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11-26-2010, 10:48 AM   #1
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Tamron 17-50mm f2.8 not as fast (bright) as Tamron 28-75mm f2.8 on Pentax K-x, PICS

I've been using the Tamron 28-75mm f/2.8 on my Pentax K-x with fantastic results, but I sometimes miss the wide angle. I planned on replacing my Tamron 28-75mm f/2.8 with its little brother, the Tamron 17-50mm f/2.8. Assuming I was happy with the 17-50mm, I would then sell the 28-75mm and buy a Sigma 50-150mm f/2.8, so I would then have 17mm to 150mm at f/2.8.

I just received the 17-50mm, so phase I is complete. After doing back-to-back comparisons of the 17-50mm vs the 28-75mm though, I'm starting to have second thoughts.

I haven't used the 17-50mm very much yet, but it seems to take nice sharp pictures with great colors and contrast, but it's not as bright as my 28-75mm at equivalent f-stops.

Comparing both lenses at f/2.8, they are very similar, but the 28-75mm is a little brighter. The difference becomes more noticeable as the aperture decreases, becoming easily visible by f/5.6 and strikingly different by f/10.

What this all means, is that I have to use a slower shutter speed on the 17-50mm than I do on the 28-75mm, which will be a limitation in some situations.

I first noticed this behavior when shooting a portrait of a friend in my home studio. I started out using the 28-75mm, and near the end of the shoot I put the 17-50mm lens on the camera to see how it compared under controlled lighting conditions. I had just received the lens in the mail a few hours earlier and had not yet used it. Both lenses were set at f/10 and 1/125 sec throughout the shoot, and the strobe settings remained constant.

I was surprised to find that the shots taken with the 17-50mm were significantly underexposed, so that my subject was very dark and my white backdrop was no longer blown out (as it should be for high key photography).

Here are a couple test shots from the shoot (BTW, it's just a coincidence that he is smiling in the first picture and looks less enthusiastic in the second):



I then took some test pictures at various apertures using both lenses. The subject was a toy car that was just lit by the light coming from my LCD monitor and dim overhead lighting. (note that I horizontally flipped the second image so that the images would mirror each other, since lighting is not even across the image).



At f/2.8 you can see that brightness seems to be similar, but on my monitor I can see that the 28-75mm picture is just a little brighter. As the aperture increases, however, the difference becomes much more pronounced.



And by f/11 there is a huge difference in brightness.



I suppose that the most important thing is that the 17-50mm still performs well at f/2.8, producing an image almost as bright as the 28-75mm. But I find it disturbing that at larger apertures the lens is clearly not capturing as much light as the 28-75mm.

Since the 17-50mm is supposed to be based on the 28-75mm, with a similar optical formula, I am surprised to see such a difference. Could this be because the 28-75mm is a full frame lens (35mm), while the 17-50mm is designed to only cover a smaller, APS-C sensor?

The way I understand it, f-stops are not fixed dimensions, but are based on some type of ratio involving the length of the lens and the width of the pupil (ie aperture) of the lens. So with the 28-75mm being a large lens (length and width), is f/10 larger on the 28-75mm than f/10 is on the 17-50mm, therefore letting in more light on the 28-75mm?

Is there any way, other than f-stop ratings, to rate the actual brightness of a lens?

Anyway, what it all boils down to is that the 17-50mm Tamron is not as fast/bright as its 28-75mm big brother. So now I'm rethinking my plan, and I may decide to return the 17-50mm, keep the 28-75mm, and still buy the Sigma 50-150mm. When I need a wide angle, I could always throw my 18-55mm kit lens back on.

This decision is made harder by the fact that I'm really happy with the results I've gotten from the 17-50mm. Image quality seems to be very close to the 28-75mm. I took some shots of my girls this morning at f/2.8 with both lenses using just natural lighting from the windows, and it looks like the 17-50mm may have a very slight edge in contrast and color, while the 28-75mm seems to be a little sharper. But the differences were very slight, and two happy little girls bouncing around on a bed is not exactly the most controlled of circumstances.

