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Posted in response to Re: Re: Re: Re: Re: Re: Specimen ID Help from Michael T Miles (Mike) on June 05, 2007 at 11:36:22:

Re: Re: Re: Re: Re: Re: Re: Specimen ID Help

Hi Mike

There is a concept in photography in general that's called "field of view" (FOV)which - basically stated - is "what's in focus" in an image. If you picture a line of trees marching away from you into the distance and take a photo of it aiming the camera along that line then, theoretical, lens optics will give you an image in which there is an area of perfect focus from front-to-back. Beyond that point going away from you into the background and also coming toward you into the foreground the image will lose focus - be less sharply defined - until, ultimately, the immediate foreground and far background are completely out-of-focus.

We don't usually see this "optical abberation" because most "typical" camera lenses compensate for it very well. The smaller an "f-stop" a lens can achieve the better an image it can focus, until you get down to a pin-hole lens, which can focus to infinity. In order to view the phenommena just take shots with - say - f-3.5 and f-16. In the shot taken at f-3.5 you will have a blurred foreground and background. At the f-16 setting you will have a focussed foreground and background.

Basically the larger the f-stop of a lens [smaller the aperature it can be set for,] the better its FOV will be. Also, the lens will cost more: Compare the cost of an f-16 lens to an f-64 lens. It is simply more expensive to grind lenses with higher ultimate f-stop capability.

As I note, it is not a major issue in general photography. But it becomes a major issue when we try to photograph smaller and smaller things. We are limited by our lens aperature's minimum size. An f-16 lens is basically good for sharply focusing on an object an inch or two in size. Anything smaller and we get blur in at least part of the image we would really like to have sharply focused. And when we use a microscope to make the images we basically end up with a spot of dead focus in the exact center of the lens's view and the rest at least slighty out-of-focus.

[By the way: I do not know how this translates from lens and film photography to digital imaging, but I do know that it does. The digital photo folks face the same or similar pitfalls of FOV as us lens-based dinosaurs. :~} ]

Over time, we have developed some technological "fixes" to help us overcome the FOV issue without resorting to more and more costly pin-hole lenses. We have macro lenses, we use extension rings, bellows, focusing rails, and so-on. With microscopes we now have what are called wide-field lenses that help extend our total field of view quite a bit compared to older non wide field lenses. None of these is the "real" solution, but they do help. For example, using a macro lens can extend your "reach" towards smaller down to items maybe 1/4 inch in size. Add a reversing mount for the lens and now the lower limit might be 1/8 inch in size. A good, if bit costly, help is to have a scope with a built in variable aperature. Its still not that ultimate pin-hole lens, but it makes quite a difference. [There is also a noticeable price jump between non-aparture scopes and those which have them. And some very cheap knock-offs of good designs trying to get your money loose without giving you anything but hype. They make claims they don't fullfill. AVOID THEM LIKE THE PLAGUE!]

To be continued...



From Alan - June 06, 2007 at 09:15:30

Message: 64234

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