Darker images with wider apertures???

[Nifkin]

I'm using a 50D in AV mode, and I've noticed, when I shoot using my Tamron 17-50 set to f/2.8, I get darker images than when I set the aperture to f/4.5, say. Of course, the shutter speed should be adjusting to compensate for the different amounts of light each aperture setting is allowing into the camera, and, although the shutter speed does shift accordingly, I'm still getting this situation where the images at f/2.8 are nticeably darker!

Does anybody know why this is?

Thanks

:)

[jacuff]

You might as well post some samples because that's what everybody is going to ask for.

Here's a guess at what is going on... At f/2.8 you usually see some vignetting compared to the lens being stopped down.

[Nifkin]

OK, both 100% crops from the centre of the frame; the first at f/2.8, the second at f4.5. The first is noticeably softer, but also darker, and vignetting wouldn't be called vignetting if it covered the whole frame like this!

[jacuff]

Got the uncropped images with EXIF info still attached?

[Nifkin]

Got the uncropped images with EXIF info still attached?

Yep, but how do I post those? they're about 4MB each!

[jacuff]

Resize them so that they are acceptable for the image posting rules. (1024 pixels max on the long side).
100% crops don't really help much here to help figure out what is going on. EXIF and seeing the full image will.

[Nifkin]

OK. In the same order as before:

[mike_d]

The exposure is different as can happen when you use an automatic mode. From the first picture to the second, you stopped the aperture down by 1 1/3 stops but slowed the shutter down by 1 2/3 stops.

[Nifkin]

The exposure is different as can happen when you use an automatic mode. From the first picture to the second, you stopped the aperture down by 1 1/3 stops but slowed the shutter down by 1 2/3 stops.

I see. Could that be as a result of the metering mode i was using (partial) reading the scene differently for each pic? They were taken using a tripod, so the positioning should be identical. Or could there be an issue with my camera's semi automatic metering modes, e.g. the camera being erratic when compensating with the correct amount of shutter speed for the aperture setting? Thanks for your help, btw ;)

[mike_d]

I see. Could that be as a result of the metering mode i was using (partial) reading the scene differently for each pic? They were taken using a tripod, so the positioning should be identical. Or could there be an issue with my camera's semi automatic metering modes, e.g. the camera being erratic when compensating with the correct amount of shutter speed for the aperture setting? Thanks for your help, btw ;)

Well 1/3 stop isn't off that far and seems to come within the margin of error to auto exposure systems. If the metering mode and composition was identical, maybe the light changed slightly. All it takes is something reflective (like your shirt) moving out of frame just enough to change the lighting and cause the camera to pick a different exposure. This is why manual mode is useful since you pick the exposure, then set the aperture and shutter speed to keep the same exposure.

[jacuff]

ISO 200 1/800 f/2.8, so to stop down to f/4.5 you would need to manually adjust the shutter speed 1 and 1/3 stops to get the same EV. That means your exposure should have been ISO 200 1/320 f/4.5. However your camera in Av mode chose: ISO 200 1/250 f/4.5. Which is +1/3 EV over what it should have been. It's not unusual for any camera to meter + or - 1/3 EV.

[chauncey]

Can't answer the exposure portion of your question but most lenses are a little crisper at f/4 as opposed to f/2.8. A review of your lens is found here http://www.the-digital-picture.com/Reviews/Tamron-17-50mm-f-2.8-XR-Di-II-Lens-Review.aspx

[yogestee]

What can happen with some lenses is when shot wide open they can often have less contrast which can appear to be darker..

Most lenses aren't at their best wide open..

Edit : I did a quick EXIF check on your images.. The f/2.8 pic shows a shutter speed of 800th sec,,the f4.5 pic shows a shutter speed of 250th sec both at 200 ISO.. Using my very ordinary mathematics the f/2.8 pic is under exposed by about 1/3 stop compared to the f4.5 pic..

Try the same test but your camera in manual mode..

[Nifkin]

Thanks guys, that's been helpful. It seems that my camera is consistently under exposing or producing darker images at f/2.8, so I guess this could be a combination of the variable exposure provided by the camera and the fact that images are less contrasty at wide apertures. I shall experiment more!

Thanks

:)

[gofer]

As this only happens when using your Tamron 17-50 it could also be that the diaphram in that lens is not behaving properly.

[Nifkin]

Hmmm. Is this the sort of thing that would be covered by the warranty? Haven't had it for long. Anybody had any experience sending a lens back to Tamron for re-calibration?

[jacuff]

...it could also be that the diaphram in that lens is not behaving properly.

That's super easy to test. Just turn the camera towards you and press the DOF preview button. Keep it pressed and adjust the aperture. If the blades don't do the same thing, then you've found a problem.

[gofer]

That's super easy to test. Just turn the camera towards you and press the DOF preview button. Keep it pressed and adjust the aperture. If the blades don't do the same thing, then you've found a problem.

But that only proves they move. It doesn't prove they move to the required aperture.

