Originally posted by Zewrak The truth. The more closed down the lens is, the more you will overexpose it using the metering.
I was intrigued by this comment because I didn't think that I noticed the same thing with the M lens as I did the Takumar. They are both manual lenses, so I should see the same effect. So, a little more testing was in order.
I took some pictures in a controlled environment (I don't have to worry about whether it was a little cloudier when such and such lens was on). Just some shots of a wall lamp. I aimed the lens right at the same middle point of the lamp for every picture. Same as before, I set the ISO to 100 for all shots, I set the aperture, and pressed the green button for the shutter speed. In all cases I used what the camera suggested for the shutter speed. I will show my calculated Exposure Value for each picture.
With the Super-Multi-Coated Takumar 50mm F1.4 lens:
F=1.4, Shutter = 1/3000 sec., Exposure Value = 12.5
F=4.0, Shutter = 1/350 sec., Exposure Value = 12.5
F=8.0, Shutter = 1/45 sec., Exposure Value = 11.5
F=16, Shutter = 1/8 sec., Exposure Value = 11.0
These findings are consistent with my outdoor shots in the original post. As the Aperture narrows, the camera's meter reads less available light and changes the shutter speed. Ideally, the Exposure Value would not budge, because I know the lamp isn't suddenly giving off more light.
With the SMC Pentax-M 50mm F1.7 lens:
F=1.7, Shutter = 1/3000 sec., Exposure Value = 13.1
F=4.0, Shutter = 1/350 sec., Exposure Value = 12.5
F=8.0, Shutter = 1/45 sec., Exposure Value = 11.5
F=16, Shutter = 1/8 sec., Exposure Value = 11.0
I see precisely the same phenomenon with the M as I did the Takumar. As the F stop narrows the camera starts metering for less available light, the EV goes down, the shutter is held open longer than it should be, and the picture is brighter than the pictures at wide apertures. Again, the subject hasn't changed at all. The EV readings should be the same. If 13 is correct at F1.7, then 13 should be correct at F16. This means my observation that I was only seeing this phenomenon with the Takumar and not the -M lens was incorrect. In fact, the effect is greater with the -M lens. A swing of 2 EV units is shown for the -M and a swing of 1.5 units is shown for the Tak. Zewrak was correct. Gotta give credit where credit is due.
To round out the picture here I decided to see if I see the same phenomenon for lenses that can talk to the camera body. I put on my FA 35mm lens and did the same test. Results below.
With the SMC Pentax-FA 35mm F2.0 lens:
F=2.0, Shutter = 1/750 sec., Exposure Value = 11.6
F=4.0, Shutter = 1/180 sec., Exposure Value = 11.5
F=8.0, Shutter = 1/45 sec., Exposure Value = 11.5
F=16, Shutter = 1/10 sec., Exposure Value = 11.3
And now some more examples with the FA 35 using sunlight, not lamplight.
F=2.0, Shutter = 1/20 sec., Exposure Value = 6.3
F=4.0, Shutter = 1/6 sec., Exposure Value = 6.6
F=8.0, Shutter = 0.7 sec., Exposure Value = 6.5
F=16, Shutter = 3 seconds, Exposure Value = 6.4
With the -FA lens on, the K10D's meter is consistent throughout the F-stop range. In both artificial and natural lighting scenerios, the metering discrepency is not more than 0.3 EV units. That is very good, considering that not all EV scores are even possible (meaning, the camera can't decide to hold the shutter open for 1/49th of a second, even if that would be perfect). For this reason, I believe that it is metering as consistently as possible with the -FA lens.
My Conclusion:
When the lens and the K10D can't talk, the camera's meter has a consistent tendency to overexpose images at narrow apertures. I've seen this with two different lenses and in two different lighting scenerios: artificial (this post) and natural (the original post). The swing can be pretty dramatic too, ranging as high as two full exposure value units (2 stops in traditional parlance). However, it appears that this isn't a problem when the lens and camera body can talk. I've only tested this so far with my -FA 35mm, so I'm not as confident of this finding, but it certainly holds true for this lens. When I get a chance I may give this kind of a test to some other auto exposure capable lenses.
I hope people find this interesting. I like it.
And thanks again, Zewrak