anyone here use a Tamron 28-75mm f/2.8?

I just ordered a LNIB used one, in the newer version that has the built-in focus motor. It will arrive on Thursday.

 

This will be my third attempt at this lens. The first one was utter, decentered garbage: an immediate return. The second was optically fine but did back-focus badly at the long end (only). If it had back-focused consistently throughout the zoom range, no problem! But it's not practical to adjust AF Fine Tune every time you change focal length on a zoom. :(

 

Those earlier failures were in the non-BIM, screw-drive version. I'm hoping against hope the BIM one—at least, this copy—will be free of focus issues. But even if it is, the lens is notorious for sample variation. I may be deluding myself that the third time will be the charm.

 

I'd like to hear from those of you who've owned, or still own, this lens. How was/is your copy/copies?

Why on earth would you be trying this for the 3rd time? Is this lens that special?

I once owned this lens, was not happy with how it focused or the photos I made with it, but I understand its attraction. It weighs half as much as the 24-70 mm f/2.8 lenses, and costs a fraction as much. If your camera falls out of your bag, this is the lens you want under it to break the fall. I now have the Tamron 24-70mm f/2.8 VC (first version) and like it, but it's big and heavy. Can't have it all.

I got an excellent sample on the second attempt. The first one had bad focus-tilt and came from - as I discovered later - an unscrupulous online dealer that I suspect was selling off returned goods or factory rejects. Getting a refund was like pulling teeth.

 

I ordered the second one from a reputable dealer with a known good returns policy.

 

When you get a good sample, this lens is excellent. It was the main lens I used for several years on my D800 until I upgraded to the Tamron SP VC 24-70 zoom. I still keep the little 28-75 around though. It's unbeatable for such a combination of specification and small size.

 

WRT needing variable focus fine-tune. In my experience that's a camera body issue. I bought a D7200 plus 18-140 kit lens. It wouldn't hold focus across the zoom range at all.... then the shutter packed up on the camera. The body was replaced under warranty while I retained the lens. Lo and behold, that same lens focusses perfectly with the replacement body.

 

It's my belief that Nikon's AF 'fine tune' option is only there to cover up sloppy factory adjustment of their AF modules.

If your camera falls out of your bag, this is the lens you want under it to break the fall.

Laughing out loud!

WRT needing variable focus fine-tune. In my experience that's a camera body issue.

 

That's an interesting theory, one that I'd not heard before. I think my camera is okay: the variable focus error hasn't happened with the other zooms I have or had.

That's an interesting theory, one that I'd not heard before.

 

- I'm surprised.

Nikon's AF, like all others, is entirely based within the camera body. All the lens has to do is motor its focussing elements to the right position. And leaving aside under/overshoot, backlash and wear, that's a very simple task for a lens to do.

 

Think about it. How many zoom lenses give focal-length dependant AF errors in Live View? That would be zero, right? So it's fundamentally not a lens-based issue, otherwise Live View would show the same problem.

 

Nope. It's either sloppy adjustment of the AF module not being exactly where it should be, or the secondary AF reflecting mirror being out of tolerance.

 

Software (so-called AF fine tune) can only apply a correction factor which is based on the difference between the sensor distance and that of the AF module. These are distances behind the lens, being computed into camera-to-subject distances in front of the lens.

 

For example: At 3m (10ft) a 0.01mm error in sensor/AF module placement results in a focussing error of about 35mm with a 50mm lens. With a 24mm lens, that same error results in a focus shift of nearly 150mm.

 

So you see that compensating the focus distance by a fixed amount can't possibly work over a range of focal lengths. The AF module error remains constant, but the effect it has on the focus-distance varies with focal length. So just 'fine-tuning' the AF servo to move the lens motor a couple of extra steps can never work if the focal length changes.

 

Basically, Nikon need to drastically tighten up their factory tolerances in AF module adjustment, or build in some method of easy aftermarket physical adjustment of the module, either mechanically or electronically.

 

The software fix just can't work without using complicated algorithms that know precisely the relationship between lens focal length, focus throw and subject distance. With current lens technology, those parameters are just not monitored to the degree of accuracy needed for precise focus correction. And probably never will be.

 

Far easier just to get the AF module in the right position to start with!

