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02-05-2010, 12:43 PM   #1
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Exposure Questions: 30mm F1.4 & 50mm F1.7

I recently acquired both a Sigma 30mm F1.4 and a Pentax A 50mm F1.7. I've been taking test shots to help me understand exposure, and I'm noticing that when in Av mode at a given sensitivity and aperture, my K-x is choosing faster shutter speeds for the 50mm than for the 30mm lens. The light in my house is not at all good today, so everything's underexposed regardless. But I'm wondering if anyone can explain to me why this is happening. Does it have to do with focal length?

Here's some data on what settings the camera used (in Av) for each lens, as well as a couple sample shots.

50mm - F2, ISO200, no flash:
1/400s 0.0ev
1/320x +0.3ev
1/250s +0.7ev

30mm - F2, ISO200, no flash:
1/250s 0.0ev
1/200s +0.3ev
1/200s +0.7ev





Also, PhotoME reports "distant view" for the above 50mm photo and "close view" for the 30mm photo. I sat in the exact same spot for each photo, so what accounts for the difference?

Thanks very much for your help!


Last edited by twokatmew; 02-05-2010 at 12:53 PM.
02-05-2010, 02:59 PM   #2
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What's happening here is that the picture is different with each lens. The camera's light meter is a complex thing, but what it boils down to is that it tries to average the whole picture into an 18% gray (it's more complicated than that, but that's a good way to look at it). So it'll try and expose the entire scene to make all the luminance average to 18% gray.

Your first shot only contains the book, but it also has a higher percentage of white. So the camera ends up under-exposing (choosing a faster shutter speed). That's why the picture looks darker.

The second shot encompasses more, but less of it is white, so the camera needs more exposure to make it average to 18% gray. That's why that image looks brighter.

If you stood farther away using the 50mm lens, and got the same composition in the frame as with the 30mm lens, then the camera would probably use a slower shutter speed and you'd get 2 pictures that are more closely exposed.
02-05-2010, 03:05 PM   #3
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The EXIF data shows that your camera is set to matrix metering. Try setting the meter to spot and then spot meter the same thing. Your exposure values should be identical (or very close) then.
02-05-2010, 03:22 PM   #4
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Thanks Frank and Steve! Tomorrow I'll do some more experimenting. I guess I'd better get reading my "Understanding Exposure" book, too.

02-05-2010, 03:26 PM   #5
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If you look at the results, the exposures are similar so you know the camera is doing it's job.
Now you just have to figure out how to do yours.

Nevertheless, it's obvious you're headed in the right direction because that was a good question to ask.
02-06-2010, 01:43 PM   #6
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QuoteOriginally posted by twokatmew Quote
Thanks Frank and Steve! Tomorrow I'll do some more experimenting. I guess I'd better get reading my "Understanding Exposure" book, too.
That book is excellent and I continue to learn things from it every time I pick it up. I did so last night, in fact. Good luck!
02-06-2010, 02:42 PM   #7
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In green, P, Sv or Av mode, the camera will also select a faster shutter speed for the longer focal length to avoid camera shake.
02-06-2010, 03:22 PM   #8
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Thanks everyone! This is starting to make more sense to me. I did more test shooting today, and I did indeed find that when I used spot metering the shutter speeds were closer and sometimes the same. The same goes for my standing back with the 50mm lens for the same composition as with the 30mm lens.

I wasn't thinking about the SR increasing the shutter speed for the greater focal length, either.

I was also playing around with the dynamic range settings, and using both the contrast and shadow correction, I get a seemingly much better exposed shot, though still not "right" due to the poor lighting. I'd rather learn to expose correctly rather than what seems like "cheating" to me by using the dynamic range settings. How/why would I use these settings, and am I being a snob by not wanting to use them?

Damn Brit, I'm glad you thought my question was a good one. I was a little afraid it might be a stupid one. But yes, I'm trying to learn.

02-07-2010, 12:11 PM   #9
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QuoteOriginally posted by twokatmew Quote
I'd rather learn to expose correctly rather than what seems like "cheating" to me by using the dynamic range settings.
Using "extended dynamic range" is NOT cheating with exposure. The sensor on the camera can only record a part of the brightness range the human eye can see. Extending the dynamic range allows the sensor (using software rather than hardware) to extend the range of brightness it can record. It still doesn't record as much as what we can see, but it brings it a step closer, but at the expense of increased noise in dark areas. Sometimes, it's the difference between seeing details in the groom's black tux or no details at all. It sure does have it's use in the right circumstances.
02-07-2010, 01:55 PM   #10
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QuoteOriginally posted by flyer Quote
Using "extended dynamic range" is NOT cheating with exposure. The sensor on the camera can only record a part of the brightness range the human eye can see. Extending the dynamic range allows the sensor (using software rather than hardware) to extend the range of brightness it can record.
Does it really, though? I'm always confused by how exactly the expanded dynamic range options work. What exactly does it do? Make different parts of the sensor behave differently depending on how much light hits it?

As you mentioned, the expanded shadow setting introduces a lot of noise. it can be pretty bad. That led me to think this was actually just software acting as a filter on what the hardware is seeing.

And the expanded highlight option limits the ISO to a minimum of 200. The only time I really need expanded highlight range is when I'm outdoors or in bright light, which is when I tend to shoot at ISO 100 anyways. I'm still not even sure what the camera is doing to "expand" the highlight range. If the sensor has more range than is being used, why isn't that always on?
02-07-2010, 02:00 PM   #11
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I really wish there were a Magic Lantern guide (or something similar) for the K-x. I've read (and keep re-reading) the manual, and although it tells me in limited detail what a given function does, being a newbie, I don't know why I'd want to use certain features. I can play with the camera and see what various settings do to my photos, but I don't know *how* they do it.

Is there any sort of book out there that expands on the manual for us noobs?
02-07-2010, 02:33 PM   #12
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QuoteOriginally posted by twokatmew Quote
I really wish there were a Magic Lantern guide (or something similar) for the K-x. I've read (and keep re-reading) the manual, and although it tells me in limited detail what a given function does, being a newbie, I don't know why I'd want to use certain features. I can play with the camera and see what various settings do to my photos, but I don't know *how* they do it.

Is there any sort of book out there that expands on the manual for us noobs?
not a set of books, exactly. Just this forum and the vast Internet

But be sure and start a new thread for different subjects, and make an effort to use the Google search feature in case your questions have already been asked.
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