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09-20-2010, 12:10 AM   #1
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Are there any F2.8 Macro lenses that are STILL F2.8 at macro distances?

I've been reading that most F2.8 macro lenses are not actually F2.8 when shooting at macro distances.

For example, the Sigma 70mm 2.8 is only 2.8 when shooting from 10' or more. At macro distances it's F4.8.

09-20-2010, 01:07 AM   #2
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QuoteOriginally posted by Unsinkable II Quote
I've been reading that most F2.8 macro lenses are not actually F2.8 when shooting at macro distances.

For example, the Sigma 70mm 2.8 is only 2.8 when shooting from 10' or more. At macro distances it's F4.8.
I think you have been reading FUD.
09-20-2010, 01:22 AM   #3
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The aperture is what it is.
I guess it's possible that some people may be getting worked up over the effective depth of field?
09-20-2010, 03:16 AM   #4
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Actually, the aperture isn't "what it is"... and I've not been reading FUD.

It seems that Macro apertures aren't what we are lead to believe. Here's a quote from one Amazon reviewer on the Sigma 70mm "2.8":

"But there is one major negative that none of the other reviews have touched upon. And it's nearly a deal-breaker for me. This is NOT a true f2.8 fixed lens. It is from 10ft to infinity, but below that the max aperture grows as the subject distance decreases. At 1:1 magnification, 3-4 inches away, maximum aperture is f4.8. This is not really what I had in mind when I bought this lens, and I may return it because it's not going to work as well as I had hoped. I bought an f2.8 lens expecting a constant aperture. I was wrong. I actually think it's a bit of deceptive advertising on the part of Sigma. Since it's primary purpose is to be a Macro lens, they should be clear that it's an f2.8 lens only at decidedly non-macro focal ranges. This is really an f4.8 Macro lens, which isn't so great. So just be advised before you purchase this lens.

Followup:

Apparently, a variable aperture at decreasing macro distances is common to many macro lenses, including Nikon's own 60mm f2.8 and 105mm f2.8 macro lenses. So I can't pick on Sigma too much for this. I'm new to the world of dedicated macro lenses, and I didn't know this 'feature' was just the way these lenses worked.

This also makes me reevaluate my comments about potentially returning this Sigma. I was strongly considering returning the Sigma and picking up the 60mm f2.8 Nikkor Macro instead. I had assumed that the Nikkors would be truly constant f2.8 lenses. I would have been wrong! Since the Nikkor's do this variable aperture thing too"



Also:

"definition of f stop: "The ratio between the diameter of the aperture and the focal length of the lens when the lens is FOCUSED AT INFINITY."
e.g., 25mm diameter aperture with a 50mm lens 1-2 ratio=f2

All lenses, not just macros, have what is called "the effective aperture" change as they focus closer. It's just that non-macros don't get extremely close where the phenomenon becomes noticeable.
In addition, through the lens meters automatically compensate as you focus closer."


09-20-2010, 03:43 AM   #5
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My observations are in accordance with your claim. The aperture doesn't shrink but hte effective aperture is actually smaller. You cant test this with metering 3 feet from uniform surface. Set focus to infinity and to closest distance.
09-20-2010, 06:16 AM - 2 Likes   #6
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Many photographic optics equations where aperture is important use focal length as the measure of distance from the aperture to the lens. This is only true when focused at infinity. When not focused at infinity the distance from lens to sensor is increased by a factor of (1+M) - where M is magnification.

This is of importance when calculating exposure for macros where the nominal F-Stop should be increased by a factor of (1+M); eg at 1:1 magnification "f2.8" for any lens is effectively f5.6 (so far as light intensity on the sensor is concerned).

It is of little practical consequence regarding exposure if through-the-lens metering or instant review is available.

But it can be important regarding the "diffraction limit" of the system... say a lens/camera combination is diffraction limited at f16 for infinity focus; the diffraction limit will be f8 at 1:1 magnification (this is because the Airy derivation uses focal length as the measure of distance from the aperture.)

This distance consideration is important for detailed DOF calculations as well, but the effect is sometimes included in the published equation: A precise equation for total DOF that explicitly includes the (1+M) factor is:
DOF=2CN(1+M)/M^2/(1-(S/S*)^2)
-where S is distance to the subject, S* is hyperfocal distance, C is Circle of Confusion, and N is f-number...the (1+M) factor is unimportant when M is much less than 1.

Dave in Iowa

PS Set your camera at constant aperture (Av) & fill the viewfinder with a blank computer screen (or sky or other constant brightness scene). You will notice the exposure change as you turn the lens' focus ring. Exposure time will increase as the lens moves away from the camera body.

