Depth of Field and Prime vs. Zoom lenses
Depth of Field and Prime vs. Zoom lenses
Written by Wednesday, 14 December 2005 17:00
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One of the most common questions asked in photography (especially by beginners) is what is the relationship of the focal length of the lens and Depth of Field (DOF for short). Everybody wants his / her pictures to be in focus and look sharp and crisp therefore this is a very important issue. Of course, sharpness is not only due to DOF but also depends on the kind of lens you have used to take the picture since all lenses are not built the same. There must be a difference between a 200 € lens and another one (with the same focal length) which costs ten times as much. True, you also pay for the big name on your lens but, again, there is a reason why this name is regarded as "big". Using a good lens is of outmost importance since the camera's sensor or film will see through it, it will not use your eyes to take the picture. Another important issue (for those using cameras accepting interchangeable lenses) is whether it is better to go for zoom lenses or fixed focal length ones ("prime" lenses). For the DOF and its relation with focal length of the lens things are quite easy. For the second question, things are a bit more complicated. However, by understanding the basics - as outlined here - you will be able to come back with sharp pictures out of your camera which can be further edited to become really nice pictures.
Apart from the theoretical part (which we trust you have already read) I thought that showing pictures would tell you much more than writing one more endless text. I chose an electricity pylon and took some shots of it with both zoom and prime lenses, at different focal lengths. The results are shown below and - hopefully answer those issues for you. However, some (elementary) theory must come first. Thus, in the end, you will get both the pictures and the endless text.
Depth of field may be defined as the range between the closest and the furthest objects which appear sharp in a picture. Despite what you may think, a great depth of field is not always desired. Thus, while in landscape photography you have to go for the maximum depth of field, in portrait photography you may prefer to go the other way around. Even in macro photography, while we want to have the whole subject in focus, a blurred (out of focus) background will show our subject better. In general, when the key element in your picture only covers part of it and you want to draw the attention of the viewer to it, it is better to go for a reduced depth of field. The general rules about Depth of Field (fortunately there are some) are the following:
The shorter the focal length (the "wider" the lens) the greater the DOF is for a given aperture. It goes without saying that telephoto lenses (large focal lengths) have a much shallower DOF. They also "compress" the distance between objects. Thus, if you use a 28 mm, a 90 mm and a 300 mm lenses all set at f/16, the 28mm will give you the maximum DOF while the 300 mm the minimum with the 90 mm falling somewhere between them.
The smaller the aperture, the greater the DOF for a specific focal length. Thus as you move from 5.6 to 22 for example, the DOF increases. Again, bear in mind that alhough f/22 will give you the maximum depth of field, this aperture rarely is the sharpest for your lens. Therefore, you may have to make a compromise in order to get an acceptable depth of fiend coupled to enough sharpness. Usually - but not always - the sharpest apertures are f5.6 - f8. You can read a review for your lens to make sure you use the optimal aperture(s) whenever possible. Please note that the words "maximum" and "minimum" apertures refer to the actual diameter of the iris opening. The numbers used refer to the ratio of the iris opening to the lens focal length, thus f/22 means that this opening is 1/22 of the lens focal length, therefore smaller than f/11 which has a 2 times larger diameter compared to the f/22. Since the light passes through the opening's surface (πR2) this means that four times more light will reach the film or sensor, thus we need 1/4th of the time to get the same amount of light with f/11 as compared to f/22.
The closer to the camera you focus, the smaller the DOF becomes for a given focal length and aperture. Thus, when you focus on a subject which is just 20 cm from your camera (e.g. macro or close up photos) the DOF will be just some mm while the same subject, when shot from a distance of 1 meter may have a DOF of several cm. If your camera / lens combination has enough resolution, it is better to shoot from a distance and crop afterwards. It goes without saying that when shooting from a very close distance, you should make an effort to bring the subject in parallel to the film (or sensor) plane to reduce the "depth" of the subject. Keep in mind that the camera focuses on a "plane" not a point.
