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PostPosted: Wed Jul 27, 2011 10:53 am 
I was reading some material on aperture and I understand that on macro lenses and telephoto lenses the aperture get slightly different as on wideangle lenses or at least it has different unfluence on the Bokeh. So my question is how does this work? (Canon users are welcome to join, since i dont think we have a general photo questions section here on the forum).

I can give you an example of my own experiences and what i noticed on two of the lenses I have and with my own photos:

I made a photo with a Nikkor 35mm prime lens at F3.5 and the distant background with cars and stuff was hardly blurred (hardly any bokeh) at all.

I made a Photo with my nikkor 70-300mm lens at F5.3 at 195mm and the background had very nice bokeh Although i didnt expect that to happen (of course I was happy that it happend).

How does this work for macro? Is that the same as telephoto even decent bokeh at higher apertures?

because then a F2.8 would not really be necessay except if you really need the light or if a Cheaper lens does not perform very good wide open (sometime you have to stop down a lens before it performs at its best, so at F2.8 you have more "buffer", i.e. a F2.8 lens stopped down at F3.5 may perform better then a F3.5 lens 100% wide open)).

So why is my bokeh different on my telephoto lenses then it is on my wideangle (35mm lens). Can somebody explain it.

So also I wonder if it is really necessary to buy a f2.8 telephoto or macro lens since I suspect at F3.5 you get almost the same results in terms of bokeh with half the price of the lens (I am talking about telephoto and macro lenses here). On wide angle lenses there would be a bigger diffrence i think.

Can somebody explain it?


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PostPosted: Wed Jul 27, 2011 12:16 pm 
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Basically depth-of-field (DoF) is controlled by three things: distance from subject, focal length and aperture

Lower f-number (larger opening)-> shallower DoF
Higher focal length -> shallower DoF
Lower distance from subject: shallower DoF

So, your telephoto lens gives you shallow DoF due to its higher focal length even though the aperture is smaller

For macro you have very shallow DoF due to the very short distance to your subject. To compensate you must stop down your aperture (higher f-number)

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PostPosted: Wed Jul 27, 2011 1:20 pm 
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I think it's also worth noting that there is no f/2.8 Macro lens from Nikon due to to the unique helical extension element (built-in zoom in plain English).

The 105mm/2.8 for example is a 210mm/4.8 at minimum focusing distance.

It seems to me that you need to learn about depth of field. Janern's reply sums it up quite nicely.


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PostPosted: Wed Jul 27, 2011 2:14 pm 
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I think you mean hyperfocal distance
http://www.dofmaster.com/hyperfocal.html

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PostPosted: Wed Jul 27, 2011 3:18 pm 
I think Canon also has not real f2.8 macro lens. Only difference is that Nikon shows you the corrected/compensated aperture in your nikonbody viewfinder and canon user have to guess it, because it says F2.8 while its actually something else.

So it doesnt really matter if you have a canon or a nikon only Canon likes to be mysterious about it.

I always thought that the aperture was just the dof in front and behind the subject. But now aparently there is something more to it then that....

I have read information that some even like to use F32 on there macro lens on other websites (but depending on your lens and camera that is really F16 then not F32). So I am a bit puzzeled why people by expensive F2.8 macro lenses. Probably some reason I dont get yet.


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PostPosted: Wed Jul 27, 2011 4:18 pm 
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The reason why many macro/micro photographers tend to use those small apertures is to have maximum depth of field, which at close focus still is a few millimeters only.

Bokeh in macro photography
Even when you stop down to f/16 or f/22 you'll often have nice background bokeh in macro photography.
The main reason is the close focusing distance and narrow depth of field. An object only an inch behind the subject (plane of focus) can be completely washed out.
Image
In this example I used a smaller aperture (f/11). The rose in the background was only 1.5 inches behind the subject.
Due to the close focusing distance it was nicely blurred out. Magnification: 1:1


Regarding sharpness...
Personally I found that you start to lose sharpness due to diffraction around f/16 and the more you stop down the more detail/contrast is sacrificed for a marginally increased DoF. Many macro specialists use the Scheimpflug principle, which allows them to alter the angle of the plane of focus (more in focus despite larger aperture). Of course, this requires the use of a lens with tilt capabilities (like Nikon's PC-E lenses) or a TS-adapter or a view camera. A cheapo alternative is focus stacking.


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PostPosted: Wed Jul 27, 2011 4:43 pm 
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On the aperture value thing, Nikon are the odd ball out there. Canon/Sony show what I call the "physical aperture". Nikon shows the "effective aperture". The two are the same at longer shooting distances (low magnification). They only diverge at higher magnifications. The effective aperture does influence exposure, so might be more useful to those using external metering for macro and save a further correction factor that otherwise would be needed. A nominal f/2.8 macro lens is effectively f/5.6 at 1:1 magnification.

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PostPosted: Wed Jul 27, 2011 5:12 pm 
As far as I know Nikon only does that with their Micro lenses, (which is probably a more correct name anyway, because they magnify the subject bigger then real life on photos which is technicaly called micro photography and not macro).

In the end it is the result that counts. With a canon you don't know what the effective aperture will be and with a Nikon you dont know the real aperture is (unless you have an aperture ring on the lens). I guess you will learn how to deal with it after a while once you get to work with the lens.


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PostPosted: Thu Jul 28, 2011 8:02 pm 
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Aperture and magnification determine the dof, not focal length.
And magnification has the much bigger influence on dof: a doubling of magnification shrinks the dof by 4. That's why you often need very small apertures (like f/16 and beyond) to get anything useful in focus at a macro-/micro-shot.

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