Why don't you just step back a bit for a crop body? compared to a FF.

Started Apr 26, 2010 | Discussions thread
OP NYPD1985 Regular Member • Posts: 130
Re: Sometimes you can, sometimes you can't, but...

Simply amazing...beautiful explanation ! thumbs up

Crocodile Gena wrote:

NYPD1985 wrote:

Okay can someone please enlighten me. For a 1.6 crop body like the Canon Rebels why don't you just take the shot with a few steps backward to get the result of a full frame? Maybe I am wrong so someone need to enlighten me. So for a full frame you take the picture and you can see a lot more than a 1.6 crop body. Okay now, so for a crop body like the 7D or rebel, why don't people just step back more in order to get everything in to the picture? For example if you take a picture from a full frame at 10 feet, wouldn't you get the same stuff in the picture with a rebel taken at 15 feet? Please tell me what am I missing, THANKS!

...when you do, a couple of things happen:

  • the perspective changes,

  • the DOF likely changes, and

  • the noise or motion blur / camera shake may increase.

The first point is somewhat obvious, since perspective, which is the relative position of objects in the frame, depends only on subject-camera distance. All the focal length does is determine which portion of the scene is recorded.

The second point is less obvious. For the same framing and relative size of the aperture diameter, the DOF will be the same. For example, the aperture diameter at 50mm f/2.8 = 50mm / 2.8 ~ 18mm, and the aperture diameter at 50mm f/1.8 = 50mm / 1.8 ~ 28mm.

If the FF camera is 10 feet from the subject at 50mm f/2.8, and the crop camera is 16 feet away from the subject at 50mm f/1.8, then the relative sizes of the apertures are the same (18mm / 28mm ~ 10 ft / 16 ft), so the DOFs will be the same. But if the FF camera is at 50mm f/1.4, then crop is SOL since it can't get an aperture 1.6x larger.

The last point is also not obvious, but very much related to the second point above. The thing is that noise is determined by three factors:

  • How much light falls on the sensor (exposure x sensor area)

  • How efficiently the sensor captures the light (QE -- quantum efficiency)

  • How efficiently the sensor processes the signal (read noise)

Since the crop camera cannot increase the sensor area, it must increase the exposure to maintain the same noise, given relatively equal sensor efficiencies. To increase the exposure, it must either use a larger aperture diameter than FF (which, if possible, would also solve the DOF issue), or a longer shutter speed (which may adversely affect motion blur and/or camera shake).

Using the same example as above, if FF were shooting the scene at 50mm f/2.8 (aperture diameter = 50mm / 2.8 ~ 18mm at 10 feet, then if we used 50mm f/1.8 (aperture diameter = 50mm / 1.8 ~ 28mm) at 16 feet with crop, then we would maintain the same DOF, and, for the same shutter speed, the same noise.

The reason the noise would be the same is because since the crop camera is 1.6x further away than the FF camera for the same framing (but different perspectives), the light reaching the lens will be 2.56x (1.6^2) times less intense. Thus, it needs an aperture with 2.56x the area (1.6x the diameter) to collect the same amount of light for a given shutter speed.

And, once again, if the FF camera is at 50mm f/1.4, the crop camera is once again SOL because it cannot get an aperture large enough to make up for the greater distance.

So, if we don't care about DOF or noise, or shoot under circumstances when crop can use a corresponding larger aperture, then the "only" issues are if there is enough room to back up, if perspective is an issue, if the crop camera's sensor will oversaturate with the larger aperture, and how sharp the image will be at the wider aperture.

Some might argue that it's best just to get a FF DSLR. Until you become focal length limited, that is.

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