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FORUMS Photography Talk by Genre Macro Talk 
Thread started 27 Nov 2017 (Monday) 20:05
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Why my lens says f stop minimum f / 22 and my camera can take pictures at f32?

 
ANDFlow
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Nov 27, 2017 20:05 |  #1

Hello,

I have a technical doubt, I have a nikon 3300 with a "Sigma 105mm 2.8 Ex Os Macro", it turns out that in the specifications of the lens it appears that the minimum stop is f / 22, but when I take pictures I can take it with f / 32 .. . How is that possible?
in the case of using a 100mm macro lens that its minimum f STOP is f / 32, could the camera increase it further? arrive at values ​​f / 42 for example?
I remain attentive, greetings




  
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Left ­ Handed ­ Brisket
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Nov 27, 2017 20:21 |  #2

Nikon does some kind of calculation that has something to do with focus distance.

It doesn't physically change anything, or result in a different image. It is just a mathematical thing.


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Nov 27, 2017 20:31 |  #3

Here: https://photo.stackexc​hange.com …tically-changes-whi/76818 (external link)


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ANDFlow
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Nov 27, 2017 20:51 as a reply to  @ Left Handed Brisket's post |  #4

for canon is the same thing?




  
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Nov 27, 2017 20:58 |  #5

ANDFlow wrote in post #18505555 (external link)
for canon is the same thing?

No. They just keep it at the standard non-macro number.

If you remember, or imagine, the days before digital, camera aperture value was set manually by clicking a ring on the lens. The kind of calculation was not possible in camera even with Nikon Macro lenses.


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ANDFlow
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Nov 27, 2017 22:22 |  #6

So for macro photography the F/stop is not real?




  
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Nov 28, 2017 02:40 |  #7

ANDFlow wrote in post #18505606 (external link)
So for macro photography the F/stop is not real?

Depends on how you look at it...

Fstop is actually a fraction, and it's the focal length of the lens (in millimeters) divided by the size of the aperture (also in millimeters). So a 100mm lens, with an aperture of 25mm, is at F4. No matter what the magnification is that physical opening in the aperture, and the ratio between focal length and aperture size, does not change.

But at 1x and higher mag there is a lot less light coming through the lens due to the limited field of view -there just isn't a lot of surface area reflecting light back into the camera. So to compensate you have to calculate exposure using a higher Fstop than you would for a non macro image. That's what Nikon is doing -just showing you what the "relative" aperture is. Handy if you're manually exposing a scene.

A lot of macro photographers take the change in Fstop calculation at macro magnifications to an extreme and apply them to diffraction effects as well. But diffraction is caused by light rays spreading out into a cone as they pass through a small opening (the aperture of a lens, for example), and noticeable diffraction is defined as light rays spreading out %50 into adjacent pixels. Since the physical size of the aperture isn't changing at macro magnifications diffraction is a constant, no matter what magnification you're shooting at. What is changing at higher magnifications is diffraction softening becoming more noticeable simply because the image softening is being magnified along with everything else in the frame. Magnify a "defect" and it's going to be more noticeable. The only way to actually increase diffraction is to increase the Fstop (make the aperture smaller).


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Lester ­ Wareham
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Nov 28, 2017 05:05 |  #8

Yes Nikon shows the effective apature, Canon only shows the set.

You should see the minimum apature change with magnification I presume.


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Nov 28, 2017 09:48 |  #9

In the perspective of historical equipment capability, what Nikon does (in displaying the 'effective' f/stop in lieu of actual f/stop) is compensating for the loss of light, and when photographers used only handheld meters could be said to help the photographer by simplifying the task of computing exposure settings.
But since TTL metering came into existence over 50 years ago, one questions the need for such 'compensation' of any displayed value for f/stop, especially with any TTL camera with exposure automation.


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Nov 28, 2017 10:55 |  #10

ANDFlow wrote in post #18505606 (external link)
So for macro photography the F/stop is not real?

The f-stop is a calculated number. The nominal aperture is a real concept, and the effective aperture is also a real concept.

At macro conditions, the nominal f-stop (as sometimes marked on the barrel of a lens) becomes significantly different from the effective aperture. It's because the aperture moves away from the sensor (in classical macro photography where you extend the lens). The further it moves away, the dimmer the light is on the sensor. The more distant aperture also affects depth of field and diffraction.

So the effective aperture is more significant than the nominal aperture, if you are interested in exposure, DOF, and diffraction.

Normally in macro photography, you want to keep the effective aperture number at around f/22 or less for crop cameras, or f/32 or less for full frame cameras. These are just rough guidelines, feel free to experiment. Keep in mind that it is the effective aperture that matters.

If you don't have a Nikon and need to calculate the effective aperture, just multiply the nominal aperture by m+1 (where m is the magnification). (This ignores the effect of pupillary magnification.) It can make a big difference. If you are shooting at a magnification of 5X, then you would need to set the nominal aperture to f/4 to have an effective aperture of f/24.


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Nov 28, 2017 11:01 as a reply to  @ Archibald's post |  #11

Silly me, in writing post 9 I neglected to consider DOF calculation issues in macro work, where the effective aperture value does matter!


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Nov 29, 2017 03:01 |  #12

Archibald wrote in post #18505851 (external link)
At macro conditions, the nominal f-stop (as sometimes marked on the barrel of a lens) becomes significantly different from the effective aperture. It's because the aperture moves away from the sensor (in classical macro photography where you extend the lens). The further it moves away, the dimmer the light is on the sensor. The more distant aperture also affects depth of field and diffraction.

I'll buy that, since the aperture moving further from the lens would allow the diffracted light to cone out further. But I don't think that the effect would be the same as making the aperture physically smaller, and effective aperture is really just a way of dealing with the reduction in reflected light coming back into the lens since the field of view at 1x and higher mag is so narrow. It actually takes a lot of light for pixels on the sensor to register it.


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Lester ­ Wareham
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Nov 29, 2017 12:48 |  #13

Wilt wrote in post #18505804 (external link)
In the perspective of historical equipment capability, what Nikon does (in displaying the 'effective' f/stop in lieu of actual f/stop) is compensating for the loss of light, and when photographers used only handheld meters could be said to help the photographer by simplifying the task of computing exposure settings.
But since TTL metering came into existence over 50 years ago, one questions the need for such 'compensation' of any displayed value for f/stop, especially with any TTL camera with exposure automation.

Correct in the first part, but if working with (constant) ambient light I miss the effective aperture which would be handy for manual exposure at different magnifications.


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Why my lens says f stop minimum f / 22 and my camera can take pictures at f32?
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