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Old Jan 20, 2009, 10:12 PM   #1
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ok i have a question about how the lenses work on each of these cameras? lets use
the 70-200mm. now supposedly on a 1.6 crop camera a lot of folks suggest that you are really getting a 112-320mm compared to the same lens on a full frame. but i read somewhere, (and sorry i can't remember if it was on this site or not, i read so much, i don't keep up where i am) that its not really the focal distance that you are getting but that the photos are cropped 1.6 smaller than a full frame. like say if you took a photo at 15 feet @ 70mm the 1.6 crop camera photo would seem a closer crop than a full frame camera photo with the same lens and distance. any help would be greatly appreciated, john

p.s. i hope i worded this correctly
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Old Jan 20, 2009, 10:52 PM   #2
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Basically, you are correct. What you get is the same field of view as a 112-320 lens, but not the same depth of field that you would get with a lens of that focal length on a full frame camera. That's because the sensor is smaller than a 35mm film frame, so it doesn't capture as big an area.
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Old Jan 21, 2009, 2:53 AM   #3
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The relationships are complex.

What MOST people care about MOST of the time is field of view. In this respect the multiplier works very well to give you an idea of equivalence.

With a 1.6 multiplier however, you lose effectively 1.3 stops of DOF equivalence.

So a 100mm @ f2 lens on an APS-C camera gives you approximately the equivalent field of view and DOF to a 160mm lens @ f2.8 on DX (35mmFF) camera.

The longer the actual focal length you are using the smaller the DOF becomes (all other things being equal).

A warning however, whenever you see words like this:
"It's not an equivalent focal length, it's just a crop!"

There usually follows a rant filled with misunderstandings about the implications.

Is the changed DOF a good or bad thing? Well that rather depends on whether you want more or less DOF. Sometimes it's very nice, sometimes it's very annoying.
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Old Jan 21, 2009, 7:04 AM   #4
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A 50mm lens on a 1.6X crop factor camera body would have an angle of view equivalent to an 80mm lens on a 35mm film camera (or a 'Full Frame' dSLR.) Butthe angle of view is not the only thing that is affected by the smaller image sensor. A 50mm lens with an f/1.8 aperture focused at 10 feeton a 1.6X crop factor dSLR has a depth of field of about 9 3/4 inches, but an 80mm lens on full frame dSLR has a depth of field of about6 inches.

So a smaller image sensor affects more than just the angle of view.

The 'Crop Factor' is a crutch for people that have a significant body of experience with 35mm film cameras. If you can look at a scene and know instictively that you should use a lens of a particular focal length, but that instinct came from years of experience shooting 35mm film SLRs, then you need ot pay attention to the crop factor of the camera body you're using, and you need to adjust that instinct for the crop factor. In that instance, the crop factor is useful to you. Ifnot, then it doesn't mean anything to you.

It is also a useful tool for comparing one camera with another. If you've already decided what to buy, then again, it doesn't mean anything to you.


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Old Jan 21, 2009, 10:38 PM   #5
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thanks everyone! tcav you are correct, because i have never shot with a 35mm camera so i have and still am learning on the 1.6 crop camera so in essence it doesn't have any effect on me, because i don't know the difference. hopefully when i upgrade to the 5dmkii 1) it may go down in price! hahahahaha yeah right 2) it won't take me to long to adjust. again thank you! john
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Old Jan 24, 2009, 4:28 AM   #6
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Sairbrushjohn wrote:
Quote:
thanks everyone! tcav you are correct, because i have never shot with a 35mm camera so i have and still am learning on the 1.6 crop camera so in essence it doesn't have any effect on me, because i don't know the difference. hopefully when i upgrade to the 5dmkii 1) it may go down in price! hahahahaha yeah right 2) it won't take me to long to adjust. again thank you! john

A compatible mount 35mm film lens will work with 1.6CF CCD DSLR, but some lenses don't project large enough image to work on a 35mm and are sold as small frame DSLR only.
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Old Jan 24, 2009, 1:00 PM   #7
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TCav wrote:
Quote:
A 50mm lens on a 1.6X crop factor camera body would have an angle of view equivalent to an 80mm lens on a 35mm film camera (or a 'Full Frame' dSLR.) Butthe angle of view is not the only thing that is affected by the smaller image sensor. A 50mm lens with an f/1.8 aperture focused at 10 feeton a 1.6X crop factor dSLR has a depth of field of about 9 3/4 inches, but an 80mm lens on full frame dSLR has a depth of field of about6 inches.
If the aperture for the latter camera (80mm lens) were also f/1.8 would not the depth of field be the same 9-3/4 inches?In other wordsgiven the sameangle of view, aperture, and subject distance (and enlargingto obtain afinished picture the same size in inches) the depth of field would be the same?





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Old Jan 24, 2009, 3:57 PM   #8
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Quote:
If the aperture for the latter camera (80mm lens) were also f/1.8 would not the depth of field be the same 9-3/4 inches? In other words given the same angle of view, aperture, and subject distance (and enlarging to obtain a finished picture the same size in inches) the depth of field would be the same?
No, not in the general case.

All of the following affect perceived DOF:
  1. Print size. [/*]
  2. Visual Acuity of the observer.[/*]
  3. Viewing distance.[/*]
  4. Size of format being enlarged.[/*]
  5. Aperture.[/*]
  6. Focal length.[/*]
  7. Subject distance.[/*]
In your scenario all except 4 & 6 are constant. You might think that changing the focal length in one direction by the crop factor and the format size in the other direction would compensate; and they do, but the formulae show that DOF is dependent on the square of the focal length. So you are actually multiplying by the square of 1.6 in the one direction and dividing by 1.6 in the other direction. So DOF changes pretty much by the crop factor too. This is why as a rule of thumb it is true that: larger formats produce images which have shallower depth of field for equivalent fields of view.

Matters are not always quite as simple as that however because as 7 approaches the hyperfocal distance it gets more complicated.

It's not very complicated when looking at the maths, but tricky to express in a natural language.

For further reading see:

http://en.wikipedia.org/wiki/Circle_of_confusion

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



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