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Forum: Pentax SLR Lens Discussion
08-30-2012, 02:25 PM
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The seller provided a very bad description. It is obviously this lens:
hxxp://www.ebay.com.au/itm/OLYMPUS-OM-FIT-28-200-F4-5-6-MC-ZOOM-LENS-2X-CONVERTOR-400MM-/200811102761?pt=UK_Lenses_Filters_Lenses&hash=item2ec1464229
(Change xx to tt).
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Forum: General Talk
05-23-2012, 09:42 AM
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I couldn't agree with you more, sir. The taboo you speak of is a result of "organized religion," and their obsession with anything that could possibly be considered sexual. Being of native American heritage, I look at the subject in a totally different light. Hypocrisy abounds in just about any religious cult, regardless of which country it exists.
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Forum: Troubleshooting and Beginner Help
03-20-2012, 11:43 AM
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just to be clear about RioRico's comment that in front of the lens macro converters (or close up lenses / diopters) do not eat light, a simple explanation of how close up lenses, tele converters and macro lenses work in general is needed.
Since the image size, is the ratio of subject to lens / lens to sensor, to get a bigger image you either need to reduce greatly the subject to lens distance or greatly increase the lens to sensor distance or a combination of both.
As the image gets larger, and is spread out over a larger area the image brightness goes down. That's life.
But depending on how you achieve magnification has a big impact on the image brightness, although it is not clear at the onset.
If you use extension tubes (to move the lens further away from the sensor plane, and allow closer focusing) the image naturally and intuitively gets dimmer because you have not changed the lens opening,
The same is true with a tele converter, because a tele converter increases the focal length, with out changing the aperture diameter, therefore the spreads the image out over a larger area (based upon its magnification ratio) and the light falls off in a similar manner to extension tubes for the same final image size.
Close up lenses, however are different. when you add a close up lens to the front of the lens, you are changing (reducing) the focal length of your lens, but you are not changing physically the diameter of the aperture or front element of the lens, so the effective F stop actually goes down,
For example your 50mmF2 with a +10 diopter close up lens becomes a 33mm F1.3 lens. extended on a 17mm extension tube.
Another way to consider this is because the focal length is reduced, to achieve the same magnification, you move closer to the subject and the lens takes in more of the total light reflected off the subject.
as a result close up lenses give brighter view finders, but regardless of the manufacture of a close up lens, they will regrade the image , especially off center) comparted to using extension tubes. FOr macro however, unless you are using the lens for true reproduction of flat objects, you may not care about off center image quality, and loss of sharpness at the edges might be creatively interesting. You can, after all crop off center as long as you consider the final cropping when taking the shot, if you want off center subjects sharp
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Forum: Pentax SLR Lens Discussion
02-22-2012, 08:39 PM
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Close one eye and tell us that again.
OK, it's not half. But we can't really equate our binocular visual system with a single lens. What happens with half-frame vs full-frame is, the sensor area is cut in half and the frame diagonal is cut by 1.5x. What happens with our visual system is... more complicated, because we move our eyeballs around to synthesize a view that's both wide and tele with deep DOF. A camera just doesn't see like a human eye does. The trick for becoming a decent photographer is to learn to see like a camera. Practice, practice, practice...
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Forum: Pentax SLR Lens Discussion
02-20-2012, 06:02 AM
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The 85mm proved to be easier to focus. Not a surprise perhaps, because of the enlargement due to longer FL, but also due to the longer throw and better damping of the focus ring.
A* 85mm f1.4@1.4
Attachment 118538
A* 85mm f1.4@2.0
Attachment 118539
A* 85mm f1.4@2.8
Attachment 118540
Pentax lens engineering at it's peak. I can't see any focus shift at all, can you? To my knowledge, there is no aspherical lens element in this lens. As I have understood it, AL is mainly used in WA lenses. Can it be that focus shift is a problem that decreases with increased focal length?
Apparently, the problem I had with getting the focus right wasn't due to focus shift, but to the Pentax matte focus screen. That's why my problem was solved when I exhanged it. Read about that here.
Looking forwards to the comments from other, and more knowledgeable, users of fast lenses.
Kjell
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Forum: Site Suggestions and Help
02-07-2012, 05:08 AM
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On the new forum camera enthusiast, there are buttons at the bottom of every post, for "like", "dumb", "agree" etc. Is it possible to have that feature on this forum?
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