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Old Jul 15, 2002, 12:36 PM   #1
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Default depth of field question

In some camera reviews it says that it has a particular type of aperture for "real" depth of field.
My question is - what does it mean "real" depth of field?
I mainly shoot portraits, so depth of field is very important to me. I want to be able to have a blurred background, especially when shooting in telephoto mode (e.g. x6 zoom).
I consider buying the Fuji S602. In max zoom, will I have normal depth of field as in a normal SLR, or is it different in digital cameras?

[Edited on 7-15-2002 by arutha]
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Old Jul 15, 2002, 4:55 PM   #2
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Depth of field is a complex issue and varies according to focal lenghth (true focal length, not "35mm equivalent"), aperture, distance and magnification. The issue with most digicams is that the frame size and true focal lengths are scaled down from 35mm. For example you're taking a portrait with a 100mm lens on a 35mm camera. You're 2 meters from the subject and have the lens opened up to f2.8 for shallow depth of field. I'll compare that to a Dimage 7 which has a true focal length and film frame of about a quarter the 35mm camera for similar angle of view. This changes the magnification which has a profound effect. I'll use a common circle of confusion size of .025mm for 35mm and a propoprtional amount for the digicam of a quarter all the 35mm dimensions. The cof for the digicam works out to about 2 pixels across. Using the following formula:

frontdepth = Ne*c/(M^2 * (1 + (N*c)/(f*M)))

Where N = aperture, Ne = aperture corrected for bellows factor, M= magnification and c = circle of confusion. Frontdepth is the depth of acceptable focus measured from the focus distance of 2000mm toward the camera. If you focus on eyes this is how big the nose can be and still stay in focus. I did the calculation on a spreadhseet I have and here are the results

35mm = 17.56mm
Digtal = 73.57

Vast difference. This is why it'sdifficult to get shallow depth of field with a consumer digicam. This is one reason I'm holding out for DLSR with a 1:1 mag ratio.

The problem is just the opposite with large format cameras. A portrait taken with a 4x5 camera at that ditance with an equivalent lens becomes a macro shot with a razor thin depth of field. If I scale all the factors including circle of conufion it's only 4mm. If I keep the same circle of confusion as 35mm which is more realistic it's barely over a milliemeter.

Short answer is that it's not inherently different for digital but for most digicams it is.

For more info on optical formulas http://graflex.org/lenses/lens-faq.html

[Edited on 7-15-2002 by padeye]
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Old Jul 16, 2002, 5:47 PM   #3
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Default hmm... more dillemmas

Thanks a lot for the detailed reply! You definitely answered my question.
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Old Jul 16, 2002, 7:41 PM   #4
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Sorry if I made your head hurt but that's what it comes down to. For the most part you will get plenty of depth of field with a digicam, you just won't be able to limit it as much as with a larger film format. Those are the breaks and we all live with them. Until they come out with a DLSR with 14-16mp and a 1:1 mag ratio (at less than $2k) I'll be using my film cameras for depth of field control.
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Old Jul 16, 2002, 9:27 PM   #5
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Padeye, I think you forgot to include one more thing into your post... some ASPIRIN!

Nevertheless, a very educated answer. It helped me out also. Thanks.

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Old Jul 16, 2002, 9:43 PM   #6
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Default Steve, you definitely made MY head hurt...

But what an intelligent, informative answer! I actually learned something... I think...


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Old Jul 16, 2002, 9:51 PM   #7
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Default The DOF rule:

The general rule is: select f stop, older cameras had dof scale, and expect 1/3 front aciptable focus and 2/3 backfocus .... but.... Not all lenses are created equal!!!!

There are many variables as to why a lens is ground and what it will do, and also, out of the same batch of lenses, one will be better!

[Edited on 7-17-2002 by M. Haner]
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Old Jul 17, 2002, 9:16 AM   #8
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Default field test results

Hi again..
I went to the store today and tried out 3 different cams: Fuji S602, Nikon 5000, and Minolta 7i.
It'samazing how important it is to actually hold the cameras in your hand.
Here are my thoughts:

1. Ergonomics: the fuji felt very comfortable when I first took it in my hand, but after playing with it for half an hour I suddently realized how bulky the right hand grip is, and that you need to have quite a big hand to hold it comfortably. Still, pretty good ergonomics in my opinion.
The Minolta has a strange design - the right hand grip is awkward, with sharp edges, but it feels quite comfortable. I didn't hold it as long as the fuji, though, so I don't know how it feels in prolonged use.
The Nikon is a different story. First, I was amazed at how small it is (too small?). Second, it has a very good feel and excellent ergonomics in my opinion, the best of the three.
Furthermore, after playing with the swiveling LCD of the NIkon it suddenly bothered me that in the other two the LCD is fixed on the back of the camera. Since you can only look at the LCD at a right angle, it means that you always have to hold the camera near yourt face. The Nikon, however, allows you to hold it any way you like, which I think is a plus.

2. Regarding the depth of field, I shot some pics in the store at max zoom and was surprised to see that I could obtain a shallow depth of field (that is, have the background blurred), which is nice when taking portraits. Maybe not as much as with a film SLR, but not as bad as I thought.

That's it. I just thought I'd mention these things since I wasn't aware of them until I had the cameras in my hand.
Bottom line..... I still don't know what to get!! :-((

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