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Old Mar 31, 2008, 7:36 AM   #1
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I just noticed that my new Tamron 90mm f/2.8 Macro (Nikon mount) does not have a constant aperture if an aperture of less than f/5.6 is selected. The maximum aperture varies from f/2.8 when the lens is focused at infinity to f/5.6 when the lens is set to the minimum focusing distance. At f/5.6 and smaller the aperture is constant regardless of focusing distance.

I previously owned a Minolta 100mm f/2.8 Macro and don't recall the maximum aperture varying with focus distance. But then, I don't recall ever trying to use it wide open either.

I was curious to know if the Micro Nikkors maintain a constant apertures over the entire focusing range - the Nikon specs just quote a maximum aperture.

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Old Mar 31, 2008, 1:06 PM   #2
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This is true of most macro lenses. If the aperature remained constant, the lens would get extremely long when focusing. Besides being awkward in handling if the lens gets too long, you could also run into problems with the lens hitting the object it's focusing on. It would also slow focus down significantly if using AF. At such close distances, it's kind of irrevelent anyway. Dof is razorextremely thin anyway, rendering you're background out of focus regardless of your aperature. You'll also be shooting stopped down because of this most of the time. You may run into issues when using this lens in non macro situations.
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Old Mar 31, 2008, 5:48 PM   #3
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My Sigma 150mm f/2.8 macro do not change aperture when focusing!
IMO any prime should have constant aperture... I can see a zoom do this but a macro that's unheard of (i.e. the aperture is a ratio of its focal lenght over its opening so the AF distance should have no bearing).
-> A zoom aperture changes as a result of its focal lenght getting longer over a fixed lens opening... if uncorrected
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Old Mar 31, 2008, 6:06 PM   #4
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NHL wrote:
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My Sigma 150mm f/2.8 macro do not change aperture when focusing!
IMO any prime should have constant aperture... I can see a zoom do this but a macro that's unheard of (i.e. the aperture is a ratio of its focal lenght over its opening so the AF distance should have no bearing).
-> A zoom aperture changes as a result of its focal lenght getting longer over a fixed lens opening... if uncorrected
I can't speak for the sigma, but I am certain my earlier statement is correct. This quote is taken from Nikon expert Thom Hogan's site in his review of the Nikon 105VR micro:

Quote:
I have news for you: all of the fixed focal length Micro-Nikkors aren't actually fixed focal length. In order to keep from being enormously long when focused at 1:1 magnification, Nikon (as well as many other macro makers) plays with the optical formula in order to keep from having an ever telescoping lens barrel. In macro work, you wouldn't want that, anyway, as a lens barrel that telescoped significantly to get to 1:1 would reduce working distance and potentially start hitting things in your scene at close working distances. Thus, at 1:1, this lens becomes about f/4.8 and does not extend even a millimeter. The aperture loss is actually a bit less dramatic at lower magnifications and the non-extension is very much welcome for macro use.
see the whole review at http://www.bythom.com/105AFSlens.htm

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Old Mar 31, 2008, 8:46 PM   #5
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OK - Thanks for the replies folks.

If there are any other Tamron 90mm owners out there I'd like to hear if they see similar characteristics with their copies. I was concerned that I might have a defective lens but Thom Hogan's review suggest that the Micro Nikkor does the same thing.

I appreciate the point about DOF at close focus distances. However, one of the reasons Ichose this lens is that it seems well regarded for portrait work. For this application a predictable maximum aperture wouldseem preferable.
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Old Mar 31, 2008, 8:58 PM   #6
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That's quite an odd statement as I'm certain the Sigma's lens barrel stay constant and do not extend any as its AF focus is internal...

Wouldn't the viewfinder 'darken' if one were to get close to 1:1 then (more than 1-stop)?
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Old Mar 31, 2008, 9:50 PM   #7
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NHL wrote:
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That's quite an odd statement as I'm certain the Sigma's lens barrel stay constant and do not extend any as its AF focus is internal...

Wouldn't the viewfinder 'darken' if one were to get close to 1:1 then (more than 1-stop)?
What's happening isn't really the aperture physically changing. The effective aperture is what is changing as magnification plays a role in effective aperature. Nikon's bodies automatically account for this change. It's not so much an indication of lack of quality or poor design...it's more simple physics (or at least as simple as physics can be). From what I understand (most of it is way over my head), this acutally happens at all levels of photography, not just macro. Technically, when you're not focusing at infinity, some compensation must occur because what you see the viewfinder does not actually totally equal the actual aperture set in the lens. Under normal (non macro) photography, these adjustments are so small they can be effectively ignored because very little magnification is involved. In macro work however, these adjustments become critical, and as magnification increases (approaching 1:1 and beyond) the effective aperture gets "smaller". In reality, this happens with all macro lenses...modern Nikon bodies build this into their programming so the photographer can make adjustments to exposure easily.

So the answer to the question "does the viewfinder darken?", is effectively yes and no. The amount of light doesn't change so the viewfinder doesn't darken. The way the light hits the sensor does change because of the extension factor as you approach 1:1. So when you use this lens normally (non macro), the lens functions normally and you shouldn't see any aperture change on the camera, as you're not dealing with high magnification. The further you are from 1:1 magnification, the compensation needed to correct for effective aperture becomes negligible.

I think:?

I admit I don't completely understand the why's and how's....I'm not a physicist, and my explanation is a summary of everything I've read on the subject.
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Old Apr 1, 2008, 4:26 AM   #8
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If I select Aperture priority mode and select f/2.8 the camera displays show the varying aperture as the lens is moved to different focus distances.
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Old Apr 1, 2008, 4:30 AM   #9
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FrankD wrote:
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I previously owned a Minolta 100mm f/2.8 Macro and don't recall the maximum aperture varying with focus distance. But then, I don't recall ever trying to use it wide open either.
Not all cameras show the effective aperture so the Minolta may not have changed or it may have changed and the camera continued to show the set aperture rather than the effective aperture. I believe Canon cameras work in this way while current Nikon cameras show the effective aperture.

As a matter of interest I tried this with a Sigma 50mm macro and that does not change aperture it stays at 2.8 throughout the focusing range.

Ken


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Old Apr 1, 2008, 4:49 AM   #10
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FrankD wrote:
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If I select Aperture priority mode and select f/2.8 the camera displays show the varying aperture as the lens is moved to different focus distances.
That is correct. Remember though, the physical aperature is not changing. The effective aperture is what is changing. The camera shows the effective aperature change so the photographer can make adjsutments to exposure. As you focus more towards infinity (away from the closest focusing distance)the aperature changes less.
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