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Old 2008-06-06, 15:12   Link #61
Join Date: Mar 2008
Location: In the Mix
Age: 35

Lens buyer's guide for digital photographers

First words

With continuing decreases in prices of digital SLRs more and more people drift into buying one. However the kit lens usually delivered with more affordable kits becomes obsolete quite quickly so here are a few things you might consider when planning on purchasing additional lenses. The guide deals with some digital specific issues and problems. Most of the information is fairly basic and provides some reference when buying you first lenses for your new DSLR. Some of it may be a bit more specific and quite often irrelevant but can be useful for a bit more advanced photographer.

As this guide mostly deals with things that are digital photography specific for information about for example lens coatings and optical corrections you'd need to resort to other resources. Internet is actually full of useful information about these subjects and they quite often receive quite a bit of attention in lens reviews as well.

A few clarifications in terms

Crop-factor: Value indicating relative size of the image sensor in comparison to 35mm film or sensor. What needs to be remembered that it's a multiplier so a camera with a crop factor of 2 has an image sensor half in width in comparison to 36mm of a full frame film.

Bayonet: Lens mount.

Full frame: The film size notated in this guide as full frame refers to 135 or 35mm film. Full frame in film photography may refer to 120mm film even as this is considered misuse of the term, so be careful and pay attention to context when you find further information about the subject.

Prime lens: Lens with a fixed focal length.

Zoom lens: Lens with variable focal length. The term is often mistaken to telephoto much because of zoom factors used in many "point and shoot" cameras.

Normal lens: Lens producing an angle of view of roughly 50 degrees. This is considered "normal" because it resembles somewhat the angle of view of the human eye.

Wide-angle lens: Lens producing an angle of view wider than 60 degrees.

Telephoto lens: Lens with focal length longer than normal. Roughly lenses with an angle of view narrower than about 45 degrees.

Macro lens: Macro lens is a relatively loose term considering that focal lengths vary from 30mm to over 200mm. Generally lenses capable of drawing a subject to the film or sensor in focus close to it's original size are considered macro lenses. Magnification ratios usually vary from 1:4 image on the sensor or film being quarter of the size of the subject to "actual" magnification where the image produced on the film or sensor is larger than the subject. Stock lenses can provide magnification ratios roughly up to 5:1 but usually 1:1 is the highest usual magnification ratio. Sometimes lenses producing an angle of view narrower than 25 degrees and incapable to focus to infinite are also considered macro lenses.

A couple of buyer's "rules"

1. Know your budget.
Photography equipment is a bottomless pit as far as expenditure is concerned so in order to buy all the nice stuff available you'll need a bottomless wallet as well. Plan ahead. Consider what you need the most and buy that equipment first. Good basic setup for general use is discussed later in this guide.

2. Be smart.
This is very closely related to the first rule. It's also not very smart to buy excessively expensive equipment. Camera manufacturer's own good quality lenses are generally quite expensive but there usually are many third party options available that are almost just as good if not better and more affordable. There are also countless nice things a beginning photographer just itches to try out. However many of these things are usually not very practical. I'll use fisheye or other extremely wide angle lenses as an example. Extremely exaggerated depth impression or perspective and very deep field of view are nice playthings, however there are very few practical uses for them so usually these lenses end up gathering dust with majority of photographers. These lenses are also very expensive or really poor quality so usually it's money wasted regardless of the situation.

3. Map your needs.
Choosing a lens is always a compromise. There's hardly ever an optimal option but usually there's the best and good enough options. If your budget is limited as it usually is the first thin to consider is what I want to photograph. This has quite an effect on the best possible equipment. Good options for different kind of photography are discussed later.

4. Read reviews.
Find things out for your self. There's plenty of reviews, comparison tests and information available on the photography sites, forums and magazines. When you've made up your mind of buying a certain type of lens. Do some digging in the internet or magazines you'll surely find some pointers which brand and model provides the best quality for your budget.

5. Think ahead.
Even as most of the digital cameras don't have a full frame sensor, it's usually smart to buy lenses for full frame cameras even for crop-bodies if possible. Many of the camera manufacturers buy in the used sensors so there's always a possibility of manufacturer changing the sensor size in future models thus possibly making lenses designed for specific crop-factor impractical or, in worst case scenario, incompatible.

6. Know your camera.
Last but not least. Get to know your camera. If you can't be bothered to find out anything else get to know the crop-factor at least. It's quite important when choosing a lens. You'll also need to know the bayonet type that your camera uses and if there's some quirks in aperture or focus control that affect the choice of lenses.

Then for some theory

Understanding Crop-factor
As said before knowing the crop-factor of your camera body is utmost important. Why? Because of the varying sensor sizes of the digital cameras the full frame is generally used as reference in many aspects of lens features though mostly due to it's effect on focal length. A 50mm lens produces a 50 degree angle of view in a full frame camera, however when the same lens is installed to a camera with for example 1.5 crop-factor the full frame equivalent focal length is actually 75mm. The maths are quite simple just multiply the focal length with the crop factor. Thus the normal objective for camera body with a 1.5 crop is 33mm (50mm divided by 1.5). Thus crop factor determines which focal length ranges are considered wide-angle, normal and telephoto.

