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10-30-2007, 06:54 AM   #1
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K100D exposure diff P, Av, Tv...

As I spent the weekend playing lens/camera comparison I noticed a couple of fairly consistent trends:

1) The 16-45 does tend to underexpose, even in P

2) I get different exposures (EV, not just a different shutter/aperture selection) when shooting the same scene in P, Av or Tv. With the two AF lenses (43 ltd, 16-45) it's usually less exposure when out of P. With MF lenses (A 28/2.8, Tamron SP 35-80) there isn't a large difference. The MF lenses tended to expose more 'correctly' than the AF did, in Av and Tv.

I noticed this fairly consistently when I used the 43/K100D as light meter for my Diacord, across many conditions. I verified it with a handful of shots. This was consistent whether I shot RAW or jpeg.

3) I use a Gossen LunaLux, in incident mode. When using the reading on the 43/K100D in manual exposure, the result is dark but very saturated. Not unpleasant, but it tells me - assuming the LunaLux is accurate (it did confirm the sunny 16) - there's something else going on in the camera/lens interface.

Some results are here:
lens_testing Photo Gallery by jussi at pbase.com


The exposure info for the bookcase shots (all camera metered in Av f6.3):
A 28: 1/2s f/6.3 iso400
16-45: 1/15s f/6.3 at 28.0mm iso400
43mm: 1/25s f/1.9 at 43.0mm iso400
35-80: 1/10s f/6.3 iso400

the 16-45 in pgm mode: 1/50s f/4.0 at 45.0mm iso400


I realise there may be other things going on, such as the metering pattern changing between lenses or modes (something I'll have to take a look at), but what's going on? Anyone else notice these patterns?

10-30-2007, 07:02 AM   #2
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Oh damnit, I didn't get the right version of the 43mm shot up on pbase. This testing stuff gets mighty confusing.
10-30-2007, 07:11 AM   #3
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I have spent a long(ish) time trying to at least understand how the metering is working with each lens. The best I have come up with is a relitively simple test method.

All you need is a cloudless sky, daylight, and some form of uniform grey to white surface. (I use either the road surface, or a concrete block wall or sidewalk.

Pick manually an ISO setting that lets you work through the entire range of apatures while keeping below maximum shutter speed.

Once this is done, start at maximum apature and work towards minimum, taking shots at your uniform surface. In any photo editor, display each photo, and record / plot the grey scale histogram average value and shutter speed as a function of apature. For Automatic Apature lenses, I use Av mode and just turn the dial, on Manual Apature lenses, I use manual mode and the green button (on my K10 or *istD)

This will allow you to check the exposure accuracy of each lens, and is much better than trying to figure out the differences when you have a wide range of exposures in your shot.

Note, on the K10D, with contrast set to minimum (shooting JPEG) , a 1EV change (1 stop) is equal to about a change of 35 in grey scale, and at maximum contrast, a 1EV change (1 stop) is equal to a change of about 50 in gray scale.
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