Sigma 105mm 2.8 that won't go down to 2.8??

LCPete said:
Hi it may not matter anyway in my opinion as at 1:1 you would stop down a bit normally say F8 or 10 to get a usable depth of field as even at F10 depth of field is small. Also the sweet spot for this lens is at around F10
Pete
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While what your saying is quite true, it does still matter if the lens supposed to stay at 2.8 if set at that setting.
 
Dogfish_magnet said:
So what your saying is if you set the lens to manual focus at infinity and AP with a aperture of f2.8 it will read F2.8
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Can you explain what you mean by infinity, as this is a 90mm prime lens so only has one focal length, or am i missing something again :thinking:
 
TG. said:
Can you explain what you mean by infinity, as this is a 90mm prime lens so only has one focal length, or am i missing something again :thinking:
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Not the focal length which is fixed at 90mm but the point of focus
- if you look at the lens there's a glass window on the top, if you rotate the focus ring until you see a symbol like a 8 on its side your focused at infinity, as this is a true 1:1 prime macro lens you will also be at the lens's minimum magnification (you change the lens magnification by altering the focus point on the lens & focus the lens by moving the camera backwards and forwards until your subject is in focus). When the lens is set at its closet focus point you get the biggest magnification 1:1 which is at 0.29 meters for a Tamron 90mm.
 
RichardtheSane said:
I don't know for sure but I would think it is more likely to be the lens that reports it's maximum available aperture rather than the camera just knowing.
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That's the point though, the lens should report f/2.8 as it's only the "effective" aperture that changes not the actual one.
 
pxl8 said:
That's the point though, the lens should report f/2.8 as it's only the "effective" aperture that changes not the actual one.
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:agree:

Put the camera in Aperture mode and set at f/2.8. It should report f2.8 no matter what, focus infinity, focus 1:1, leave the darn lens cap on it should still say f2.8 in Aperture mode, if it does not report that setting then the lens or camera is faulty.

Take it back and get another one.
 
And then.... they all lived .... happily ever after ..tune in next week ....same time .... same place THE END :D
 
i have used the Nikon 105mm f/2.8 micro lens on a D300 and a D700 and they both exhibit this behaviour. Set at 2.8 focusing at infinity it is at 2.8 as soon as i started focusing closer the f/# increases,

the simple matter of fact is who would want to shoot macro @ 2.8? wouldn't the dof be ridiculously small? i have always got the aperture around f/16 to get the most dof.
 
whiteflyer said:
:agree:

Put the camera in Aperture mode and set at f/2.8. It should report f2.8 no matter what, focus infinity, focus 1:1, leave the darn lens cap on it should still say f2.8 in Aperture mode, if it does not report that setting then the lens or camera is faulty.

Take it back and get another one.
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I'm with WF here. And also pxl8 and RtS whom I think have said much the same thing. When set at f/2.8, the camera should always report f/2.8.

Apart from this being the correct way of the camera reporting f/number, since I am 99% certain no current D-SLR knows the focusing distance anyway, it has no way of 'correcting' the aperture even if it wanted to.

Maybe some kind person would check the Exif data of a few different cameras/lenses and see if focusing distance is ever recorded. I'm thinking Sony might have the most up to date system perhaps? Or Pano? Can't see focus distance recorded on any of my Canons.

Richard.
 
whiteflyer said:
:agree:

Put the camera in Aperture mode and set at f/2.8. It should report f2.8 no matter what, focus infinity, focus 1:1, leave the darn lens cap on it should still say f2.8 in Aperture mode, if it does not report that setting then the lens or camera is faulty.

Take it back and get another one.
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HoppyUK said:
I'm with WF here. And also pxl8 and RtS whom I think have said much the same thing. When set at f/2.8, the camera should always report f/2.8.

Apart from this being the correct way of the camera reporting f/number, since I am 99% certain no current D-SLR knows the focusing distance anyway, it has no way of 'correcting' the aperture even if it wanted to.

Maybe some kind person would check the Exif data of a few different cameras/lenses and see if focusing distance is ever recorded. I'm thinking Sony might have the most up to date system perhaps? Or Pano? Can't see focus distance recorded on any of my Canons.

Richard.
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Agree with both. Tried it last night to make sure. Popped the Sigma 105 on the A700 and set the aperture to f2.8 and no matter how close I was to the subject and whether it was AF or MF, the aperture read as f2.8 without deviation.
 
cannockwolf said:
the simple matter of fact is who would want to shoot macro @ 2.8? wouldn't the dof be ridiculously small? i have always got the aperture around f/16 to get the most dof.
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Yes your quite right who would shoot macro at 2.8 as more than likely you wouldn't get the whole subject in focus, but that's not the point i'm trying to make, i can't get my head around why the aperture is changing as you get closer to the subject as it's a fixed focal lens which should in theory stop down to 2.8 and stay there whether in focus or not :shrug: anyway on a different note does anyone know where i could get this lens tested properly, as i can't find any contact for Tamron in the UK.
 
mxfun said:
And then.... they all lived .... happily ever after ..tune in next week ....same time .... same place THE END :D
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Not quite :)
 
TG. said:
...i can't get my head around why the aperture is changing as you get closer to the subject
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Inverse Square Law, which works something like this.

Shine a torch on the wall. Measure the width of the circle of light. Move back to double the distance, and measure again. The circle is now 2x as wide, which is 4x the area, and therefore 1/4 as bright. That's two stops difference.

In macro, the lens is the torch.

Richard.
 
HoppyUK said:
Inverse Square Law, which works something like this.

