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09-29-2013, 02:01 AM   #1
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sigma 70mm macro / K30 metering

I've just been experimenting with my new Sigma 70mm f2.8 macro and generally getting some nice sharp well exposed images. However I ran into a problem when shooting upwards at a sunflower head. The image has come out significantly overexposed. I thought it might be due to metering so switched to spot metering, but the same problem again. Taking a similar photograph of a different head with a larger proportion of sky and the problem went away. Exposure is as I expected. (OK I know I should compensate for the sky).
I reset all the settings in the menu, set the camera back to spot metering and tried again to replicate the first photograph, but still the same overexposure problem. I'm stumped, anyone have any ideas?
For reference he sun was approximately behind me, and light cloud in the sky.

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09-29-2013, 03:38 AM   #2
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I think its just that spot meter and center weighted both only look at the area that is very dark on that sunflower - the centre dark circle. In that case, the camera will try to make that dark circle look 16% grey (what the camera thinks is a good exposure). For this kind of shot you should use matrix (so it takes into account as much of the frame as possible) or simply M mode and manually input the aperture, ISO, and shutter.
Another problem might be flare, if you are not using a lens hood, or if you are using a filter. This can cause a drop in contrast, but this is probably not the cause in this case.

You will get similar problems if you are taking a photo of a black car or a wedding dress that fills up most of your frame. The camera will not understand that these objects are supposed to be black or white, and will try to make them look "bright enough for humans to enjoy." In those cases, use EV compensation (+/-), manual mode and ignore the meter, or bracketing/HDR (if the object is not moving and you have a tripod)

tl;dr: cameras are not super smart and they will try to make white or black objects appear darker or brighter, which will look odd and make the rest of the scene even more under/over exposed. Automatic metering is great, but different subjects require different metering modes. And you dont necessarily need the automatic metering - just input your own settings in M mode.

Edit: Also, when you upload photos for us to help with, please include the exif data, so we can see the shutter speed and aperture and ISO and metering mode. This data can help us give better advice, or see that you are already doing the right thing and look for problems elsewhere.
04-29-2014, 07:45 PM   #3
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I was having the same problem with this image:

I pointed the center spot at the shadows in the yellow petals, hit the Exposure Lock button, composed and focused. Added +0.24 in post, perfect exposure.
07-24-2014, 07:45 AM   #4
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Light Metering with K-30

I am very confused by metering. Can someone explain to me how to successfully meter with the K-30 mounted with the pentax DA 55-300 mm lens. Thanks

07-24-2014, 09:00 AM - 1 Like   #5
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QuoteOriginally posted by audiodoc Quote
I am very confused by metering. Can someone explain to me how to successfully meter with the K-30 mounted with the pentax DA 55-300 mm lens. Thanks

First, your camera is doing the metering and the lens that's attached to it doesn't have an affect on the meter itself. The focal length can make it easier or more difficult to meter of specific areas of a scene. If for example, you're using a wide angle lens, you won't be able to spot meter off small areas. However, using a normal to telephoto lens enables the camera to meter off smaller, more specific areas of any scene. This enables the photography to know the brightness values of even the smallest areas such as the highlight on the model's cheek.
The problem that you've experienced with the sunflower is a result of metering off of the black part of the flower. All cameras, regardless if we're talking about an ME Super, a Spotmatic, K3, D4, 5D Mark III or what have you - it doesn't matter, they all are based on the same premise. And that is that they all meter to render the area metered as 18% gray. So when you spot metered off of the black seeds of the sunflower, the camera produced an exposure to render them, not black, but middle gray by overexposing the scene by 1.5 stops.
If you had spot metered off of something that had a reflectance value near or at a middle-gray tone, (the green leaves for example) then the black areas would be black and the bright areas would have been bright.

When I started out, I carried an 18% gray card with me so all I had to do is meter off of the gray card to get an accurate exposure. It was accurate because the camera's meter is calibrated to render whatever it sees as 18% gray. Well, if you point it at something that has a reflectance value of 18% then it (along with the rest of the scene) will be properly exposed. I soon learned the reflectance value of various things such as green grass (which very close to 18% and is a great substitute for carrying a gray card), blue sky and snow.

The key to making an accurate exposure is to be able to understand what brightness values are in the scene and where they are relative to 18% gray. Then once you've got an exposure to render the 18% gray areas as they are, metering off of other areas will show either an increase or decrease in exposure. In effect, this will allow you to pre-visualize how the scene will be rendered. This isn't easy at first, it will take a little bit of practice.

The next thing you need to understand is that all cameras have difference dynamic ranges. You may find that increasing the exposure of white snow by 2 stops renders the snow very bright, but with detail. However, your friend using a different model needs to increase the exposure by 1 1/2 or 2 1/3 stops to render the scene the same way your camera had. You need to know the limits of your camera. Just how much over or under middle gray can your camera render detail? When I was shooting film, this was one of the first tests that I always did when using a new film. I had to know the film's dynamic range in order to get correct exposures, especially when using slide film which required an exposure accuracy of +/- 1/3 of stop.

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