1

I am shooting with a Canon 6D and an f4 24-105 L-series lens. I am pretty certain I set the aperture at f4.

After inspecting my pictures later, I am noticing that the aperture goes to 5.6, 6.3, 7.1, and 8 in some cases.

I have IS (Image-Stabilization) on, but that shouldn't affect the aperture setting, right?

Additionally, the lens should support f4 throughout the range. I have exposure safety shift enabled

I did NOT manually adjust the exposure with the jog dial and accidentally bump the aperture setting, so I must be doing something wrong every time or there is another piece to this puzzle.

Any ideas?

  • 2
    Does your camera have Exposure Safety Shift? Is it enabled? – mattdm Aug 2 '17 at 2:56
  • Yes, it is enabled, let me read up on that. – Walter Aug 2 '17 at 3:01
  • Okay, I remember reading up on these years ago ... This shouldn't be a problem though because if I were to shoot at f4, I'd have more light, so I'd need less exposure. The picture was shot at around 1/200, well within limits. I used a flash, and believe I had high-speed sync on, so again, I would think the camera should be able to handle it, right? – Walter Aug 2 '17 at 3:03
  • Okay, I was wrong, I had it on first-curtain sync, so that would probably limit the shutter speed to 1/250s. – Walter Aug 2 '17 at 3:06
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I believe the answer to the question is that I needed to change the flash from first curtain sync to high-speed sync.

I thought I changed that setting before I started shooting, so that would limit the shutter speed of the camera and hence why the aperture was locked down more.

Also, I have the setting Exposure Safety Shift enabled, so that is what was used to compensate for an unattainable shutter speed, change the aperture. Thanks for mattdm for the pointer.

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A Zoom lens (eg. 24-105) has a variable Focal Length.
In 'simple' terms- Aperture (f-numbers) are a ratio of [ Lens Diameter : Focal Length ]
When you change the Focal Length (zoom) and the lens aperture diaphragm (diameter) remains the same, you are then changing the f-number.
So you are seeing the f-number set on the camera but the "effective" f-number (in your photo metadata) shows differently.
Some 'professional' lenses are constructed to maintain a constant "effective" f-number when changing focal length.
eg. If the diaphragm diameter is 12mm-
at 24mm zoom the f-number = 24/12 = f2
at 105mm zoom the f-number = 105/12 = f9

  • This is true, but clearly not the answer to the question as the OP is using a 24-105 f/4 which already has constant aperture. – Philip Kendall Aug 2 '17 at 7:41
  • The EF 24-105mm f/4 lens has a constant f-number over the entire zoom range. The true f-number is derived from the diameter of the entrance pupil (the aperture as seen through the front of the lens) and includes any magnification provided by lens elements between the front of the lens and the physical aperture diaphragm that does not change in size. For more, please see: How do zoom lenses restrict their widest aperture at the telephoto end? – Michael C Aug 3 '17 at 6:48

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