Believe or not I got one from KEH one of the ugly ones for $50.00 bucks, when I received it everything worked but the

lens element's had it's share of fog and white stuff. So I took it apart and cleaned it out and it's a really nice lens on my

D700. I don't blame you on trying to find a good one, there a nice light sharp lens.

If your camera falls out of your bag, this is the lens you want under it to break the fall.

 

Heh. I've just made pretty much that argument for the 28-80 f/3.3-5.6 G - it's featherweight so it's not adding inertia to the camera, and it's got a plastic mount so that'll break before the camera does. Oh, and it costs $60 to replace it. Though you do need to protect the front element.

 

Think about it. How many zoom lenses give focal-length dependant AF errors in Live View? That would be zero, right? So it's fundamentally not a lens-based issue, otherwise Live View would show the same problem.

 

Wait, what? The AF module has a different light path from the route to the sensor. Some of the issue is certainly ensuring that everything matches, but there are absolutely lenses that have different focus error at different zooms, and many lenses (notably including my Sigma 35mm Art) that have different focus adjustments at different focus distances. That's why the Sigma dock (and I assume the Tamron one) lets you program in multiple adjustment points, it's why the 80-200 f/2.8 AF-N/AF-D tended to miss focus at short range, and I believe compensating for this is a difference between the D800 and D810 AF modules.

 

If it were just the camera, you wouldn't need a different AF adjustment for each lens. I still maintain that Nikon should allow programming at different focal lengths, different focus distances, and different AF points (the docks don't support the last one, but field curvature is a thing...) - and I can't believe it's all that hard to do (or, mostly, automate, since it would be very tedious to do by hand - as those of us who've done the on/off camera shuffle with the docks have discovered).

...but there are absolutely lenses that have different focus error at different zooms, and many lenses (notably including my Sigma 35mm Art) that have different focus adjustments at different focus distances.

 

- If you read the rest of my post, it explains in long-winded detail why a rear-focus error can't be cancelled out at all focal lengths and subject distances simply by tweaking the focus throw on the lens.

 

Live View AF might be fooled by stop-down focus shift, but what you see is what you get WRT primary focus. An AF module stuffed in the bottom of the mirror box and fed via a hinged mirror is a different kettle of wobbly fish.

 

"If it were just the camera, you wouldn't need a different AF adjustment for each lens."

 

- Yes you would. The formula for conjugate focii shows precisely that.

 

1/F = 1/v - 1/u

 

Complicated by the fact that the rear nodes of lenses can be in an entirely different plane from their focal length, and that the rear node of a zoom might also change throughout the zoom range.

Edited November 28, 2018 by rodeo_joe|1

- If you read the rest of my post, it explains in long-winded detail why a rear-focus error can't be cancelled out at all focal lengths and subject distances simply by tweaking the focus throw on the lens.

 

Sorry, I saw the start and panicked.

 

"If it were just the camera, you wouldn't need a different AF adjustment for each lens."

 

- Yes you would. The formula for conjugate focii shows precisely that.

 

1/F = 1/v - 1/u

 

Complicated by the fact that the rear nodes of lenses can be in an entirely different plane from their focal length, and that the rear node of a zoom might also change throughout the zoom range.

 

Hmm. Interesting; I'd thought of there being a more indirect relationship between the light paths, but that perspective gives me something to think about. And I'm perfectly prepared to be schooled in this.

 

I do think bringing in the degree of error in the real world (scene side) is confusing things, but I do see how the motion of the rear nodal point of the lens can shift the amount by which the mirror/sensor relationship is "wrong". I'm not sure that practical tolerances will ever support just getting everything perfect by design, but it's a nice dream.

 

A while back I had thoughts about adding some worm drives behind the sensor, with a view to supporting rear movements. Much less complicated in a mirrorless system than a dSLR, since the mirror system can't really be expected to keep up. Doing this would allow an in-the-field way of calibrating alignment, although it still might not help with off-centre AF points...

Sorry, I saw the start and panicked.

 

 

 

Hmm. Interesting; I'd thought of there being a more indirect relationship between the light paths, but that perspective gives me something to think about. And I'm perfectly prepared to be schooled in this.

 

I do think bringing in the degree of error in the real world (scene side) is confusing things, but I do see how the motion of the rear nodal point of the lens can shift the amount by which the mirror/sensor relationship is "wrong". I'm not sure that practical tolerances will ever support just getting everything perfect by design, but it's a nice dream.