Here are some of the shots from this morning with both lenses. These pictures are straight out of the camera, with absolutely no color or exposure adjustment, and no sharpening. The only thing I did is crop the two vertical images from 3:2 aspect to 4:3 aspect (the native 4:3 aspect is something I seriously miss from my old Olympus Four-Thirds DSLR).

Click on the image for the full-size original, but be prepared to wait if you don't have a fast broadband connection.











Last edited by Edgar_in_Indy; 11-26-2010 at 11:10 AM.
11-26-2010, 11:27 AM   #2
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Oops, I'm new around here and didn't realize there was a lens discussion forum. Can a moderator please move my thread to the proper area.
11-26-2010, 11:34 AM   #3
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moving to the lens forum
11-26-2010, 01:05 PM   #4
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Thanks for sharing this.
First, I assume that you have applied no exposure/brightness adjustments in PP to any of these images, and that lighting of the scenes comparing the two lenses remained absolutely constant. If not, these cannot be indicative of the lenses' apparent difference in 'brightness'.

But if so, there is a clear difference in the first test but not so much in the following two. Nevertheless, the subtle differences in exposure settings in the third test, as well as different lighting and scene background/content in the images makes comparison in this way impossible.
The second test is more robust in controlling confounding factors but the 'huge difference' you note at f/11 is reasonably acceptable to my eyes, notwithstanding the obvious WB difference that may be contributing to the overall exposure of the image.

11-26-2010, 01:06 PM   #5
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It is not uncomom for some lenses to fool lightmeters in camera. Why is that, I don't know. Maybe due to the shape or aperture blades or due to the wrong comunication between lens and body.

My sigma 10-20 underexposes for about 0.7 EV, but I don't know is that due to her wide nature (high contrast in scenes) or due to the shape of aperture blades. I never bothered much about that and I just "overexpose" images for 0.7 to 1.0EV
11-26-2010, 01:13 PM   #6
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Was the first test under fluorescent light? If so, that could easily account for the difference in exposure and color cast. Shutter speeds over 1/30th second under fluorescent (and some other types as well) can give very strange results.
11-26-2010, 01:13 PM   #7
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This is not an issue of camera light meter variance - the exposure settings in tests #1&2 are the same between the two lenses and the same lighting conditions apply between the two images (I'm assuming). So the only other variables are the operator with the camera position, ambient light effects, white balance (which would be good to confirm here that it wasn't set to AWB but rather kept constant) and lens glass light reflection/transmission.
11-26-2010, 03:00 PM   #8
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Correct, I almost always shoot manual, so metering is not an issue. I included exposure info showing that the settings were identical between comparison shots.

The first test (the studio portrait session) was conducted in my home studio in my basement using four Paul C. Buff White Lightning strobes: one on each side of the back drop and one on each side of the camera. The white balance was set for flash the whole time, and I shot from pretty much the same position. The only thing that changed is the glass.

The second shots were also conducted in my basement, with the toy car sitting on my computer desk right in front of the monitor. There are no windows in my basement so the only light was coming the monitor and the 60w can lights in the ceiling. Lighting remained constant the whole time. White balance was set to auto, but I realized afterward I should have set it manually since it did change a little from shot to shot. I do not, however, believe that accounts for the difference in exposure I'm seeing.

And no, nothing was done to the pictures in post processing, other than to resize them and add the exposure info.

Now that I'm at work and looking at the pictures on my laptop, the difference is much less apparent. But at home on my 24" LCD monitor, which has a much higher contrast rating (ie it displays darker blacks and brighter whites), the difference was much easier to see. So depending on how contrasty your monitor is the difference may not be quite as apparent. But even on this laptop screen I can still easily see the exposure difference in the first test (the portrait) and the last test (the car at f/11). In the car shot, I can still see that the car hood is a much brighter white on the 28-75mm lens, and the shadows on the Toy Story 2 blu-ray case in the background are much deeper in the 17-50mm shot.

