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DPChallenge Forums >> Hardware and Software >> Basic Level Question about Lenses ???
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04/29/2008 03:04:16 PM · #1
Hi all,

I dont have a DSLR but recently used my friends EOS 400d for a day or two and then ended up doing a lot of research and study on the lenses. I have got all the basic concepts. What telephoto and wide lenses are. Zoom and Prime lens. Focal length, F-Number, Cropping factor, Aperture and all that. Most of these things i already knew but i came to know about them in more detail. And some things i knew wrong.

But after so much research two things are still not clear. Can someone please clear out my 2 doubts.

1. How is Manual Focus implemented in a lens. I know what manual focus is (that is what is does). But it seems to me that manual focus would be like changing the focal length. Yet if take a lens say 18-55mm to max focal length (55) and then focus the subject manually and then again go to wider smaller focal length by rotating the lens barrel, I dont loose focus at all. This has been confusing me. Also shouldn't the focus be achieved by changing the focal length. But that just affects the zoom. So when i rotate the lens for manual focus, what actually happens in side the lens. Please explain ???

2. Macro Lens: I use S3IS. And to take macro and super macro shots, I hold the camera VERY close to the subject. Lens is almost touching the subject.So after learning about lenses today I searched for different lenses. And I thought that a lens with small focal length would server good for macro. I mean as Telephoto is good for objects far away. And using S3Is I take macro shots on minimum focal length ... And so I thought that small focal length (wide) lenses should server good for macros. but on the contrary i found that it is the Telephoto lenses that are used and preferred. 200-300mm lens for extreme macro ... insect eyes and all. But isn't that a telephoto lens used for far away subjects. If I keep the subject close to such a lens, wont it be out of focus ??? Also is there a difference between Telephoto normal and Telephoto Macro lens. If yes, what is the difference. Can i use normal Telephoto for macro and can i use Macro Telephoto for long distance shots ????

Apart from these two points, almost all my doubts are clear.

Thanks in Advance.

BTW for those of you interested, this is the link:
//photonotes.org/articles/beginner-faq/lenses.html

04/29/2008 03:20:10 PM · #2
I don't have an answer for you, but those are some very good questions that display an understanding of the concepts involved. I'm interested in knowing the answers as well, if anyone else can explain it.
04/29/2008 03:29:30 PM · #3
I'm thinking ' . substr('//www.dpchallenge.com/images/user_icon/21.gif', strrpos('//www.dpchallenge.com/images/user_icon/21.gif', '/') + 1) . ' Bear_music will be an excellent source of info for this topic (and others), as well as others here. I'm mainly posting so I can remember to check back for the answers also.

Message edited by author 2008-04-29 15:29:39.
04/29/2008 03:36:33 PM · #4
I can't give you the scientific answers, but I can give a general explanation.

1. Manual focs. Lenses have 2 focus rings, one that moves the focal length and the other that does fine focusing. If its a prime lens, the focus ring is only doing the fine focus because there is no change in focal length hence prime.

The fine focus is not changing the focal length, just the focus w/in the set focal length. For example, on a 70-200mm lens, you may want to use 200mm to shoot a bird in a tree or a mountain in the distance. At 200mm you will need to adjust the fine focus between the 2 subjects.

2. Macro. Macro lenses are made special/have special features to shoot macro. They will let you get closer to a subject than their respective non macro lens. For instance, a regular 70-200 won't let you shoot closer than say 6ft. A 70-200 macro will let you get within 1 ft.

Most of the science behind it is in the positioning of the front and rear elements inside the lens and their relationship to each other.

Thats the 101 version...
04/29/2008 03:39:56 PM · #5
I am probably wrong, but maybe close, I remember a chapter about refraction in highschool. But that was 25 years ago. I think the lens is devided into a few sections. one section is for focus one section for zoom, which only enlarges the image like a magnifying glass. Not sure how it all functions, Waiting for Bear's arrival.
04/29/2008 03:42:07 PM · #6
I'll answer the macro question. Most macros are 1:1 meaning at the closest focus distance the object shows up on a 35mm negative the same size as it is in real life. Therefore a 60mm macro and a 180mm macro magnify the object on the sensor the same (there's no difference in how much you can zoom). However, there is a large difference in what is known as the "working distance" (the distance from your subject to your sensor). The higher the focal length, the longer that distance. I use a 180mm macro and I have roughly 19" between my lens end and my subject. This helps to keep bugs and such from spooking and flying away before you can shoot them.

If you are looking at hugely magnified images you are probably looking at someone who has reversed a lens. What they do is typically take a 50mm lens and turn it around and attach it backwards to the end of another lens. With this method you can get magnification that is greater than 1:1. Because I am unaware of any true macro lenses with a focal length beyond about 180mm, you might be seeing this when people list a 200 or 300mm lens.
04/29/2008 03:48:42 PM · #7
Hey doc, are you getting my PMs? just say yes or no, because I sent two, and never received an answer yet. I am used to be ignored, so if it's not the PM system that I should blame, than I know what's going on ;)
04/30/2008 12:38:37 AM · #8
Thanks a lot everyone, I guess my second question is somewhat clear now. But the first question I am still confused about. Now i got the concept of macro lenses. More the Focal length, greater the working distance. And in macros it is the ratio (magnification factor) that matters more than the focal length.

But please someone explain the Manual focus in terms of what happens inside the camera. That thing is really troubling me. As I really want to understand the concept as well as possible. Full knowledge never hurt anyone.

Thanks
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