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09-10-2008, 12:29 PM - 1 Like   #1
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Inconsistent metering at different focal lengths with 18-250mm

Is this normal? I took a series of similar pictures with three lenses, 50mm f/1.4, 35mm f/2, and 18-250mm f/3.5-6.3. Program mode, EV +0.3 for all, matrix metering.

While the 35mm and 50mm seemed to meter very similarly, metering at higher zoom on the 18-250mm seems to be inconsistent. The image gets darker as the focal length is increased, yet no other settings have been changed. While the aperture has changed, the camera should have compensated by reducing shutter speed, etc. Please ignore the color shift on the 18-250mm pics, I forgot to remove a warming filter.

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09-10-2008, 12:48 PM   #2
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QuoteOriginally posted by krypticide Quote
yet no other settings have been changed.
did you crop or did you walk
09-10-2008, 01:13 PM   #3
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QuoteOriginally posted by krypticide Quote
Please ignore the color shift on the 18-250mm pics, I forgot to remove a warming filter.
Did you have a warming filter on the 35 & 50? if not then take it off the 18-250 and shoot again.
09-10-2008, 01:28 PM   #4
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You didn't note the shutter speeds used.

Clearly the composition is somewhat different among the 18-250 images--none of the three are that close to the FA35 & FA 50 shots, maybe the 77.5mm is the closest.

Second, was auto ISO enabled? It is likely that the shutter speed (and possibly ISO) were both altered between shots to compensate for the reduction in available light.

Thirdly, there appears to be a fair amount of falloff on the 18-250 (dark corners) that probably exaggerates any difference in exposure--if you compare the middles of the frames, the difference is probably smaller. Stopping down should reduce the falloff somewhat--all your shots were wide open. The APS-C-designed 18-250 has about 1EV falloff (corners are 1EV dark) when wide open. Stopped down to f/11, it's about EV 1/2. The two full-frame-designed primes only have about EV 1/4 at f/2.8, barely noticeable.

So there may be some difference in metering, but maybe not as big a difference as you think.

09-10-2008, 01:33 PM   #5
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QuoteOriginally posted by Venturi Quote
Did you have a warming filter on the 35 & 50? if not then take it off the 18-250 and shoot again.
The filter might affect the resulting aperture/shutter/ISO settings but the resultant capture should be of similar luminance. You could put a 2-stop ND filter on and it still shouldn't change the brightness of the image. The different color may affect perception somewhat though.
09-10-2008, 01:52 PM   #6
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Just for "fun" I took the grey scale average at the same point in all 5 clippings

the results are

with the 35mm lens, I read 181
with the 50mm lens I read 189

With the zoom
set to 18mm I read 194
set to 77 mm I read 150
set to 250mm I read 140

I think part of the problem is the metering itself interpreting the images differently as a function of focal length.

I note that yoou have been asked already regarding composition and whether these are crops or you physically moved to get the same relitive image, this is important to evaluate the exposure, because matrix metering will change with composition. It is always better to use spot metering.

If you consider the 18mm, 35mm and 50m m these are very close together, the spread from 181 to 194 for the grey scale value (between the regulator gauge and the gas handle on the next tank) represents 1/3 of a stop.

similar comment for the 77 and 250mm shots, they are within 1/4 stop.

I think you are getting tricked by differing composition and matrix metering.

Try just the wall, or use spot metering and see how it works.
09-10-2008, 01:53 PM   #7
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The proportion of light and dark in the scene changes as you zoom. The camera will try to compensate according to the metering algorithm. I find matrix metering particularly easy to get exposure variations.
09-10-2008, 03:53 PM   #8
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QuoteOriginally posted by creampuff Quote
The proportion of light and dark in the scene changes as you zoom. The camera will try to compensate according to the metering algorithm. I find matrix metering particularly easy to get exposure variations.
Creampuff gave you the best answer.

On the 18-250, the proportion of light background to dark foreground increased for each step (esp. around the center). This was not the case for the two primes. In fact, the 50mm is slightly lighter than the 35... guess what? it has proportionately more foreground in it's composition. The difference is only very slight - resulting in only a very slight difference in exposure.

09-10-2008, 04:45 PM   #9
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This is very simple, the further you zoome dout the closer you came to max apeture and every lens I ever owned underexposes by half a stop at max apeture compared to f/8.
09-10-2008, 05:47 PM - 1 Like   #10
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Original Poster
Thanks for the replies. I did not crop.

I did take three consecutive shots of a blank wall at the same angle and lighting (just walking closer or farther depending on focal length), and the exposure differed as well. I deleted those shots though, so I'll have to redo it again.

It shouldn't have to do with aperture or shutter speed, as I did not bump up against the max or min ISO, aperture, shutter speed for any of these shots.

I'll redo it with the wall again and post the results.
09-10-2008, 06:08 PM   #11
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At 250mm, f/6.3 is the max aperture
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