# Please tell me I am correct & the pro Is wrong???

Started 4 months ago | Discussion thread
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 Re: I'd be happy to. In reply to miketuthill, 4 months ago

miketuthill wrote:

Great Bustard wrote:

happysnapper64 wrote:

I read a thread a week or so ago, & thought I had at last got the understanding of the difference between an image taken with a crop v FF. It was a "Eureka" moment for me, or so I thought. The difference, I learned, was in the FIELD OF VIEW, & NOT the focal length. That a 70-300 was a FOCAL LENGTH of 70-300 on both, but the FF allowed a WIDER view of the scene but NOT a longer view, eg. it didn't make the subject appear closer. Right, to the point. I have just been reading a UK photography mag which was testing various 70-300's. Reading the review of the Tamron it read, "The lens gives a FOCAL LENGTH of 110-420 on a crop frame camera". Please put me right, it is the FOV that changes, right? NOT the focal length? If I am correct, it is no wonder us newbies have a hard time getting to grips with things, when the pro's & teachers can't explain it propperly.

Neither the focal length nor the f-ratio of a lens change just because they sit in front of a different size sensor. However, the effect of both the focal length and f-ratio do change. Thus, 50mm f/2 = 50mm f/2 no matter what sensor the lens sits in front of, but:

http://www.josephjamesphotography.com/equivalence/#introduction

50mm f/2 on 4/3 (mFT) is equivalent to 62.5mm f/2.5 on 1.6x (Canon APS-C), 67mm f/2.7 on 1.5x (APS-C for everyone else), and 100mm f/4 on FF (FX), where "equivalent to" means:

• The photos all have the same AOV (diagonal angle of view) and aperture (entrance pupil) diameter: 50mm / 2 = 62.5mm / 2.5 = 67mm / 2.7 = 100mm / 4 = 25mm.

• The photos all have the same DOF (as well as diffraction softening) when they have same perspective (subject-camera distance), AOV, aperture diameter, and display size.

• The photos all have the same motion blur and the same total amount of light falls on the sensor when the aperture diameter and shutter speed are the same (e.g. ISO 100 on mFT, ISO 160 on APS-C, and ISO 400 on FF).

• The photos all have the same same noise when the same total amount of light falls on the sensor if the sensors are equally efficient (less noise if the sensor is more efficient, more noise if the sensor is less efficient).

• Other elements of IQ, such as resolution, bokeh, flare resistance, etc., as well as elements of operation, such as AF speed/accuracy, size, weight, etc., are not covered in this use of the term "equivalent".

Hope that helps, and is not overly technical. But it's about more than just AOV.

Now we're really getting into the twilight zone:-)

Nah; we've been there for a long time (and well we should be).

--
gollywop

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