Another thing which may be of great importance to you is Hyperfocal distance. What is that? It is a distance which is calculated for any lens / aperture combination. The equation which gives you the Hyperfocal distance involves the focal length of the lens (in mm), the f/number you set on the lens and the circle of confusion (which is - simplified - the diameter of a circle which will simply look like a dot to our naked eye. Anything larger than that diameter will look as a circle and our eye will render it as an "out of focus dot"). The formula will give you one distance which is called "hyperfocal". Why should you care ? For a very simple reason. If you know this distance you can focus at half of it and everything from that point to infinity will be in focus ! Nice, isn't it? Thus, if you use a 50 mm lens and f/16, then the hyperfocal distance is 17 feet (5.1 meters). If you focus at 8.5 feet (2.55 meters) then anything beyond that point will be in focus. Of course, you don't need to make those calculations yourself. You can get some printouts of the hyperfocal charts or even a nice piece of software which will create the charts for you which is offered free at www.dofmaster.com. I have printed out the charts for the lenses I use most and laminated them so they can be used in the field even when raining. While being at this site, you may browse it and find almost everything you need to know about Hyperfocal distance.
An old rule of thumb says that depth of field extends from 1/3rd in front to 2/3 behind the focus point. Although this is true in some applications involving subjects at normal distances from the camera, it is not to be universally accepted. In fact this fluctuates a lot starting from almost 1/2 in front : 1/2 behind for macro photography to almost "all behind" when shooting landscapes using the hyperfocal point.
There are many more terms and techniques related to Depth of Field like "bokeh", "selective focusing", "far and near focus" etc. but you can find a lot more about them in the list of references given at the end of this article, so there is not reason to bother with them. After all, I guess that by now you know the basic elements that can increase or decrease the depth of field and give you the desired result.
It is quite hard to answer the question about the advantages and disadvantages of zoom lenses vs. prime ones. There are many parameters involved and - unlike the DOF issue which is a mathematical formula - it involves subjective elements. What do you want to shoot ? Are you involved in a particular kind of photography (e.g. landscapes or close up photography) or you just like to take pictures of everything around you ? Do you shoot still life or fast moving objects ? Do you want it for nature photography or sports ? Do your shoot in the controlled environment of a studio with tripods and controlled light or are you a photo journalist ? How much weight are you willing to carry in your camera bag? What is your budget ? Do you want to keep lens changes to a minimum to avoid dreadful dust on your sensor (or mirror) ? Those are just some of the questions asked and even if I had all the answers, still the verdict wouldn't be an easy one. For the sake of argument, I will assume that the dilemma you are faced with refers to lenses of "equal" quality. You must keep in mind that every lens you buy is a compromise in some way. The exotic Nikon VR 200 mm f/2 prime that is targeted to photojournalists who need to take sharp pictures in dark halls will be much faster than a 200 mm f/4 but it will weight three times more and cost five times as much. If a fast 80-400 f/2.8 zoom would extist for wildlife photography, it would cost a fortune and weigh a ton. A Tamron 200-400 f/5.6 will be very cheap and light but you will always need a tripod to take a photo with decent sharpness at the long end. Of course you can buy the Nikon 80-400 f/4.5-5.6 with the VR (vibration reduction) option. Good news for sharpness since you can take really sharp photo's while handholding the camera at low speeds which you would never imagine before, but it is expensive, still weights quite much and has a very slow auto focus. So, which is the right lens to buy ? I am afraid there is no answer to this question. Apart from your budget, you need to define yourself the parameters which are going to be taken into account, forgetting the rest. All I can do is to list some things that I have found out during the last 25 years of taking pictures and you are going to pick the points you want and make a decision.
Fixed focal length lenses are easier to be made and most companies do their best to ensure that you will get the best optical quality from them. Zoom lenses are more complicated to be made and are usually heavier and bulkier than their fixed focal length equivalent. Zoom lenses must perform two actions (zoom and focus) while prime lenses only one (focus). Moreover, prime lenses have fewer glass elements so the internal light reflections are kept to a minimum which means a better control of flare. Moreover, light scatter may reduce contrast, which, along with resolution are key elements in what we evaluate as "optical quality".
