Practical evaluation of the recording systems

You are here: Nature Science Photography – Visual acuity – Image sharpness II

In the end, it seems time to make a summary based on the numbers: The final image typically lacks optimal sharpness in two key areas. In the first respect, the final image’s sharpness falls significantly short of the visual system’s capabilities. Its resolution limit is 0.0582 mm at a viewing distance of 20 cm. Based on this, we should only allow a circle of confusion diameter of 0.0582/8 = 0.0073 mm at 8x magnification and would have to achieve a resolution of 1/0.0582 = 17.2 L/mm = 8.6 Lp/mm*8 = 69 Lp/mm in the negative. Scheimpflug’s focus expansion is the only way to achieve an acceptable depth of field with such a small circle of confusion diameter. This is only possible in large format and with a few special lenses for 35 mm and medium format. Even a large format lens cannot achieve the high resolution value. We remember the combined resolution values from the section
„Resolving power, viewing distance and print size„:

35 mm
Lens 400 lp/mm, Film 80 lp/mm
= 1/120 + 1/80 = 1/0,021 = 48 lp/mm

Medium format 6 x 7 cm
Lens 90 lp/mm, Film 80 lp/mm
= 1/100 + 1/80 =1/0,0225 = 44,4  lp/mm

Large format 4x5“ (10,1 x 12,7 cm) and 8x10“ 20,3 x 25,4 cm)
Lens 70 lp/mm, Film 80 lp/mm
= 1/70 + 1/80 = 1/0,027 = 37,3 lp/mm

In the second respect, the industry’s use of a 0.032 mm circle of confusion diameter falls far short of what is practically achievable with high-quality equipment. Let’s look at how sharp the different circle of confusion diameters really are. We can measure the sharpness of these circles of confusion using their modulation transfer function. Thankfully, David Jacobson has derived the calculation for this in his Lens Tutorial (https://www.largeformatphotography.info/archives/JacobsonLensTutorial.pdf). For a circle of confusion with a diameter z = 0.032 mm, the following values for the spatial frequencies MTF = 50%, 20% and 10% result without diffraction effects:

f50 = 0,72/Z = 22,5 lp/mm
f20 = 1/Z = 31,25 lp/mm
f10 = 1,11/Z = 34,69 lp/mm

For the physiologically based maximum permissible circle of confusion diameter of 0.0073 mm results:

f50 = 0,72/Z = 98,6 lp/mm
f20 = 1/Z = 137 lp/mm
f10 = 1,11/Z = 152 lp/mm

For the maximum permissible diameter of the circle of confusion of 0.025 mm, which is to be used as a practical average value, the following results:

f50 = 0,72/Z = 28,8 lp/mm
f20 = 1/Z = 40 lp/mm
f10 = 1,11/Z = 44 lp/mm

When we compare this information to the previously repeated combined resolution values for various image formats, the significant discrepancy with the industry standard becomes evident. The practical average value of 0.25 mm is somewhat more favorable, and the physiologically based circle of confusion diameter again sets a technically unattainable mark.

However, the presented figures highlight two additional aspects. When some people rave about the super sharpness of their pictures taken with large format system XY, they are right to do so because the figures show quite clearly that, assuming the same final format, a greater impression of sharpness can be achieved with it than with a high-quality 35 mm system. To make a 20×30 cm print from 35 mm, 8.3x enlargement is required and 48 lp/mm/8.3 = 5.78 lp/mm. To go from 4×5″ format to 20×30 cm requires only 2.2x enlargement and 37.3/2.2 = 16.95 lp/mm. Of course, the 8×10″ format performs even better. Some individuals, who perceive a sharpness difference between two images taken with high-quality systems, have the same right to do so as those who do not. Individuals‘ visual abilities differ, and a person with a visual acuity of 20/10 can distinguish a larger area than one with a visual acuity of 20/30. From a personal perspective, it makes sense to invest more in recording technology and financially support reputable manufacturers, without resorting to guilt-tripping in Usenet discussions.

Next

Main Visual acuity

Previous The mathematical short path to a sharp image

If you found this post useful and want to support the continuation of my writing without intrusive advertising, please consider supporting. Your assistance goes towards helping make the content on this website even better. If you’d like to make a one-time ‘tip’ and buy me a coffee, I have a Ko-Fi page. Your support means a lot. Thank you!