Macro DOF vs Resolution

[Lord Vetinari]

Following on from a thread on DPreview about whether a specific lens was no good at reduced apertures, I did some test shots at F8 and F16. I'll leave you to work out which is which.
AFAIK the rate of loss of resolution due to diffraction effects is exactly the same as the rate of gain of DOF with reduced apertures ie for one whole F stop the DOF doubles and the resolution halves. This of course applies to any type of optical instrument/lens or any type of photography.
Should add the pics are all 100% crops no resizing & no pp apart from the same settings in RAW conversion and a little levels adjustment.
Hmm think I might take more pics at F8 and focus stack them-lol

http://homepage.ntlworld.com/valenb00/CRW_8597c.jpg

http://homepage.ntlworld.com/valenb00/CRW_8599c.jpg

http://homepage.ntlworld.com/valenb00/CRW_8601c.jpg

http://homepage.ntlworld.com/valenb00/CRW_8604c.jpg

http://homepage.ntlworld.com/valenb00/CRW_8610c.jpg

http://homepage.ntlworld.com/valenb00/CRW_8612c.jpg

[BigAl]

Brian, those weavils are really nicely done. :thumb

I've done a bit of googling, and found quite an interesting page (http://www.rags-int-inc.com/PhotoTechStuff/DoF/). If you scroll down to "Diffraction limits" and look at the table (most folks won't understand the maths & physics preceding it anyway :evil), you will see that going from f8 to f16 will cause the circle of confusion to double as a result of diffraction. Hence, the softening of the image. I have a feeling that the 1.6x smaller factor of the detector won't help either. His pictorial example doesn't exhibit the problem as well as your weavils do though!

regards
alan
ps: thanks for the PMs

[Lord Vetinari]

Brian, those weavils are really nicely done. :thumb

I've done a bit of googling, and found quite an interesting page (http://www.rags-int-inc.com/PhotoTechStuff/DoF/). If you scroll down to "Diffraction limits" and look at the table (most folks won't understand the maths & physics preceding it anyway :evil), you will see that going from f8 to f16 will cause the circle of confusion to double as a result of diffraction. Hence, the softening of the image. I have a feeling that the 1.6x smaller factor of the detector won't help either. His pictorial example doesn't exhibit the problem as well as your weavils do though!

regards
alan
ps: thanks for the PMsThanks Al,
Just to confuse me even more a paragraph from your referenced article . The shots above are all done at 1:1
"Close-up Photography:

When you focus on very close objects (macro) there are other considerations that need to be factored into the formulas above. These are based on the reproduction ratio, usually a number from zero to one. For example at a reproduction ratio of 1:1 an f/8 setting becomes an effective aperture of f/16. Calculations for DOF, diffraction, depth of focus and even exposure are all affected because the lens focal plane has moved significantly away from the image plane. You need to use the effective aperture instead of the selected f-stop. At normal focusing distances this effect of lens movement is insignificant. "

[Lord Vetinari]

Had to try this- this is a crop and resize of a garden spider pic I just did focus stacked from 4 pics taken at F8. The pic has had the equivalent of level 1 sharpening in focus magic- I tried level2 as I normally use and it was way over the top so faded it to 45%.

http://lordv.smugmug.com/photos/32569843-L.jpg

This is an approximate 50% crop of the original pic and resize to give an idea of the scale above.

http://lordv.smugmug.com/photos/32569933-L.jpg

[BigAl]

Thanks Al,
Just to confuse me even more a paragraph from your referenced article . The shots above are all done at 1:1
"Close-up Photography:

When you focus on very close objects (macro) there are other considerations that need to be factored into the formulas above. These are based on the reproduction ratio, usually a number from zero to one. For example at a reproduction ratio of 1:1 an f/8 setting becomes an effective aperture of f/16. Calculations for DOF, diffraction, depth of focus and even exposure are all affected because the lens focal plane has moved significantly away from the image plane. You need to use the effective aperture instead of the selected f-stop. At normal focusing distances this effect of lens movement is insignificant. " Brian, during my searches, I found this on more than one site, also without any theoretical foundation. I'll try and look into it and see if I can shed any further light, but I'm also in the dark (says he who has lectured lens optics to land surveying students!).

regards
alan
ps: the 50% crop of the spider looks really cool.

[Higgmeister]

Hi Brian,
I just wanted to say thanks for the info and links. I too am trying to digest this info:huh, but I can see it's going to take several readings to get get a grasp.

I appreciate the post and links:thumb,
Chris

[DavidTO]

I find it funny that the article linked above is called DOF: Demystifying the Confusion. What follows is the most abstruse article...

I just gave up.

abstruse |ab?stro?s| adjective difficult to understand; obscure : an abstruse philosophical inquiry..

[Lord Vetinari]

Kinda agree with you, but it all goes back to the saying you don't get something for nothing so increasing the DOF by using smaller apertures does significantly reduce the resolution. I didn't think this would be that significant at F16 on a macro lens but it obviously is possibly due to the comment above that in reality you are operating at F32.

Solution obviously is to use as large an aperture as you can for the shot you are going to take.