Prime lenses are also usually sharper than zoom lenses and can focus at a shorter distance from the camera. Prime lenses do not need to zoom but only focus so their design may be optimized by the manufacturer to a much greater degree. This means - among other things - that the physical length of prime lenses changes to a much lesser degree when running their whole focusing range (from the closest point to infinity) while zoom lenses can easily double their length when going from wide angle to telephoto. In addition, prime lenses with IF (internal focusing) do not change their length at all - which is practically impossible for zoom lenses. This may be regarded as a trivial point but when you are using a bellows and work at a distance of some cm from your subject, this lens extension is the last thing you need.
The front of most prime lenses doesn't rotate during focusing while the same is not true for most zoom lenses. This is a clear disadvantage when you use filters that have a varying effect depending on their position in relation to the film or sensor plane. This kind of filters include some really common and useful filters like circular polarizers (used to minimize reflections from non metal objects - like water - and darken the blue of the sky) as well as half - filters (like the ones which are half painted or offer a neutral density in their bottom half). If you use this kind of filters on a zoom lens, you will have to re-adjust the filter's position after you focus or whenever you change your focal length, not very practical.
Prime lenses are usually "brighter" than zoom lenses. Thus, while it is not rare to find prime lenses with a maximum aperture of f/1.4 or f/1.8 (which won't break your bank account) the maximum aperture for zoom lenses is rarely larger than 2.8 (and even this is the exception rather than the rule). When shooting with little available light this half - one f/stop can make a huge difference. It affects the minimum light level which will allow you to hand hold the camera and - equally important - the minimum light level which will allow you to shoot without a flash unit thus preserving the natural "feeling" of the scene. Moreover, most zoom lenses have a varying maximum f/stop which means that the lens becomes darker as you zoom in and - equally important - any manual settings you have done at a particular focal length need to be re-adjusted when you zoom in. Again, this is not important if the only setting you use on your camera is fully and unconditionally "Auto" but it is extremely important in situations when you need to have a complete control of the photo - for any reason.
Another thing I find particularly annoying with zoom lenses (especially the wide-angle zooms) is the vignette (black edges or - in the worst case scenario - a black circle at the edge of the photo) caused by the lens hood, which although perfect at the long end of the zoom range, becomes part of the photo in the short one. This is one more thing you have to take into account when you are in the field taking pictures and I prefer to focus on my subject instead of changing the hood or cropping later. Needless to say, prime lenses don't suffer from this.
Even when advertised as "specialized" (e.g. macro) zoom lenses are inferior to prime lenses designed for the same kind of work. Apart from the optical characteristics even the magnification is not the same. Thus, while prime lenses designed for "macro" photography will give you a true life image (1:1 magnification) most zoom lenses will not even give you 1/3 of that (at best). Some specialized lenses (e.g. fisheye lenses) always come as prime lenses.
Fixed focal length lenses of moderate optical quality are much cheaper than zoom lenses of the same overall quality. Price is not always a measure of quality and this should be clearly understood. I have found excellent optics at very moderate prices, an excellent example being the Nikkor 50 mm f/1.8D which is a truly remarkable lens at an affordable price.
However... (yes, there is always a "but" in this life).. zoom lenses have some clear advantages too.
Zoom lenses, especially the new generation of lenses which come with a 10X or even more zoom range may "cover" the focal length of many prime lenses and thus be a far more economic solution. To give you an example, the 28-300 mm zoom used in this article covers the following key focal lengths: 28mm, 35 mm, 50 mm, 90 mm, 135 mm, 200 mm and 300 mm. Those focal lengths cover the medium wide angle, normal, short telephoto, medium telephoto and telephoto range. Therefore, with just one lens, you can eliminate the need for at least 7 prime lenses and - as an additional bonus - you get all the focal lengths between them. In short, there are very few things out there that you can't frame and shoot at your ease. If you couple this lens with the other zoom lens used in this article, the 12-24 mm one, then you have everything from wide angle to telephoto with just two lenses. You may also add a good 2X teleconverter in your bag and there you go. The whole range from 12 - 600 mm is now available to you. This sounds like a real bargain - and to some it (justifiably) is.
