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Tamron 70-300 VC exposure issues
#21
>> Well, maybe they (finally) do. There have been reports of Di 60 lenses which no longer show underexposure, either purchased new just recently or repaired by Tamron. That's why I just sent mine in again.

> While we're at it, just a quick update on this one: the lens is back from Tamron, repair report says aperture has been adjusted, but it seems unchanged to me.



Have they adjusted metering perhaps?

My guess regarding the exposure issue with this lens is that the aperture works as designed, but that there is another rim in the lens blocking the light more than the aperture diaphragm at inf and intermediate focus (not at close focus, you get F/2 there). And that the autoexposure issue is due to the lens "thinking" it meters at F/1.8 (the metering aperture position is a bit wider than that of F/2), which in fact gives no more light than say F/2.5.

If this is so it should be possible to adjust the meter to the reality so +1EC would not be required anymore. Don't know if the underexposure in M is fixable because this is likely a construction thing.

Notice that the Nikkor 50/1.8D seems to have a similar issue but at the close-focus end (less likely to be noticed when testing MTF and such).
#22
I´ve been reading all over internet tamron lenses on nikon body tends to give overexposure shots.



I would just compensate exposure a little but using a tamron lens.



I believe this is not a faulty lens, but a quirk of the lens.
#23
[quote name='_sem_' timestamp='1287995098' post='3769']

Have they adjusted metering perhaps? [/quote]



According to my tests so far, it doesn't seem so.



[quote name='_sem_' timestamp='1287995098' post='3769']

My guess regarding the exposure issue with this lens is that the aperture works as designed, but that there is another rim in the lens blocking the light more than the aperture diaphragm at inf and intermediate focus (not at close focus, you get F/2 there).[/quote]



Certainly a point to consider. So far I did all formal exposure testing at infinity. However, the issue was clearly visible to me in the field, too. At any distance.



[quote name='_sem_' timestamp='1287995098' post='3769']

And that the autoexposure issue is due to the lens "thinking" it meters at F/1.8 (the metering aperture position is a bit wider than that of F/2), which in fact gives no more light than say F/2.5.

[/quote]



This is the part that I don't understand, really. In my opinion, all "thinking" should be done by the camera. And, actually, there's not really much "thinking" to it anyway, the camera simply has to meter the light coming through the lens with wide open aperture (and it doesn't really make any difference if it does so with a single cell light meter or a thousand dot matrix metering ... it's the camera in the end that meters and matters). It actually doesn't really need to know what that aperture is used (that's how stopped down metering works with old manual lenses, right?), unless it needs this information to "scale" the metering for other apertures (read: any modern lens).



But to be honest: I have no clue how the metering and information and parameter exchange between lens and camera actually works. I just find it strange that especially Tamron seems to struggle quite a bit in this regard, but only with F mount.



To be fair, I just got the Sigma 70-300 OS back from service. It seems to show very similar behaviour as the Tamron 70-300 VC ... which is significant overexposure especially at the short end.



-- Markus
Editor
opticallimits.com

#24
[quote name='hackmann' timestamp='1288183660' post='3816']

I´ve been reading all over internet tamron lenses on nikon body tends to give overexposure shots.[/quote]



It's not that simple. I have used Tamron lenses that showed underexposure (Di 60/2), overexposure (70-300 VC) and some seemed to be spot on (17-50 VC).



[quote name='hackmann' timestamp='1288183660' post='3816']

I would just compensate exposure a little but using a tamron lens.

[/quote]



That only works if the exposure offset is static. Which is unfortunately not the case with the 70-300 VC (and the Sigma 70-300 OS, it seems), which shows a varying amount of exposure offset with different focal lengths and apertures.



[quote name='hackmann' timestamp='1288183660' post='3816']

I believe this is not a faulty lens, but a quirk of the lens.

[/quote]



In this case I would call it faulty by design <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='Smile' />



-- Markus
Editor
opticallimits.com

#25
[quote name='_sem_' timestamp='1287995098' post='3769']

>> Well, maybe they (finally) do. There have been reports of Di 60 lenses which no longer show underexposure, either purchased new just recently or repaired by Tamron. That's why I just sent mine in again.

> While we're at it, just a quick update on this one: the lens is back from Tamron, repair report says aperture has been adjusted, but it seems unchanged to me.



Have they adjusted metering perhaps?

My guess regarding the exposure issue with this lens is that the aperture works as designed, but that there is another rim in the lens blocking the light more than the aperture diaphragm at inf and intermediate focus (not at close focus, you get F/2 there). And that the autoexposure issue is due to the lens "thinking" it meters at F/1.8 (the metering aperture position is a bit wider than that of F/2), which in fact gives no more light than say F/2.5.

If this is so it should be possible to adjust the meter to the reality so +1EC would not be required anymore. Don't know if the underexposure in M is fixable because this is likely a construction thing.

Notice that the Nikkor 50/1.8D seems to have a similar issue but at the close-focus end (less likely to be noticed when testing MTF and such).

[/quote]

Actually many macro lenses have extra shades built into them, even normal ones. Some even have extra diaphragms. All this to prevent flare amongst others, especially when closing down the diaphragm.



Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
#26
[quote name='mst' timestamp='1288186210' post='3818']

It's not that simple. I have used Tamron lenses that showed underexposure (Di 60/2), overexposure (70-300 VC) and some seemed to be spot on (17-50 VC).







That only works if the exposure offset is static. Which is unfortunately not the case with the 70-300 VC (and the Sigma 70-300 OS, it seems), which shows a varying amount of exposure offset with different focal lengths and apertures.







In this case I would call it faulty by design <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt='Smile' />



-- Markus

[/quote]

Markus,



have you tried spotmetering as well, by any chance, to see if that gave the correct results?



Kind regards, Wim
Gear: Canon EOS R with 3 primes and 2 zooms, 4 EF-R adapters, Canon EOS 5 (analog), 9 Canon EF primes, a lone Canon EF zoom, 2 extenders, 2 converters, tubes; Olympus OM-D 1 Mk II & Pen F with 12 primes, 6 zooms, and 3 Metabones EF-MFT adapters ....
#27
[quote name='wim' timestamp='1288187195' post='3820']

have you tried spotmetering as well, by any chance, to see if that gave the correct results?

[/quote]



Yes, I tried all three metering modes. Made no difference.



-- Markus
Editor
opticallimits.com

#28
Have you tried center weight metering with exposure lock feature? I am pretty sure it is not human error, but it just a test. I have bad timming using matrix, I dont like it.



I have 3 nikkor lens and two sigma



All my nikkor lens give at least -0.5 EV (18-105, 50mm 1.8d, 70-300)and a little cool tone (more in 18-105), at least for my eyes.



both sigmas (150mm ex macro and 24-70mm) nail the exposure and color tone, at least for my eyes.



What I am trying to say is that tamron does not follow a pattern and sometimes provide "bad copy" lens. But I only read about that, i dont know for sure.



Maybe you have a bad copy, I dont know. I never had any tamron. I would like to have some to give it a try. But I live in Brazil and here I cannot find those lens.



Does your F number change when u try to have a close focus??



<img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/smile.gif' class='bbc_emoticon' alt='Smile' />
#29
Sorry for the delayed response...



[quote name='mst' timestamp='1288185934' post='3817']

According to my tests so far, it doesn't seem so.



Certainly a point to consider. So far I did all formal exposure testing at infinity. However, the issue was clearly visible to me in the field, too. At any distance.

[/quote]



I think in the field there are two things involved. First is the naked effect of the aperture. Second is the meter (autoexposure, and also M mode indirectly via you watching the meter).



I can clearly see the aperture blades move looking from the front using the DoF preview button. It certainly does get wider when decreasing the F stop. The metering position is even a bit wider than F/2.0. But at inf focus, watching the resulting histogram, there is no more light when going from F/2.4 to F/2.0. But there is at close-focus. Watching through the detached lens from the rear playing with the aperture lever observing the rims seems to confirm this.



[quote name='mst' timestamp='1288185934' post='3817']

This is the part that I don't understand, really. In my opinion, all "thinking" should be done by the camera. And, actually, there's not really much "thinking" to it anyway, the camera simply has to meter the light coming through the lens with wide open aperture (and it doesn't really make any difference if it does so with a single cell light meter or a thousand dot matrix metering ... it's the camera in the end that meters and matters). It actually doesn't really need to know what that aperture is used (that's how stopped down metering works with old manual lenses, right?), unless it needs this information to "scale" the metering for other apertures (read: any modern lens).

But to be honest: I have no clue how the metering and information and parameter exchange between lens and camera actually works. I just find it strange that especially Tamron seems to struggle quite a bit in this regard, but only with F mount.

[/quote]



I'm sorry if I can't write clearly <img src='http://forum.photozone.de/public/style_emoticons/<#EMO_DIR#>/wink.gif' class='bbc_emoticon' alt=':blink:' />

I think the camera is aware at what aperture it is metering, which is at a bit wider than F/2.0, say F/1.8. But I assume the exposure will be miscalculated if what the camera in fact sees at position F/1.8 is no brighter than at say F/2.5 (the point where the aperture diaphragm actually becomes the limiting rim - its movement earlier being a kind of a dead-zone, onyl with some effect on bokeh). I would assume this could be a problem with adapting the lens to the F-mount, and the kind of thing Tamron could have a problem admitting.



Another thing is the light meter which seems "rigged". I mean, the lens is obviously metering always at the same widest aperture position. But changing same-exposure combination in M mode (dec F-number and inc shutter time, both 1 stop at a time) with the same white-wall scene the meter readout changes when approaching wide-open (not taking a shot at all, so the aperture stays in place all the time). It seems to cover up for the less light at widest aperture. I was very surprised to see such behaviour and I'd kindly ask someone knowledgeable to have a look into it. Also noticed this with Nikkor 18-200VR but not with 50/1.8D. I guess this could be reasonable for more user-friendly experience with superzooms, but a questionable practice with primes.
#30
[quote name='hackmann' timestamp='1288217672' post='3823']

Does your F number change when u try to have a close focus??

[/quote]

Certainly it does, but that's effective aperture, it decreases at close-focus normally ("bellows factor"). The nominal aperture (physical hole) is probably the same throughout the range. Can't tell for sure due to the lenses between my eyes and the aperture diaphragm, but the aperture blades surely do move at least approximately as expected at both focus ends.
  
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