 

A while back I had thoughts about adding some worm drives behind the sensor, with a view to supporting rear movements. Much less complicated in a mirrorless system than a dSLR, since the mirror system can't really be expected to keep up. Doing this would allow an in-the-field way of calibrating alignment, although it still might not help with off-centre AF points...

 

- In view of the fact that AF and main sensor alignment needs to be pretty much micron-accurate with current sensors; I was thinking more of piezo positioners being used. You'd fix a decent prime lens to the camera, hit "AF Align" and sit back while the camera compared a Live-View focus with the output of the AF module and made electronic adjustments to the AF sensor position - might be a bit noisy with the mirror zipping up and down though.

 

WRT field curvature errors. I don't think you're ever going to fix that. There's an optical law that states the field can only be part of a large radius spherical surface, and not truly plane, if other important aberrations are to be fully corrected.

Whether that law still applies with today's magical use of aspherical surfaces and extra-extra-extra low dispersion materials, I don't know.

Yes, my worm drive thought was based on significant rear movements. I've seen enough TV shows that appear to have a tilted lens locking in a weird focal plane that I think it might have some relevance - and it would be a significant step up if rear movements allowed tilt-shift to autofocus (while adjusting shift). If it's just for fixing alignment, it can probably be a smaller mechanism; I don't know how far the SBIS shifts the sensor.

 

And yes, some day I'll get the chance to read up properly on modern optical design. :-)

Received the lens yesterday. To my relief, it doesn't exhibit variable focus error at different focal lengths.

 

It does, however, have a different kind of variable focus error: distance-dependent focus error! With no AF fine tuning, the lens focuses perfectly at shorter distances, up to about 25' / 8m by rough estimation. At much longer distances, it front-focuses quite a bit, so that an AFFT value of +18 or so is required to make it focus well again.

 

Thoughts, anyone? Is this kind of error also more the fault of the body than the lens? I never saw it with other lenses.

How does the lens focus in Live View?

 

You also have to remember that the AF rectangles shown in the viewfinder have absolutely no coupling to the AF module. Again it's down to factory adjustment how well they line up with the true AF position.

 

By trial-and-error, I determined that the illuminated viewfinder AF rectangles in my D800 were slightly offset to the left of the actual AF area.

 

it's getting rid of this sort of crap that I see as the major advantage of going mirrorless!

Edited November 30, 2018 by rodeo_joe|1

How does the lens focus in Live View?

 

You also have to remember that the AF rectangles shown in the viewfinder have absolutely no coupling to the AF module. Again it's down to factory adjustment how well they line up with the true AF position.

 

In Live View there's no problem.

 

Regarding (PD)AF rectangles, I am testing long-distance focus using isolated subjects, such as the top of a telephone pole with nothing but the sky in the background. Thus, I don't think the issue is that the camera is focusing on the wrong thing: there is nothing else around the target.

 

I've been noticing that the lens just isn't all that sharp at long distances, whether using CDAF or fine-tuned PDAF. This despite being tack-sharp up close. I picked up a Nikkor 35-70mm f/2.8D at the same time as this Tamron, planning to keep the better lens. The Nikkor has the opposite problem: it is bitingly sharp at long distances but somewhat soft with close subjects. In the middle distance (say around 6–20 ft), where I shoot most often, the lenses are about equally sharp.

 

The Tamron has more spherical aberration wide open—ironically, since it has aspherical elements. The Nikkor has more longitudinal CA. Both lenses' flaws are pretty much gone at f/4.

 

Overall, the Nikkor wins this contest because it focuses equally well at all distances (save perhaps very close ones, for which I have other lenses that are good). The Tamron's variable focus errors are really regrettable, because otherwise the lens has some enviable qualities: it focuses very quickly and more quietly than the screw-drive Nikkor, and it is significantly lighter.

I've been using the version without the built in motor,

My only complaint is that images arn't as sharp at f2.8 as when the lens is stopped down to f4 or smaller. I'm using it on a cropped sensor DSLR, and don't notice any chromatic aberration over the limited image area of the DX cropped sensor.

That's been my problem lately with the other Tamron lens I received the 17-50mm f2.8 soft focusing at f2.8.