SLR v. Fixed-Lens Camera Apertures

Discussion in 'Photography Beginners' Forum' started by fatsheep, Jan 5, 2008.

  1. fatsheep

    fatsheep TPF Noob!

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    According to "Understanding Exposure", larger apertures (small F-numbers) on fixed-lens cameras are equivalent to much smaller apertures (large F-numbers) on SLR cameras. "Your fixed-lens digital camera is hopelessly plagued with the uncanny ability to render a tremendous amount of depth of field." (page 46)

    He says that f/2.8 on a fixed-lens is equivalent to f/11 on SLR. He goes on to say that f/4 is equivalent to f/16, f/5.6 -> f/22, f/8 -> f/22, f/11 -> f/64. "Those of us who use SLRs can only dream of the vast depth of field that would result from apertures like f/64."

    So apparently an aperture of a fixed-lens camera is equivalent to an aperture four stops higher on a SLR where depth of field is concerned. Why is this? I would think that SLR cameras which cost hundreds more then typical fixed-lens cameras would be superior in all ways.

    My camera is a fixed-lens and goes up to f/13.6 so my depth of field at that aperture must be really large?
     
  2. Helen B

    Helen B TPF Noob!

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    The S700 has a sensor with a diagonal of about 6.5 mm. The diagonal of a 35 mm frame is about 43 mm. They are different shapes (aspect ratios) but we can ignore that for this simple comparison.

    That means that the sensor of the S700 is about 1/7th the linear size of 35 mm. Therefore, if you stand at the same place and take the same picture with the same angle, the focal length of the lens on the S700 will be about 1/7th of the focal length of the lens on the 35 mm. If the 35 mm camera had a 50 mm lens, the S700 zoom would be set to about 7 mm.

    The depth of field depends on the inverse of the square of the focal length of the lens, if everything else is held the same. Therefore the S700 could have about fifty times (about seven squared) the depth of field of the 35 mm camera.

    However, the image from the S700 is going to have to be enlarged seven times more than the 35 mm image - so to keep the same sharpness it needs to be seven times sharper to begin with (ie it must have 1/7th of the amount of acceptable blur in the 35 mm image).

    That works against the gain made by using a shorter focal length, so that the S700 has seven times the depth of field as the 35 mm at the same aperture (ie 7 squared divided by 7).

    The depth of field also depends on the aperture - it is directly proportional to the F-number. That means that the S700 has a depth of field equivalent to the 35 mm with an F-number seven times higher. F/4 is like f/28. (All approximate)

    How does that sound?

    Best,
    Helen
     
  3. Mav

    Mav TPF Noob!

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    It's because of the much larger sensor. I'd have to break out my old Optical Engineering textbook to explain the math, which I'm not about to do. :p Nothing is ever superior in all ways to something else. There's always trade-offs, and there's no such thing as a free ride.

    Yes, P&S cameras do have very large depths of field, but that's exactly what you don't want if you're trying to get a 3D look to your photos or some background blur or you're trying to isolate your subject. A 1.5x crop factor DSLR will have better subject isolating ability than a P&S. A no crop DSLR (Canon 1Ds / 5D, Nikon D3) have better isolating ability than the 1.5/1.6x DSLRs. Medium format has better isolating ability than any 35mm based DSLR, etc. As far as P&S cameras, the depth of field is nice for landscape work, but it's much more limited in other ways. The tiny sensor results in very low resolution and poor sharpness compared to even the cheapest DSLR. And the small sensor also makes it impossible to get any sort of a wide angle setup without external adapters which will rob light and probably sharpness too. All other factors being equal/equivalent, medium format is far sharper and has better resolution than any 35mm based DSLR also.

    I was looking at some image samples from some P&S cameras over Christmas wanting to get a backup camera I could just slide in my pocket, but the image quality was just so poor that I couldn't pull the trigger on anything. I'm spoiled by the superior image quality of my DSLRs now and can't go back. I'll still probably get something though, gotta be cheap and under $200.
     
  4. Sideburns

    Sideburns TPF Noob!

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    They are superior. How are they not?

    More depth of field is not always a good thing...thus you would use a small F-number. On a fixed lens camera, you're kinda screwed....

    This is because of the size of the sensor. I could get more into it, but that's about the most important piece of info.
     
  5. fatsheep

    fatsheep TPF Noob!

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    Thanks for all the responses everyone. I'm really new to photography so this information is a bit confusing. As such I have many many questions. :)

    Thanks for the explanation but I'm getting a bit confused here. The "35 mm" in a 35 mm camera is the diameter of the lense correct?

    The "focal length" is the distance in mm between the center of the lense and the sensor which affects how zoomed in you are?

    Is this 35 mm camera in the example a film, digital, SLR, ... or does that matter? What exactly is the "frame" of the 35 mm camera?

    I'll take your word for the optics but I'm not visualizing this really well and it doesn't make much sense at the moment. Any links for further details and explanations and such on this would be greatly appreciated. :)


    So crop factor for a DSLR is the 35 mm frame diagonal divided by the diagonal of the sensor in the digital camera? I'm assuming a "no crop" DSLR would have the same diagonal length in its sensor as a 35 mm camera's frame? "Medium format" is film larger than 35 mm?

    In "Understanding Exposure" Bryan Peterson makes it sound as if fixed-lens cameras have a big advantage over SLRs in being able to use larger apertures and still get great DOF. On page 47 there is a picture he took handheld with a point and shoot. He mentions in the caption to the right how he would have had to use a tripod on an SLR to get the DOF he needed. However, on the P&S, he set the aperture wide open and still got good DOF which allowed for faster shutter speeds. On the other hand, he does also mention that SLRs are much better for situations where shallow depth of view is needed.

    He didn't mention any difference in picture quality that I can remember or pick out from scanning it. I think he was just trying to make the readers with P&S cameras like me feel better. ;)
     
  6. RKW3

    RKW3 TPF Noob!

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    Yeah it's not always a good thing to have a big DOF. On my old fixed lens I could never get the cool "bokeh" I can with my new nikon.
     
  7. Mav

    Mav TPF Noob!

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    Crop factor: http://www.kenrockwell.com/tech/crop-factor.htm
    tons more on formats here: http://www.kenrockwell.com/tech/format.htm

    Gigapixel large format film: http://www.gigapxl.org/ (crazy! :mrgreen:)
     
  8. Sideburns

    Sideburns TPF Noob!

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    He's right. They DO get crazy DOF...

    But what if you don't want a lot?

    There are times when you only want little...and he covers that.
     

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