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09-12-2009, 02:23 PM   #1
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macro lens question

Hi guys,

I was shopping for macro lens and explained the salesman that I will be shooting objects 1mm in size. I said I needed macro lens that will capture these at almost full screen.

They've suggested and sold me a Sigma 105mm 2.8 macro lens.

The problem is, even with extension tubes, the object is NOWHERE NEAR the size I need it to be (at full focal length of course).

Please tell me what determining parameter (and value) do I look for in a macro lens, (or how do I calculate the value needed) when I have an object that is 1mm in size that needs to be full screen.

Or you can just suggest specific lens models to consider.
It is for a Pentax K7.

I hope I'm explaining this right,
Thanks very much!

09-12-2009, 02:48 PM   #2
Ash
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Mr Pink, a 1mm subject is extremely tiny.
Capturing this kind of subject to fill the frame requires much more than a 1:1 macro + extension tubes.
The closest thing you may get is a 2:1 or 3:1 macro + extension tubes (where you may find one of these macros, who knows...) but even then I'm sceptical if it will effectively fill the frame with a 1mm subject.

Looks like it's a high magnification light microscope with camera fitting you're gonna need...
That's specialised stuff, found in the medical division.
Don't think this has helped, but hopefully you'll find what you're after...
09-12-2009, 02:52 PM   #3
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buy a nice prime lens around 28mm and a bellows, and you'll get close to 5:1 magnification which is about as good as its going to get unless you do what ash suggested.
09-12-2009, 03:01 PM   #4
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thanks guys

I'm just reading that Canon makes a 5:1 macro lens, the MP-E 65.
If I can adapt it to K, I should be all set, plus I can add extensions.
What do you think?

09-12-2009, 03:23 PM   #5
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Im going to suggest switching to canon than as Im not sure you can make than lens usable on your K-7. That lens is the only reason I would ever use 2 systems.


What I would give if pentax made an equivalent lens

BTW at 5:1 mag. there is little to no light meaning a off camera flash is a necessity
09-12-2009, 03:35 PM   #6
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switch

Yes, I think I'll need to switch to Canon guys, thanks for your unbiased opinion.
09-12-2009, 06:23 PM   #7
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No problem man. Feel free to stick around, we have a part of the forum devoted to non pentax camera's now, and you can still post pictures in the "post your photos" section
09-12-2009, 06:36 PM   #8
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An APS-C sensor is 16mm tall (more or less). That means you'll need 16:1 magnification to get a 1mm object to fill the entire height of the frame. That's not a macro lens; that's a microscope.

You don't say how large a print you'll be making - or if you'll be printing at all. But if it's just for screen viewing, the K-7 captures images over 3000 pixels tall, and you only need maybe a third of that at best to get your 1mm object fill the screen at 100% view. Meaning you really "only" need 5:1 magnification to do what you want. Which is to say, a short prime plus extension tubes and so forth would do the trick - as would the 5:1 macro lens, of course.

But my guess is, you'll find out that in practice, actually getting a usable image of an object that small is far more difficult than you are realizing, and once you spend all the money it will take to get the equipment needed to pull it off, you might end up questioning whether it was worth it, and start wondering if there isn't a different way to ahcieve whatever the final goal is.

09-12-2009, 11:01 PM   #9
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A microscope would be appropriate for a 1mm subject (10-20X magnification.) Otherwise you will not have the stability, lighting, and fine mechanical positioning adjustment you will need. Just taking a photo through the microscope's ocular may be sufficient.

Dave in Iowa
09-13-2009, 02:16 AM   #10
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You could always enlarge and crop final image.
09-17-2009, 08:43 PM   #11
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A male to male filter connector might allow you to put a 28mm lens reversed onto a 300 mm lens for about 10:1 magnification (I think). But good luck getting it in focus and lighted.
ETA now I am tempted to try...
09-18-2009, 05:21 AM   #12
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QuoteOriginally posted by Nick Siebers Quote
A male to male filter connector might allow you to put a 28mm lens reversed onto a 300 mm lens for about 10:1 magnification (I think). But good luck getting it in focus and lighted.
ETA now I am tempted to try...
That's a good idea. Add to that some $30 focusing rails and it is a reasonable zoom photo-microscope.

Making a setup that would rigidly hold the camera + stacked lenses at a fixed distance (about 40mm) from a lighted subject would work well, but not as well as a setup involving focusing rails.

Without focusing rails and non-zoom lens the magnification will vary with exactly how far the camera is from the subject - from about 11X to 40X as you adjust the focusing rings, but for a particular in-focus distance there will only be one magnification possible. Add the focusing rails and both distance and magnification can be adjusted independently.

Such a setup would be as good or better than a low powered microscope ONLY if it is rigidly constructed and stable.

Dave

PS here's a simple example of a constant stand-off reversed lens arrangement; a translucent polyethylene cone holds the reversed lens a constant distance above the subject - this allows reasonable lighting on the subject:


Later, I replaced the cone with a ring light:

The ring light is positioned above the table by some plastic stand-offs glued to it.

Finally, here's a setup involving focusing rails attached to an adjustable tilt table-top "tripod".

Maybe these examples will generate some ideas suitable for your needs.
09-18-2009, 06:38 AM   #13
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QuoteOriginally posted by mrpink Quote
....
Please tell me what determining parameter (and value) do I look for in a macro lens, (or how do I calculate the value needed) when I have an object that is 1mm in size that needs to be full screen......
For a full screen width of 1mm on a K-7 you need an optical magnification of 24X. There are no out-of-the-box 24X macro lenses.

As others have pointed out, you probably will be happy with less than this as you can crop.

A reversed lens setup with focusing rings set at infinity will give a magnification of :

magnification_minimum = prime_lens_focal_length/reversed_lens_focal_lengths

As the focusing ring on the primary lens is turned, the magnification will increase by:

magnification_increase=(primary_lens_extension/Primary_lens_focal_length)(1+minimum_magnification)

so for a 300mm primary lens and 28mm reversed lens:

minimum_magnification=300/28=10.7X

If turning the 300mm lens focusing ring extends it by 30mm the magnification will increase by:

magnification_increase = (30/300)(1+10.7)=1.17X

So if you *really* want a magnification of 24X you'll need to use a 300mm and reversed 12.5 mm lens or maybe add a close-up lens to the stack. It'll be hard to handle, but possible I think.

Last edited by newarts; 09-18-2009 at 01:06 PM.
09-19-2009, 07:38 PM   #14
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Well, I dummied up a connector with two filter rings that happened to snap inside each other a little (I was careful!). So here's the feather I will use for these test shots, the center spine is 1-2mm max width.


For comparison, here is the FA 50mm macro lens at about 1:1



Now here is my M 28mm f/3.5 reversed onto my K 80-200mm f/4.5 zoom. Initially I tried at 80mm , but obviously it vignettes.



Zooming in to about 110mm



And at 200mm, for about 7:1 mognification



Now this was all very rough, camera sitting on the bed, hand held feather, flash on a cable backlighting it. But, it does show the magnification possibillities, no?
09-21-2009, 02:12 PM   #15
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I like it! I want to try out macro as well and that looks great to me
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