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01-05-2011, 04:06 AM   #16
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QuoteOriginally posted by Stiv Quote
...
The only thing I'm not sure of is how you worked out the 1:1 working distance. Isn't 208mm the working distance for x1.67 not x1.0?
Thanks for pointing that out. It was a misleading, big error on my part.

Actually, the 208mm working distance I quoted is what one would expect when a 208mm lens is added to an existing lens focused at infinity.

In general, when a lens of focal length f' is added to a lens with focal length f" already focused at infinity, the "new" lens has a magnification of m'=f"/f' and a working distance of f'.

When this "new" lens is extended to increase the magnification, the actual working distance decreases, so an add-on close-up lens' focal length is the maximum expected working distance.

A useful formula describing the ratio of working distances when a lens is extended to magnification m from m' is:

w/w' = (1+1/m)/(1+1/m')

For example, when a 208mm Raynox 150 is added to your 70mm macro lens focused at infinity, the magnification is 70/208 ~ .33x and the working distance is about 208mm.

When the 70mm lens is fully extended (to 1x) and the close-up lens is added, the magnification is (1+1)(1+.33)-1=1.66x and the ratio of the working distances is:

w/w'=(1+1/1.66)/(1+1/.33)=0.4

Wow! I was way off in my earlier statements about working distance with the combined lens at a total mag of 1:1, which would be

w/w'=(1+1/1)/(1+1/.33) = 0.5 or a working distance of 104mm, less than that of the 70mm at 1:1 (140mm).

I apologize for my error and hope I've not lead too many people astray,

Dave


Last edited by newarts; 01-05-2011 at 07:40 AM.
01-05-2011, 10:40 PM   #17
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Thanks for clearing that up! I think I still have one last question though...

If I understand correctly, the working distance for x1.66 magnification would be 0.4 x 208mm = 83mm.

Where is working distance measured from? If it's measured from the end of the add-on lens that's ok, but if it's measured from the end of the first lens before the focusing ring (which is massive on the Sigma when extended for x1.0) then maybe I'll never be able to achieve x1.66 since 83mm would be inside the lens?
01-06-2011, 05:02 AM   #18
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QuoteOriginally posted by Stiv Quote
Thanks for clearing that up! I think I still have one last question though...

If I understand correctly, the working distance for x1.66 magnification would be 0.4 x 208mm = 83mm.

Where is working distance measured from? If it's measured from the end of the add-on lens that's ok, but if it's measured from the end of the first lens before the focusing ring (which is massive on the Sigma when extended for x1.0) then maybe I'll never be able to achieve x1.66 since 83mm would be inside the lens?
The equations are all derived from "Thin Lens" theory which treats a lens as a plane with no thickness. A real camera lens is quite thick and is assembled from a number of simpler lenses. Such a lens still obeys the thin lens equations with respect to its "Principal Planes" (one towards the front & one towards the rear of the physical lens assembly.) Sometimes these principal planes coincide with the actual glass at the front & rear of the lens but usually not. Sometimes they are inside the physical lens & sometimes outside.

So all I can say is that the working distance is measured from the front principal plane which is somewhere near the front lens in the assembly. To make things worse, the front lens & front principal plane is often recessed by the lens barrel, a hood, etc. However it makes sense that the front principal plane of a lens with a close-up lens attached is somewhere near the physical close-up lens.

Don't let this worry you too much, the thin lens results are still valid, but the distances resulting will be offset from the glass a bit depending on where the actual front principal plane lies with respect to the real lens.

Dave

PS I have yet to find a reference that addresses this point in a concise, clear manner.
01-06-2011, 05:23 AM   #19
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Once again, thanks for all the great info!

01-06-2011, 05:48 AM   #20
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QuoteOriginally posted by Stiv Quote
Once again, thanks for all the great info!
Thank you!

