the D3x). When 100% crops of the two photos were compared side by side, it became evident that the D3x lacked detail which the D2x didn’t.

This analysis provided an explanation for this finding. Simply put, the D3x recorded the image using much less pixels than the D2x because of the two factors we analyzed in this section. It has a lower pixel density (1.18 times less) and a lens with a shorter focal length (resulting in 1.37 times less pixels). Overall, the difference is 1.6x in favour of the D2x. For every Mpixel the D3x used to record the image, the D2x used 1.6 Mpixels.

We concluded that it is better to use your long telephoto lens on a camera with a cropped sensor and a high pixel density. To put it simple, all 12 Mpixel DX Nikon cameras (D2x, D300, D90, D5000) outperform the Nikon flagship by a good margin.

8.  Lens weight, construction and design 

This appears to be easier than it is in reality. A good quality, weather protected, sturdy lens is bound to perform better in the field and last longer than a flimsy plastic counterpart. Yet such a lens is a burden to carry, particularly if a photographer enjoys walking around to find appropriate subjects. More often than not the size and weight of a lens determine its usability. Again, it all depends on the type of photography you enjoy, the length of your photo sessions and the conditions you are likely to find yourselves under while shooting. Remember that the important thing is to take good photos, not expensive or exotic gear. The right balance is different for each person depending, amongst others, on physical built, condition, shooting habits, preferred subjects etc. Make sure that your lens will give you good photos rather than shaky hands and a back ache from carrying it.

The design of the lens is equally important. Check that all the controls are easily accessible and that switches between different modes of operation can be made quickly and effortlessly (fiddling with the lens in the field can be very frustrating and the cause many a lost shot). It is not the lens that will take the photos, you will. The lens is just a tool you will have to use correctly to capture what you want. To achieve this you must be comfortable using it and familiar with it.  Ideally camera and lens combination should be an extension of your own body.

Top of the line telephoto lenses, both zoom and primes, are really heavy lenses. The lighter of them will easily surpass the 1.5 Kg mark (which becomes 2.5 Kg with the camera, straps etc.), while the really big teles start at 3.5 kg (see table below, in the Price section). These lenses, in order not destroy the camera mount, must be properly supported – and that adds to the price as well as the flexibility of the photographer.  In order to reduce the weight of the lens some manufacturers have used magnesium alloy for the construction of the body of the lens (the Canon EF 800mm f/5.6 L IS USM is one of these lenses). This results in a relatively lighter lens but a much higher price.


Flexibility is not the name of the game with exotic teles and the more exotic the lens the less flexible it becomes. They are difficult to carry around, they need a special technique to be used correctly, they must be mounted on a tripod or other kind of support; they are not the kind of lens you just drop in your bag and off you go. They are meant for those who need them and know exactly what to do with them.

9. Focusing characteristics

By focusing characteristics we refer collectively to the way in which a lens focuses. Some lenses can focus only manually, others have an autofocus function (usually indicated as AF on the lens). Other characteristics include IF (internal focusing), S = silent (usually indicated as AF – S). Manual focusing is required on certain occasions however a fast autofocus lens is preferable particularly when shooting without the aid of a tripod or other support as both hands are required to keep your camera and lens steady. We find that Internal focusing is better for shooting wildlife; the movement of the barrel of the lens is bound to frighten and drive away your subject, particularly at close distances. For the same reason silent lenses are way preferable to their noisier counterparts.

10. Vibration Reduction (VR)

Also referred to as Image Stabilization (IS) by Canon, vibration reduction  (Nikon) refers to a number of techniques used by camera and lens manufacturers to counteract camera shake while shooting and therefore allow photographers to get sharper images. (8)

VR comes in handy in a number of cases, particularly with long focal lengths (tele lenses):

  • Minimizes or counteracts the movement of the photographer’s body, including shaking hands (all the more so when carrying heavy tele lenses),
  • Allows you to stop down a variable amount of stops (usually 2-4) depending on the vibration reduction system of the lens (for Nikon lenses VR allows you to stop down 2 stops while the company claims that VRII allows you to stop down four stops); (9) this is particularly useful when you need depth of field in relatively low light conditions or
  • Allows you to use your lens in light conditions in which it otherwise would be unusable, i.e. use lower shutter speeds, which is particularly useful with long teles, (10)
  • Allows you to take better photos of moving subjects by panning.

