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07-13-2011, 10:24 AM   #1
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Strange light readings with prime vs zoom

I have a question. I was just using my Gossen Luna Pro light meter and checking exposures against what the K1000 camera says I need. When I use a 50mm 1.4 or 28mm 2.8 lens, the readings are very close to the meter. When I use the 35-105 zoom, the camera says I have more light than the other lenses and meter and recommends a higher aperture. Do you know if the zoom somehow affects the light readings compared to a prime lens?

07-13-2011, 10:43 AM   #2
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aperture is not a direct measure of light transmission, that would be a T-stop. Just cause 2 lenses have the same aperture does not mean they can transmit the same amount of light, since aperture is just a ratio of the focal length to the aperture opening thingy
07-13-2011, 10:45 AM   #3
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That would explain it. I tried another zoom on the camera and obtained similar readings to the Pentax zoom. If this is the case, would that mean that a Gossen light meter would be relatively useless for a zoom or telephoto lens?
07-13-2011, 10:51 AM   #4
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no, usually the amount of light you get from light readings is pretty damn close. Just remember to add however many stops of light you need to compensate for any shortcomings of the lens itself. Think of it as mentally applying exposure compensation

07-13-2011, 10:54 AM   #5
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I am assuming that the zoom lenses are giving the correct readings to the camera's meter based on the fact that the camera gets the actual light through the lens. The readings must be affected by the lens itself.

I tried the Pentax 35-105, a Tamron 60-300, and Vivitar Series 1 70-210 and had about the same readings at 3.5 aperture. The two primes seemed to have the same readings (different than the zooms). I guess I'm going to take the camera's word for the readings.
07-13-2011, 10:58 AM   #6
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the camera meter works differently from an external meter. It looks through the lens and sees the available light in order to give you the reading. that's why the meter doesn't appear to over or underexpose because of different lens transmission rates. External meters read either the reflected or the incidence light available and give an approximation of the light reading based on how most lenses perform at certain apertures.
just remember that meters are not gods, they can be fooled too
07-13-2011, 11:39 AM   #7
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QuoteOriginally posted by adpo Quote
the camera meter works differently from an external meter. It looks through the lens and sees the available light in order to give you the reading. that's why the meter doesn't appear to over or underexpose because of different lens transmission rates. External meters read either the reflected or the incidence light available and give an approximation of the light reading based on how most lenses perform at certain apertures.
just remember that meters are not gods, they can be fooled too
To clarify a bit. Incident light meter measures the available light. Incident meters cannot be 'fooled'. Reflected light meters (including the camera meter) are easy to fool.
07-14-2011, 01:11 AM   #8
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All hand held meters have a wide angle of view where as the TTL light reading would show a more spot reading of the scene. This is another take. This also could be another reason, average reading with the hand held and center weighted, multi segment or spot with the camera. All will give different readings of the same scene as well as focal length of the lens on the camera.

07-14-2011, 10:34 AM   #9
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Now for something completely confusing

As the title implies, there is another confusing thing to consider.

first of all what are the lenses exactly that you are using, K, A, F FA etc... i.e. are they auto aperture or manual aperture, etc.

some cameras meter oddly with manual aperture lenses, especially the K10D and K20D, and especially at large apertures, where they tend to under expose, and at smallapertures where they tend to over expose.

your 35-105 is probably an F5.6 ens which is magically where the cameras meter the best.

what you may wish to try is take a series of shots, at each aperture letting the camera meter, and then measure the grey scale value in a photo editor. a uniform surface like a wall or paved road is best. 120 greyscale is correct exposure and each 40 greyscale variance from this is 1 stop.,

compare this exposure test to you r hand held meter.
07-14-2011, 08:22 PM   #10
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Strange Metering

I am using the Pentax 35-105 f 3.5 on a K1000 vs. the readings on a Gossen Luna Lux meter.
07-14-2011, 09:51 PM   #11
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And yet another factor: different AOV's gather different amounts of light from non-trivial views. This is most extreme with a full-circle fisheye vs a long tele -- the fisheye image includes fast regions of black at the sides. But you should see this with the 35-105. Set metering mode to matrix or center-weighted and shooting mode to M(anual). Set the aperture to f/8. Aim at a far object. Meter at 35mm and again at 105mm. I betcha they aren't the same! And they'll differ from the handheld meter too, which probably sees a different AOV than the lens at either extreme.

Last edited by RioRico; 07-14-2011 at 09:57 PM.
07-15-2011, 04:52 AM   #12
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QuoteOriginally posted by njpentax Quote
I am using the Pentax 35-105 f 3.5 on a K1000 vs. the readings on a Gossen Luna Lux meter.
the 35-105 F3.5 is it constant aperture or is it a variable aperture?

typically I would expect this to be F5.6 at the 105mm focal length.

wide open, the lens will meter correctly on the camera, but you have no way of knowing at other than minimum, focal length the true aperture.

The aperture markings are set identified only for the minimum focal length,
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