[Nifkin]

That's super easy to test. Just turn the camera towards you and press the DOF preview button. Keep it pressed and adjust the aperture. If the blades don't do the same thing, then you've found a problem.

Yes, I did try that; by default the lens sits at f/2.8 (obviously), so you can only see movement from the diaphragm with the DOF preview button depressed when you stop the aperture down; it seems to be fine in this respect, unless... it could be a case of, when the lens is at f/2.8, it's not sitting as wide as it should? the camera is reading it at f/2.8, and adjusting the shutter speed accordingly, but the lens itself is sitting at some interim stop, like f/2.9 or f/3?? Does this sound plausable?

EDIT: I did a test with the set up being 35mm, f/2.8, Picture Style set to neutral (so no sharpening) camera on tripod, so no VC needed. I then took five shots, each time changing the 50D's AF microadjust: +20,+10,0,-10 and -20. And, when the AF point is viewed at 100% in DPP, I can safely say that when it's at the 0 setting the AF is bang on. So it seems the softness is not down to front- or back-focusing.

[DStanic]

I had a Sigma 70-200 macro which was also quite dark at f/2.8 compared to f/4 and especially f/8. It was very soft too (compared to other pics I've seen on here). I know those lenses are known to be soft but I think there may have been something wrong with it (diaphram?). I sold it a while back so no way of knowing now.

I'm not sure what to say about your 100% crops, there is the obvious sharpness difference however it's with 15mp image larger then what I'm used to.

edit: does your Tamron under expose at all focal lengths? My Sigma was not so bad at the wide end- terrible at 200mm.

[Rubi Jane]

I don't think I saw this suggested, so sorry if it's already been suggested.

Shoot in manual, set your exposure for 1/2 stop increments. Setup on a subject and adjust aperture and shutter speed so you have correct/even exposure. Then open up the lens 1/2 stop and stop down the shutter 1/2 a stop and take another shot. Keep doing that until you arrive at your larges aperture and see if there are any differences in exposure. Doing this eliminates any camera metering and will help you determine if there is a problem with the lens as 2.8

[yogestee]

I don't think I saw this suggested, so sorry if it's already been suggested.

Shoot in manual, set your exposure for 1/2 stop increments. Setup on a subject and adjust aperture and shutter speed so you have correct/even exposure. Then open up the lens 1/2 stop and stop down the shutter 1/2 a stop and take another shot. Keep doing that until you arrive at your larges aperture and see if there are any differences in exposure. Doing this eliminates any camera metering and will help you determine if there is a problem with the lens as 2.8

Exactly right.. I doubt it's the lens and/or camera that's the problem but the way the subject was metered..

[jacuff]

I agree with Lindsey. I actually don't think there is anything wrong per say, you just need to get used to how the camera meters using this lens. It's just one of those oddities that may or may not affect others because it is a third party lens.

[Nifkin]

I don't think I saw this suggested, so sorry if it's already been suggested.

Shoot in manual, set your exposure for 1/2 stop increments. Setup on a subject and adjust aperture and shutter speed so you have correct/even exposure. Then open up the lens 1/2 stop and stop down the shutter 1/2 a stop and take another shot. Keep doing that until you arrive at your larges aperture and see if there are any differences in exposure. Doing this eliminates any camera metering and will help you determine if there is a problem with the lens as 2.8

OK, I tried this, hand held in my office. I adjusted the shutter speed/aperture 1/3 stops from f/5.6 down to f/2.8. When comparing the results at f/4.5 and f/2.8, there is a small but noticeable darkening of the image, maybe not as pronounced as in the shots that I posted earlier, but it's there. Is this likely to be a contrast issue usual to wide aperture settings? the sharpness also drops off below f/4, but I understand this is to be expected to some degree, and is only really dicernable in a 100% crop viewed from laptop-screen distance away. I just feel that my issue is so on the border between what might well be the probable expected performance of the lens and a recalibration issue.

EDIT: Tried the above manual test again a few times, and not only does the darkening seem less aparent at f/2.8 when compared to f/4.5, but the sharpness at fully open seems to have improved too (bearing in mind I'm using the LCD on the back of my 50D to check this, but it would still be noticeable). Is it possible for the automatic exposure to have an effect not only on the brightness but the sharpness of the image as well???

[PhotosGuy]

Is it possible for the automatic exposure to have an effect not only on the brightness but the sharpness of the image as well It might if...
1. You're hand holding & it gives you a slow shutter speed.
2. Your focus was a bit off & it gives you a wider aperture.

Especially in a situation like this where nothing is moving & you have bright, unimportant reflections in the glass, go full manual & use this: Need an exposure crutch? (http://photography-on-the.net/forum/showthread.php?t=89123)

Why?
Post #47 (http://photography-on-the.net/forum/showpost.php?p=5191658&postcount=47)

[WT21]

Just to cross-link your issue with my similar issue: http://photography-on-the.net/forum/showthread.php?t=781156

[Tixeon]

Hmmm. Is this the sort of thing that would be covered by the warranty? Haven't had it for long. Anybody had any experience sending a lens back to Tamron for re-calibration?

