This is a continuation of a report on new ways to look at depth of field. The series starts here:
In the preceding post, I discussed a new way of calculating hyperfocal distances in which the criterion was degradation in resolution for objects at infinity relative to the MTF50 values that would have been obtained if the lens had been focused at infinity at any given f-stop.
Now I’ve going to go for absolute sharpness, not dialing back expectations as you stop down and diffraction lowers the bar.
It’s all in this chart, computed for a modeled excellent 55 mm lens on a Sony a7RII. I can present it and explain it, but I can’t reduce the hoops I jumped through to make it quickly, and it would be a really nerdy, dense paragraph, so I’ll skip it for now:
The horizontal axis is the absolute resolution, measured by MTF50, in cycles per picture height. The vertical axis is the hyperfocal distance in meters. If you focus at this distance, objects at infinity will have resolutions that are shown by the various lines, which are one whole f-stop apart.
The way to use this chart is as follows. Decide on how what MTF50 you can tolerate at infinity. Find that place on the horizontal axis. Run your finger upwards, stopping at the first line you encounter. Note the color of the line, and set your f-stop appropriately. Note the distance along the vertical axis corresponding to where you first encountered the line. Set your lens to that distance.
Avoid the apertures and distances that are on the soaring end of the hockey stick curves.
All of the above assumes that you’re trying to get an image that has acceptable (as defined by you) sharpness at infinity and that same sharpness as close to the camera as possible. It is not a substitute for thinking and planning, and can probably be abused as much as the present hyperfocal distances are. But it does provide an integrated way to think about diffraction and defocus blur when planning a photograph, something that I think has been lacking up ’til now.
There’s a fly in the ointment, though. I generated tha above curves for on simulated lens on one camera. How do we get to similar curves for your particular lens on your particular camera? That’s going to take some thought.
I’m really looking forward to the short paragraph summary of this series!
Is there any rhyme or rhythm as to the ratio of the closest distance that is achieving the same MTP as infinity does to the distance the focal scale is set to? For example, is the close distance with the infinity MTF always about half (or two-thirds, or 60%) the focus distance?
That’s a good question. I think there is, at least in the case of my modifications to the hyperfocal distance. I’ll be doing some experiments.
Jim
Preliminary results are that the infinity sharpness is reached at half the focused distance, even in the presence of some diffraction. I’ll bet this is part of the CoC canon. However, there are settings to produce certain MTF50s at infinity that have no peak at all for some f-stops. I’ll post something about it tomorrow if the results hold up to more scrutiny on my part.