PhotoLife

Lens Hoods

2006-12-30T18:12:00.000-08:00

The purpose of a lens hood is to prevent flare, which can seriously degrade the image quality of photographic lenses. A longer hood offers better flare protection than a short hood, but when the hood exceeds a critical length vignetting sets in. Therefore, a lens hood needs to be carefully chosen. The optimum hood depends on the philosophy behind the hood, with the shape and size as the key parameters, and on the lens aperture and subject matter. Several lens hood considerations will pass in review in this article. I will not discuss issues such as choice of material - I leave it to the imagination of the reader that an effectively blackened hood is more useful than one whose interior shines as a mirror. Rather, I concentrate on size and shapes. Although I am the first to admit that the contents are partly of an academic nature, the material could be vital for those readers who strive after the best possible image quality.

Size of the hood

A wide, long hood is a better choice than a narrow, short hood. Figure 1 exemplifies this statement for the Sonnar 135/2.8. The imageforming light collected by the lens is indicated with the gray beam that strikes the front element at the lens field angle. Vignetting sets in when a lens hood penetrates into this beam. Both indicated lens hoods, the short, built-in hood and the larger Contax metal hood #5, just clear the front element, i.e. they do not obstruct the gray beam and hence do not cause vignetting. However, the small hood allows the slanting black ray to hit the front element whereas the large hood blocks this ray, preventing it from contributing to lens flare (indicated by the reflections in red).

Figure 1. Two lens hoods on the Sonnar 135/2.8. The larger hood is the better choice as it blocks more nonimageforming light.

Influence of hood length on vignetting

Vignetting by a lens hood is called mechanical vignetting. It is usually associated with black image corners and an abrupt transition from bright to black. However, a lens hood can also give rise to a gradual corner darkening in a similar fashion as natural and optical vignetting. The influence of a lens hood on vignetting will be illustrated with a series of sketches with an increasing hood length.

Figure 2 shows the Planar 50/1.4 as is, without lens hood. The green part denotes the rim surrounding the front element. The red bars mark the lens entrance pupil, which is the image of the aperture stop (the black bars) seen by an observer looking into the lens from the front. Finally, the purple bars correspond to the image of the rim around the rear element. Each of these elements is a circle and the clear aperture is given by their common area. The circle plot at the right of the lens is the situation relevant for the image corner. A corner object point at infinity sees the circles in these mutual positions and is confronted with a clear aperture marked by the orange area, viz., the common area of all circles. There are also more complicated lens designs which require the inclusion of internal rims to determine the clear aperture, but the principle of the common area remains the same. For the image center all circles are concentric and the smallest one defines the entrance pupil, which is round.

Figure 2. The Planar 50/1.4 without lens hood at full aperture.

At full aperture the lens accepts a broad light beam. For the image center this beam is colored yellow. For the image corner the accepted beam is narrower. The darker orange beam is the beam that would be accepted by the entrance pupil if the lens barrel weren't present and if the lens elements were larger. The brighter orange beam is the part of the darker orange beam that is not obstructed by the lens barrel. It is delimited by the common area of the green and red circles in the equivalent circle plot. Further clipping of the oblique beam occurs by the rear rim and what remains is a clear aperture that is substantially smaller than the aperture for the image center (optical vignetting).

Figure 3. The Planar 50/1.4 without hood at f/11.

When the lens is closed down to f/11, the entrance pupil becomes small. Figure 3 shows that optical vignetting is no longer a concern: the oblique beam accepted by the entrance pupil is narrow and no longer clipped by the lens barrel.

Note that figures 2-6 do not show refraction of the yellow and orange beams. In this regard it should be realized that the lens elements in the sketches merely serve as a guide to the eye. Refraction is indirectly taken into account by the position and size of the entrance pupil (Zeiss data) and the image of the rear rim (calculated). The beams are correctly drawn up to the point where they hit the front element, and should further only be considered relative to the colored circles. In passing, figure 3 does away with the myth that a lens employed at a small aperture uses only a small part of the front element. The intersection of the yellow pencil with the front element marks the section needed for the image center, the intersection of the orange pencil with the front element marks the section needed for the image corner. So although each image point uses only a small part of the front element at a small aperture, the image as a whole still relies on a large part of the front element.