In some cases, getting closer to your subject may be dangerous, difficult, impractical or even impossible. Your subject might be a big cat (I refer to lions and the alike), a bird high on a tree, or a boat leaving the port. The same kind of problems may appear when you need to get further away from it (there may be a rock or a cliff behind you). Therefore, in order to compose your photo the way you want, moving your camera back and forth may not be possible. In this situation you can either use your zoom lens and get that photo or simply get the photo with one of your primes and then work in your darkroom (or the image editing software).
Another consequence of the "all in one" modern zoom lenses is that you don't have to change lenses every once in a while to get a picture of that distant object and then a panoramic view and even later a portrait. The benefit is a reduced risk of getting dust particles in your camera. This may sound as not important, unless you already have this experience. Getting this dust particles out of the camera (and ALL your photos) is a real problem.
You decide to go for a photo session. All you have to do is pack your camera, your two zoom lenses, your teleconverter and your flash unit and off you go. Packing seven prime lenses is an entirely different issue. You can ride your bike or even walk for hours carrying your two zoom lenses while carrying seven prime lenses means that your car must be able to reach the photo session location - or at least very close to it. The difference in weight and size is very big and it will definitely make you feel tired before you even shoot your first picture. After a couple of this kind of photo sessions you will start to be more "selective" and leave half your primes at home.. until you come face to face with the best photo opportunity in your lifetime while the best lens to shoot it is resting in your home, some (or many) miles away. Usually I travel with my son so we take our two cameras with all our lenses (primes and zooms), flash units, filters etc. with us, but we carry two bags with us, each weighting more than 5 Kg. Just to give you an idea, a 600 mm prime lens will outweigh your 2 zoom lenses, your teleconverter, your bag and perhaps even the tripod!
The discussion about interchangeable lenses is perhaps of limited interest since it is addressed to those using SLR cameras (film or digital). I know many photographers, me included, who have taken some really nice pictures with compact digital cameras, especially the ones with extended zoom range and macro capabilities. However, once you use a digital SLR you will find out that the results can be much better and the control you will have over the final results makes the difference.
During a recent visit to Brazil I decided to travel the fast (and light) lane and I just got a prosumer (a compact digital) camera with me. The results speak for themselves. I lost at least 30 great photos because the camera wouldn't focus (where is my manual focus ?) or couldn't get close enough (where are my lovely macro lenses?) or the flash unit was too weak (where are my powerful Sunpaks?) or completely ruined the image with too much or too little light (where is my i-TTL SB800?). You want more? Most of the pictures (about 500 of them) were a bit soft (no shutter speed indication) or the depth of field was unacceptable (where is my DOF preview button?) or .... the list is endless. In short, a (photographic) disaster. Next time I will just pack my D70, my macro lens, the two zoom lenses and there I will be. I will travel the slow lane but I will come back with 20-30 breathtaking photos.
Thus, nobody can tell if zoom lenses are better or worse than prime ones. It just depends. Under controlled conditions at home, I will almost always use a prime lens. While travelling, I will rely on my zoom lenses.. perhaps with the addition of a macro lens. As I said at the beginning, it depends on many parameters and it seems that zoom and prime lenses are both essential "glass" for any photographer. The advantage of zoom lenses is that you can buy one and start shooting a great variety of photos right away till your budget allows you to get some prime lenses for the kind of work you really prefer to do. If your only lens is a prime one, you will be far more limited as to what you can frame and shoot.
The photos below will give you an idea of the different focal lengths and the depth of field which goes with them.
Notes: Large boldface numbers indicate actual focal length of lens on a digital camera. Red numbers indicate equivalent focal length for 35 mm film cameras. Lenses appearing in yellow are prime lenses (fixed focal length). Lenses in normal typeface are zoom lenses.
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