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Depth of Field same with digital v. film w/same lens?
Old 01-30-2004, 02:05 PM   #1 (permalink)
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So, I certainly see the differnece in digital v. film cameras when I strap my 50mm lens on my Fuji S2 versus my Nikon N80. Much wider angle in the film camera due to the size of the ccd in the S2. I think the number is somehting like a 1.5 times difference.

So I understand that using my 50mm on my S2 is really like using about a 75mm. Now, what does this do to the depth of field calculations?

Do you look up DOF using a 50mm or a 75mm focal length then?

Here's one of the many online DOF calculators if anybody is interested:

http://www.shuttercity.com/DOF.cfm

I imagine this is pretty obvious to some of you, but it kept me awake for awhile last night, so I though I'd ask.

Do you get the compression that a 75mm would give you? Feel free to retitle this thread if somebody figures out a better way to ask if a 50mm still has the characteristics of a 50mm when it's being used on a digital where the ccd is smaller than a full 35mm.

-Toby
 
 
Re: Depth of Field same with digital v. film w/same lens?
Old 01-30-2004, 02:50 PM   #2 (permalink)
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The depth of field depends on the true. not effective, focal length of the lens and the size of the sensing element. So you should use 50 mm, not 75 mm, for determining the depth of field.

However, the compression you're refering to is a perspective phenomenon, so you'll see the scene as if you were using a 75 mm lens on a 35 mm camera.

In other words, use 75 mm for estimating compression, but 50 mm for depth of field.

According to this calculator, the larger the film (sensing element), the greater the depth of field for a given focal length and aperature. Thus, the sensor in your S-2 (slightly smaller than a 35 mm frame) will have somewhat less DOF than the calculator shows.

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DOF vs APS
Old 01-30-2004, 03:21 PM   #3 (permalink)
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Toby:

Stuff like this makes ya crazy don't it.

It make me wonder if the two or three people who bought a Nikon Pronea or Canon EOS IX (I'll confess to owning an IX) asked these same questions. Maybe they did and I just missed them.

Joe



(Sabrina with my racing team's T-shirt. Spanish speakers will get the humour (?) on the team name.)
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Re: DOF vs APS
Old 01-30-2004, 05:10 PM   #4 (permalink)
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A 35 mm frame is 24 x 36mm.

The sensor on most digital imagers is smaller than that.

The lens which is "projecting" the image (whether on to a sensor or onto a piece of film) doesn't change in character just because you are taking a smaller "section" of the projected image.

Imagine that you shot a 4x6 photo (normal 35mm dimension)

Now imaging you cut an inch off the photo all the way around and then blew the photo back up to 4x6.

If you held the two photos side by side, you would think that the image that you cropped and blew up had been "magnified", but if you compared the the exact same portions of each image, the characteristics are the same.

Using a film lens on a digital camera produces a "multiplication" factor, not a MAGNIFICATION factor.

The same would hold true if you put a 35mm lens on a 4x5 camera....the projected image would contain the full circle seen by the lens...but that doesn't mean that your lens is a wider angle lens....you are just getting more than you get when on a 35mm camera.

If I shoot a 300mm lens on film and a 300mm lens on my D1x, it seems like my lens is longer (450mm), but actually, I am getting the exact same image that I would get with a 300mm on film, but the sensor is only capturing a smaller portion of it which makes it seem like I am getting more.

Same concept with using a 35mm lens on an APS camera.

Hope this makes sense.

Mark
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A circle of confusion
Old 01-30-2004, 07:09 PM   #5 (permalink)
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Depth of field for your Fuji will be smaller at the same distance with the same focal length lens. This is because the acceptable circle of confussion used in the calculation needs to be smaller and because the resulting image will be blownup more due to the crop factor. For 35mm pretty much the standard CoC used in the calculation is .032 for acceptable sharpness when viewing a 8x12" photo at a foot. You have a 1.5 crop factor with your Fuji. If the picture is taken at the same distance with the same lens as 35mm the resulting picture from the Fuji S2 will be a blowup when printed 8x12". DoF is not some fixed distance. It is how much blur we think is Ok away from the one perfect plain of focus. The more you blowup or the closer you view a picture more blur will show. So it appears the DoF gets smaller.

Also if you move back so the image appears the same size in the view finder then the DoF will be larger when using the Fuji. Because Dof is a function of aperture, focal length, distance, output size and viewing distance of the photo.

So when shooting with the Fuji you need to use a smaller CoC in the calculation. Instead of the a CoC of .032 you need to use a CoC of about .019 and if you are making large prints this number needs to be even smaller. I don't think the calculator your using will allow you to change the CoC so you might try this one at Max Lyons site.
http://www.tawbaware.com/maxlyons/calc.htm

Hope that helps,
Bill
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Re: Depth of Field same with digital v. film w/same lens?
Old 01-30-2004, 07:16 PM   #6 (permalink)
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I suppose it is related to compression, but I've always avoided using less than a 50 mm lens when photographing people, since a wider angle tends to "round" features a bit more than is usually desirable.

If this is a goal, I suppose we now have to stand 1.5 times further away to get the same shot?

...just a thought.
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There is some confusion here, that is clear
Old 01-30-2004, 07:20 PM   #7 (permalink)
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Well, from the answers so far it is clear that "confusiion" is the key to all of this. Perhaps someone will want to take a look at this article
and see if it clears some of this up. The rule is interesting and the info below it will clarify some of the confusiion. There are many other good sites on this. Just search Google for the words Digital photography DOF.
Cheers,
rfs


DOF Example
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Re: There is some confusion here, that is clear
Old 01-30-2004, 08:36 PM   #8 (permalink)
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Yup, the rule works pretty well. So if you take the 35mm aperture that gives you the desired DoF and multiply by the crop factor of 1.5. You will get the aperture needed to get the same DoF with the Fuji.

And because we are using lenses of the same focal length as 35mm cameras at the same focal length, distance and aperture the APS size sensor will have a smaller DoF.

So with our digital slrs we can switch to wider lens, move back or stop down to increase the DoF to what we need to equal 35mm.
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Re: There is some confusion here, that is clear
Old 01-30-2004, 08:36 PM   #9 (permalink)
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Good article! Thanks for everybody's opinions.

-Toby
 
 
funny, we\'ve had that same...
Old 01-31-2004, 11:37 PM   #10 (permalink)
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calculator on GG with permission since 1999 in our tips section. If you want to see the DOF calc, go here....

You can also find the Golden Hour Calc and many more by looking in our tips section...thanks, rg sends!
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