Tamron 90mm macro won't go to f/2.8

sometimes and I can't figure out why.
Bought barely used and no manual and can't find one on-line.
It will go as large as 3.2 consistantly but won't go to f/2.8 except once in a while and I can't figure out why nor when.
Any ideas?

i believe it has to do with focus distance. its been a while but im pretty sure i'm nikon macro lens does it to. when your at minimum focus distance you can go higher f stops than normal but not as low. its something like it equates to the same amoutn of light its just keeping the numbering consistant. you normally want larger f stops when your that close anyways.

I used to have that lens (for years, did tons of macro with it). I have never noticed this before and I've never heard about the focus distance thing that Dylan mentioned. I would like to express my polite skepticism in that regard.

I think a more likely explanation is that your camera is doing it. There are some exposure protection options in the camera when used in certain shooting modes. I don't remember what the options are called. And maybe it depends on your camera model. But check for any custom functions that try to be smart about aperture/shutter speed to protect from over/under exposure and also try shooting in manual mode (with that option disabled) and see if you still have the problem.

The only time I've ever been aware of lenses changing possible aperture dynamically is when it's a variable aperture zoom lens, which the tamron is not (unless there's some old version of the lens I'm not familiar with)

Dylan, I'll try the distance thing later today, thanks.

Trevor, I'm shooting with a D80 and in manual so I'm not sure that's it but I'll look at my menus again and see if there's something to do.
The fact that I'm shooting manual is what mystifies me as to why I can't get an f/2.8

Thanks for the update. I don't have any knowledge about Nikon's so can't contribute any further and retract my skepticism :) I did just check my Canon though and the feature I was talking about is only enabled for Tv or Av shooting modes. I couldn't recall if it's used elsewhere (I always have it turned off)

yeah i just checked my nikon 60 mm macro (afs version)

its definitely the distance thing. set your camera to apature priority mode and manual focus. then focus at infinity, and move it down to f2.8. then slowly back the focus closer to you.

my lense goes from

distance f stop
infinity 2.8 - 32
1m 3 -36
~0.32m 3.2 - 40
~0.28m 3.3 -45
etc

minimum focus
8 inches 4.8 - 57


i can't find my lens booklet (if it even came with one) so i forget the exact reason. but its something like the effective f stop is the same

Ok, I'll try the distance thing. Seems strange though that can't use any aperture I want at any distance.
Thanks for replies.

I take it that both the Tamron 90mm and the Nikon 60mm lenses are non-IF (lenses physically grow in length as you focus closer).

If that is the case, I can see why the lens or camera would compensate in the manner described. Non-IF lenses focus closer in the same way that extension tubes allow a given lens to focus closer. But adding extension means there is the compromise in terms of some light loss.

The lens is still an f/2.8 lens (as calculated by the focal length divided by aperture diameter) only now you have some built in extension to compensate for.

An internally focusing (IF) lens won't have these issues unless you actually use it with an extension tube.

the nikon 60mm AF-S lens IS an "if" lens (the one i tested above). the previous non afs version was not.

also note the max f stop goes up as well (from 32-57).

Quote:

---

Originally Posted by Dylan8i

the nikon 60mm AF-S lens IS an "if" lens (the one i tested above).

---

OK, then that's weird (scratches head).

accordign to ken rockwell http://www.kenrockwell.com/nikon/60mm-afs.htm#perf


"The 60mm f/2.8 AF-S alters its optics as it focuses. It becomes slower as it's focused more closely."





and from
http://www.slrgear.com/reviews/showp...ct/1175/cat/12




The lens is marked as a constant ƒ/2.8 lens, but in fact the effective aperture will be smaller as you focus at shorter and shorter distances. The following table lists the aperture to distance differences:
<table border="1"><tbody><tr><td>Subject distance</td><td>6''</td><td>7''</td><td>8.5''</td> <td>9''</td><td>10''</td><td>1' 3''</td><td>3'</td> <td>infinity</td></tr> <tr><td>Largest aperture</td><td>ƒ/4.8</td><td>ƒ/4.0</td><td>ƒ/3.8</td> <td>ƒ/3.5</td><td>ƒ/3.3</td><td>ƒ/3.2</td><td>ƒ/3.0</td> <td>ƒ/2.8</td></tr> <tr><td>Smallest aperture</td><td>ƒ/57</td><td>ƒ/51</td><td>ƒ/45</td> <td>ƒ/45</td><td>ƒ/40</td><td>ƒ/36</td><td>ƒ/36</td> <td>ƒ/32</td></tr></tbody></table>



so still not sure WHY it does it. but that is the factor (at least for nikon)




heres a link to the tamron 90mm macro ( not sure if theres different versions) manual, i didn;t see anythign about it in a quick glance though.

http://www.tamron.com/assets/pdfs/272E.pdf

The change in the F stop is due not to the change in F stop but to the magnification factor.

Didn't have a chance to check today but I'll let you all know the results when I do.

I have the Tamron 90mm macro, will have to check this myself (the EXIF data will reveal the truth)!