Perspective impression
Depth or perspective impression is only dependent on the distance from the subject so it's generally irrelevant. So a camera regardless of the crop factor produces the same perspective impression when used with lenses with an equivalent focal lengths in relation to the crop-factor. This is because of the distance of the subject stays the same when the angle of view is the same. Say you're using a full frame body with a 50mm lens and a 1.5 crop-factor body with a 33mm lens. Both have equal angle of view due to crop and thus to achieve the same the same arrangement in the scene your distance from the subject stays unchanged.

Effects of crop and equivalent focal length in depth of field
Effects of the crop go deeper though. As aperture value or F-number is dependent on focal length so the depth of field is actually affected by crop as well. This is because the depth of field is dependent on absolute aperture size not the relative F-number. Generally this is not very important but is to be noted if you want to take pictures with shallow depth of field. Olympus' four thirds system has one of the highest crop factors in the DSLR market. It has a crop-factor of 2. I'll use it as an example because using 2 as a crop-factor makes maths quite easy and as far as DSLRs are concerned it's the worst case scenario considering the following example.

Crop-factor 2 means that to achieve a similar depth of field and and angle of view to a picture taken with full frame camera with 50mm lens and F8 aperture you'd need to use 25mm lens and F4 aperture value. Due to change of the relative aperture size shutter speed or ISO value needs to be changed as well. A sensor with a crop factor of 2 is roughly a quarter in area compared to full frame sensor. This means that the actual light density on a full frame sensor is quarter of the one on a crop 2 sensor as the absolute amount of light is the same but it's divided on an area four times larger. So if you used 1/60 second exposure time for your shot with full frame body you can actually use 1/240 in crop 2 body if you use the same sensitivity value. The same goes with ISO values. You can use an ISO of an quarter of the full frame's value in crop 2 body with unchanged shutter speed. So if you used ISO value of 200 on a full frame you can use ISO value of 50 in a crop 2 body. Naturally this is a case of one or another so if you change both you'll get a picture with exposure of a quarter. Naturally you can meet in the middle and use shutter speed of 1/120 and ISO 100 to produce the same exposure.

Depth of field is determined by the absolute amount of light that is delivered to the sensor or film which again depends on the absolute aperture size. Sensor or film size is actually irrelevant to depth of field but very important determining sensitivity settings and shutter speeds.

However with a crop factor of 2 really shallow depth of field is somewhat an impossibility using relatively short focal lengths. Take 50mm F1.2 for an example with a full frame body. This would require a 25mm lens with aperture value of F0.6 which is to my understanding a physical impossibility. This naturally doesn't apply to macro photography which is really a world of it's own as far as field of view is concerned.

However, as said this is generally not that important but you may find it useful if you're studying photography tutorials made for full frame cameras or in specific cases like studio photography.

A quick recap on the maths using the previous scenario as an example.

Focal length required for full frame equivalent angle of view:
Focal length of the full frame camera in this case 50mm divided by crop factor: 50mm / 2 = 25mm

F-number producing similar field of view with a crop 2 body using a equivalent focal length:
F-value of the full frame camera divided by the crop factor: F8 / 2 = F4

Sensitivity (ISO) value with a crop 2 body:
ISO value of the full frame divided by the square of the crop factor: 200 / 2^2 = 50

Vignetting is a decrease in brightness or saturation at the periphery compared to the center of the image. This is an optical feature of the lenses and is universal to all photography so I'm not going to go very deep into the subject. However there's a digital specific form of vignetting. This is called pixel vignetting and it's caused by physical features of the image sensor.

Most of the image sensors are made out of grid of light sensitive pixels. The problem is that the sensor itself doesn't recognize the color of the light. Therefore a single pixel is formed out of sub-pixels that are sensitive to different colored light. This is done by installing filters to the image sensor's sub-pixels that filter out unwanted wavelengths. So in effect a single pixel in a picture is formed depending on the sensor out of one red, one blue and one or two green sub-pixels. The filters are installed in a mask on the sensor and to avoid light bleeding to the other sub-pixels or pixels these filters are confined with small ridges or walls between sub-pixels and sub-pixel groups.

This causes a problem with very short focal lengths. As the light hits the sensor in a very steep angle at the periphery these walls can cause shading in the image. There's not much that can be done to avoid this so if it occurs the only option is actually to correct it later in a photo retouching software. This on the other hand can cause more visible noise at the periphery of the image than the center so I'd advise to carefully asset how disturbing the effect is.

What to buy?

