Real World Aperture

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• Angle of View and Houdini Focal Length
• Aperture Science Radio Gets the Real World Treatment - Technabob
• Aperture Science Radio Gets the Real World Treatment
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An aperture is not a part of the simplified lens model here, but many real lenses in have apertures. It is a variable-sized disc within the lens assembly that blocks light from the outer parts of the lens, effectively making the lens behave like a smaller one. For our purposes, we will assume that the aperture is fully open blocking nothing , so we can refer to the lens size and the aperture size interchangeably to mean the same thing. The focal length of a lens is a property of a lens itself unrelated to the scene , determined by the curvature of its surfaces and the refractive index of its material.

It is defined as the distance where parallel rays from an object infinitely far away converge to a single point. Lenses with longer focal lengths produce a larger image for the same object at the same distance. For objects that are sufficiently far away, the size of the image is approximately proportional to the focal length. For example, a mm lens will produce an image that is twice as large as compared to a 50 mm lens.

Angle of View and Houdini Focal Length

However, the law of conservation of energy comes into play here. The amount of light gathered by a lens is proportional to its area. If the outgoing projected image is enlarged, then the image must be dimmer at each point because the total energy of the image must remain the same. This situation is analogous to moving a lamp farther from a wall or moving a video projector farther from the screen, both of which make the lit surface dimmer.

Therefore, if two lenses have the same aperture diameter but different focal lengths, then the longer focal length lens will produce a dimmer image. This effect can be counterbalanced by making the aperture diameter bigger in proportion to the focal length, which brings us to the topic of relative aperture sizes. If we divide the aperture diameter by the focal length, we get a dimensionless number. The lowercase italic f is a variable that denotes the physical quantity called focal length.

The slash indicates division — e. This also explains why the aperture number seems to get bigger as we make the physical aperture size smaller — because the aperture size is the focal length divided by this number. In real-world products, Nikon gets the notation right whereas Canon gets it wrong. We have seen that if we hold the absolute aperture size constant while increasing the focal length, the image becomes dimmer.

In particular, due to the inverse square law for radiation in 3D space, the image brightness is inversely proportional to the square of the focal length. For example, doubling the focal length will make each point a quarter as bright. At the same time, doubling the absolute aperture size will quadruple the area of the lens. Putting these two facts together, if we double the absolute aperture size and double the focal length then there will be no change in the image brightness. Therefore, we conclude that the square of the relative aperture size is proportional to the image brightness.

This is why relative aperture sizes are so useful to the photographer. But it is absolute aperture sizes that explain the physics going on and justify the need to express in terms of relative apertures. As you zoom the lens in, the aperture opening as seen from the front appears to increase in size, just as predicted by the equation: Another consequence is that we can estimate the physical size of a lens package based on its focal length and aperture specifications. As for teleconverters, we can see why they make the image dimmer. A teleconverter fits behind the lens, magnifying the image and thus increasing the effective focal length.

As a result, the relative aperture becomes smaller. I found the star burst section very helpful. A while back I took some night photos of a local port. I thought that the starbursts from the lights of ships, trucks, streetlights and cranes made the picture look twee. I had used f Last night I retook some pictures at f8. I have had a dslr for just over a year and have learned most of my stuff from articles on this web page and on you tube as well as by trial and error.

My question is why ever go over f16 in landscape photography? Oh yes I had to leave a comment as a reply as for some reason there is no comment box at the top of the article. Wow thanks for this, clarifies so much I was wondering about! Nice to have this all in one article.. I wonder if they could come up one day with a reconstructing algoritm to deal with diffraction at small apertures to bring back or reconstruct actual pixel info from information spread over few pixels….

Even then, and even if consumer algorithms improve significantly, it will always be better to minimize diffraction whenever possible. I think you made a mistake when you said to divide the f numbers by 1.

It should be multiply the F number. Hi Sherman, thanks for watching my back! In this case, I believe the article is correct, unless I am misunderstanding your comment. You are correct, my brain was thinking backward. Anyhow, another great artical. Thanks for taking the time to do these, they are very helpful.

For example, my Sony RX10ii is capable of F2. Still an excellent camera, tho.

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Yes, quite true, glad you mentioned this! And yes, the chart in 11 does help. Nice way to summarize all this info. One of the biggest headscratchers for me is exactly what this article discusses. One buys a 1. I find that the 35mm 1. It differs greatly from lens to lens. Judit, you are doing a good job describing the headache of capturing sharp photos! Wide aperture lenses are quite helpful in many situations, but those apertures are never perfectly sharp, and the extra depth of field difficulties are another problem. This is a most useful summary. One factor influencing Depth of Field is pixel size [ie sensor specs] which influences the circles of confusion.

Thus, the camera model influences DOF. This was discussed recently with respect to timelapse photographing using the Nikon D also in some of the comments that followed under a most informative blog republished by NikonRumours:. The first thing I noticed when shooting with the D, was that I had to stay further away from my closest subject than I was used to with my D Making of Hercules Rising.

That is a tricky one. The extra detail of high-resolution sensors does let you notice errors more easily. At the same time, for a given print size, you will always see more detail with more pixels all else equal , and the out-of-focus background blur will be identical in size. Depending upon your interpretation of depth of field, there are arguments either way. Thanks for adding this! And now add a speedlite into the mix and you get some new problems to fix.