11. Resolution

This refers to the amount and quality of information which a lens is capable of recording. To identify it lenses are formally tested; test results are available on the internet.

It is beyond the scope of this article to discuss or even present such tests; it is of little use to duplicate information already available. The point to remember is this: it is important to make sure that the resolution of the sensor of your camera is similar to this of your lens. If you get a lens which has less resolution than your camera you downgrade your camera (essentially not exploiting its full potential); the photos will only have as much information (quantity and quality – wise) as the lens can capture. If, on the other hand, the resolution of your lens far exceeds this of the sensor of your camera you are most probably throwing good money down the drain; the sensor of your camera will only record at its maximum capacity, ignoring the remaining information which the lens captures. You may be well advised to invest on higher quality lenses, however, if you plan to upgrade your camera.

It should be noted that differences of the type discussed here will, more often than not, not be visible in small resolution prints. However the differences become obvious when cropping or larger prints are required.

12.  Price

Cameras come and go but good lenses stay. In fact, the arsenal of good lenses is what binds a great many photographers to a specific brand name.

A good lens will offer you very high image quality, very high resolution, little or no vignetting, minimal color aberration and flare and no ghost images. A reasonable photography enthusiast will more likely want to be ready for all eventualities. In practice this means a long, fast tele with nano crystal coating, extra low dispersion glass and VRII.  And the price of it is? Get ready for it:

Lens Weight Price Lens Weight Price

Canon EF 200mm f/2L IS USM

300mm f/2.8 ED-IF AF-S VR NIKKOR

Canon EF 400mm f/5.6L USM

AF-S 400mm f/2.8G ED VR NIKKOR

Canon EF 600mm f/4 L IS USM Lens

Af-S II 400mm F2.8 If-Ed NIKKOR

Canon EF 800mm f/5.6 L IS USM Lens


Canon EF 1200mm f/5.6L USM


Sigma 300-800mm f/5.6 APO EX DG HSM Lens

Nikon 200-400 mm f/4G ED-IF AF-S VR Zoom-N

Sigma 200-500mm f/2.8 EX DG APO IF


Do you get the drift? These toys are not cheap. Neither are their peripherals, should you ever need to replace them (the hood for the Nikkor 300mm f/2.8 currently costs £260). As most of us have to scrape and save to invest on our hobbies serious thought is required to determine what exactly it is we need and what we can do without. While a lens with nano-crystal coating, extra low dispersion glass, 17 elements and VRII is a great piece of kit, if the price of it is out of reach you will never benefit from it. The VR system alone can add up to £800 on the price of a lens; maybe a lens without it (or generally, with lower specs) is enough for your needs – and well within your budget.

Start by being reasonable: you do not need a 600mm lens to shoot sparrows. It makes more sense to look for a friendlier bird which will come closer to you or improve your technique for approaching your subjects. Long lenses may give you that extra magnification but how do you plan to use it? How often do you go on photographic safaris or trips? Are you prepared to spend a photo session using one lens only or do you prefer the flexibility different lenses (and subjects) offer? How familiar are you with using teles?

Talk to people who have used or are using long teles to find out their limitations. This is easy enough to do even if you do not personally know such a person: register in a reputable photography forum. Consider what impact these limitations will have on you and your photography. While additional focal length comes in handy on certain occasions it is totally useless on others. For example, when you need to approach close to your subject anyway because of barriers in the way (reeds, trees, rocks etc) or when the weather conditions interfere with the shot (haze, heat rising from the earth, wind etc.  The longer the lens the more likely it is that its performance will be affected by these conditions.).

