Saturday, December 31, 2022

Making pragmatic sense of the Depth of Field scales on manual prime lenses

In previous posts I have discussed how we can use a split, thin lens approximation (image below) to create a depth of field scale for any lens, at a given focal length, aperture and assumed circle of confusion.

The expression for the ‘length’ of the near and far depth of field scale (d=J/h) being: ((MFD-X)*L*N*C)/(F*F). Where: MFD is the minimum focus distance, as measured from the sensor; X is the position of the front principal, as measured from the sensor; L is the ‘rotational distance’ travelled, in mm, that is the lens throw, as measured from the minimum focus to infinity; N is the aperture number; C the circle of confusion; and F the focal length of the lens. 

In this post I’ll show how we can apply the model to manual prime lenses, where there is usually a very good depth of field scale on the lens, but we don’t know what circle of confusion (C) was used to create the scale.

Knowing the value of C is useful for landscape photographers, when: hyperfocal focusing to a specific CoC; setting a specific infinity blur; or ensuring a specific overlap blur when focus bracketing.

To find an estimate of the DoF scale’s value of C, we simply rearrange the expression for d, to obtain the equation for C. Namely C = (d*F*F)/((MFD-X)*L*N).

As discussed in a past post, we can estimate the value of X, the distance from the sensor plane to the front principal, by looking up the value, say, on Photons To Photos, or, if PTP lens information isn’t available, then we can estimate X by locating the entrance pupil, eg by using parallax, and measuring/guesstimating the pupil magnification.

As for d and L, these can be directly measured from the lens by placing a strip of masking tape on the lens and marking the location of the MFD, ie 0, the infinity location, oo, and the length of the depth of field scale at the maximum depth of field location, eg f/22.

The following image shows three measurement records, for a Mamiya 645 35mm Sekor lens, the Irix 11mm, and the Pergear 14mm: ignore the faint text as this is print through from another page.

In the above the MFD is at 0, the infinity location on the far right, with the length of the near to far depth of field shown at 22 or 16, with the left hand (near) DoF being placed at 0. Thus L is the length from 0 to oo and d is half the length of the 0 to f/22 or f/16 length.

Having got a model and a process to gather all the data, let’s look at a these lenses.

The first lenses we will look at is the Mamiya 645 lens, which I personally mount on my EOS R via a Rhinocam Vertex adapter, to create a quasi medium format camera, ie using sensor bracketing to create a ‘square 645’ image, equivalent to a sensor of about 45mm square, eg about 8500x8500 on the R.

In the case of  my Mamiya 645 35mm, on the R, the measured MFD = 420mm, with L = 110mm, d = 29mm and X estimated at 80mm. Thus the circle of confusion used for the depth of field is about 43 microns.

Now some may be saying, so what, I didn’t need to know this to use the depth of field scale. The manufacturer did all the work for me. But, as we will see, you can’t always trust the manufacturer, or, put another way, don’t assume the CoC: always confirm the DoF scale’s CoC; especially if you are seeking to set focus to a specific optical blur.

To illustrate this, let’s look at the other two manual prime lens.

First, the Irix 11mm Blackstone, where the MFD is 270mm, with L = 100mm, d = 18mm, an estimated (but not confirmed) X of 90mm; giving the circle of confusion used for the depth of field on the Irix at about 8 microns. That is not the ‘standard’ CoC that some may assume for a full frame camera, ie around 30 microns.

Finally, let’s look at the Pergear 14mm using an MFD of 430mm, with L = 53mm, d = 16.5mm, an estimated X of 60mm; the circle of confusion used for the depth of field on the Pergear 14mm is also about 8 microns. 

So what does all this mean?

First, it looks like there is a difference between the older, film lens depth of field scale and the two, new digital lenses. That is the Mamiya appears to use a ‘standard’ CoC for the 645 format, ie around 43 microns; whereas the two modern lenses seem to use a CoC much less than the 'format, standard CoC' of around 30 microns, ie instead around 8 microns.

Taking the Mamiya as an example, and knowing the CoC that has been used by the manufacturer to construct the depth of field scale, we can now use this information to ‘adjust’ focus settings in the field.

For example, if I was focus bracketing with the quasi medium format setup, using the Mamiya, I would be seeking an overlap blur that is, say, half to 2/3 of the standard format CoC. Thus, if my aperture is f/16, then I would use the f/8 depth of field markings to focus bracket, knowing this will result in a (medium format) overlap blur of around 20 microns. 

In a similar way, if I wish to set the infinity blur to, say, 20 microns, once again I would place the infinity against the f/8 mark, with an aperture set at f/16.

However, on the Irix and Pergear lenses, I would adopt a different strategy, knowing the CoC that has been used to construct the depth of field scale is around 8 microns.

All this assumes I’m using the Mamiya lens as a film camera, but I’m not. Hence, knowing the value of C that has been used, ie 43 microns, on my R I might be tempted to use an aperture of, say, f/16, but use the f/4 depth of field mark. Having said this, I think there are a few more experiments I need to make, before I’m confident about things ;-)

In future posts I’ll explore the above with further examples, but, for now, I'll simply finish this post here.

As usual I welcome any comments on this post or any of my posts.

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