The brochure lens hood for the Planar 50/1.4 is metal hood #4. It is a wide hood which just clears the front element: figure 4. Since the hood does not affect the clear aperture for obliquely incident light (cf. figure 2) it does not lead to vignetting. Flare protection is offered without the slightest compromise to the design whatsoever.

Figure 4. The Planar 50/1.4 equipped with Contax metal hood #4. Look here to examine the real thing.

If the length of the hood is increased by 15 mm, the situation of figure 5 is established. At full aperture the clear aperture for the oblique beam is reduced and the image corner receives less light than it would in the absence of the hood. Mechanical vignetting sets in. By contrast, the lens hood has no effect at f/11. At this aperture the light beams accepted by the lens are not hindered by the hood. Thus, figure 5 represents a situation where mechanical vignetting is cured by stopping down the lens. An alternative view is given in figure 7, which shows illumination curves for the scenarios sketched in figures 2-6. At f/1.4 mechanical vignetting manifests itself by a corner illumination that goes down from 30% to 20%. The decline is gradual however and may not even be noticed in real-life images. As a matter of fact, depending on the application a small amount of (additional) vignetting may even be tolerated in favor of a better flare prevention. The curve for f/11 is identical between the upper left and upper right plots in figure 7: the hood has no effect on the image illumination at small apertures.

Figure 5. The Planar 50/1.4 with a lens hood that is 15 mm longer than lens hood #4. Vignetting now sets in at f/1.4.

When the length of the hood in figure 4 is increased by 30 mm (which happens to correspond to metal hood #5), figure 6 results. At f/1.4 only a small area survives to illuminate the image corner. The corner is dark but not black as it does receive some light. However, at f/11 corner blackening is a fact. Here, the lens hood completely obscures the entrance pupil (the small circle in the circle plot) and no light is passed on to the image corner. Where a small aperture cured the vignetting in figure 5, it worsens the vignetting in figure 6. The lower left illumination charts in figure 7 corroborate the corner blackening at f/11. The sharp kink in the f/11 curve implies an abrupt brightness transition from the image towards the corners, which are completely black.

Figure 6. The Planar 50/1.4 with a lens hood that is 30 mm longer than lens hood #4. Vignetting is manifest at all apertures.

Still longer lens hoods lead to black corners at all apertures and the main effect of the f-stop is found in the abruptness of the transition. This is illustrated by the fourth graph in figure 7, which results from the addition of yet another 15 mm to the hood.

Figure 7. Calculated image illumination for the Planar 50/1.4 in combination with several lens hood lengths.

From figures 5 and 6 it appears that the optimum length of a lens hood depends on the aperture. When the chief ray (the ray that goes through the center of the aperture) is not obstructed by the hood, stopping down the lens cures mechanical vignetting. When the chief ray is clipped, a small aperture leads to black corners. Unfortunately it is quite cumbersome to put this knowledge in practice. A lens that is regularly used at various apertures requires hood adjustment each time another f-stop is chosen. Impractical, but it can be done with a continuously variable, compendium type lens hood. A nice description to figure out the optimum length, by inspection of the exit pupil rather than the entrance pupil, is available as a pdf file [1]. The author allows some 20% pupil area obscuration by the hood because he considers protection against flare more important than a small, gradual decrease in corner illumination which is not noticed in the image anyway.

A simple, practical approach to determine whether a certain hood (or filters, or a combination) causes vignetting on a certain lens consists of a series of test exposures. The subject should be an evenly illuminated object at a large distance. Vignetting is less of a problem at close range than it is at infinity, so when infinity poses no problems a nearby subject is also safe. A brick wall on an overcast day will do fine. Four exposures are required, two at the lens full aperture (with and without hood) and two at the smallest aperture (again with and without hood). Slide film is preferred because of its short dynamic range, which will reveal vignetting more convincingly than print film. If the slides taken with the extension(s) show no additional corner darkening in comparison with the slides taken without, you are completely safe. If there is a slightly increased, gradual corner darkening at full aperture, you probably won't notice the presence of the hood with other subjects than a brick wall or a blue sky and you are also safe. Black corners however are generally considered gruesome and the extension (or combination of extensions) is just not suited for the lens.

