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10-28-2011, 07:40 PM   #1
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Magnification difference between Macro and Non Macro Lens

I took this 2 pictures from the exact same distance to the subject and the one taken with the Macro lens (Pentax 100mm f/2.8 WR Macro) seems to have greater magnification than the one taken with the Pentax 18-135mm Lens.

The Pentax Macro Lens is Amazing, the images are REALLY SHARP.

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Last edited by brosen; 10-28-2011 at 08:05 PM.
10-28-2011, 09:32 PM   #2
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The difference is probably because of the way zoom lenses work. You will usually only get the maximum of 135mm when the lens is focused at infinity. When it is focused closer the effective focal length is sometimes much less. This is what you are no doubt seeing here.
10-29-2011, 01:30 AM   #3
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Zoom lenses are nearly always out-performed by Single focal length or prime Lenses. Prime lenses are simpler to construct - easier to correct for aberrations and they tend to have less glass which helps make them lighter and more compact.

from what I have seen of the optical performance of the pentax 18-135mm lens there is certainly much room for improvement at focal lengths over 75mm
10-29-2011, 02:00 AM   #4
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Indeed. Lenses with internal focusing (IF) elements (such as many zooms) will change focal length at different focus distances. And primes are usually sharper than zooms. Some primes are also IF and show the same FL shift, or so I have read. So a zoom needn't suck just because it's IF.

To find the actual focal length of a lens... well, that's not easy. We can't just measure distances from the objective, not with a complex camera lens, because the lens' optical center is inside there somewhere. So, measure angles. Point the camera at a ruler, achieve focus, and measure the AOV (angle of view). Yeah, use a protractor. Do this with the zoom at both close-focus and infinity-focus. (That may require a BIG ruler!) Knowing the AOV and the sensor size, calculating the actual focal length is simple. Only minimal trig. A pi here, an arctan there, no problem.

Or, just live with it.

10-29-2011, 04:55 AM - 1 Like   #5
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Actually, measuring out FL at any focusing distance isn't that difficult. Let's take a practical and real example:

With my Tamron SP 35-80 mm Model 01A CF Macro lens, I set the FL on the lens to 80mm (the lens sets itself at that position at closest focusing distance) and photograph a ruler at the closest focusing distance, which I shall call S.

Now I don't know where the optical centre of my lens is but I can measure the distance S from the ruler to my sensor plane. I find S=27 cm.

Next, I study the image of my ruler. I take 1 cm (called b1) in the following and find that the image (hereafter referred to as b2) of that one centimeter is 660 pixels long on my K200D sensor. That sensor has pixels that are 0.00606 mm wide and thus, my image, b2 is 0.4 centimeters long. I have M = b2/b1 = 1:2.5 as Tamron has also promised me in their lens sepcification.

Now I have enough - measured! - data to compute my FL:



In this figure, I know S = 27 cm, b1=1 cm and b2=0.4 cm. From the figure you can see (from the principles of equidistant triangles) that M = b2/b1 = s1/s2 and S = s1 + s2. Thus, you have two simple equations with two unknowns and you find:

s1 = S/(1+M) and s2 = S*M/(1+M) where I can measure both S and M.

I finally need to know the general lens equation that tells me:

1/FL = 1/s1 + 1/s2.

Inserting my actually measured numbers, S=27 cm, M = 0.4 I find that the focal length of my Model 01A at closest focusing distance at the "80mm" setting is actually 55 mm !

And all I needed to find out was a ruler and the pixel size of my sensor.

Last edited by Stone G.; 10-29-2011 at 05:02 AM.
10-29-2011, 05:32 AM   #6
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Here is a schematic diagram showing the optical description of a typical lens:

http://www.giangrandi.ch/optics/lenses/focalcalc.html

There are in effect two effective locations of the lens - one in the front and one in the rear - these are called the "principal planes". The distance between these principal planes is usually unknown and it is not used in the thin lens optics equations; therefore extra measurements must be made to determine this distance.

1/f = 1/xi +1/xo
m = xi/xo = yi/yo

Measurements of at least two image/object distances and associated magnification are needed to determine the distance between the principal planes.

Say "D" is the distance between principal planes for a lens; the total image-object distance for that lens is then:

Total.distance = focal.length(1+m)^2/m + D two measurements can determine D and focal length

Internal focus or zoom lens' principal plane locations move as the lens is focused or zoomed so instead of using focus or zoom controls the whole lens should be moved between measurements.

Unfortunately there doesn't seem to be any way to make these measurements for IF/zoom lenses when the lens is mounted directly onto a camera designed for the lens; a bellows or extension tube is needed. Here's a potential measurement scheme using a bellows:

1) set the bellows to minimum & set the lens to a particular focus and/or zoom - measure the magnification & total distance - call these m1 & td1
2) increase the bellows extension and repeat the measurement (do not change the lens focus/zoom) - call these m2 and td2.

focal.length = (td2-td1)/[(1+m2)^2/m2 - (1+m1)^2/m1)]

Last edited by newarts; 10-31-2011 at 01:25 PM.
10-29-2011, 09:39 AM   #7
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Not to mention those rare lenses where the rear principal plane is in front of the front principal plane. There are some out there, but not many. It is fairly common to find the node of a lens outside the physical lens. Telephotos (as a subset of long focus lenses) will have the node in front of the lens, and a lens with a focal length less than the registry distance (mount to sensor/film plane) has the node inside the camera. Weird stuff, optics.
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