I also noticed that when I use the "green button" on my K-x to let it set the appropriate shutter speed, it sets the speed slightly lower with the 17-50mm lens than with the 28-75mm lens. I tried this several times with both lenses pointing at the same subject. This tells me that the K-x is seeing less light from the 17-50mm lens, so it is reducing the shutter speed to compensate.


Last edited by Edgar_in_Indy; 11-26-2010 at 03:32 PM.
11-26-2010, 03:09 PM   #9
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QuoteOriginally posted by Parallax Quote
Was the first test under fluorescent light? If so, that could easily account for the difference in exposure and color cast. Shutter speeds over 1/30th second under fluorescent (and some other types as well) can give very strange results.
I think what you're seeing as a change in color cast in the first set of pictures is just the fact that the white background is going from being blown out/overexposed to being visible. The lighting remained constant, as did the camera's white balance.

The fact that the difference is great enough in order to make the background became visible is indicative of how great the exposure difference really is, even if it's not as apparent on everybody's monitor.

If I get a chance tomorrow I might throw the Pentax 18-55mm lens into the mix and do some more tests to compare it to the other two lenses at identical settings.
11-26-2010, 03:24 PM   #10
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This would be consistent with a couple of posts noting underexposure in auto modes with smaller apertures I have seen here.
11-26-2010, 03:33 PM   #11
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i have the Tamron 17-50mm as well, and have pretty much the same experience as you. When used in Av mode mine seems to overexpose at f2.8 and progressively underexpose more as i stop down. End up using the EV adjustment dial a lot.

Nice lens otherwise. Do end up having to manual focus at the wide end on distant subjects, but that seems to be a camera AF deficit.

Have the kit lens too and am looking forward to your findings.
11-27-2010, 05:17 AM   #12
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QuoteOriginally posted by jolepp Quote
This would be consistent with a couple of posts noting underexposure in auto modes with smaller apertures I have seen here.
Yep. This week I tried several copies of Tamron 17-50 and noticed the underexposure when using smaller than 2.8 apertures. My uneducated guess is that the aperture level mechanism sets a slightly smaller aperture than it should (it stops at the wrong position) for some reason, thus less light hits the sensor.

Unfortunately all of my copies had other issues as well, so today I gave up finding a good copy and just bought the DA*.
11-27-2010, 01:50 PM   #13
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I took a new set of test pictures this morning, and this time I included my 18-55mm kit lens. The kit lens and the Tamron 28-75mm produced identically exposed pictures, while the 17-50mm continued to produce underexposed pictures, with the problem getting progressively worse as aperture decreased.

I think the previous poster who speculated that the aperture of the 17-50mm is not being correctly set and/or reported is probably right. At the minimum this seems to be a common problem on this lens, but I wouldn't be surprised if every 17-50mm exhibited the same behavior.

I can post pics from the latest test if people are interested.

I would also like to see some other people compare their Tamron 17-50mm exposure to one of their other lenses.
11-27-2010, 02:41 PM   #14
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I have not had time to do any testing, but my gut feeling is there is something to all this. I have taken well over 10,000 shots with the Tammy 17-50mm, but sensed something was amiss, akin to the discussion above. The pics it produces are great, but its economy with light does not seem to par with other lenses. I hope to get some time to look further into this, perhaps even a call to Tamron is in order. Thanks all for the discussion.
11-27-2010, 03:04 PM - 1 Like   #15
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This would not surprise me, really. Almost every lens I have sucks in different amounts of light at a given aperture. The difference between my DA 40 2.8 and A 28mm 2.8 at 2.8 is noteworthy (with the DA 40 being brighter).

Light transmission is dependent on optical construction and the thickness of lens elements. You can see the difference in transmittance, for example, between the DA 35 2.4, and the FA 35 2.0, with the DA 35 being brighter at an equivalent aperture.

I don't know enough to go into detail, but this finding does not look extremely unusual. It's a little unfortunate though, as it keep the 17-50 from being a true low-light wonder zoom.
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