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09-16-2013, 04:30 AM   #1
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Pentax 18-55mm vs Tamron 17-50mm lens

Right, I got my replacement Tamron 17-50mm f2.8 lens and the sharpness is now great, wheras the first copy I had was terrible. But i've noticed that when I set the mode to Aperteur Priority (same with any mode) and set the aperteur to the same aperteur and same ISO while using both lenses, that with the Tamron the shutter speed indictaed to to get a correct exposure is 2 stops slower than the 18-55mm kit lens. I set the aperteur to f6.3 and ISO to 250 (I was indoors) and while using the kit lens a shutter speed of 1/13 was indicated, wheras when using the Tamron lens, a shutter speed of 1/8 of indicated. Has anyone else noticed this? It's harder to get sharp photos with the Tamron 17-50mm f2.8 than it is the 18-55mm kit lens, my thoughts are that the Tamron lets less light in, am I right maybe?

09-16-2013, 04:51 AM   #2
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Yes and no. The aperture is not the measurement of light transmission. But a lower f-number is still brighter than a higher one (f2.8 is brighter than f5.6). Two different lenses, at same focal length and same f-stop and same ISO, should require a very similar shutter speed. The difference in brightness, at the same aperture, should be pretty small.

I suggest you make sure the lens and camera contacts are clean, so the camera will recognize and operate the lens properly. Then take a photo with the Tamron and look into the lens, so you can see if the aperture blades close down properly.
Because by most measures, the Tamron should be the better lens.

Oh, and try toggling shake reduction (SR). And turn off highlight correction for these tests.
It would also be great if you could upload two sample photos with exif data, so someone can take a look at the photos and help you further
09-16-2013, 04:59 AM   #3
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Your difference 1/13th to 1/8th is approx 2/3rd a stop, not two stops. There is a difference between lens in terms of light transmission for a given F stop. This is known as the T value and is less than the aperture setting - and measures the combined effect of the aperture and how much light is lost on the way through the lens due reflections, absorption etc. A third of a stop variation between lenses wouldn't be uncommon. The Pentax with its SMC coatings may have slightly less reflection losses. Number of lens elements would also play a part. Rounding of shutter speeds by the camera to nearest 1/3rd stop may play a minor part in metering differences also (eg Tammy rounding up, Pentax down), and finally slight composition changes in the scene will also play a part in the light being metered.

The Tamron used at or near wide open will be more demanding to focus exactly as you have less depth of field at F2.8 vs the widest available on the 18-55 for the same focal length. This will be most noticeable at the longer end where the widest aperture varies the most between lenses and the longer focal length also reduces DOF the most.

I have the Tamron 17-50. I did need to play with the focus correction adjustment on my K-x (via a small mod file to allow access to the debug program) to get best sharpness. Once done, it generally focuses well in good light. Low light coupled with low contrast can be more difficult for my K-x to get accurate focus. If you can reliably get good focus in bright light and not so good in poor light, it may well be the auto-focus capabilities of your camera that is the issue. And the smaller aperture of the 18-55 may have hidden minor mis-focus somewhat. Also at low speeds like you are quoting, are you satisfied that camera shake is not contributing to what you perceive as less accurate focus? My ability to handhold successfully gets marginal at 1/15th and below.
09-16-2013, 05:43 AM   #4
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Well it's not really focus so much that i'm worried about, it's ust that I was wondering why the Tamron needs a slower shutter speed to achieve the same exposure. Another thing is, I mainly 98% of the time keep ISO at ISO200 or maybe ISO400 absolutely maximum when in low light and then lower shutter speed to get the correct exposure when I need a set aperteur of say (just for example) f6.3. Is this doing it wrong? Or should I ramp up the ISO in order to get a faster shutter speed? I worry about going over ISO500 because the noise does start to show up on the K30, and any ISO higher than that noise starts to become a problem. I know I have noise reduction turned off 98% of the time, but it's only because when I turn it on, the images get much softer.

09-16-2013, 06:31 AM   #5
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That's not something I would be terribly concerned about. Just get used to it and deal with it. But, I just checked it out with my 18-55 and 17-50 and mine's worse. It's a full stop on mine... 1/125 to 1/60 @ f4. In fact my 17-50 does not give me as high a shutter speed wide open as my 18-55 does @18mm.

When shooting landscape, I always shoot 100 ISO for increased dynamic range... when shooting wildlife ( or moving children) I always shoot 800-1600 ISO for increased shutter speed. 800 ISO on a bright day is good to excellent. 1600 on a dark day can really bring up the noise.
09-16-2013, 06:38 AM   #6
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I guess maybe me never ever using any ISO higher than 400 and then complaining about the shutter speed being too low to get a sharp photo is just me expecting my K30 to be better than it can be. What settings do you use for high ISO noise reduction? I've got mine set to 'off' because I just want my photos to be as sharp as possible, but then I pixel creep a hell of a lot so mayb I should just stop doing that because by doing that it's obvious i'm gonna find flaws.
09-16-2013, 06:50 AM   #7
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QuoteOriginally posted by richardstringer Quote
I guess maybe me never ever using any ISO higher than 400 and then complaining about the shutter speed being too low to get a sharp photo is just me expecting my K30 to be better than it can be. What settings do you use for high ISO noise reduction? I've got mine set to 'off' because I just want my photos to be as sharp as possible, but then I pixel creep a hell of a lot so mayb I should just stop doing that because by doing that it's obvious i'm gonna find flaws.
On a K-30 800 is just fine... I don't use noise reduction. 1600 might be fine, but sometimes gets ruined by noise, but usually, only when under-exposed. Watch your histogram really carefully at 1600, make sure you're using all the DR you have. USe your EV button. Expose to the right edge of the histogram.