It is relatively easy to estimate where the front principal plane lies for your macro lens. Set the lens at 1:1 magnification, place the camera on a table & slide it towards a target until the target is in focus. Now you can measure the actual working distance & compare it with the theoretical 140mm (which is correct with respect to the front principal plane of your 70mm lens by definition).

The difference between measured and theoretical distances will tell you the location of the front principal plane.

This is a valid procedure in-so-far-as you actually know the focal length of your lens... which opens yet another can of detail worms...

If you actually do this test I'd like to hear the result! Especially with and without a close-up lens attached!

Dave
01-06-2011, 11:28 AM   #21
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I can vouch for the Raynox-150 macro "filter" on top of the DA L 55-300. $42 from Adorama. Last night was my 1st session with it and I think I came away with some acceptable photos.

Raynox directions say to set zoom and focus to max. That gives max. magnification. I calculated magnification to be 1.4:1. I can also adjust the zoom and focus if I want less magnification.

I get a decent focus distance so the lens doesn't cast a dark shadow over my target. These photos were taken with normal room light, no flash. No cropping. I used f14 for good DOF to minimize my focusing errors. Tripod mandatory for that. I hope to try handheld in bright sun sometime soon.

Click for larger versions.

01-07-2011, 01:10 AM   #22
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QuoteOriginally posted by newarts Quote
If you actually do this test I'd like to hear the result! Especially with and without a close-up lens attached!
Since I don't have a close-up lens I can't test that, but I can test the straight up Sigma macro at x1.0

View Picture EXIF
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So working distance from the end of the lens is around 60mm, 140mm is measured from the focusing ring. Very interesting!
01-07-2011, 01:17 AM   #23
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QuoteOriginally posted by DeadJohn Quote
I can vouch for the Raynox-150 macro "filter" on top of the DA L 55-300. $42 from Adorama. Last night was my 1st session with it and I think I came away with some acceptable photos
Impressive magnification!

I was taking some more shots last night at straight up x1.0 and got this one

View Picture EXIF
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Size:  375.1 KB

This little guy was only about 10mm long, maybe I'm not as desperate for more magnification as I thought? Although this has been cropped by about 2/3...

01-07-2011, 05:37 AM   #24
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QuoteOriginally posted by Stiv Quote
Since I don't have a close-up lens I can't test that, but I can test the straight up Sigma macro at x1.0

Attachment 80063

So working distance from the end of the lens is around 60mm, 140mm is measured from the focusing ring. Very interesting!
Thank you very much for reporting your experimental work.

I am surprised the working distance is so small compared to predictions. Sigma says the working distance is 112mm at 1:1 - http://www.sigmaphoto.com/shop/70mm-f28-ex-dg-macro-sigma -very misleading!

Either the front principal plane is well inside the lens or the lens is an "Internal Focus" type - and not really 70mm at full extension.

According to the Sigma site the lens is an internal focus design so is likely not 70mm at full extension. Further the Sigma site is inconsistent regarding minimum focusing distance*; the text says 7.4" and the tabulated specs say 10.1" - quite a difference and consistent with your data.

I suspect the lens is 70mm when focused at infinity and decreases in focal length when extended.

Viva la measurement!

Dave

* "minimum focusing distance" is measured from the subject to the image plane; for an ideal 70mm thin lens it would be 280mm, 11".

Last edited by newarts; 01-07-2011 at 06:02 AM.
01-07-2011, 06:27 AM   #25
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The glass is actually quite indented at the end of the lens so I'd estimate there is around 80mm from glass to object, but definitely not 112mm.

Looking inside the lens there seems to be another layer of glass another 30ish mm in so maybe they are calculating from that. I don't really know the components so that's just a guess.

Either way I agree it's misleading!
01-07-2011, 06:35 AM   #26
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Interestingly, I took a photo of a ruler like DeadJohn and consistently measured 22.5mm across.

If the K-7 sensor is 23.4mm wide, I guess I'm actually getting x1.04 magnification. At least they're not lying about that!
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