Macro and f-stop

Tiziano

So I got my Tokina 100mm f/2.8 Macro the other day and have just begun playing with it.

What struck me initially was that the f/2.8 setting was not constant like on my other primes. When I power up the cam it'll go to 2.8 but as soon as I focus on something it jumps up. At times the lowest I can set is f/4.

I read through the flyer-type manual (it's a one size fits all for all of their current lenses) and found a reference to the aperture changing based upon whether one is in macro mode or not. At least I think I read that last night at 2AM. I don't have it with me ATM.

I've 2 questions.
Is there something amiss with my lens?
If not, would someone explain why this macro lens is not able to constantly keep it's aperture at f2.8?

ziggy53

I believe that your lens is compensating for the loss in efficiency at very long extensions. Extension tubes have the same impact when they work to expand the image circle and reduce the efficiency of the lens.

Tiziano

OK, so that's the result of the front element physically extending so far out when focussing close. Interesting. Is this a normal thing? I mean, would the Sigma 105mm Macro do the same thing?

Brendan

I believe almost all (if not all) macro lenses do this. My Nikon 60mm f/2.8 Micro does the same thing, and my old MF macro lens even has exposure compensation values listed on the lens barrel to account for light loss.

The good news is that it shouldn't really matter. DoF will be ridiculously shallow at large apertures and close focus anyway, so there should be no need to open up further. You should probably be using a tripod anyway, so the loss of light shouldn't matter. And you can still use the lens as a 'normal' lens with the aperture set ~f/2.8.

Hope this helps.

—Brendan

Tiziano

Thanks Brendan. Between your response and Ziggy's I think I've finally got it!

Tosser

Canon's 100 f2.8 macro does not do that.

It's also internal focusing so the front element does not move.

pathfinder

Macro lenses are not used for macro purposes wide open.

Not an absolute truth, maybe, but as close as you are going to get in photography.

Look through the Macro forum for an image shot wide open, by Lord Vetinari or other posters.

Tosser

pathfinder wrote:

Macro lenses are not used for macro purposes wide open.

Not an absolute truth, maybe, but as close as you are going to get in photography.

Look through the Macro forum for an image shot wide open, by Lord Vetinari or other posters.

True if you are trying to accurately depict a bug or something.

I'm often trying for something a little more "creative" and frequently use the lens wide open to get a shallow DOF. It's a challenge though because at 2.8 and 2" the DOF is razor thin.

Qarik

I have my fstop at f/16 on my sigma 105mm macro and always use a flash. I don't think any GOOD macro setup is going to use f/2.8 (that's like 1 billionth of an inch dof heh) and no flash

Brendan

Tosser wrote:

True if you are trying to accurately depict a bug or something.

I'm often trying for something a little more "creative" and frequently use the lens wide open to get a shallow DOF. It's a challenge though because at 2.8 and 2" the DOF is razor thin.

Agreed. I shoot wide open with my macro lens quite often to pursue something a bit more 'creative' (or at least that's my goal). For example, this photograph was shot wide open to achieve a specific goal:



—Brendan

jfriend

Biarien wrote:

Agreed. I shoot wide open with my macro lens quite often to pursue something a bit more 'creative' (or at least that's my goal). For example, this photograph was shot wide open to achieve a specific goal:



—Brendan

If you are shooting at 1:1 macro distance, I challenge anyone to be able to tell the difference between a shot at f/2.8 and f/4. We're talking about the difference between 0.01" and 0.02" inches of DOF. It's razor thin no matter which wide aperture you pick.

These macro lenses are designed for typical uses at their macro distance which will be shot at at least f/8 and often f/11-f/22 to get some semblance of depth in focus so anything other than a completely flat subject can be seen as in focus.

Tiziano

I spent two hours shooting flowers last night with this lens.
AMAZING is the word!

It did have a learning curve however.
One thing I was used to with the Sigma zoom/macro was getting real close. At the extended 70mm I often just about had the front lens element touching the flower in order to fill the frame as I intended. With the 100mm I had to keep backing up a bit.

Also, I always shoot these things handheld. I know, I know that I need to use a tripod but I'd rather wing it. With the 100mm it's even more sensitve to movement that the 70mm. Still, manually focussing I was able to shoot 200 shots in 2 hours and only a small handfull are blurry.

Finally, the extra distance I mentioned in point 1 above also really helped with the DOF. It is noticeable to me compared to the 70mm.

pathfinder

Biarien wrote:

Agreed. I shoot wide open with my macro lens quite often to pursue something a bit more 'creative' (or at least that's my goal). For example, this photograph was shot wide open to achieve a specific goal:



—Brendan

What you are doing is a perfectly valid use of a mild telephoto lens, but does not require a macro lens to do.

The use of telephotos, focusing closely to the film plane, with a wide aperture, can offer very shallow DOFs. Extension tubes can even be used to increase the shallowness of the DOF, but this is not necessarily a real macro photograph, until you get close to a 1:1 ratio of image size at the film plane to subject size.

Tiziano

Here's one of the shots I'm happiest with (also crossposted in the Macro forum).
Handheld, ISO200, 1/200 at f/18, minimal Lightroom 2 post processing, no cropping.

Tosser

Here's one at 2.8 just putzing around in the yard. And, this was at macro distance of about 4" from the subject.

Tosser

The same subject at f13 just doesn't appeal to me.