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09-21-2010, 03:33 PM   #1
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DOF vs lens zoom (& reversed lenses) -technical discussion

Hi. I was reading this site here:
Understanding Depth of Field in Photography

And came across something I never knew. According to the calculator:
-The Tamron 90mm macro at 1:1 (0.29m) and f22 will give a DOF of 0.002 m
while
-The Pentax DA 35mm macro at 1:1 (0.139m) will give a DOF of 0.012m

And when the Crop factor is taken into account:
-The Tamron 90(135)mm macro at 1:1 (0.43m) and f22 will give a DOF of 0.007 m

So my first question is, does this mean that shorter zooms make for better macro photography due to increased DOF?


My second question is how to calculate magnification and DOF on lenses reversed filter-filter(facing each other). I know that the more zoom the first lens(the one attached tot he camera) has, and the more wide-angle the second(the one coupled filter-filter) is, the higher the magnification.
The lens I use coupled to the camera is a bellow-takumar 100mm (4 elements in 4 groups for minumum amount of glass and size), while the second is a DA AL 18-55 II lens. Here is the setup:

From personal testing, I assume the magnification to range from 1:1 (17mm across the frame) to 1:3 (7mm across the frame).

How can I find out the DOF and magnification based on the zoom of the second lens, and how would the aperture of either effect the DOF?

09-21-2010, 06:09 PM   #2
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Well, I'll leave most of this (Notably calculating ) to the real macro experts, but where you start comparing magnification and focal length in that way, much of the real difference, regarding depth of field, especially, will be about *working distance.* Notice how that a 90mm achieves 1:1 magnification a lot further away from the subject than a 35 needs to be.

If you look at the focusing scale on a lens with depth of field scales on there, (like maybe that Tammie, if you have one, most old manual focus ones have these) you'll notice that as you focus closer to a subject, that the depth of field constitutes a shorter linear distance, (such as shown in those very short distances you cited) (for instance, there's a bigger space on the scale between two feet and three feet than there is between, say, ten feet and fifty.)

It's very like the difference between focal lengths outside of macro, just that the close distances exaggerate the same phenomena.


As for reversing lenses, as a rule you want the lens you've reversed to be shorter in FL than the one you mount it on. Zooms actually don't tend to be the best choices for this, but if i works, it works. You'd likely get better results with any ol' 28 or 50 that you can mount on there than with any zoom.
09-22-2010, 04:37 AM   #3
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I realise the working distance varies greatly, but what I was surprised to see is that at 1:1 magnification, the 35mm had a larger *calculated* DOF than the 90mm. Is this correct?
09-22-2010, 07:46 AM   #4
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QuoteOriginally posted by Yaro Quote
I realise the working distance varies greatly, but what I was surprised to see is that at 1:1 magnification, the 35mm had a larger *calculated* DOF than the 90mm. Is this correct?
I believe it is incorrect. There's an error in the computation somewhere; perhaps in the interpretation of "distance to the subject".

There is theoretically a VERY small increase in DOF at 1:1 magnification with a shorter lens (due to the hyperfocal effect) but it is VERY VERY SMALL*, at least using conventional thin lens theory.

Dave in Iowa

*conventional thin lens theory (after some algebra) says total DOF for a particular magnification M, F-Stop, focal length, and a display width resolved by the viewer into N pixels is:

FieldDepth/FieldWidth =2(F-Stop/N)(1+FieldWidth/SensorWidth)/(1-((F-Stop/N)(FieldWidth/FocalLength)(1+M))^2)

The denominator includes the contribution due to "Hyperfocal Distance". Note that magnification M is (SensorWidth/FieldWidth)

Say the Magnification is 1:1, display is 1000 pixels wide, field width is 25mm and f-stop is 10; the ratio of DOF's comparing a 50mm lens with a 25mm lens is:

DOF_25/DOF_50 = (1-(2*25/(50*100))^2)/(1-(2*25/(25*100)^2))= 1.0003


Last edited by newarts; 09-22-2010 at 08:16 AM.
09-22-2010, 12:18 PM - 1 Like   #5
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Whether working at macro or major scales, lenses have the same characteristics. Shorter FL or wider aperture = thicker DOF. Longer FL or narrower aperture = thinner DOF. A 28mm macro has thicker DOF than a 105mm macro, at the same f-stop and focus distance on each lens. Non-technically, DOF at macro scales is always pretty damn close to zero. Narrowing the aperture always gives increased DOF, but it's still pretty damn close to zero. Wait till you get down to f/100... That online DOF calculator can give nonsensical answers so don't take it seriously.

It's easy to calculate magnification when stacking (reversing) one lens on another. The lens on the body is the PRIMARY; the stacked (reversed) lens is the SECONDARY; M=P/S. If I stack a 50mm secondary onto a 100mm primary, magnification is 100/50= 2x. Reversing them gives severe vignetting, all the way to an image circle. Hey, I can get that in PP!!

A word about working distance: non-reversed, longer lenses always give a longer WD. In fact a non-reversed lens cannot focus closer than its focal length, so a 24mm lens lets you get down to one inch, whilst a 105mm lens keeps you at least four inches away. Reversed, whether stacked or not, the working distance is always the lens registration, which for Pentax-type lenses is ~45mm, under two inches.
09-24-2010, 01:33 AM   #6
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Wow, thanks for the info guys, lots of stuff I didn't know.

As in regard to DOF at the same aperture for varying focal lengths, I'll have to test myself if it's much of a difference. If I can achieve 2 or 3 times the DOF at the same aperture with my DA 35 then I'll probably put up with the non-existent focal distance inconvenience and use that as my main macro lens...

Will report back with some findings this weekend!
09-24-2010, 08:51 AM   #7
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Withdrawn... My experiments don't seem to agree with well established theory backed, I presume, by experimental evidence.


Last edited by newarts; 09-24-2010 at 01:49 PM.
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