Read reviews; there are plenty available on the internet, some of them by seasoned professionals. If at all possible, try renting or borrowing the lens you intend to buy for the weekend; nothing can tell you more than the actual use of the lens in situ.  (We have checked the price of renting the Canon EF 1200mm f/5.6L USM for a week in the UK out of curiosity: it was around the £2800 mark. One could buy an excellent lens for this amount of cash.)

A word of warning: there are considerably cheaper lenses available for the same focal lengths by third party lens manufacturers. The only thing these lenses have in common with the lenses of leading lens manufacturers is, in fact, the focal length. The differences in terms of construction and the technology used are massive and that is reflected in the price differences – as well as the performance of the lens. When budgeting for a lens remember that you get what you pay for. (12)

Zoom or fixed focal length lens?

There are those who swear by either type of lens and this is true among the authors, too. Most of the pros and cons of each type have been discussed in full length elsewhere. In this article you can also see the image you get using lenses of different focal lengths so you get the impression of the magnification – all shots were taken from the same spot. (13)

To us, the main advantage of top of the line zoom lenses is that they replace a couple of primes thus reducing the weight of gear to be carried around and that they offer flexibility in the composition in situ. The primes offer a bit of extra image quality and are usually a bit faster.


Zoom lenses come in a great variety and are consequently classified as “all around”, “good” and “top of the line”. In general, all around zoom lenses have a focusing range of 6x or more (like the Nikkor 18-200 mm VR lens) and offer a mixed bag as far as the optical quality is concerned. Some focal lengths / aperture combinations work pretty well some may produce mediocre results. Usually they suffer from distortion which may be of a different type at various focal lengths. This is even more so for zoom lenses which cover the wide angle – telephoto range. Lately zoom lenses with a 15x range (Tamron 18-270 mm) have appeared in the market and, although they seem to sell well, the image quality they deliver is, as expected, average. In fact all around zoom lenses are a compromise between convenience, price and image quality. In some cases, when working under perfect conditions (lots of light and the lens used at its optimal aperture and focal length) you may come up with a really nice photo. However, this will be the exception rather than the rule and you can’t expect low budget gear to give you results comparable to lenses which cost 4 or 10 times more.

Good lenses usually offer a 4-5 x zoom range like the Nikkor 70-300 mm VR or the Nikkor 80-400 mm VR lenses. These are usually slower than the top of the line zoom lenses both in focusing as well as the maximum aperture they have (usually f/4.5-5.6). However, under the right light conditions, excellent results can be obtained, which is very rarely the case with the “all around” zoom lenses. Optically they are closer to the top of the line zoom lenses but suffer from their small maximum aperture which, apart from slower shutter speeds results in slower focusing and a lot of hunting. They are very hard to be used to track birds in flight – especially in a cloudy day. However, once they lock on, they can produce comparable results. 

However, this may not always be the case. In the following photo, a “good” zoom lens simply fails to nail the hawk sitting on the electricity pylon, despite the high contrast between the bird and the bright sky.


Finally, there are the top of the line zoom lenses which offer a 2-3x zoom range and are extremely fast. In this category fall the Nikkor 70-200mm  f/2.8 VR, the 80-200mm  f/2.8, the 200-400 mm f/4 and the two monsters made by Sigma namely the 300-800 mm f/5.6 and 200-500mm f/2.8 – the latter requires a minivan to carry it to the shooting location. It should be noted that, although we refer to Nikkor lenses, most SLR camera manufacturers (Canon, Pentax, Olympus, and a limited range by Sony) offer comparable lenses both in quality and money. Independent lens manufacturers offer some great lenses, too, which will save you some money. We have not tried either of the Sigma monsters yet, but we are sure they are very well built and offer excellent image quality.