Should I Buy A Telephoto Lens Or Use A Teleconverter?

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Oliver (Auckland, New Zealand) asked: I like doing nature and wildlife photography and considering my options whether to buy telephoto lens or teleconverter. Please suggest what should I do?

Hello Oliver! If the budget is not an issue, getting telephoto lens for your need is the best choice by all means. If you are serious into sports photography or nature and specially wildlife photography, telephoto lens will make its way into your camera bag sooner or later. And if you can't afford good telephoto lens yet but have mid-range zoom lens, 70-200mm for example, your option is to get teleconverter and extend its range.

Why do we need a Telephoto Lens?

Telephoto lens is a specific type of long-focus lens and an essential tool to have if you are into wildlife photography and considering to make it your profession or serious hobby. You cannot always get closer to your subject and telephoto lens is the only way to capture them. But good telephoto lens comes with big price and might be bulky for some of us to carry around all day long. As of today, the most expensive and long range Nikon super-telephoto lens (AF-S NIKKOR 800mm f/5.6E FL ED VR) costs around $16,299.95 and weighs around 10.1 lbs (4.5 Kg) and mid-range telephoto lens (AF-S NIKKOR 200-400mm f/4G ED VR II) costs around $6,999.95 and weighs around 7.4 lbs (3.3 Kg). Not everyone needs and can afford these beasts but that doesn't mean we should give up.

What should we do then?

Where there is a will, there is a way, and, this is where teleconverter comes into our discussion. Teleconverter is a cheap way to extend your lens range from its normal range. As name suggests itself, teleconverter multiplies your lens range by its x factor. Let's say you have a 70-200mm lens with f/2.8 max aperture and you bought 2.0x teleconverter. Once you fit this teleconverter to your lens, it will extend range from 70-200mm to 140-400mm and reduces maximum aperture to f/5.6, by half, allowing only half amount of lights and hence slowing down the speed which might be crucial for wildlife photography specially for capturing birds and fast moving animals. If you were thinking little earlier why telephoto lenses were that much expensive when you can achieve the same range with cheaper teleconverter, you may now have the answer. Yes, it not only extends the zoom range by x factor but also decreases the aperture by same factor. So, if your goal is to get greater range with maximum aperture possible (to produce nice bokeh effect in the background and faster shutter speed) then you may have no choice but buy expensive telephoto lens. Sometimes you may be able to get nice blurry background even with smaller aperture if your focusing distance is greater because depth of field is affected by your distance to subject as well. Another disadvantage of using teleconverter is distortion effect. Your image might be little distorted because teleconverter adds different sets of glasses behind your lens adding an external components to the lens whereas telephoto lens is made up of same quality glasses incorporated inside single barrel to produce better result. Distortion might be little less noticeable or even unnoticeable if you use teleconverter from same manufacturer as your lens.

To summarize our discussion into points,

1. Telephoto lens can be heavy and expensive but it is the best option if the budget is not an issue and you do not want to compromise with quality of pictures.

2. Teleconverter is the best alternative and cheaper option to get extended range from your normal zoom lens.

3. Teleconverter extends the range of your lens but also decreases the max aperture by same factor and slow down your lens by allowing only half of the lights than the lens without the converter.

4. And that is why getting telephoto lens or using teleconverter depends on what you want to achieve and how much money you are willing to spend for it.

Understanding An MTF Chart For Lenses

If you own DSLR or planning to buy one in near future, you will be looking for lenses sooner or later and when time comes, you have to be very careful while choosing lenses. There are limited types of camera bodies (which are upgraded pretty frequently) but there are wide varieties of lenses and stay longer in the market before it gets upgraded to newer version. Usually camera bodies get upgraded in couple years whereas lenses get upgraded in 5 to 6 years or even more in some cases. Having said that, investment on lenses is long term than bodies and hence you have to be more careful when choosing correct lenses. Choosing lens depends on what kind of work you do in photography. If you are doing sports photography, you may want to choose fast telephoto lenses; for macro photography, you need macro lens; for landscape photography, you may want to choose wide angle lenses and for portrait, you need medium range prime lenses. Now you may be wondering which particular lens to choose among so many varieties and how do we know which lens is good over the other. It is very important to know that you have to compare lenses within its own category. That means you have to compare wide angle lens from one brand to wide angle lens of another brand because every lens is made with different components and materials to serve different purpose. OK then, do we have a tool to compare lenses? And the short answer is yes, we do. There is something called an MTF chart which gives us graphical model of the lens performance and describes behavior of particular lens in different conditions. You may find this article bit technical but I will try to explain as simple as I can.

What is an MTF Chart?

MTF is an abbreviation for Modulation Transfer Function and is widely used standard measurement to evaluate the performance of the lens. In a generic language, Modulation means the process of modifying signal to result an output from given input. In ideal case, lens should transmit all the lights it receives as an input but no lens is ideal and can't transmit all the lights because glass is not 100% transparent. So with the help of an MTF chart, we see graphical representation of how much light one lens can modulate (transmit) to the sensor in different scenarios.

