User talk:Jerry-va

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[edit] User_talk for jerry-va

Hi, please leave new comments at bottom.

I am concerned that Wikipedia's increasing erudition sometimes brings obscurity and impenetrability. I would welcome "Getting Started" sections that kick off more entries with an overview. I would like to see more sentences at the end of paragraphs that begin with words like Summary: or In conclusion,.

Is the world of hobbies neglected?

Wikipedia is understandably concerned more with "What is this thing?" than with "How to perform this process", accomplish this task, acquire and practice this skill, get something done or make it work. Where does this leave hobbies? If you know of some Wikipedia "hobby" and "how to" pages that might be a good example for us all, please list them so we can all have a look. How about these: "How to sharpen a handsaw" "How to build a Universal Power Supply" "How to tell if a transistor is blown" "How and why to prune fruit trees" "Tips for growing tomatoes" "Practical composting"

Thanks!!   —Jerry-va 20:19, 26 February 2006 (UTC)

I believe this was a policy decision. See Wikipedia:What Wikipedia is not, where it says:
Instruction manuals - while Wikipedia has descriptions of people, places, and things, Wikipedia articles should not include instruction - advice (legal, medical, or otherwise), suggestions, or contain "how-to"s. This includes tutorials, walk-throughs, instruction manuals, video game guides, and recipes. Wikibooks is a Wikipedia sister-project which is better suited for such things. Note that this does not apply to the Wikipedia: namespace, where "how-to"s relevant to editing Wikipedia itself are appropriate, such as Wikipedia:How to draw a diagram with Dia.
Srleffler 17:34, 27 February 2006 (UTC)

[edit] Welcome

Welcome!

Hello, Jerry-va, and welcome to Wikipedia! Thank you for your contributions. I hope you like the place and decide to stay. Here are a few good links for newcomers:

I hope you enjoy editing here and being a Wikipedian! Please sign your name on talk pages using four tildes (~~~~); this will automatically produce your name and the date. If you need help, check out Wikipedia:Where to ask a question, ask me on my talk page, or place {{helpme}} on your talk page and someone will show up shortly to answer your questions. Again, welcome! --Srleffler 21:38, 26 February 2006 (UTC)

[edit] Lens aperture

Can you provide some enlightenment about both effective lens aperture (entrance pupil) and effective focal length? You might mention "nodal point" "retro-focus" and other concepts from both modern lens design magic and basic optics. Here's the problem: it has become impossible for me to tell if a lens's stated specs are either plausible or consistent with one another. I can understand that focal length is not the distance from the rear element to the image plane in a 13-elements-in-7-groups lens. The optical "middle" of the lens (presumably a nodal point from a ray diagram point of view???) is, well, somewhere in middle. But the aperture is even more baffling. For example, most zoom lenses today do NOT become as slow as they "should" be at longer focal lengths. What is left at the practical level for an every-day check anyone can perform? I presume that, if one accepts that one camera lens has a 60mm FL than an unknown lens producing an image in which the same objects have half the linear size has half the focal length. Any other sanity checks? Thanks, the technology is really getting away from me. -- Jerry-va 18:53, 26 February 2006 (UTC)

I understand your confusion. I may be able to help some, but I'm not a photographer nor do I design camera lenses. (I do design lasers, so I know something about optics.) The focal length of any lens is the distance from the rear principal plane of the lens to the focal plane (where the film or CCD goes when you're focusing at infinity). For a simple thin lens, the rear principal plane will be near the center of the lens. For a complicated multi-element lens, the principal plane could be anywhere. In particular a telephoto lens is, technically, one where the optics are designed to put the principal plane somewhere out in front of the lens, which means that the lens can be shorter than its focal length. Note that it has become common to call all long-focus lenses "telephoto" but this is not technically correct. It is possible to make a non-telephoto long-focus lens. It would just be very long. A retro-focus lens is the reverse of a telephoto lens: the rear principal plane is closer to the film than the nearest element, allowing the rear element of the lens to be further from the film than would otherwise be possible. This allows the construction of very short focal length lenses that can be used with a standard SLR body.
If you're taking photos in air and there is air inside the camera, the rear nodal point is at the same location as the rear principal plane. Nodal points don't appear to be well understood in photography, though. There seem to be a number of misunderstandings about what happens to light at the nodal point. I don't think I can address these here.
Another thing to be cautious of: manufacturers of digital cameras don't always report the focal length accurately. For example, the digital camera I have here says "37mm-111mm (Equiv)". The "Equiv" is there because the focal length range of the zoom lens in this camera is not actually 37-111 mm. Since the CCD in a digital camera is much smaller than a 35 mm film frame, the focal length required to take a photo with a normal field of view is much smaller. Rather than report the actual focal length of the lens, though, they give the focal length of the lens that would produce the same field of view on 35 mm film, and mark it "equiv". Personally, I think this is terrible and leads to unnecessary misunderstandings. I guess photographers like it, though, because it helps them quickly decide whether a lens is normal, long-focus, or wide-angle.
I'm not sure exactly what is confusing you with apertures, but I can guess. Besides misreported focal lengths on digital cameras, many photographers believe that the f-number is the ratio of the focal length to the diameter of the physical diaphragm opening. This is not true. The f-number is the ratio of the focal length to the diameter of the entrance pupil, which is an image of the diaphragm, formed by the lens elements in front of it. If you can open the shutter and see the diaphragm, you should be able to get a rough idea of the entrance pupil size by looking into the front of the camera and observing how big it appears. It may appear much bigger or smaller than the actual opening, due to the magnification of the lens elements. It just depends on the design of the lens.
Hope this helps. Feel free to ask more questions.--Srleffler 21:35, 26 February 2006 (UTC)