If you bought your camera as a kit you'll probably already have one quite versatile zoom-lens. Kit lenses typically have a focal length in the range of 14-80mm. Camera bodies with roughly 1.5 crop-factor are quite often delivered with a 17-55mm or some similar kit-lens. Cameras with higher crop-factors are often delivered with a shorter zoom and vice-versa. Quite generally the basic kit lens has a full frame equivalent focal length of roughly 30-90mm. So what's next. This depends on your individual needs.

Jack of all trades
For general photography with very versatile subjects I recommend buying a prime lens in a 50mm focal length range. With high crop factors you may want to go for a little shorter one but a 50mm is a good choice in a crop range of roughly 1.7-1. Canon uses a censor with a crop-factor of 1.6 apart from full frame options and Nikon cameras generally have a crop-factor of 1.5. This would make a 50mm lens 80mm in Canon and 75mm in Nikon when converted to full frame equivalent focal lengths.

Not more than 20 years ago there were many professional photographers that used solely 50mm prime with 35mm film cameras. So it's a lens with a lot of history and development to go with it. These lenses are usually really cheap, provide large apertures and have a razor sharp draw. A good example is Canon's EF 50mm F1.8 II lens which costs roughly $100. It's not particularly fast focusing but otherwise really superb.

50mm lens is often a great choice for portraits in a crop body. Even as a 50mm lens produces roughly the same angle of view as the human eye in full frame body the perspective impression is actually somewhat different. Many professional photographers use lenses in 75-100mm range with 35mm cameras for portraits. The same is range is achieved with practically all crop body DSLRs with a 50mm. Though many professionals were merciful enough to use soft-draw lenses or filters for portraits but in the digital age that's not a problem anymore. You can always play around with photoshop to clean up the pictures. 50mm lenses usually pick up all the little details and defects you don't necessarily want to show in a portrait.

Then what?
It really depends on your needs. Usually replacing the kit lens is a good idea. Find a relatively affordable third party lens with fixed relative aperture value throughout the focal length range. Here's where reviews come in handy. For an example Tokina 16-50mm F2.8 has received quite a bit of praise. Versions are available for Canon and Nikon bayonets. These are not exactly cheap but not hideously expensive either. Roughly $500.

Getting what's further closer
A longer telephoto lens may come in handy as well if you're for example a bird watcher. High quality ones are generally really expensive and cheaper ones can have quite a bit of problems. Maximum aperture size becomes increasingly important when focal lengths increase. As a bit of a rule you could say that the maximum exposure time you can use without a camera stand can be determined by using a fragment of a second with an equal numeric value as the full frame equivalent focal length. So 1/50s for 50mm, 1/100s for 100mm, 1/300s for 300mm etc. The cheaper telephoto lenses usually provide a maximum aperture size of F5.6 at the far end so with a 200mm telephoto lens in a 1.5 crop body the maximum overall exposure you can get is F5.6 and 1/300s. Needless to say you'll need a lot of light unless you want to use an insanely high sensitivity value which always increases the amount of noise.

There's help available though. There are many lenses available with optical image stabilizers. These can allow you to use up to roughly four times longer exposure times than ones without stabilizers. These lenses are naturally more expensive as they are more complex. What you need to note using long telephoto lenses is that depth of field and the perspective impression are both really shallow. You can help with the depth of filed by decreasing the aperture size but then you might run into a problem with the amount of light. Perspective impression is something that just can't be helped.

If you're not sure whether to buy a telephoto lens there are lenses that allow to try it quite cheaply. Tamron and some other manufacturers have a 70-300mm F4.0-5.6 telephoto lens with macro option. These generally provide 1:2 magnification ratio at 300mm and macro option is usable in focal length range of 180-300mm. Picture quality wise these are usually really far from greatness but you can kill two birds with a one stone and get yourself a long telephoto and a macro lens all at once. Price range for these low end macro telezooms is roughly 100-250 bucks.

High quality long telezooms with fixed relative aperture value can cost up to tens of thousands of dollars so you'd really need to need one to buy one. Price range of good enough ones is roughly 500-1000 bucks.

What about wide-angle?
This was somewhat discussed in the basic "rules". Generally they're useless. There are very few practical applications for them so for most they're just playthings. These playthings have a habit of costing $500 or more so once again think hard whether you really need one. These are useful if you need to photograph in really confined spaces but for the most photographers they're just expensive paper weights. Crop bodies give you trouble with wide-angle. There are some fisheye lenses available that have close to 180 degree angle of view in a full frame body but I'm yet to come across one that's capable of doing that in a crop body. As far as playthings go these are really nice ones but I strongly recommend you consider the return of your investment. Barrel and perspective distortions are massive. So the photos are quite often in dire need of much labor intensive retouching. Generally it's better to take a few steps back and use a longer lens.

Final words

Read reviews, examine example photos and map your needs. Buying lenses is actually not that scary when you're well prepared. Don't buy anything just because it may be fun because they're usually fun for a quite limited time. So plan ahead and most importantly have fun.

Last edited by escimo; 2008-07-10 at 16:19.
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