2.8 Macro Lenses don't actually shoot at 2.8?!

N

NickRummy

New member
Well at least I've been doing some reading up on an "issue" I found with my Nikon D200 and a Tokina 2.8 100mm Macro lens I have.

When shooting in manual mode I noticed that at times I could use a 2.8 aperture setting but other times the camera would self adjust to a lower aperture setting (like f/4) . After messing with it longer I noticed that for some reason it had something to do with what I was focusing on.

So I started doing some reading up and searching.

Turns out Macros lenses actually lose about 2 stops when focused at 1:1. So even though my 100mm 2.8 lense says 2.8, at 1:1 magnification it'll actually read 5.6 for aperture. I guess Nikon cameras will display the "effective aperture" on the screen which is why it would change based on my focal point. Canons on the other hand will not show the effective aperture and show what the camera is set to.

Effective Aperture = Set Aperture times (Magnification + 1)

Effective Aperture = 2.8 x (1 + 1)
Effective Aperture = 5.6

Effective Aperture = 2.8 x (1:2 + 1)
Effective Aperture = 2.8 x (1.5)
Effective Aperture = 4.2

This has probably been covered but I found it interesting :D

EDIT: Ugh! Thread title was supposed to read "won't"
 
As well there is a curious aspect to true macro lenses - those that can focus as full 1:1 macro. When you start approaching macro magnification the apparent focal length shortens. So at full 1:1 macro a 100mm macro will actually be functioning closer to somewhere around 70mm. I'm not sure, never heard of any mathematical calculations to figuring it out, but this is why a full stack of tubes on a 100mm macro will give you around 2:1 magnification. :)
 
That really isn't news. True macro lenses achieve their minimum focusing distance by moving the optics away from the camera body (think internal extension tube). That causes light loss. The lens still has all of the bokeh and depth of field of a f/2.8. It's sort of like putting a lens on a cropped sensor. It's still 100mm lens and will behave as such, it just gives you the effective magnification of a 160mm lens (assuming a Canon 1.6x crop).

You're still metering through the lens, it's not a big deal.
 
Yep, everything you've posted jives with what I've read. The 105mm Nikkor I've been wanting actually indicates the change in effective f-stop in the viewfinder as you approach 1:1, whereas I understand from talking to Canon users that Canon lenses leaves the info in the viewfinder at the set and not effective value. I'm not sure which method I prefer, since I have read here and elsewhere that you get the DOF effect of the 'set' aperture but the light transmission of the effective aperture. Of course on an f/2.8 macro this only applies if you are close to 1:1 and have the aperture set smaller than f/5.6.
 
beerguy said:
Ken Rockwell is a moron. The aperture doesn't "diminish" but you do get light fall-off as you approach 1:1 magnification. The two things are not the same. Clearly he's not smart enough to figure that out.
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ROTFLMAO!! "Ken Rockwell" and "moron" are a few words I've often heard together in the same sentence.

Interesting read though guys...thanks for stretching my brain tonight. I've not delved much into macro so this was good info.
 
It's just another excellent example of why you should take this dork with a grain of salt.
 
I feel like Ken Rockwell is Doug's arch nemesis!!

Do many people really shoot 1:1 at f/2.8 anyways?? I feel like in most cases you would have a depth of field of like .5mm...
 
Brett9917 said:
I feel like Ken Rockwell is Doug's arch nemesis!!

Do many people really shoot 1:1 at f/2.8 anyways?? I feel like in most cases you would have a depth of field of like .5mm...
Click to expand...

It just grates on my nerves when folks post his stuff without question. To call him a nemesis would mean that I think he's relevant. I don't.

Depends really, on the type of shot. I've shot at 2.8, with my 100 macro and 60mm of extension tubes. DOF is about the width of a hair at that point but it can be used artistically. For a documentary type shot of a coral, sure, you need more DOF then that.
 
beerguy said:
It just grates on my nerves when folks post his stuff without question. To call him a nemesis would mean that I think he's relevant. I don't.

Depends really, on the type of shot. I've shot at 2.8, with my 100 macro and 60mm of extension tubes. DOF is about the width of a hair at that point but it can be used artistically. For a documentary type shot of a coral, sure, you need more DOF then that.
Click to expand...

Hahaha Touche!

I guess I was more in the coral (or even insect) mindset than an artistic one.
 
Aperture isn't a measurement. It's a fixed ratio based on the lens focal length, optic size and size of the opening through the lens.

Even if there is light falloff because of magnification, the lens is still a f/2.8 and behaves as such in regard to bokeh and depth of field.
 
Aperture isn't a measurement. It's a fixed ratio based on the lens focal length, optic size and size of the opening through the lens.
Click to expand...
Agreed. I at least understand this.

Even if there is light falloff because of magnification, the lens is still a f/2.8 and behaves as such in regard to bokeh and depth of field.
Click to expand...
This is where I am getting confused with all the things I am reading on the web. My understanding is that in order to focus, the lens elements move internally (IF) and that this changes the actual focal length. This also causes a larger circle of light to be cast on the sensor, accounting for the light fall-off. I am just wondering how DOF and aperture cannot change if the focal length is in fact changing. I read different things about this different places. An example discussion here:

http://photo.net/nature-photography-forum/00Ipot

These guys seem to be saying that, with the Nikons and most macro lenses I would assume, that the actual focal length shrinks as you get to 1:1, and that makes no sense to me if the lens opening is staying the same and light is actually decreasing on the sensor. I think maybe I need to stop worrying about all this, because frankly the science of the optics is beyond my desire to comprehend as an amateur, and I probably just need to man up and by the macro lens I've been looking at. The optical magic they work to get lenses to 1:1 or more kinds screws up the things I thought I knew.
 
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