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02-16-2010, 03:15 PM   #1
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Working distance versus object size and lens selection

Howdy, rookie question here :-)

I have a K20 D with DA35mm Limited Macro and Tamron 90mm Macro lenses. Love 'em both but I seem to be photographing smaller and smaller subjects.

Does anyone have a lens setup recommendation given that I desire a working distance of 3-6 inches, objects can be as small 1/8" across and I want the object image to fill the sensor.

Thanks for your thoughts

02-16-2010, 03:17 PM   #2
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Bellows. Extension tubes. Lens reverser.
02-16-2010, 04:26 PM   #3
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for distant subjects, (more than 10 x focal length

image size = oject size * focal length / subjet distance

when you get close this becomes much more complex but note

1:1 is always when image distance = subject distance = 2x focal length.

If you put close up lenses on, you modify focal length and focus point.

effective focal length 1/(1/focal length in meters + diopter)

1 diopter is 1 meter focal length, 2 diopters is 0.5 meters focal length etc.

maximum focusing distance with a close up lens = 1/diopter in meters

In re reading your post, if you want to get 1/8 inch across the sensor you are really discussing 6x magnification.

I am not sure how easy this will be to achieve at 6 inch working distance


For example you would need about 300 mm of extension tubes on a 50mm lens, but focusing wouldbe at 60 mm from the front element, less than 1.2 your desired distance.

you could put your 24mm lens on a bellows and get a good magnification but with probably a working distance approaching the focal length of the lens

I don't know how reversing rings would work.

Last edited by Lowell Goudge; 02-16-2010 at 04:35 PM.
02-17-2010, 12:42 AM   #4
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For a thin lens, working distance is:
Working_Distance=Focal_length(1+1/m)

1/8" is a magnification of 8.

So for your 90mm lens,

Working_Distance=90(1.125)~100mm

Lens_Sensor_Distance=Focal_length(1+m)=90(9)=810mm!!

It's going to be a challenge!!

With a 24mm lens, the sensor-lens distance would be more reasonable, but not easy.

For such magnifications a microscope is usually more suitable.

02-17-2010, 12:13 PM   #5
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While you could look into specialized equipment to give you the ~8X magnification you need to fill the frame with an object 1/8" across, you might find it more expense and trouble than it is worth unless you are making large posters of these images. Otherwise, a simple 1:1 macro set up on a high resolution camera (which you've already got covered) combined with aggressize cropping on a reasonably high resolution camera should give dequate results for "most" purposes (eg, viewing on screen, smaller prints) with *far* less expensse or hassle.
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