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04-03-2014, 03:35 PM   #1
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Question about lightmetering when changing lenses on K30

Hello,

I have just bought my first prime - a DA* 55 mm and I have played around with it for the past few days. I have noticed a slight difference in the brightness / histogram level when taking a shoot using the DA* 55mm and comparing it with a similar shoot (under the same lighting conditions) using the 18 - 135mm WR kit lens. At first I thought it might be due to differences in the optical quality of the lenses, but should the camera lightmeter not be able to make a correct exposure and thus change the shutter, aperture or ISO accordingly? Thus I have made a few follow up tests where I compare an inside shoot taken at a fixed aperture of F5.6, ISO100 and focal length of about 55mm on the kit lense.

The first photo is taken with the DA* 55 mm while the second is taken with the 18-135mm at about 55mm. Both pictures are taken at ISO100 and F5.6. Just by visual inspection the brightness difference is noticeable - the observation is confirmed by comparing the respective histograms on the camera.

It should be noted that I have not updated to the latest firmware, so I will try to do that and take a few follow up shoots under the same lighting conditions. Should I be concerned with my new lense or camera?... or is it to be expected that you have to change the exposure compensation when changing lenses?

Best regards,
Michael

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04-03-2014, 04:01 PM   #2
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It isn't that far off, about 1/3 of a stop. There is going to be some variation, especially when you consider the 55mm was metering at f1.4 and calculating the exposure needed for f5.6 while the 18-135 was already somewhere near f5.6. With the stock focusing screen metering tends to be more accurate at wider apertures. I wouldn't even worry about compensation at that point if you shoot RAW, just bring it up in post.

A bigger problem is someone appears to have painted your aebleskiver pan and hung it on the wall as a decoration.
04-03-2014, 04:11 PM   #3
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I'm not sure of the exact reason but I have some ideas. One is the difference in the shape of the aperture. The DA* 55/1.4 has 9 blades and the DA 18-135 has 7. Both are trying to approximate a circular opening. They might get close but not entirely equal. The second idea is transmission losses. The DA* 55/1.4 has 9 elements and the DA 18-135 has 13. You have them set to the same aperture, which is the opening in the aperture blades. But the zoom is less efficient at transmitting all the light.

I have a couple of ideas that aren't really happening here. You can compare two primes of the same focal length and one will vignette (darker corners than the center) more. That's just lens design. You might also see contrast or color differences. In both cases, the histograms look a lot different. In fact, you can see these differences with the same lens at different apertures. Wide open histograms will look different than at a lens's best apertures. That's not as dramatic with a really good lens.
04-03-2014, 06:10 PM   #4
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Every lens will be slightly different. As it was said, your picture with the DA* was metered at f/1.4 and exposure to f/5.6 was calculated from there. On your zoom lens (being a variable aperture), the true aperture opening is measured at only one focal length and will vary (f/5.6 becoming let's say f/5.9) but is still reported as the same. When the camera calculates proper exposure the variance between reported aperture and actual aperture may cause a lens to over/under expose a bit.

The complexity of the lens (# of elements) should not affect metering since it is performed TTL. The same is true regarding the light transmittance of the lens. Better lens will tend to allow faster shutter speed at the same aperture because of better light transmittance. But, the more light in, the more precise the metering will be, an advantage of fast lenses.

Vignetting as Just1MoreDave mentioned may also have a slight impact. But the DA* doesn't vignette all that much and the 18-135 does vignette quite a bit. Regardless, the vignetting should have driven the shot with the 18-135 toward overexposure because of the darker corners. Using spot metering will help if heavy vignetting causes headaches.

Indirect light (i.e. flares) could also affect the metering. Affecting the reading when wide open (to meter) but eliminated when stopped down (to shoot). But then again, if it was the case, your shot with the DA* should have underexposed and since the 18-135 was already almost at the shooting aperture, indirect light shouldn't have impacted.

All and all, it's normal. I have lenses that over/under exposed by a full stop. With time you'll learn their secrets and it'll become automatic to compensate in camera or just fix in post if the difference is minimal.

04-04-2014, 12:07 PM   #5
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Thank you all for the great response.

I guess it makes sense that there are small variations in the exposure calculations. Especially when considering the 4 stop difference between f1.4 and f5.6. I feel reassured that it's not a technical problem with the new lense or camera.

elliott: The ębleskive pande is actually not painted but is made of copper but it is used as a decoration
04-04-2014, 12:18 PM   #6
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QuoteOriginally posted by mln83 Quote
elliott: The ębleskive pande is actually not painted but is made of copper but it is used as a decoration
Ahh, every one I've seen here is either aluminum or cast iron. People in the southern US have an annoying habit of painting old cast iron pans and hanging them on the wall.
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