Shine a torch on the wall. Measure the width of the circle of light. Move back to double the distance, and measure again. The circle is now 2x as wide, which is 4x the area, and therefore 1/4 as bright. That's two stops difference.

In macro, the lens is the torch.

Richard.
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Ok thanks Richard i see your point, but what i'm not getting is why some are saying there setting is not changing while others are, would this be just down to the make of lens or camera :shrug:
 
TG. said:
Yes your quite right who would shoot macro at 2.8 as more than likely you wouldn't get the whole subject in focus, but that's not the point i'm trying to make, i can't get my head around why the aperture is changing as you get closer to the subject as it's a fixed focal lens which should in theory stop down to 2.8 and stay there whether in focus or not :shrug: anyway on a different note does anyone know where i could get this lens tested properly, as i can't find any contact for Tamron in the UK.
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Tamron are dealt with by Intro2020, I have emailed them asking them this question, just waiting for a response now.

My Tamron will only attain f5.6 at the close focus/macro end.
 
If the lens is reporting f/5.6 as the widest value do you gain 2 stops at the other end?
 
well you learn something every day
I would have thought that this was crap too...
I had a sigma 105mm f2.8 on a canon mount.
it always reported on the 350D and 40D bodies that it was running at f2.8 if I requested...
However, close up you will find less light within the field of view naturally as it can't suck in light from outside the frame...is this what you guys are talking about??
 
Bazza155 said:
I have emailed them asking them this question, just waiting for a response now.
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I will be interested to hear what their response is, if you wouldn't mind posting what they say :)
 
RichardtheSane said:
A quick google brought this up

http://www.epinions.com/reviews/pr-Tamron_90mm_f_2_8_1_1_SP_Macro_for_Nikon

It looks like it is a design 'feature' of the lens to report the effective aperture rather than the actual aperture as there are a number or reviews showing the same behavour.

edit, and a review or two n FM suggests the same thing
http://www.fredmiranda.com/reviews/showproduct.php?product=249&sort=7&cat=44&page=1
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Yep certainly seems that it's the norm then.
 
Response from Intro2020/Tamron

Please see instructions ref Fig 12:

"Where the lens is moved outward to increase the magnification ratio for shooting close-ups, the actual brightness on the film or CCD decreases and the effective F-number changes therefore from f2.8 to f5.6"

Regards,
Andrea Takahashi
Service Department
 
Well this thread certainly got lively!
After the first reply I did go away and have a play with the lens.

If I sit on the sofa using the camera in full manual mode and focus on an object on my lap then the F number changes to F5.6. If without adjusting anything, I just point the camera at an object on top of my tv the F number automatically decreases to F2.8. However when i was tking shots with this lens at college even though it said F5.6 the depth of field still showed as though I was using F2.8.

If I aim at an object close up but do not focus I can get it at F2.8, however as i focus on that close up object the aperture changes to F3, then 3.2, then 3.4 and so on. This is also the same if it is on aperture priority instead of manual.

Going from alot of replys on here I am taking it that this is normal :LOL:
 
HoppyUK said:
Hmm. So that's crap is it. Or maybe not. Take a seat at the front of the class ruvor :) The rest of you doubters sit in the corner. Facing the wall. We'll come back to you later.

For all lenses, the f/number and focal lenth only hold absolutely true when focused on infinity. (That's what the figure 1 is referring to, when you see 1: 2.8 on the front of the lens, since you ask ;) ) When you focus closer, things change. They don't change much at normal focusing distances, but when you get down under say 1:4 magnification ratio they start to change a lot, and quickly. Normal depth of field calculations go wonky too. Macro is a whole different world in more ways than one.

When you get down to 1:1, which is where the OP is heading, the light loss is 4x, or two stops. Your f/2.8 lens is now effectively f/5.6. And this is regardless of whether the lens is internal focusing or not, and also holds true regardless of what that link to lowendmac appears to imply. The way I read it, that's not quite right either.

Not only does the f/number change, but focal length reduces a bit too. Especially on internal focus macros, but that is really very slight. It's a purely technical point anyway, and has no real world significance. The reason for the f/number changing is the Inverse Square Law - a fundamental of physics. It's the bane of flash photography too, but I won't go into that, and neither is this the thread to explain magnifications ratios.

The fact remains that when you get down to a magnification ratio of 1:1, even though the viewfinder may well still say f/2.8, it is effectively f/5.6. For those of you facing the wall, which is evenly illuminated I hope, set your camera to Av, select f/2.8 and focus on infinity. Note the shutter speed. Let's say it reads 1/125sec. Now focus the lens right down to 1:1 and check the shutter speed again. It will now read 1/30sec. Two stops different.

DogfishM, take 100 lines. "At a magnification ratio of 1:1, the marked lens aperture is effectively reduced by two stops and my camera is telling porkies."

Richard ;)
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you took the words out of my mouth:LOL::LOL::LOL:
 
:thinking: Aha! Nicola, you have the camera set on manual and it's maybe telling you what you should set, and not what actually is set. The camera doesn't know you've got a macro lens fitted.

Set the camera to A (aperture priorty) and the lens to f/2.8. With the focus on infinity, stand close to an evenly illumated wall and note the shutter speed. Now without moving, focus the lens down to 1:1 and the shutter speed will drop down two stops while the f/number remains at f/2.8.

This is as it should be. While the f/number is a technical measurement and does not change, the amount of light that is passed at 1:1 is two stops less than at infinity focus, so the camera is adjusting the shutter speed to compensate.

Problem solved? Let us know ;)

Richard.
 
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