Just to satisfy curiosity, I would run this by Tamron. I believe they would be willing to help. They have a good (& long) warranty & my experience with them on the phone has been positive.

[PhotosGuy]

Also, every lens has a sweet spot, that is the aperture at which the lens is the sharpest, usually 2-3X stopped down from max aperture. I would find out what apertures give you the sharpest images for each of your lenses.

[gkarris]

I get this all the time with my adapted manual lenses in aperture priority. You just have to learn what f stop works best with the lens.

[WT21]

The last two posts are about finding the aperture that works best, but on this Tamron, the OP and I are seeing the same issue -- strong halation and soft focus at f/2.8. Sure, it disappears at f/4.0, but then what's the point of the heft and cost of a 2.8 lensif it's not usable wide open?

[Nifkin]

The last two posts are about finding the aperture that works best, but on this Tamron, the OP and I are seeing the same issue -- strong halation and soft focus at f/2.8. Sure, it disappears at f/4.0, but then what's the point of the heft and cost of a 2.8 lensif it's not usable wide open?

Exactly. There's plenty of glass available that's lovely from f/4 down. But a HUGE point of this lens is that it has a constant f/2.8 across a very very useful focal range. I've been using it a lot for the last few days, and have been trying to decide whether it's as good as the non-VC version but with VC, or that the VC has come at the cost of good IQ at apertures larger than 4. A hell of a decision to make when you haven't got both lenses side-by-side... :rolleyes: :D

[WT21]

Exactly. There's plenty of glass available that's lovely from f/4 down. But a HUGE point of this lens is that it has a constant f/2.8 across a very very useful focal range. I've been using it a lot for the last few days, and have been trying to decide whether it's as good as the non-VC version but with VC, or that the VC has come at the cost of good IQ at apertures larger than 4. A hell of a decision to make when you haven't got both lenses side-by-side... :rolleyes: :D

I called Tamron in the US. It'll be 3 weeks to repair. They agreed to look at some photos and tell me if they think they are acceptable or not. If they feel it's "within acceptable limits" then I'm returning the lens. If not, then I'll have a decision to make.

The complicating factor is that I was just asked to shoot some indoor wide angle shots, so I had to pick up an UWA. Trying to also decide if I can afford both lenses right now :shock:

[yogestee]

The last two posts are about finding the aperture that works best, but on this Tamron, the OP and I are seeing the same issue -- strong halation and soft focus at f/2.8. Sure, it disappears at f/4.0, but then what's the point of the heft and cost of a 2.8 lensif it's not usable wide open?

Lenses most lenses aren't at their best wide open.. This is not just Tamrons but Canon, Nikon, Sigma and every lens manufacturer on the market.. Stopped down a bit, things heat up..

You are comparing two lenses with the same focal lengths.. One lens is a f/2.8 the other a f/4.. Both lenses wide open are softer than say at stopped down a stop.. The f/2.8 lens will be sharper at f/4 than the f/4 lens at f/4 giving you a 1 stop advantage when light levels fall..

This is why pro-photographers spend the big bucks on fast aperture lenses..

I have a Tamron 17-50mm f/2.8.. I'll admit at full aperture it's not at its best but the images wide open are very much usable with a bit of sharpening during post processing..

Edit : A f/2.8 will always be sharper at f/2.8 than a f/4 lens at f/2.8..

[Nifkin]

Edit : A f/2.8 will always be sharper at f/2.8 than a f/4 lens at f/2.8..

Umm... not sure I get you... of course an f/4 lens (assuming you're talking about a given lens's max aperture) is not going to be very sharp at f/2.8; we'd never find out coz it can't open up that wide! :lol:

[yogestee]

Umm... not sure I get you... of course an f/4 lens (assuming you're talking about a given lens's max aperture) is not going to be very sharp at f/2.8; we'd never find out coz it can't open up that wide! :lol:

That was a tongue in cheek statement..I'd rather have a lens that's slightly softer at f/2.8 than a lens that doesn't open up to f/2.8 at all..

[PhotosGuy]

That was a tongue in cheek statement I saw that, too. A ;) would have been nice?

[Nifkin]

That was a tongue in cheek statement..I'd rather have a lens that's slightly softer at f/2.8 than a lens that doesn't open up to f/2.8 at all..

Oh I see, haha! Coz you didn't use of an emoticon I thought you were just talking b0110cks! :lol::lol::lol:

[yogestee]

Oh I see, haha! Coz you didn't use of an emoticon I thought you were just talking b0110cks! :lol::lol::lol:

People who know me well in these forums know I always talk b0110cks ;)

[20droger]

True! He's very fluent in b0110cks! And, he uses good b0110cks grammar!

[yogestee]

True! He's very fluent in b0110cks! And, he uses good b0110cks grammar!

Now Rodger,,that's b0110cks ;)