Excellent lesson with detailed information. Beginners should print your article and use it in the field. Spencer, Another interesting and informative article for those of us who the only way I know to describe it need them. Have you considered putting some of your articles together and publishing them in a book? I get out and take pictures, but not as often as I would like. So even if I have read your articles or am familiar with the basics a refresher that is easy to find is good to have to review once in a while.

When you publish a new article on Photographylife I always seem to end up clicking former articles and reading them again. Allen, we will be working on a few eBook releases soon. For landscape photography in particular, we already have an eBook published — we are currently working on releasing our Level 3 Landscape Photography video. Right now, we are working on creating almost exactly what you mentioned — essentially, a front-to-back set of articles on landscape photography, functioning as a book, which also will have a downloadable eBook version.

Migration of dust into the camera can occur during lens changes.

Out of precaution, I always place the plastic cover on the camera when removing a lens. I do this as quickly as possible and screw the cover down. I do not remove the cover until I have the white dots lined up between the camera body and the new lens. Remove the cover and insert the lens almost simultaneously.

Then clean the sensor. I do this every time no matter where I am indoors or outdoors. The process seems to work — I have had few dust problems. But then I have only been doing serious photography with my D for 18 months. I enjoyed reading this article very much. Photography is at its best when it is combined with understanding the workings of it and practicing it. I am glad to be connected to this site. Enjoyed this article and really looking forward to getting out and practicing with this information.

Thanks for allowing me to participate! This is very helpful and written so that it is easy to understand or clearer than most posts on this subject. You write with such clarity and precision. That, and the added benefit of your wonderful photos, is a real treat! Very nice article, however, AS a physics major optical engineer to be precise I have to comment on your Sunstar chapter, specifically this line: It is actually the straight part of the blade that causes the diffraction and not where they meet or imperfections. Sharp edges cause diffraction of light perpendicular to the edge direction.

In our camera, the aperture blades acts like these edges and cause diffraction. If the blades are straight the contribution is larger for each specific angle whilst rounded blades spreads the diffracted light more. See link for photo.

Note in my illustration I have only drawn the diffraction in the outwards directions. Thanks for adding this, I learned something new! Are you totally certain about it? Very glad you added this, thank you.

I will change around the diagram in the article later today. The quirky relationship betwixt aperture blades and number of sunbeams is esp. Glad you liked it! If you take your focal length divided by f-number, the resulting value in millimeters is the physical diameter of your entrance pupil which is how wide across the aperture appears to be when you look through the front of the lens. The way you understood it is a clever learning aid, though, and not one I had thought of before — thanks for adding this. Perhaps in the near future you can do an article on how fill flash effects some of these issues.

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I tend to shoot groups of people and find eliminating shadows is important to me so that everyone is happy with the shot. I use my camera for personal not professional and find my friends and family are easy to satisfy as long as they are visible without any detraction. Tilting the flash with a defuser for me works best, I will admit that I am always shooting different setting to protect the shot, easy to do for staged shots, when I get into action and or movement shots all that goes away.

Then there is the introduction of ISO and speed. In other words, you can change the brightness of your fill flash by altering aperture, ISO, and flash power, but not by changing shutter speed, since the flash duration is too quick. However, you can change the relative brightness of the flash exposure and the regular exposure by altering shutter speed which brightens or darkens the regular exposure independently or the flash power which brightens or darkens the flash exposure independently.

To put that in more concrete terms, most of the time, you would set an aperture that gives you the proper depth of field, your base ISO, a good shutter speed to capture your regular exposure, and then lastly a fill flash for the flash exposure — and if that brightens things too much, try using a quicker shutter speed to cut down on your base exposure.

This tends to be easiest in manual mode with your automatic TTL flash, along with flash compensation, although aperture priority with manual ISO also can work well. Your use of the diffuser seems quite helpful. Thank you for the feedback, Arthur, much appreciated! Excellent article and thanks again!

I have marked this for re-reading and further study. You never know, if I live long enough, someday I might understand it all. Mervyn, I believe in you, and have no doubt that it will make more sense as you continue in photography! Very well written article — fantastic summary that is not only balanced in depth but also makes the knowledge so usable. Indicates the points from night time streetlights, etc.

Nicely presented set of photograph illustrated examples — very informative, thank you Spencer. Moving that through the wider aperture range, this means diffraction remains lower in the range f8 — f16 too. This must be one of the contributing factors for full frame being sharper, apart from the obvious. It means that for a given aperture, the diffraction effects are less. I never realised that before, that one thing alone is a valuable thing to keep in mind — I have thus far fought shy of using f19 — f32 on my full frame because I associated these apertures as potentially damaging to image quality with the diffraction they might bring.

You have the best and such a nice way to explain it. Finally something I can understand. Appreciated Also thank you the a gentleman a BestBuy that recommended me to your site. Simple, straight to the point, yet very powerful and knowledgeful. This is among one of the best articles here at PL, as it is a nice asset for beginners and veterans in photography, with examples accompanied by the deep enough technical explanations. A very helpful article and well illustrated and described, even for an old person like me, to understand and absorb.