Rebel T7i vs 5DS for Macro Photography using 65mm Macro Lens

Started 5 months ago | Discussions thread
John Sheehy Forum Pro • Posts: 19,139
Re: Your maths is wrong, and macro photography is not easy.

2ndact scene1 wrote:

John, thanks for the reply. Obviously, I claim no expertise - just trying to improve my comprehension.

My question originated from the DLA (diffraction limited aperture) listed in the The Digital Picture's camera reviews. The DLA on the 5D is f13.2 and on the 70D is f6.8 ( this thread compares FF to crop).

It sounded more like a pixel size thing, the way you presented it: using bigger pixels to combat diffraction. All that bigger pixels combat is resolution. They do not remove diffraction; they just give a bigger problem than diffraction, so you don't notice the diffraction. You fail to notice the bigger problem, when you fall lockstep into a rut of viewing the results at a lower level of magnification, dictated by the larger pixels, at 100%.

The DLA figures are a lot less profound than you might assume; they are nothing more than a fixed proportionality between airy disk size and pixel size, at an arbitrary, standard maximum pixel contrast. They should not and do not imply that it is not worth using smaller pixels at that f-number. That interpretation is totally absurd, because high pixel contrast is a NEGATIVE thing when it is due to low pixel density.

For macro, f22 is apparently not uncommon because DOF is so narrow. At that aperture, diffraction is an issue.

The diffraction of f/22 over a FF sensor is very small, compared to the entire frame. You shouldn't even see it in an image reduced to web size. Even looking at it from your perspective of matching pixel size to f-number, to get the higher pixel acuity, it is your final, displayed virtual sensor pixels that are crisp from being large. If you look at an image from a 5Ds, for example, resampled on a standard HD monitor, you are basically looking at 1.75MP, so your viewed pixel can actually have 5.35x the f-number of the original 5Ds pixels with the same potential pixel contrast. In fact, the original pixels being soft is good for a downsampled image, not bad, and crafty use of median-like filters before downsampling can remove things that don't translate well in downsizing, and appear as jitter or noise.

My thought process was that if the object fills the frame, 12MP should be sufficient for a reasonably sized print at 300 ppi. Mitigating diffraction might therefore be of value but I am unclear on the benefit of significant additional MP.

The more MPs you start with, the less artifacts are forced upon you in PP such as rotation, perspective and CA correction, and resampling to other pixel counts. It is best to think of resampling as a necessary evil - necessary because we are currently forced to use displays with fixed pixel sizes or print dither sizes. Ideally, a display would use the original pixels, all equally sized and weighted, and the PPI would vary. Any change of pixel count is destructive, especially to "illegal" points of acuity due to poor sampling, which must be softened and/or spatially displaced (the only way to maintain pixel acuity).

I should have a EF mount macro lens shortly so I can do my own comparisons.

You could still be fooled by display technology to prefer the inferior capture, only to be disappointed later when you present your image upsampled on an 8k monitor, after seeing a series of native 8k images. Or, you could just show your image on a fraction of the 8k screen to make it look "sharp and detailed".

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