Why does a manual Rokinon 16mm f/2 meter dark on my Canon 70D, especially for night sky shots?
Asked 5/18/2016
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I’m using a fully manual Rokinon 16mm f/2 on a Canon 70D for astrophotography. Since the lens has no electronic contacts, I set focus and aperture on the lens and use the camera meter to choose shutter speed. The issue is that in Manual mode, an exposure that reads “0” on the meter comes out noticeably underexposed, and I often need to shoot around +3 on the meter to get a more centered histogram. This seems to happen regardless of metering mode. In Av mode, the results seem more reasonable. Is this normal for a manual lens, or could it indicate a faulty lens or camera?
Originally by Photography Stack Exchange contributor. Source · Licensed CC BY-SA 4.0
Photography Stack Exchange contributor
10y ago
2 Answers
1
If I'm not wrong it's a totally manual lens, so what the camera meter it's exactly what you'll shoot. And in this sense no, you shouldn't have this problem at all.
But given that, again, is totally manual and there is no electronic contact at all, there is nothing the lens can do to confuse the camera meter. So the only explanation that comes to my mind is that your metering is set in a way that prevent the camera to correctly measure the light, maybe like having the metering set to central spot and pointing the camera to a star...but you already excluded that.
Apart from that...it happened to me to have taken just a few shoots of the sky at night (not in sky photography at all), and they obviously have the histogram all to the left...I can't think a way to shoot at something so dark like the sky at night and have the istogram perfectly in the center. Are your shoots underexposed, or is you who are expecting something unrealistic?
Finally: no point in using Av mode, you have to shoot in M.
Update after OP comment
You can try to do a small test:
- Choose an easy subject
- Mount the kit lens; Av mode, choose an aperture, shoot
- Go to View mode and note down the shutter speed used. While you are there, just because "never say never", check the recorded aperture
- Mount the manual lens, M mode, set the same aperture and shutter speed as before with the kit lens, shoot
The two images should be nearly the same, and the parameter stored in the images too.
If they are not...the lens must have some weird problem.
Originally by user38944. Source · Licensed CC BY-SA 4.0
user38944
10y ago
0
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A faulty lens is unlikely. With a fully manual lens that has no electronic contacts, the camera is simply metering the light actually reaching the sensor, so the lens itself usually isn’t “confusing” the meter.
What’s more likely is that you’re shooting a scene the meter handles poorly: the night sky. Astrophotography is an extreme high-contrast, mostly-dark subject, and camera meters often underexpose these scenes to protect bright highlights. That’s why a reading at “0” can still look too dark, and why adding exposure beyond the meter reading can be normal.
Metering mode can matter, and wide-open lenses may also behave a little differently, but for night sky work the practical solution is to ignore the meter as a strict target and use test shots, histogram, and exposure compensation/manual adjustment.
So yes: for this kind of subject, needing to expose above the meter’s zero point can be normal and does not by itself mean the lens or camera is defective.
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