In the last two posts I introduced the Poor Man's Optical Bench (PMOB) tool and showed how you can estimate the location of the on-axis entrance pupil of your lens using a cheap laser leveller.
Knowing: the position of the entrance pupil, but as previously mentioned, assuming it is at a single on axis location; the minimum focus distance and the focus distance of interest, both from the sensor plane; the pupil and optical magnifications; and the aperture number at infinity; allows you then to create your own 'thick lens spec sheet' at a given focus.
In this post, I'll illustrate how to do this, using my 645 Mamiya-Sekor 35mm C N lens, at infinity focus. I will also show how to 'calibrate' your lens depth of field scale.
From the lens manufacturer we know the minimum focus distance (MFD) is quoted as 450mm (on this occasion I used this, rather than measure this myself). We also know the infinity focal length is 35mm. Finally, we known the flange distance of the 645 Mamiya system is 63.3mm.
From taking a couple of snaps on a light box, and aligning the images in Photoshop, we can estimate the pupil magnification at infinity as 2.1:
The ‘entrance pupil’ location is found using the laser leveller technique, giving an estimate of 35mm from the lens rear flange surface. Which tells us the location of the entrance pupil, relative to the sensor, is at 35 + 63.3 = 98.3mm:
In the above Photoshop composite, ie I don’t have two laser levellers, we see the two laser lines clearly locating the entrance pupil, relative to the lens rear flange surface.
Having got all the input data, we then open up the Poor Man's Optical Bench (see link RHS) and input all the information:
In the screen grab above we see all the input data and that focus is at 'infinity', ie greater than 10 hyperfocal distances, ie magnification is 0. The other magnification, 0.104. is an estimate at the MFD, but assuming the focal length remains fixed. Once you adjust focus to, say, the MFD, you will need to tweak the focal length, after remeasuring the entrance pupil location and pupil magnification, and getting the optical magnification at that focus, eg as quoted by the manufacturer, ie 0.11 in this case, or measured by yourself.
We also see an estimate of the hiatus (44.9mm), and the hyperfocal distance from the sensor, here based on a circle of confusion of 20 microns.
Finally, we also see a dotted line showing where the flange distance sits relative to the sensor.
For the pano photographer, the above gives us the location of the no parallax point of the lens, ie the entrance pupil.
The next thing we can do is calibrate the depth of field scale.
The 645 Mamiya-Sekor 35mm, being a prime manual lens, has an unambiguous depth of field scale, unlike a zoom lens, but, of course, we don't know what CoC the manufacturer used.
Using the PMOB we can get an estimate by simply counting the number of DoF rotations to go from infinity, ie focused at the hyperfocal at a given aperture, say f/16, to, say, the MFD. This can be accomplished by putting some masking tape on the lens and marking the DoF rotations needed to get from the hyperfocal to the MFD.
Using the f/16 marks on the DoF scale, I estimated the number of lens rotations to get from the hyperfocal to the MFD at about 3.1:
Having set focus in the PMOB to the MFD, we first need to repeat the process of locating the entrance pupil with the laser leveller and measure the pupii magnification, and optical magnification if you wish or take the manufacturer’s value, and enter these values into the PMOB. Once entered, we need to adjust the focal length in the PMOB until the magnification matches the manufacturers 0.11, or your measured magnification.
Which means, after adjusting the CoC slider in the PMOB, to match the number of brackets to about 3.1, tells me the manufacturer used a CoC of around 48 microns when laying out the scale, which compares well with taking a full frame CoC value of 30 microns and factoring it by the 645 crop of 0.62.
Knowing the DoF scale CoC, it is a simple matter to dial in any hyperfocal infinity blur you wish, or any overlap blur when focus bracketing. For instance, if you wanted to use a CoC of 24 microns, instead of 48, with an aperture set to f/16, you would use the f/8 DoF scale.
Finally, knowing the position of the flange and the overall length of the lens, we can overlay a pretty picture of the lens, say in Photoshop, to enhance the look of our 'lens data sheet':
So, there you have it, using the PMOB and a laser leveller, you can characterise any lens at a specified focus, eg infinity or at maximum magnification.
As usual I welcome any comments on this post or any of my posts.
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