The quality of lens depends on its resolution and contrast which are closely related to each other. If the lens can transmit high contrast, it can have better resolution power. But lens resolution is also dependent to other factors and that is why an MTF charts are preferred method to study optical performance but not the only one measure to determine its quality. An MTF chart consists two axis; X-axis and Y-axis.

The X-axis (horizontal axis) shows the distance from the center of the image (based on film or 35mm equivalent censor) toward the edges. In the graph, "0" represents the center of the lens and the different numbers represent the distance from center towards the edge of the lens in mm (millimeter).

The Y-axis (vertical axis) represents what percentage of light the particular lens can transmit. It is represented in percentage with maximum value of 1 meaning 100% transmittance of light (which is ideal case only). So in real case scenario, higher the Y-axis value better the lens performance will be (lens having 0.9 Y-axis value is better than lens having 0.7).

How to read an MTF Chart?

Below is an example of an MTF chart.  It has two pairs of lines (two solid and two dotted) plotted in different number values. Those different lines and numbers indicate how well the lens performs with different measurements.  When measuring the lenses performance for an MTF chart, the test is carried out with the lens working at its maximum aperture value.  An MTF chart consists of measurement for the Sagittal (Solid line) and Meridional (Dotted line) lines at both 10 lines per millimeter and 30 lines per millimeter hence producing a chart with 4 separate lines. In this case, you can think of 10 lines or 30 lines sketched inside 1 mm size area and we are going to see how well any lens can transmit the contrast between each lines so that quality of image can be produced in the sensor; better the contrast, better picture quality will be.

MTF Chart for AF-S NIKKOR 14-24mm f/2.8G ED lens

Wide (14mm)                                                                         Tele (24mm)

MTF Chart Wide Nikkor 14-24mm

MTF Chart Tele Nikkor 14-24mm

MTF chart for AF-S DX NIKKOR 35mm f/1.8G lens

MTF Chart NIKKOR 35mm

Here are the few key things you have to know to understand an MTF chart. The red line (10 lines per millimeter) indicates the lenses ability to reproduce low spatial frequency or low resolution.  This line indicates the lenses contrast values and the higher and straighter this line is the better.  The higher the line appears the greater the amount of contrast the lens can reproduce. Similarly, the blue line (30 lines per millimeter) indicates the lenses ability to reproduce higher spatial frequency or higher resolution. This line relates to the resolving power of the lens and again the higher the line the better.

As the line starts on the left of the chart this represents the center of the lens and as the line moves to the right it indicates the edge of the lens.  So you can see how much the contrast and sharpness of the lens decreases from the center to the edge of the image.

Using an MTF chart to determine the Bokeh effect of the lens

Another factor that can be read from the MTF graph is the 'bokeh'. Bokeh is a term used to describe the quality of the out of focus areas a lens produces. The bokeh effect varies between lenses and the effect is influenced by the quality of the lens elements used and also the number of aperture blades in the lens design (more blades produce a better circle and therefore a better 'bokeh' effect). The closer the solid line and the dotted line are together, the better the 'out of focus' effect will be on a particular lens.

Nowadays all the major lens manufacturers include MTF Chart along with their lens specification.

Reference: Nikon Europe and Nikon USA

Understanding Depth Of Field Preview

Depth of Field (DoF) is the distance between the nearest and farthest objects in a scene that appear acceptably sharp in the image. It is widely used term in all types of photography and specially popular in Portrait and Landscape photography. We use small DoF (small aperture value), also called shallow depth of filed, in portrait photography to emphasize the subject and throw an unwanted background away from viewer's attention by blurring it; whereas, large DoF (larger aperture value), also called deep depth of filed, is widely popular in Landscape photography where everything from the foreground to the background should be in focus.

What is Depth of Field Preview and When do we need it?

Let's say your lens's widest aperture is f/2.8 and you want to shoot with f/11. In this particular case, when you close down your aperture value to f/11 from f/2.8, your camera will not close down aperture blades, also called diaphragm, immediately after you change an aperture value. This is because all camera bodies focus at the widest aperture of the lens and then stop down to the chosen aperture (f/11 in this case) when you press the shutter release button all the way down to take the picture (after focusing your subject obviously).

Note: I noticed that if you are using Nikon D90 and the D lens which has an aperture ring on it's barrel, you have to set the ring to f/22, lock it and set the actuator (at the end of the lens) which allows opening the aperture from the camera body.

DOF Preview Button on Nikon D90 and D80

Today's most advanced DSLRs come with a dedicated button (DoF preview button) to preview the Depth of Field and allows you to close down your aperture to the selected value by stopping down the lens's diaphragm. So, when you set an aperture to f/11, pressing DoF preview button closes down the aperture to f/11 and darkens your viewfinder by allowing the less light through an aperture. It is a really helpful tool when you are doing Landscape photography, and most importantly if you are using Hyperfocal distance formula. In often cases, you will get deceived by the widest open aperture and will be difficult to judge what is in focus and what is not through the viewfinder.

DOF Preview Button on Canon EOS 40D

Canon EOS 40D has a Depth of Field Preview button right below the lens release button. Every camera manufacturer has a different body design and their choice of placement for this button also differs by camera models (even on the camera from the same manufacturer).