tamron 90mm - advise/help

[danny]

Hi all,

I'm desperately in need of some advise. I've just bought a new tamron 90 macro and I can't get it to go up to f2.8. Am I doing something wrong or have I got a doggy copy?

[mongo]

how far will it go and how are you getting it to go there ?? does it have a manual/physical aperture ring at the base of the lens or is it purely electronic ??

if it is electronic, there should be a little "keyway" on the aperture ring to be able to "lock" it into place. From Mongo's recollection, you must also set the aperture to the smallest setting (largest aperture number) when doing so.

when we get some feedback to these , we can take the next diagnostic steps

[danny]

Thanks mongo. No ring, all electronic. It just didn't seem to go to f2.8 no matter how I try. Tried every focus range setting and tried focusing close and across the room.

[Boo53]

I'm a smidgeon confused. How is moving the focus ring likely to alter the aperture.

Are you shooting in manual, aperture or speed priority?

[mongo]

Mongo agrees with Boo53's comments. You have not told us how you are changing the aperture. Are you able to move it at all electronically and if so, from what aperture to what aperture ??

Also, what aperture range does your lens have ?

If you camera is set to aperture priority, you should be able to vary the aperture from either the front wheel or the back wheel on your camera body - are you trying that ??

[arthurking83]

The problem isn't a problem as such.
This is common for these lens types.
EDIT: lens types = macro lenses.

Focus to about 3m or so and set aperture to f/2.8 and all should be good. (once again this is normal)

As you focus closer, with the aperture at f/2.8 still!) the aperture value recorded will slowly decrease(increase number).
That I'm aware of, this happens with Nikon (and Pentax, I think) cameras.
I don't think it happens on Canons tho .. but cant' be sure(actually can't remember ).

[Mary Anne]

Canon User here.. I own the Tamron Di SP AF 90mm macro lens not the newer USD VC model.
Put it on my old Canon 40D, tried it on Full and Limit at different distances and Apertures on Manual and Auto Focus with no problems at all.

[I @ M]

Danny, it is perfectly normal for that lens to do that as Arthur has explained ^ above. We have the Tammy 90 and the max apertures that you will see will be either F/3.5 or F/3.2 under close up focus conditions.

Canon User here.. I own the Tamron Di SP AF 90mm macro lens not the newer USD VC model.
Put it on my old Canon 40D, tried it on Full and Limit at different distances and Apertures on Manual and Auto Focus with no problems at all.

I hate to tell you this Mary Anne, but Canon are the VW of the camera world. What the camera reports and what is really happening are 2 different things.
I noticed that with a Tamron 90mm macro some years ago that it was reporting an aperture of F/2.8 at close focus but it simply wasn't so as Canons report the aperture that they are set to and not the aperture that results from close focus.

[danny]

Thanks guys. You have put me at ease after thinking that I had got a doggy lens. Should have known better then to jump to conclusions. You are right. At infinite distance it does go to 2.8 and varies as you bring in closer.

Thanks guys!!!!

[arthurking83]

I'll try to keep it quick and simple(which is a hard task in itself )

You have two different aperture values to deal with: Effective aperture and nominal aperture.

Effective aperture is, as the name implies the actual aperture when the magnification factor is taken into account.
Nominal aperture is what the lens's aperture has been set too(in a sense).

So if you set an f/2.8 lens to f/2.8, you are reading the nominal aperture setting .. that is, what the lens opening diameter actually is relative to it's focal length.
Apparently(as I understand it), this is what Canon bodies report.

Effective aperture is a little different tho, especially as you focus closer and closer.
As with focal lengths reported .. 1/. they are rounded to a degree, and 2/. they only hold true when the lens is set to infinity. Many lenses shorten focal length as you focus closer, especially zoom lenses.

If we call effective aperture E and nominal aperture N, there is a formula to work out an effective aperture if you have a reading of a nominal aperture.

The formula is: E = N x (M+1) where M = magnification. Because it's a mathematical formula, you can of course work out nominal aperture if effective aperture is reported too. {N = E / (M+1)}

So if Danny sets his lens to say about 0.29m or whatever to achieve 1x magnification(1:1), then if the camera is reporting f/4.8(as I think it would), you can work out that N = 4.8 / 2 which then makes N(nominal aperture) = 2.4

You're reading this and thinking what a wild rounding assumption being made here, but it's not as bad as it looks. The other thing being rounded off(or misreported) is focal length as well!
Again, the focal length may be accurate(as well as rounded) off when the lens is set to infinity, but when focused closer it will almost certainly change by an amount. That amount of variance in focal length will account for the large rounding off of the aperture values.

