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[OM] Having 90/2, should I buy 50/3,5?

Subject: [OM] Having 90/2, should I buy 50/3,5?
From: Chuck Norcutt <chucknorcutt@xxxxxxxxxxx>
Date: Thu, 10 Mar 2005 09:41:25 -0500
Olli said:
I have understood that in macro photography the depth of field is not 
dependent on the focal lenth, but on the magnification. Therefore there 
is no NEED for me to buy a shorter macro lens. But the angle of view 
does change according to focal length even for macro shots (am I 
right?). Therefore I might have some use for a 50/3,5.

I really would like to see an example of what does the angle of view 
look like for these two lenses (if the photos are taken of the same 
subject). Does anyone have some examples to share? Is there any 
difference in perspective?

What about sharpness and contrast? Is 90/2 so superior to 50/3,5 that I
shouldn't even consider it? I bet that the difference is not that huge...
--------------------------------

I'm sure I have stated several times before that the best way to get a 
good understanding of DOF is to play with a DOF calculator.  There is a 
good one free at <http://www.dl-c.com/Temp/>

Go to the "downloads" tab and select the 4th item down, the DOF 
calculator.  After installing and starting the calculator be sure to 
press the "help" button which will give you a great deal of information 
about DOF, hyperfocal distance and resolving power of many films.  Pay 
special attention to the discussion of resolution as it relates to your 
mention of magnification.  Simply put, the more we magnify an image the 
sharper the original image needs to be in order to appear sharp on the 
final print.

Short focal length lenses are commonly thought to have greater depth of 
field than longer lenses but this is not true if the final image size 
and magnification are the same.  Let's take an example using the dl-c 
calculator above and accepting the default 30 lines/mm resolution.

Let's assume you are using the 50/3.5 set at f/11 and focused at 10". 
The near focus is reported as 9.71" and the far focus as 10.31" for a 
DOF of 0.6".  Now, if you switch to the 90/2 also set at f/11 and 10" 
focusing distance you'll find that the DOF is only 0.14".  However, the 
90mm lens has a narrower field of view than the 50mm.  If we had been 
shooting a flower before and filling the frame with the 50mm we now only 
see a central section with the 90mm.  In order to maintain the same 
field of view and magnification we need to back the 90mm away from the 
subject by the ratio of the focal lengths.  90/50 = 1.8 so we'll back 
the 90mm away from the subject until it's 18" away. At 18" the 90mm's 
linear field of view is the same as the 50's at 10".  Except for 
perspective*, we now see the same thing in the viewfinder no matter 
which lens we're using.

And we also get the same DOF (within rounding error).  Plugging in focus 
distance of 18" with the 90mm set at f/11 the calculator reports near 
focus of 17.71" and far focus of 18.30" for a DOF of 0.59".  Close 
enough to 0.6" that we'll attribute the difference to rounding in the 
calculations.

So, why would you want a 50 vs 90mm lens?  There is a difference in 
perspective but that is probably not very important.  What is important 
is that the 90mm gives you greater working distance for the same 
magnification.  If you are shooting skittish or venomous critters you 
may choose to be further away.  On the other hand, the 90/2 is no light 
weight.  The 50/3.5, however, is very small and light and makes a very 
comfortable, general purpose walk-around lens which also happens to be 
able to focus very close.  If I'm traveling light with just a single 
body and lens I'm much more likely to take the 50 rather than the 90.

Of course, if you are a Zuikoholic you need to have both.

*(Perspective is different since it varies with distance... place your 
head at different distances from your monitor and note what you can see 
behind it both near and far)

Chuck Norcutt


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