[Xastoul]

I've tried the Sigma 17-70 and I've been suprise how is was more bright than my other lens. I've took the same picture my 18-55 and switch to the 17-70, with the same seting I end up with a over exposed picture. Is this just because the 17-70 have bigger glass in front? ( 72mm vs 52mm on filter size)



Which would mean that the bigger to glass in front, the better the lens will act in low ligth situation ( since you need to step down to have the same picture )



I'm not sure what the explanation for all this...

[Klaus]

[quote name='Xastoul' timestamp='1326329102' post='14779']

I've tried the Sigma 17-70 and I've been suprise how is was more bright than my other lens. I've took the same picture my 18-55 and switch to the 17-70, with the same seting I end up with a over exposed picture. Is this just because the 17-70 have bigger glass in front? ( 72mm vs 52mm on filter size)



Which would mean that the bigger to glass in front, the better the lens will act in low ligth situation ( since you need to step down to have the same picture )



I'm not sure what the explanation for all this...

[/quote]



This shouldn't occur actually. The amount of incoming light should be measured and compensated accordingly via shutter speed. If you are seeing substantial overexposures this would suggest a problem with the aperture mechanism - at least if this occurs after stopping down.

[Guest]

Unless it was on manual setting and aperture set to max? Just guessing <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/rolleyes.gif' class='bbc_emoticon' alt='' />

[frank]

[quote name='Klaus' timestamp='1326353568' post='14787']

This shouldn't occur actually. The amount of incoming light should be measured and compensated accordingly via shutter speed. If you are seeing substantial overexposures this would suggest a problem with the aperture mechanism - at least if this occurs after stopping down.

[/quote]



I am curious. Can overexpose happen to a lens when the lens is wide open?



Frank

[wim]

[quote name='Frank' timestamp='1326357905' post='14795']

I am curious. Can overexpose happen to a lens when the lens is wide open?



Frank

[/quote]

Not normally. Only if something is wrong with the lightmetering system, or when the lens reports something to the camera it shouldn't.



Having said that, overexposure can occur for other reasons. For one there is the fact that the metering system in camera only guesses at whatever it thinks is correct, and circumstances and metering mode may cause overexposure (or underexposure) indeed. It really is up to the photographer to judge the circumstances correctly, and adjust the exposure accordingly. Do note that the metering system is based on average settings for exposure - it treats the metered parts on average as a scene with 18% reflection. If it is in fact a very bright scene, it will therefore underexpose, relatively speaking, and when it is a very dark scene, it will overexpose, relatively speaking. It will try to bring everything back to this 18% average.



F.e., shooting a large, bright sky without any adjustments will result in underexposure of everything that is not sky, generally speaking, and similarly, when shooting a nightscape, everything will be overexposed and look as if it is day when one leaves it to the metering system. In short, one should use one's discretion in correcting exposure to obtain the exposre one desires or aims at. But that is nothing new, this was valid with film as well <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/biggrin.gif' class='bbc_emoticon' alt='' />.



Kind regards, Wim

[Xastoul]

Both picture were take a 50mm f8, the camera on manual, Wb in manual. They were take will the camera was stable on a desk. If this can help.

[Guest]

I've seen this before with some of my lenses. In my case, a Sigma was about a stop or so darker than the equivalent Nikon. I guess that's the difference between f-stops and T-stops...

[wim]

[quote name='BG_Home' timestamp='1326553225' post='14858']

I've seen this before with some of my lenses. In my case, a Sigma was about a stop or so darker than the equivalent Nikon. I guess that's the difference between f-stops and T-stops...

[/quote]

No, it isn't, actually. It may be a difference between aperture number reported to the metering system prior to closing down the aperture, and actual aperture when closed down, provided of course the lighting didn't change between taking the shot and metering. This because light is measured prior to taking the shot with any automatic aperture lenses, while the shot is taken with aperture close to the selected aperture number.



Kind regards, Wim

[Guest]

[quote name='Xastoul' timestamp='1326378423' post='14810']

Both picture were take a 50mm f8, the camera on manual, Wb in manual. They were take will the camera was stable on a desk. If this can help.

[/quote]



that sounds more like the sigma lens has some miscalibration or even hardware issue, the f-stop not closing down to f8 as it should, maybe only to f5.6.

If you increase ISO, so that you can use faster shutter speeds, does the effect get worse?? (In case the mechanical shutter of the Sigma does not close fast enough - an unlikely scenario, but the above test is easy).

[Xastoul]

[quote name='photonius' timestamp='1326633685' post='14880']

that sounds more like the sigma lens has some miscalibration or even hardware issue, the f-stop not closing down to f8 as it should, maybe only to f5.6.

If you increase ISO, so that you can use faster shutter speeds, does the effect get worse?? (In case the mechanical shutter of the Sigma does not close fast enough - an unlikely scenario, but the above test is easy).

[/quote]



I do not have the lense with me rigth now, but I'll do that test as soon as I get home



Thanks



and thank to all the other