eg. for that 90mm lens situation for Danny, at 1:1 focus distance, the actual focal length may be more like 70mm or maybe even less.
So if Tamron say the lens is a 90mm f/2.8, then all they have to do to make this accurate is create a lens with a 90mm focal length and 32mm entrance pupil(front entry point for the light).
But if the focal length is shortened (to allow close focusing without massive lens extension, as many lens makers do) then focal length may be more like say 70mm.
The actual entrance pupil's physical size doesn't change(nominal aperture) .. so the lens may actually be a nominal 70mm f/2.4 lens at 1x magnification. If we divide 70mm(or whatever the shortened focal length may be) by the physical size of the entrance pupil(32mm), then 70/32 = 2.18 .. ie. f/2.2
If we want to calculate an actual shortened focal length of the lens at 1:1 close focus, if we assume that the f/2.4 is close to correct, then Focal length divided by 2.4 to achieve a 32mm entrance pupil(F/2.4=32mm => 32x2.4= F) gives us a value of 76.8mm. So it's easy to now assume that the 90mm Tamron lens at 1:1 is actually a 77mm lens.
My guess of 70mm was obviously wrong!

I'll work it out with the Nikon 105mm lens too:

105mm and f/2.8, so we assume a entrance pupil of about 37.5mm.
At 1:1 it reports an f/4.8 aperture(effective).
using the formula to determine effective vs nominal aperture too, I should have an f/2.4 lens at closest focus too.
So using that f/2.4 value knowing that the entrance pupil is 37.5mm, the focal length has probably changed at MFD.
Using that, the focal length should be 90mm(37.5x2.4). From reading various articles on the net, the 105mm VR micro actually has a focal length just over 85mm at 1:1 magnification(or 0.32m focus distance). Taking rounding errors and such, this all makes perfectly good sense.
Once again, this is normal, and almost all lenses do this as they focus closer.

There do exist some old macro lenses that don't shorten focal length as they focus closer and closer, but these lenses extend quite a large amount, as they do focus closer.
Internally focusing lenses all show this focal length shortening issue due their overall design. This complicates the process of not only reporting aperture value(s) .. and which one to report, but also the ability to keep an accurate track of it all too.

[danny]

Arthur. Thanks very much for the explanation! I've read it a couple times and will no doubt read it a few more before I fully understand but I'm starting to get the idea.

Cheers

[arthurking83]

If you think my description is a bit confusing, the real technical explanation is like a LSD trip if you try to understand all the math involved.

Took me a while to find the link to some sites I have(my organisational skills are in the subpar category!) .. but have a peek at this one ...

Focal length and Magnification explained.

Don't worry too much about the techy stuff, and the maths .. the reason I posted this one is simply for some of the nicely created diagrams this guy has taken the time to create.
For you're purposes, the two diagrams you may want to look at closely are about half way down that page(or just before half way down).

You want to look for the Nikon 180mm f/2.8 ED lens, and then directly below that is the Nikon 85mm f/3.5 Micro lens.
As you mouse over the two respective diagrams, they change to illustrate the change in their optical properties.

Note how the 180mm f/2.8 ED lens gets longer as it focuses closer to minimum focus, but the actual focal length doesn't change between infinity and close range.
Note tho that for that lens MFD isn't very close .. it's 1.8m!
For a macro lens that can also go to infinity tho, things get a little more difficult. The focusing range is more extreme.
The Nikon 85mm, and also the 105mm that I have don't change length at all. I don't know which model tammy you have, but the old model did get longer as you focused closer.
But even with that in mind, and considering the situation of the Nikon 180mm ED lens .. it's a matter of extemes.
That is, even tho the Tammy does extend at MFD, Tamron obviously had to come to a balance between the extension design of the lens and focal length loss.

The key point to note between the two different lens types is that with the 180ED lens, the entire group of lens elements move forward and backwards for focusing.
This design is called unit focusing.. literally the entire unit of lenses elements moves as a group. These lens designs are older type, usually seen in early model lenses, and most(that I've seen) don't have a very close MFD design. So imagining of it in this way, try to comprehend how much extension would be needed for the 180ED lens to get to a close in focus distance(ie. macro).

By comparison tho, the only lens elements that move in the 85mm micro lens diagram are just a few small internal lenses. Two lenses from the from the front area move back and two elements from the rear move a little forward at the same time. This obviously changes the lens dramatically in terms of optical properties.
And it's this reason why many lenses act the way they do .. i.e. why your aperture gets numerically smaller as you focus closer and so on.

If you had a Canon camera, you wouldn't have seen this issue. But that is a less accurate way to reported the lens specs.

[MissionMan]

I had the same issue with my 90mm Tamron. This forum saved me an embarrassing return visit to the store.


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[Nick Cliff]

Thanks Arthur for that great info re the formulas and I can see the appeal of mounting lenses on bellows with this information.

[danny]

Glad to see Im not the only one

[mongo]

the ansawer is completely wierd and very VW but it is good to know. thanks Arthur and Andrew