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03-29-2012, 01:56 AM   #1
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macro / min focusing distance

i have another question i hope you fine folks can answer for me...

so i'm about to order my FA43 for a project, i love every photo i have seen on the forums taken with it (though it's also by some decent photographers) and think that its teh best rendering lens with the best perspective @ 43mm.

but one thing troubles me, it says on teh pentax website that the 43 has a min focusing distance of 1.5 feet.
so i grabbed my trusty 18-55, set it to 43, and measured 1.5 feet in front of the lens.
problem is, that's not a very tight crop at all, in fact, its almost unusable if thats supposedly the closest it can focus.

so then i thought i could maybe get a 35 macro for the same cost, but i did the same thign with my 18-55... set it to 35mm and went to the 6" min focusing distance, which hardly seemed macro at all.

so, i must not understand macro.

what i want is the FA43, and i have seen a few photos (the famous green frog for example) that show it with the close-up functionality i need. i dont need macro i assumed, jsut a tighter crop than 1.5 feet at 43mm (this is on a k5 sensor). can someone a) explain if my little field test was accurate and b) assure me that this lens can in fact take close up photos of a green frog? i have the a50 1.7 and 1.4 and tehy seem to max out around 1.5 feet. are the frog shots taken withthe lens reversed? if so does reversing it effectively make a max focusing distance instead of a min?

phew, sorry i guess thats more than one question... sorry

03-29-2012, 02:26 AM   #2
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QuoteOriginally posted by dominikkolendo Quote
so i grabbed my trusty 18-55, set it to 43, and measured 1.5 feet in front of the lens.

Don't quote me, But I believe that the MFD is measured from the sensor plane and not from the end of the lens.

QuoteQuote:
problem is, that's not a very tight crop at all, in fact, its almost unusable if thats supposedly the closest it can focus.

so, i must not understand macro.
The FA43 isn't really a Macro lens at all, it's MFD is not especially great, it's something I wish my FA77 was better @ is the MFD

You could always make it do some macro works, buy getting some extension tubes or screwing a Raynox on the front of it, which will increase the Magnification. This way most of the time it's a great prime lens for general shooting, and a macro lens, when you want it to be.
03-29-2012, 03:02 AM   #3
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QuoteOriginally posted by Freak Quote
MFD is measured from the sensor plane and not from the end of the lens
Correct. So that'll make a big difference with the 35 ltd! Not so much with the non-macro lenses.

QuoteOriginally posted by dominikkolendo Quote
i have the a50 1.7 and 1.4 and tehy seem to max out around 1.5 feet
Specs show that both of those have the same MFD as the 43, 450mm. So the 50mm lenses will have slightly greater magnification at MFD.

Frogs come in a large size range. The 43 magnifies to about 1:8 at MFD, meaning you could fill the frame with a subject about 190mm (7.5 inches) wide. Bullfrogs, anyone?

Read the famous article on cheap macro for more info on reversing/stacking lenses, extension tubes, close-up filters, and more:

https://www.pentaxforums.com/forums/pentax-lens-articles/152336-cheap-macro-b...lose-work.html
03-29-2012, 03:14 AM   #4
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tahnk you!

baronite, that link is wonderfull.
i jsut held my 50mm up to my camera backwards and took a shot, its perfect!
i am ordering the 43 and a reverse mount adapter right now, that totally fixes my worry.
awesome!

03-29-2012, 03:20 AM   #5
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Just so you nderstand macro correctly, lenses give 1:1 macro when the image to lens distance is equal to the lens to focal plane distance and when this condition is met, the image to lens distance is 2x the focal length.

When you consider the focusing helix of many short lenses they only move a few mm to focus from infinity to the MFD, but to achieve 1:1 macro would need to move the equivalent of their on focal length, so unless the lens is specifically designed for macro the MFD will typically be in the 12-18 inch range.

For some lenses, and zooms especially, they achieve close focus by modifying the focal length, and only hold their true focal lengths at infinity. So replicating the focal length with a zoom and then close focusing, may be a little misleading. If you want to get closer, you can add close up adaptors, which modify the focusing range. These lenses are typically rated in diopters, 1, 2 etc... A diopter is 1/focal length, and when attached to a lens make infinity focus of the lens equal to the focal length of the close up lens, so adding for example a 4 diopter lens in front of your lens means that it is impossible to focus on anything more than 250mm away. Is that close enough?
03-29-2012, 03:37 AM   #6
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that info helps me understand more than i even wanted to know. thanks.
this deos bring up another question though:
with diopters and reversal, i assume you get the same characteristics of teh original lens, but with a macro add on would that alter your lens characteristics as its another lens on top of it essentially?

oh, nevermind, its not exactly a lens (the macro adapter)
03-29-2012, 05:36 AM   #7
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QuoteOriginally posted by dominikkolendo Quote
that info helps me understand more than i even wanted to know. thanks.
this deos bring up another question though:
with diopters and reversal, i assume you get the same characteristics of teh original lens, but with a macro add on would that alter your lens characteristics as its another lens on top of it essentially?

oh, nevermind, its not exactly a lens (the macro adapter)
It is a lens and adding it to the original lens creates a new lens with a new focal length, shorter than either. The formula for stacking two lenses, focal lengths, f' & f" with a separation distance d is:

f = f'f"/(f' + f" - d)
03-29-2012, 06:15 AM   #8
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QuoteOriginally posted by dominikkolendo Quote
that info helps me understand more than i even wanted to know. thanks.
this deos bring up another question though:
with diopters and reversal, i assume you get the same characteristics of teh original lens, but with a macro add on would that alter your lens characteristics as its another lens on top of it essentially?

oh, nevermind, its not exactly a lens (the macro adapter)
as others have indicated, it is a lens.

the entire impact is as follows

Focal length of your lens changes and the change is as follows

New Focal length = 1 / (1/FL + Diopter)

this new focal length is to be considered in addition, on a lens extension of FL - New Focal Length, which causes the lens to now have a Maximum focal length equal to the focal length of the diopter

and also has a new aperture where the F number is



F new = Forigonal x New Focal lentgh / Focal length

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