Hood shape

So far the discussion involved circular lens hoods. Indeed, a circular lens hood has the same rotational symmetry as the lens and aesthetically matches the round image formed by a photographic lens. However, the round image is not fully used as the presence of a field stop, a 36x24 mm mask in case of a 35-mm camera, crops the image to a rectangular section. This has important consequences for lens hood design and the optimum lens hood is not round. The accepted light cone that is used to illuminate the frame is pyramidal. At full aperture, going from the lens towards infinity, the cone starts out circular at the front element and converts to a rectangular cross section at some distance. At small apertures, and depending on the design, the cross section of the cone is already rectangular at the position of the front element. The pyramidal cone is illustrated in figure 8.



Figure 8. No hood.		Figure 9. A circular hood.

In the illustrations that follow a variety of hood shapes pass in review, designed not to introduce additional vignetting. The length of the round hood in figure 9 is such that it touches the light cone at four corner points. Voids in the plane of intersection evidence the shortcoming of a round hood: there are gaps where nonimageforming light may enter the system and introduce flare. One method to fill these holes is to extend the round hood to create the hood in figure 10. This so-called tulip hood is shaped by the intersection of a cylinder with a pyramid. Occasionally the designation butterfly hood is encountered.



Figure 10. Tulip-style hood.		Figure 11. A rectangular hood.

Another strategy resorts to a rectangular shape. Figure 11 exhibits a rectangular hood with the same cross sectional area as the hoods in figures 9 and 10. Both the tulip hood and the rectangular hood are more effective than the round hood. Not only because they are longer, but also because their shape is matched to the pyramidal cone and leaves no holes. A rectangular hood reduced to the same length as the round hood in figure 9 would still be more effective.



Figure 12. A chopped tulip hood.		Figure 13. Chopped and capped tulip hood.

Although the tulip hood in figure 10 is very effective with respect to flare prevention, it won't win a compactness popularity poll. To sacrifice some effectiveness for convenience, the two longer butterfly wings may be clipped to yield the chopped tulip hood in figure 12, which requires significantly less space in the camera bag. Finally, some of the effectiveness may be regained by filling the two gaps that arose in the clipping procedure. The 'chopped and capped' hood in figure 13 is relatively compact and still offers an excellent protection against flare. Zoom lenses are often provided with a chopped tulip hood that offers reasonable protection at the wide end, but which is inadequate at the tele end. Nonetheless, it is better than nothing.

Final remarks

Lens hoods are often undervalued and considered impractical because of the space they require in the camera bag. Too often I notice photographers with the best lenses money can buy, but who employ them without lens hood - or tripod for that matter. They will either say that a lens hood is impractical or that their lens is so good that it does not need a hood. As to the last reason, that one is plainly wrong. There are many occasions where a lens hood does not add to the image quality, but there are also many occasions where it does - even with the best lenses. IMHO, a proper lens hood should be among the standard equipment of the serious photographer. An adjustable bellows lens hood (compendium) is a very flexible solution for field work with a tripod, when prompt action is of no concern. One compendium hood serves a battery of lenses. In a ready-to-shoot shoulder bag outfit each lens is best equipped with an individual hood. Any hood, even if it is imperfect, is better than no hood at all.

This lens hood page has some overlap with the flare and vignetting pages, which are recommended reading to recognize the usefulness of a lens hood as well as the danger of overdoing it.

By PA van Walree

Studio Photography and Digital Backgrounds

2006-12-30T18:08:00.000-08:00

The article given here was written for Adobe Photoshop 5,6,7,CS, and Adobe Photoshop Elements 2 and 3.

