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I recently bought a Yongnuo 50mm for my Canon 5D MkIII. I'm having problems adjusting the aperture, it seems like it always shoots at ƒ/1.8.

I adjust the aperture in the camera up to ƒ/22 but it doesn't seem to take any effect on the lens. In the live view mode the image gets darker, but when I shoot the picture has exactly the same light as if I shoot at ƒ/1.8.

I'm wondering if this is a common issue on the lens or if maybe I'm doing something wrong or something else needs to be done (I'm really just checking with you guys before sending the lens back).

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    \$\begingroup\$ What shooting mode are you using? The only time one would expect adjusting the aperture to result in a darker image is if there's no equal and opposite adjustment to either shutter speed or ISO. If you're shooting in auto modes, the camera will be adjusting these for the new aperture value, so the shot itself will have the same exposure. What WILL be different is the depth of field...not overall "brightness". \$\endgroup\$
    – OnBreak.
    Nov 16, 2017 at 21:12
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    \$\begingroup\$ Seriously, the much better EF 50mm f/1.8 STM isn't that much more than the Yongnuo knockoff of the EF 50mm f/1.8 II. \$\endgroup\$
    – Michael C
    Nov 16, 2017 at 22:11
  • \$\begingroup\$ @corey I'm shooting in manual mode \$\endgroup\$
    – petekaner
    Nov 17, 2017 at 7:13
  • \$\begingroup\$ @petekaner Do you have ISO set to Auto? If so, then even though you control aperture and shutter speed, the camera will still adjust ISO to achieve its idea of best exposure. \$\endgroup\$
    – twalberg
    Nov 17, 2017 at 17:03
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    \$\begingroup\$ When you hold the depth of field preview button, can you see if the aperture blades are closing \$\endgroup\$
    – Crazy Dino
    Dec 17, 2017 at 17:31

3 Answers 3

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So, because of your live-view test, we know that the diaphragm is functioning. But on other shots, it isn't.

I'm wondering if there's a timing problem. As in, the lens is stopping down - just after the shutter has already exposed the frame. To test this, use bulb mode and take some frames in the range of a few seconds. Look into the front of the lens - do you see the blades coming together at any point? Does it feel delayed?

Whether that answer is yes or no really doesn't change the outcome. It's a throw-away lens that's giving you issues. It's time to throw it away. Or tear it apart to learn what the inside of the lens looks like. Either way - it'll be cheaper to just replace it.

I've never shot a Yongnuo and have only used the Canon 50mm f/1.4 and 50mm f/1.8 STM. So I don't know if this is a Yongnuo QC issue or not. With the Canon 50's - I used to take them rock-climbing, and they survived all manner of weather and chalk infestation. Given the price point on the STM, that's the one I'd recommend.

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I would add that it could be a sticky aperture problem. Whilst in DOF mode the aperture blades did close, during the fast shutter moment, it may not have enough time to react. I had a similar problem with one of my yongnuo lenses. I could temporarily remedy the problem by repeatedly doing a DOF preview until the aperture blades had loosened. When I took images following this procedure the image was correctly exposed. The next day however, the problem returns...

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  • \$\begingroup\$ To me, this does offer a bona fide answer in the first two lines: The aperture blades are moving too slow. It's the most feasible explanation offered in any of the answers submitted. \$\endgroup\$
    – Michael C
    Dec 21, 2019 at 1:54
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... when I shoot the picture has exactly the same light as if I shoot at ƒ/1.8.

If you are using an autoexposure mode, this is expected behavior because the camera will compensate for the smaller aperture with shutter speed or ISO.

You should determine whether you really have a dysfunctional aperture. You can look into the front of the lens to see if the aperture is moving when you fire the shutter. You can switch to manual mode with fixed shutter speed and ISO to see if changing the aperture setting in isolation has any effect.

I'm shooting in manual mode – petekaner

Assuming ISO is also not set to auto, there may be firmware incompatibility since you are using a third party lens. Updating your camera firmware may help. If your lens has firmware that can be updated, you should try that as well.

I had similar problems with a different third-party lens and camera body. In my case, a camera firmware update improved lens function, although it did not entirely fix the problem.

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