f test on sigma 105 macro

[tpe]

Not sure if this should really be posted here but i thought it was interesting, well new to me at least when recently a real photo buff told me to avoid higher appature settings if i wanted sharp photos.

It is a collage of a seriese of shots taken with a 105mm sigma macro of a ruler. For each shot from left to right the apperture was halved. You can see the depth of field change nicely. What you can also see if you have a look at the f36 f45 strips is that although the DOF does increas ever so slightly the sharpness of the picture gets reduced because of internal refraction? Please correct me or explain as i cant say i have any idea why it should start refracting at small appertures?

The second pic is a 100% crop of the middle right and bottom right hand corner showing f36 and f45 sections, please excuse the jpging, i hope the effect is visable anyway, especially in the upper middle section where the f36 is sharper than the f45?

tim

 

[Alex_B]

well, i cannot see the pictures, but what you say is right ... if the opening gets smaller ... diffraction gets more obvious and kills the sharpness... you get more even sharpness through the depth of your image, but it is less sharp than say with a larger aperture.

for many lenses an f-stop of 8 is great .... but with 20 the problems start to settle in ...

 

[Alex_B]

that is fun .. NOW i can see the images .. that is actually a nice illustration of the story

 

[LaFoto]

This is really interesting, Tim, and I like how you put all the results together into that one collage here ... I never knew you lost in sharpness when the aparture is really small (and its corresponding number bigger! - nice for our newcomers in photography to see) ... looks like for macros such as this one you fare best (if you want some more DOF) between f16 and f22, I think.

I wonder whether there is a forum in the Foundations of Photography where this thread might be placed, too ..... :scratch: ?