Most photographers who are serious about improving their portrait taking skills have realized that a beautiful background can greatly enhance their photographs.

Muslins are a very popular choice among professionals. However, they can get very costly at several hundred dollars each. For the professional who is well established, this may not seem too much, but for the hobbyist, this may be a stretch.

A less costly alternative is a digital background. With a digital background, one can get the look and feel of a professional looking portrait without the additional cost of muslin.

Many Photography websites also sell digital backgrounds on cd. In some cases…as low as $15.00 dollars for an assorted collection. As with anything else…you usually get what you pay for. Some of these pre-made digital backgrounds are not worth the cd they’re printed on, but some of them are very beautifully made. Even so, you may be wondering how YOU can make your very own digital background. That being said, the following information will help guide you along the way.

Okay, let’s create a digital backdrop using Adobe Elements 2.0. We will create an 8x10 300dpi (dpi = dots per inch or ppi = pixels per inch) digital backdrop. For printing purposes, 300dpi is strongly recommended for a high quality print. This will also create a large file approximately 20.6megs.

Having a fast processor (Pentium 4 or Athlon XP) and at least 256megs of ram will definitely help. Also having your screen resolution on your monitor set to its highest setting 24 or 32bit will give the best results.

Okay let’s begin! If you haven’t already opened Photoshop Elements, do so now.

1. Click on File <>

2. Make a copy of the background layer you just created by pressing Control-J on your keyboard. If you look at the bottom right corner of your monitor, you should see an exact copy of your background layer named layer 1.

3. Select the “foreground” color for your background by clicking on the foreground square on your toolbar… the black one. (Bottom left)

4. The Color Picker window should now be open. Select a light red by clicking on the top right corner of the color palette. Click ok.

5. Now select the “background” color by clicking on the background square on your toolbar… the white one. (Bottom right)

6. The Color Picker window should open again. Select a darker red by clicking halfway below the lighter red you previously chose. Click ok.

7. Now comes the fun part! Let’s put some clouds in there. On the top of your screen, click on Filter

8. First, make sure that layer 1 is not locked! Look at your layers palette on the bottom right of your screen. Do you see the little black “lock” icon? Make sure that the little white square to the left is NOT checked!

9. Okay, now press “m” on your keyboard. At the top left corner of your backdrop, click and drag your mouse to create a 2x3 inch square. If you do not see the ruler along side your backdrop, press “control-r” on your keyboard.

10. Press “v” on your keyboard. Your selection should now have little handles on all sides that you can click on to pull to any size you want. If you do not see any handles, click on “Show Bounding Box” at the top.

11. Click on the lower right handle in the corner and drag and stretch it to the bottom right corner of your backdrop.

12. Double click anywhere on the red backdrop to apply the resizing. Now press “L” on your keyboard then “control-d”.

Okay, now you should have a beautiful looking backdrop ready to use, but wait, there is still more we can do to take it to another level. You will be simply amazed at the results you will be getting in the next few steps. You may have noticed that many types of muslins on the market have painted on them what’s known, as a “hot spot”.

What is a hot spot? A hot spot is a circular area near the center of the backdrop that is lighter in color. Why is this there? It allows the subject to stand out and creates a vignette at the borders. This will greatly enhance your backdrop. Are you ready? Okay, let’s do it!

13.On the top, click on Filter-Render-Lighting Effects. Select “Omni” for your “Light Type” and move the slider for your intensity to 25. Now click on the middle handle in the circle and raise the circle just above the halfway point to where the person’s head will appear.Click ok.

Now you should have an amazing looking backdrop!

By Fabian Barajas

Photography Jobs: Do You Have a Future in Photography?

2006-12-30T18:02:00.000-08:00

There is a wide world of photography. It touches each of us in our lives on a daily basis in some form or another. Photography is so much a part of our culture now that we hardly even notice all the places that it exists. When you watch television, look at a magazine or even view a billboard on the highway, this is all because of photography. There are so many ways that photography crosses our lives each day. There are a lot of opportunities for someone looking for photography jobs.

