Raynox 250 strange image sharpness at wide aperture

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macjack
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Joined: Mon Jun 15, 2020 2:16 pm

Raynox 250 strange image sharpness at wide aperture

Post by macjack »

Hi, I've been focus stacking with my raynox 250 at f/8 (Nikon D750, 105mm macro and raynox 250 with flash, and using an electronic focus rail). The sharpness of my stacks are terrible. However when I stack at f/18 the stacks are much sharper, which seems counter intuitive because you'd think diffraction softening would kick in. Does anyone have any idea what is going on here? I'm shooting with my lens focused at 1:1. I've included some test shots, the first is f/8, second is f/18. I have friends with similar setups who regularly shoot at f/8 and get fantastic results, so I'm completely lost. The only thing I can think is that perhaps focusing the lens at 1:1 is a bit much and I should aim more for 1:2


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Scarodactyl
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Post by Scarodactyl »

I suspect it will be hard to tell exactly what's going on without some 100% crops of the image--at web resolution it's hard to make out details.

rjlittlefield
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Post by rjlittlefield »

macjack, welcome aboard!
I have friends with similar setups who regularly shoot at f/8 and get fantastic results
My first thought is to wonder exactly what "similar" means.

Depending on exactly what model of camera and what model of lens are being used, a setting of "f/8" at 1:1 can easily mean an effective aperture anywhere from f/8 to f/16.

My guess is that you have one of the combinations where "f/8" means effective f/8, and your friends have combinations where "f/8" means effective f/16.

Raynox 250 lenses have quite a bit of spherical aberration, which scales as aperture raised to the fourth power. So, a factor of 2 in effective aperture turns into a factor of 16 in severity of aberration. Stopping down is a very powerful method of taming spherical aberration.

You're correct that effective f/18, on full frame D750, is starting to get into diffraction territory. But that amount of diffraction is still not enough to add a lot of softening, compared to the havoc that spherical aberration can wreak on the contrast of fine detail.

Definitely do post some 100% crops (and see https://www.photomacrography.net/forum/ ... hp?t=36168 to be sure what we mean by that).

Also please clarify whether the two images were shot at exactly the same focus point. When I magnify the images posted here, I see a shift of focus as well as a difference in sharpness. That is consistent with different amounts of spherical aberration at the same focus arrangement, but of course it's also consistent with a slight shift of camera, lens, subject, or focus ring.

--Rik

kutilka
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Post by kutilka »

Can't the blur be front focus?
(Google translator :? )

Chris S.
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Post by Chris S. »

Macjack, welcome to the forum! :D
macjack wrote:I'm shooting with my lens focused at 1:1. I've included some test shots, the first is f/8, second is f/18. I have friends with similar setups who regularly shoot at f/8 and get fantastic results, so I'm completely lost. The only thing I can think is that perhaps focusing the lens at 1:1 is a bit much and I should aim more for 1:2
Any micro-Nikkor 105mm lens should be very good at 1:1. (My micro-Nikkor 105mm AF-D is surprisingly close at 1:1 to the quality of the vaunted Printing Nikkor, which is optimized for 1:1.)
rjlittlefield wrote:My first thought is to wonder exactly what "similar" means.

Depending on exactly what model of camera and what model of lens are being used, a setting of "f/8" at 1:1 can easily mean an effective aperture anywhere from f/8 to f/16.

My guess is that you have one of the combinations where "f/8" means effective f/8, and your friends have combinations where "f/8" means effective f/16.
I strongly suspect Rik’s guess is correct. In the days of manual cameras, when you turned the aperture ring on your macro lens to f/8 and focused to 1:1, your nominal f/8 became effective f/16, due to loss of light from the extension of the focusing helicoid—something called “bellows factor.”

With modern electronic cameras, bellows factor is still reducing effective aperture as we crank into macro range, but we’re often less aware that this is happening. We look to the camera’s LCD display to tell us our aperture, and this can be deceiving: Nikon cameras display effective aperture, and many (most?) other cameras display nominal aperture. Canon, for one, displays nominal aperture, so far as I know. So if your friends cameras are not Nikon, and are saying their 1:1 aperture is f/8, this is equivalent to your Nikon camera’s display of f/16. And of course this disparity holds for other f/stops as well, at 1:1 and other close-up ranges.

If that difference in reported f/stop exists between you and your friends’ rigs, then Rik’s point about spherical aberration is the likely culprit.

--Chris S.

macjack
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Post by macjack »

Guys, thankyou so much. This all makes so much sense now! I feel like an idiot but it's fantastic to finally get an explanation. Just for good measure I'll show you the cropped images (using the method from the link). I used stacking software so apologies if these comparisons aren't very good, the positioning of camera and subject did remain constant in both tests however. The reason you see areas out of focus below the head in the second f/18 image is because I stupidly excluded this part in the stuck, hopefully when observing the head alone you can see the differences though.

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