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10.3x close up photography sharpest when set at f/8~f/11

 
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jmseight



Joined: 25 Jul 2013
Posts: 11

PostPosted: Fri Jul 26, 2013 1:29 pm    Post subject: 10.3x close up photography sharpest when set at f/8~f/11 Reply with quote

Hi,

I recently got a Nikon PB5 bellows an Nikkor 24mm f/2.8 lens which allowed me to take 10.3x photos at full 190mm extension.

Nikon recommends using f/8 for the best images, but I feel they look blurry when pixel peeping. I am thinking this is because of diffraction limit.

I read some of the articles on this forum and found that the effective aperture is what is important, not the set aperture. So at ~10x, the effective aperture is the set aperture multiplied by 11. With aperture set on the lens at f/8, the effective aperture is f/88. With aperture set on the lens at f/2.8, the effective aperture is f/30. This indicates that set f/2.8 should be better than set f/8.

I took a series of close up photos at 10.3 by changing the aperture. I foudn that Nikon was correct. The IQ is poor at f/2.8 and f/4, best at f/8 and f/11, and very blurry at f/16 and f/22.

This seems contrary to what effective aperture would indicate.

Does anyone have similar experience?

Thanks,
James
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ChrisR
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Joined: 14 Mar 2009
Posts: 8427
Location: Near London, UK

PostPosted: Fri Jul 26, 2013 5:51 pm    Post subject: Reply with quote

Did you reverse the lens?
You'll be getting blur from many sources, I guess it just depends which is the most dominant.
Blur at f/90 effective, is pretty bad, so the optical aberrations must be awful!
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jmseight



Joined: 25 Jul 2013
Posts: 11

PostPosted: Fri Jul 26, 2013 9:06 pm    Post subject: Same result with Nikkor 50mm at 4.3x Reply with quote

Yep. I did reverse the 24mm lens.

I did more testing with Nikkor 50mm reverse mounted which gave 4.3x magnification. This lens performed the best set at Nikon recommended f/8, which is equivalent to f/40 effective aperture.

Thanks,
James
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johan



Joined: 06 Sep 2011
Posts: 1004

PostPosted: Fri Jul 26, 2013 11:57 pm    Post subject: Reply with quote

"effective aperture" gives a theoretical 'best' based only on diffraction calculations, but doesn't necessarily give an 'actual best' because that's also based on lens optical qualities. And lenses aren't always that great wide open Smile
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ChrisR
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Joined: 14 Mar 2009
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Location: Near London, UK

PostPosted: Sat Jul 27, 2013 3:29 am    Post subject: Reply with quote

I haven't kept records, but from memory using a few trials with normal f/2.8 camera lenses, my best results were had about 3 stops from maximum.
Enlarger lenses are quite a lot better though (Eg El Nikkor 50mm f/2.8 ). Running to about effective f/30-40, though it depends on your sensor, that can be ok (or better) for a web-sized image.

If you think about the field of view the lens is designed for, you're only using tiny part of it (angle) in a macro application, eg when reversed at 5:1. Also the angle of incidence of the rays from the image and object, are way off what it was designed for. EG, if you use a 24mm reversed, it's focusing at say 240mm where it's designed to be a large distance.
Even enlarger lenses may not be optimal, but they're closer. If you use a 50mm reversed at 3:1, it's outside its best operating zone, and they aren't designed for wide aperture normal use. Some of them are very nice though.

To get somewhere near theoretical performance at wide aperture you usually have to use a lens near the way it was designed to be used. That's where bellows-macro and microscope lenses come in.

A lens Combo, eg a 50mm reversed onto a 200mm, uses them both focused at infinity, & sometimes works very well.


Blur is like light.
Light + dark + dark + dark = light
Blur + sharp + sharp + sharp = blur
You don't get a sharp image by making the "sharp" contributions better, only by reducing all the "blur" influences!
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ray_parkhurst



Joined: 20 Nov 2010
Posts: 2482
Location: Santa Clara, CA, USA

PostPosted: Sun Jul 28, 2013 7:41 pm    Post subject: Reply with quote

It's likely the best aperture recommendation from Nikon is based on a combination of improved field flatness (corner sharpness) and depth of field. This is due to the way systems like you've described were used in the past with film. But with focus stacking, neither of these criteria are useful in determining best aperture. What is important is finding an aperture where the lens is sharpest at the corners INDEPENDENT of the center. This may be at a different focal plane than at the center, but this is not a big problem as long as you are able to adjust your focal distance to compensate for field flatness problems, and then focus stack to achieve the best final image.
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_sem_



Joined: 04 Aug 2011
Posts: 47

PostPosted: Mon Jul 29, 2013 4:23 am    Post subject: Reply with quote

ChrisR wrote:

If you think about the field of view the lens is designed for, you're only using tiny part of it (angle) in a macro application, eg when reversed at 5:1. Also the angle of incidence of the rays from the image and object, are way off what it was designed for. EG, if you use a 24mm reversed, it's focusing at say 240mm where it's designed to be a large distance.
Even enlarger lenses may not be optimal, but they're closer. If you use a 50mm reversed at 3:1, it's outside its best operating zone, and they aren't designed for wide aperture normal use. Some of them are very nice though.

To get somewhere near theoretical performance at wide aperture you usually have to use a lens near the way it was designed to be used. That's where bellows-macro and microscope lenses come in.


Reversed wide-angles usually have relatively short min focus distances. So when used on bellows, with focus set to close-focus (or, so that the object is exactly at the flange distance), they are not so much off what they are designed for. Notice the resulting magnification is quite high.

...It is true that only a small portion of the lens is used due to the high magnification. Also a lot of stray light may be bouncing around inside, unless you cut it off (veiling flare). And the effective aperture gets high, leading to diffraction blur. These lenses are mostly not best at their widest apertures, and also not at min focus distance.
F/8 is the recommendation for normal use; you should try wider to reduce diffraction.
You should also spend some time sharpening. Proper sharpening may be quite efficient with pure diffraction blur (but don't expect miracles).
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