Last edited by newarts; 09-20-2010 at 06:44 AM. Reason: clarification of details
09-20-2010, 06:39 AM   #7
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The formua for effective aperture: EA= NA*(M+1) where EA is effective aperture, NA is nominal aperture (the lens setting), and M is magnification.

Suppose a lens is cranked out to 2:1 magnification. If the aperture is set to f/5.6, then EA= 5.6*(2+1)= 16.8, almost f/17. Yes, 2:1 magnification means a loss of just over 3 f-stops of light, so the shutter must be 8 x slower than when focused to infinity. And 1:1 magnification means a loss of 2 f-stops, for a 4x slower shutter. Magnification eats light.
09-20-2010, 06:49 AM   #8
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This is NOT an issue with this lens or for that matter a fault of any lens in particular.

the fact is that all the lens characteristics, i.e. focal length and aperture are based upon infinity focus.

09-20-2010, 07:49 AM   #9
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QuoteOriginally posted by Lowell Goudge Quote
This is NOT an issue with this lens or for that matter a fault of any lens in particular.

the fact is that all the lens characteristics, i.e. focal length and aperture are based upon infinity focus.
Agreed. Ask wikipedia, you'll see that aperture, as defined by photographers, is related to the diameter of the pupil (fixed) and he focal length (varies a bit with focus distance). Nothing spefial here, it happens with all lenses, it's physics.
09-20-2010, 09:02 AM   #10
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Seems to me the real question is, so what? DOF is going to be paper thin whether you call it f/2.8 or f/4.8, and while I suppose there might be some small percentage of shots where you want extremely shallow DOF but the difference in shutter speed between f/2.8 and f/4.8 is the difference between a sharp and blurry picture, I can't see that really being a major issue.
09-20-2010, 10:43 AM   #11
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QuoteOriginally posted by Marc Sabatella Quote
Seems to me the real question is, so what? DOF is going to be paper thin whether you call it f/2.8 or f/4.8, and while I suppose there might be some small percentage of shots where you want extremely shallow DOF but the difference in shutter speed between f/2.8 and f/4.8 is the difference between a sharp and blurry picture, I can't see that really being a major issue.
Losing nearly 1.5 stops of light that you thought you were paying for is an issue.

Also, keeping with the Sigma 70mm "F2.8" as an example, it's not even F2.8 until 10 feet, meaning you don't get the full DOF advantage with headshot portraits.
09-20-2010, 10:50 AM   #12
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Seriously, this is a non issue... and there is no such thing as a macro lens which retains its aperture at very close focus. And anyway, you don't want wide aperture when doing macro shoots.
09-20-2010, 11:09 AM   #13
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QuoteOriginally posted by Unsinkable II Quote
Losing nearly 1.5 stops of light that you thought you were paying for is an issue.

Also, keeping with the Sigma 70mm "F2.8" as an example, it's not even F2.8 until 10 feet, meaning you don't get the full DOF advantage with headshot portraits.
You don't seem to either understand the real issue or want to accept the facts.

NO LENS actually performs the way you would like, i.e. a constant F stop (i.e. no light fall off) as you focus close in. All lenses perfrom the same way, and all lenses are tested for aperture at infinity regardless of manufacturer.

If you do enough research on the subject, you will probably fond some form of international standard on the issue, which details exactly the tests made.

As for light fall off in macro mode, that is a published issue, It is unfortunate that you seem to have been caught unaware of the problem, but we can't fix that on this forum. The same holds true of lenses and depth of field, they all work the same way so you are not being "cheated" by getting a "smaller" aperture than what you expected.

I appologize if I am sounding a little abrupt here but you have to accept the realities of physics.


As for the sigma, how are you determining the apertures it is providing? Is this in the EXIF data, or displayed on the camera display in any way? or are you calculating it from the change in shutter speed with a "preset" aperture on the lens?
09-20-2010, 11:40 AM   #14
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The question, in general, is: Is there any lens that has max. aperture of F/2.8 at infinity and also has max. aperture of F/2.8 at closest focusing distance?

The answer is no. It's just a law of physics.

For a "normal" (non-macro) lens, the difference can be 0.5 stop. Macro lenses are just extreme case because the closest focusing distance is so small.

Take a look at the aperture marking of this Mamiya 60mm macro F/2.8. At closest focusing distance (1:1 mag. ratio), the actual aperture is 2 stops slower than the aperture setting.

09-20-2010, 11:50 AM   #15
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QuoteOriginally posted by SOldBear Quote

Take a look at the aperture marking of this Mamiya 60mm macro F/2.8. At closest focusing distance (1:1 mag. ratio), the actual aperture is 2 stops slower than the aperture setting.
That's because the lens diameter does not change, but at 1:1 the lens extension equals the focal length, and as a result you have
2 * focal length / aperture

or 2 stops slower.
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