What Photography Jobs are Available?

If you enjoy photography and are thinking of it as a career, there are actually many different directions you can choose from. Obviously, there is professional photography but even in that choice there are many other smaller options that you have as well.

You can become a photographer in a special field such as wedding or family photographer. You can choose to make money taking pictures of things you love such as animals, nature or ships. If you enjoy scuba diving, you can become an underwater photographer. Pretty much anything you can think of, there is room to take and sell pictures of it.

If you love taking pictures and the idea of a career in photography sounds good to you, just how do you begin finding photography jobs?

How Do You Find Photography Jobs?

There are different ways of finding photography jobs, depending on the type of job you are looking for and your experience. You can begin by creating a resume and portfolio of your work. Then you can search on the internet at freelance photography job boards or photography websites and message boards. Get specific into types of photographing you have done and look on the internet for those.

You can go locally and look for internships or local firms or companies that may need photographers. You can get the word out by doing small events such as birthday parties and such. Get your name out as someone who will work these functions. Show friends and family samples of your work and ask them to spread the word. You may even want to participate in a charitable or non-profit event to boost your portfolio.

You can also begin submitting your work to contests and magazines to get yourself known and build your portfolio. Placing or winning in contests and getting published in magazines can help you build clips that you can use for getting bigger and better jobs.

Lastly, you can actually apply to jobs directly. It is best to do this only if you have the proper qualifications for the job. You don’t want to set your goals so high that you get let down but there is nothing wrong with going for what you want. Most importantly, get as involved with what you love as possible. Learn as much as you can about photography and what makes good photography. Subscribe to magazines, read books, look at winners of photography contests.

By Colin Hartness

Angle of View in Photography - The Intimate Landscape

2006-12-26T01:43:00.000-08:00

When you are outdoors with your camera and looking for new subjects to photograph, try considering the intimate landscape. This is how some landscape photographers call landscape pictures where the horizon is not present. This is unfamiliar as a landscape photograph, because usually we include the horizon, be it the skyline of a mountain, a city or the sea. It does not have to be necessarily like that, however.

Actually, it is much easier to be impressed by a great classical panorama than it is by an intimate landscape. A classical panorama is easy to be described. It is where we zoom out as much as possible with our camera, so to fit in the frame a large amount of objects. Mountains reflecting in lakes, distant sand dunes, a bell-tower on a hill are all examples of classical landscape photography. It is easy to realize when we are in presence of a beautiful landscape like that. I think the main reason for this is probably due to our angle of sight. Our eyes have more or less a 180 degrees angle of sight. So, it is natural for us to look at a grand view, encompassing all we see around.

However, if you look more carefully, you will find that landscape photography and nature in particular have much more to offer. You should train yourself to limit your angle of view, restricting it at will. Imagine your eyes have the ability to zoom in as an ordinary camera can do. Limit your attention to details in landscape, let your brain crop the image that you see with your eyes. If you act like that, a completely new world will start opening up to you. The intimate landscape has no horizon. It conveys the attention to something unusual, or something too usual to be considered worth photographing. Fine details, enchanting textures, curious juxtapositions, trinkets offered by Mother Nature... They are everywhere, all around us. But it is hard to notice them, at first. You must practice. You must see something first, if you want to photograph it! Maybe, the best advice I can give you to start training yourself with is: look downward. Look near you and downward, don't be distracted by what is happening above. If you are a nature photographer, remember that Nature lies everywhere, up and down.

Intimate landscape differs from macro photography, too. Macro photography concentrates on just one detail, like one flower or an insect. Conversely, the intimate landscape is something between macro photography and grand panoramas.

If you start noticing intimate landscapes, you will get more opportunities than ever to take original and very personal pictures. Grand panoramas are few, while intimate landscapes are countless. They are limited only by your ability to see and discover them. I can give you some examples to start with: pebbles on the ground, fallen pinecones, a small pond, bushes, the bottom of a waterfall or a hill, tree trunks in a forest.

So now grab your camera and start shooting!

By Andrea Ghilardelli