ISO 800


ISO 1600

Last edited by normhead; 09-16-2013 at 06:58 AM.
09-16-2013, 09:06 AM   #8
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There isn't an EV button on the 30. Plus if you mean exposure compensation, I never ever use it because all it does it make the shutter speed slower to compensate for you wanting a lighter exposure. Maybe I get a lot of noise because I have the sharpness set on full in camera.


Last edited by richardstringer; 09-16-2013 at 09:17 AM.
09-16-2013, 09:11 AM   #9
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QuoteOriginally posted by richardstringer Quote
There isn't an EV button on the 30.
It's button no. 1 in the picture
on page 18 of the manual.
09-16-2013, 09:19 AM   #10
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QuoteOriginally posted by richardstringer Quote
There isn't an EV button on the 30. Plus if you mean exposure compensation, I never ever use it because all it does it make the shutter speed slower to compensate for you wanting a lighter exposure.
I use the EV button on 65% of my images. I don't know how you'd ever get an optimum exposure without it. Changing your ISO doesn't change your exposure. There's been an exposure compensation button on every Pentax I've owned and it's on one of the dials on top of my film cameras. Even if you increase your ISO you may still have to slow your shutter speed down with the EV to get a correct exposure.

You might be interested from the sound of it in TAV mode. Set your Aperture and Shutter speed, and the camera will select the ISO. That doesn't get you out of fine tuning with EV. By the way, I often turn the EV down to -1 or -2 which would increase the shutter speed. It works both ways.
09-16-2013, 09:38 AM   #11
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Another difference between the two lenses is that the camera may be using a different AE metering scheme for each lens.

While not everyone accepts this, according to the manual of every recent Pentax camera I've read, full 77 segment matrix exposure metering basically only works on Pentax A lenses and newer. Third party and older manual lenses use centre-weighted average metering.

Hence if a scene had elements with varying levels of illumination scattered around the frame, you would expect to see a different set of exposure parameters chosen by the camera for each lens.
09-16-2013, 09:40 AM   #12
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Ah yeah I know which button it is now, yeah I don't use it because it changes the shutter speed to slower or faster which I can do anyway if I need to in order to get a correct exposure. By the way, this test shot was taken kist about 10 minutes ago to see how much noise there is :

http://farm3.staticflickr.com/2831/9774865393_8e20fdcae3_o.jpg

I took it using ISO800, I used the 'Natural' preset and set all parameters to default '0' meaning the middle position, even sharpness which I normally have set to full. Does this look ok for noise? I can see noise but i'm right in thinking that noise shows up the lower the amount of light there is right? So at night noise would show up a lot more than say, 1pm in the afternoon on a sunny day. Also, I was under the understanding that no-one with half a brain uses an ISO above 200 in the daytime, I always thought ISO400 and above were only used normally at night or in low light. Amd I right in thinking now, that people DO use ISO800 and 1600 in the daytime to get the shutter speed they need?
09-16-2013, 09:52 AM   #13
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QuoteQuote:
Ah yeah I know which button it is now, yeah I don't use it because it changes the shutter speed to slower or faster which I can do anyway if I need to in order to get a correct exposure.
Only if you are shooting in manual mode. If you change the shutter speed in TV mode because you're image is over-exposed, it will adjust the Aperture accordingly and you will still be over-exposed, even though you changed the shutter speed. Same with TAV. In AV you can't change the shutter speed directly. If you go to manual mode, you can change the exposure whatever way you choose and slowing the shutter will over-expose.

QuoteQuote:
that people DO use ISO800 and 1600 in the daytime to get the shutter speed they need?
Exactly. At least in AV mode.
09-16-2013, 10:08 AM   #14
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I never shoot any fast moving objects or even really any moving objects apart from maybe my kids walking sometimes so I stay in AV mode 99% of the time. If I take a landscape photo using a tripod then i'm in manual mode but that's hardly ever.
09-16-2013, 10:18 AM   #15
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QuoteOriginally posted by richardstringer Quote
I never shoot any fast moving objects or even really any moving objects apart from maybe my kids walking sometimes so I stay in AV mode 99% of the time. If I take a landscape photo using a tripod then i'm in manual mode but that's hardly ever.
Then 99% of the time, you have to use the EV button to change your Exposure Value, there's no other way to do it.
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