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Old 08-30-2012, 05:55 PM   #124
xluben
Scooby Guru
 
Member#: 261612
Join Date: Oct 2010
Location: Minneapolis, MN
Vehicle:
2002 WRX - 750WHP
9.5s @ 150mph

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Quote:
Originally Posted by vision.dynamix View Post
Quote:
Originally Posted by vision.dynamix View Post
For instance, I plugged in:

5D, 50mm, f/1.2.

At 3', the total DOF is 0.07'
At 10', the total DOF is 0.86'
At 25', the total DOF is 5.47'
At 50', the total DOF is 22.7'
This is the calculator I used to determine that 135mm f/4.5 on full frame is about the same as 85mm f/2.8 on a crop body. It is a very useful tool, but simply looking at DOF can be a bit misleading for the topic at hand, which is background blur. Many people think DOF and background blur are one in the same. They are not. They are related, but not the same thing.

Depending on focal length (which control camera to subject distance) and subject to background distance (controlled by how you set up the shot), the background blur can greatly change, even with the same DOF. Here is an example in which the focal length and camera to subject distance are changed (in order to keep the subject the same size in the frame, and to keep the DOF exactly the same). You can clearly see that the compressed perspective of the longer lens makes for more background blur.



Here is the article that goes with the photo (NOT written by me):

http://www.bobatkins.com/photography...ound_blur.html

And here's a good synopsis:

Quote:
If the background is far enough away (well outside the depth of field) and the subject if fairly close (well inside the hyperfocal distance) the degree of blurring is related to the absolute physical size of the lens aperture. For a 56mm f5.6 lens that would be 10mm and for an 85mm f5.6 lens it would be 85/5.6 = 15mm. So you'd expect the far distant background blur of the 85/5.6 to be about 50% more than that of the 56/5.6, and, in fact, it is!

Based on this you can see why a 600/4 lens wide open (physical aperture = 150mm) will blur the far distant background far more than a wide open 50/1.4 lens (physical aperture = 35.7mm). In fact it will blur it 4.2x more. However if shooting the same subject at the same magnification (let's say 0.01x, which would by typical for a full body shot of a person), the depth of field of the 50/1.4 will be about 1/3 of that of the 600/4 and so will blur close in objects more.

There's no really simple "rule of thumb" that will tell you whether relative aperture (f-stop) or absolute aperture (size of the aperture in mm) will be most important in determining the degree of blurring at a given distance behind (or in front of) a particular focus point. Hence the need for this calculator!
I wouldn't get too tied up in the calculator at the bottom of the page, but it can be fun to play with. I've found that once you become more familiar with setting up the shots, you should be able to estimate what is best for each situation just by looking at it.

Here's another good example (from another one of his articles) on how the same f-stop will give you more background blur with a longer lens (if you back up so that the subject framing is the same):



For car shoots you're going to want to have a large subject to background distance and a large physical aperture. Usually this means you're going to want a fast telephoto over something like a Nifty Fifty. Even a 50mm f/1.2 probably isn't going to have the look you want. It will have a razor thin DOF, but not that much background blur. This is really the opposite of what you want. You'd rather have the whole car in focus, but the background completely blurred. This is exactly the job for a telephoto.

For some good advice on getting a lens with a lot of blur, here is another article:

http://www.bobatkins.com/photography...ound_blur.html

The first section just proves the point that a fast prime will have way more blur than an f/2.8 zoom, but we all knew that already. The bottom section goes back to the point about choosing the lens with the largest physical aperture (focal length / f-stop), in order to have the most background blur.

To try and put it in perspective, I came up with a table of some common lenses to consider. I calculated the physical size of the aperture as well as the DOF at a given distance. I came up with the distance based on how far you would have to stand in order to get a 20 foot wide field of view. I figured this would be useful for a car photo. I used the "Dimensional Field of View Calculator" section of this site to calculate the distance to subject:

http://www.tawbaware.com/maxlyons/calc.htm

And then I went back to this calculator to get the DOF:

http://www.dofmaster.com/dofjs.html

What I didn't calculate is the background blur. Why? Because that depends on how far away the background is! The DOF in the table being small has nothing to do with the background blur, it is the physical size of the aperture that matters. Adding the DOF into the table just proves the point that shorter, fast lenses have a shallow DOF and small physical aperture (usually not what you would want for blurred backgrounds in car photos), while a longer lens has much more useful DOF and a larger physical aperture, therefore more background blur with the correct setup.

Here's the table I came up with:



Looking at a specific example for INKMAN...



The top and bottom are ones you currently have. The others are "affordable" options you may be considering. Between the ones you have, you can see that the 250mm end of the zoom is much better than the nifty fifty. The only problem is that you need over 200ft of working distance (and you'd want that much behind the car too)!

The 70-200 f/4 really isn't going to gain you any background blur at all (very little), but it will make it so you can stand 50ft closer. It may be the best lens for you all around (including motorsports, walkaround, people, etc), but in terms of background blur for set up car shots, I think the 85mm f/1.8 may be your best option. Anything longer on a crop body starts to require huge amounts of space to use properly.
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