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'Effective' Focal Length & Crop factor Explained
This question seems to come up over and over, so I though I would give a go at explaining it. As they say a picture is worth a thousand words....

In the image below the tip two frames show an ilustration of a full frame camera image. the squares represent an individual picture element (pixel). at 100mm I am taking a picture of a green box with a blue box around it. if I switch to a 160mm zoom lens, I "zoom" in to fill the sensor/image with only the green box..

Now if I take the same picture with a 1.6 crop body, it has a smaller sensor so the blue box spills over the sensor and the resulting image on the bottom right looks like I was zoomed in with 160mm zoom. but notice that the squares are now bigger, ie lower quality, because the image is "cropped" and expanded to the same size. the benifit is that you leave off the part of the lens that tends to suffer from distortions, the edges, but you are not captureing the resolution, or image quality, unless you use a smaller sensor with 1.6 times as many pixels..

Image control:Zoom outZoom 100%Zoom inExpand AllOpen in new window..

Comments (11)

Photon:.

That's a good description of "digital zoom" but not of cropped sensors. Your example presumes that someone has taken a scalpel and cut out the central portion of a full frame sensor and plopped it into another camera. Were that the case, the cropped sensor would indeed have fewer pixels. But while a cropped sensor MAY have fewer pixels it could also have the same number, or even MORE pixels. Your example would not hold water in that case. The concept of a cropped sensor has ONLY to do with relative physical size of the sensor and has nothing to do with the number pixels..

Now, a better point, and the one that perhaps you were leading to, is that if the smaller sensor has the same number of pixels, the pixels are more crammed together on the sensor. And the closer pixel pitch does indeed affect image quality all things being equal there will be more noise present in the cropped sensor image..

Dave.

Http://www.pbase.com/dsjtecserv..

Comment #1

Now, a better point, and the one that perhaps you were leading to, isthat if the smaller sensor has the same number of pixels, the pixelsare more crammed together on the sensor. And the closer pixel pitchdoes indeed affect image quality all things being equal there willbe more noise present in the cropped sensor image..

There was along (and bad-tempered, in places) thread recently on this, and many people who seemed to know what they were talking about expressed the opinion that this doesn't actually make any difference. Of course smaller pixels have more noise *individually* than larger pixels, but the fact that there are more of them means that, *in a given image size*, the greater number of samples per unit area reduces noise, and the effects more or less cancel out..

A good test of this would be the Nikon D40 (6MP) vs. D40x (10MP in the same size sensor), with all other things (lens, software etc) being the same. If you looked at '100% crops' of both the 10MP image would have more noise for the reason you mention, but that equates to printing an image 66% larger. If you printed the two pictures at the same size there should be no difference..

Best wishesMike..

Comment #2

PhotonFiend wrote:.

This question seems to come up over and over, so I though I wouldgive a go at explaining it. As they say a picture is worth athousand words....

I fully understand effective focal length and crop factor, and your description left me confused! LOL!.

If you want to demonstrate this pictorially, all you need is three captures of the same scene. The first would be 100mm full frame, the second would be 160mm full frame, and the third would be 100mm with a 1.6x crop. The first two, you can imagine what theyll look like. The third image should be physically smaller than the other two (by 1.6x) and have the subjects the same size as the FF 100mm image, with the FOV of the FF 160mm image...basically take the FF 100mm image and crop it but leave it smaller than the FF 160mm image to show that theyre not really the same. That will clearly demonstrate whats happening with an APS sized sensor..

I never really liked the term effective focal length because its not. Youre getting the same FOV but resolution (lens, not sensor) isnt as good so its not really the same as a larger focal length...

Comment #3

Mike703 wrote:.

There was along (and bad-tempered, in places) thread recently onthis, and many people who seemed to know what they were talking aboutexpressed the opinion that this doesn't actually make any difference.Of course smaller pixels have more noise *individually* than largerpixels, but the fact that there are more of them means that, *in agiven image size*, the greater number of samples per unit areareduces noise, and the effects more or less cancel out..

A good test of this would be the Nikon D40 (6MP) vs. D40x (10MP inthe same size sensor), with all other things (lens, software etc)being the same. If you looked at '100% crops' of both the 10MP imagewould have more noise for the reason you mention, but that equates toprinting an image 66% larger. If you printed the two pictures at thesame size there should be no difference..

That's an interesting point and no doubt valid, at least to an extent. But it boils down to saying that you don't have to enlarge the noise elements as much to make a given size print, and thus they are less visible than if they were larger. But that begs the question of whether more noise is in fact created. The degree of enlargement is moot if in fact the noise is not even there. If the 10 mp sensor makes more noise than the 6 mp sensor, then there is at least a high likelihood that it will affect the image to a greater extent, even though it may not be enlarged as much..

It would, of course, come down to the actual amount of noise created by two sensor under comparison, and there are, of course MANY other factors that affect both noise and other elements of image quality. The conventional wisdom, which is supported at least to some extent by actual cases, is that greater pixel density tends to cause some degradation in image quality. Since the original poster was noting a decrease in image quality with a smaller sensor (but for the wrong reason), I wanted to show where his point did match the common understanding, which I think is still useful in that context..

Davehttp://www.pbase.com/dsjtecserv..

Comment #4

Graystar wrote:.

I never really liked the term effective focal length because itsnot. Youre getting the same FOV but resolution (lens, not sensor)isnt as good so its not really the same as a larger focal length..

Agreed, effective focal length is the wrong term. Equivalent focal length, or more precisely 35 mm equivalent focal length should be the proper term. Lens focal lengths can be compared and made "equivalent" between any number of sensor and film formats, it's just a matter of simple arithmetic. Those of us who shot 4 x 5 years ago were used to comparing the equivalent field of view of lenses used for that format and 35 mm. We are just doing the same thing now in the other direction, still using 35 mm as a benchmark..

But I'm a little mystified by your assertion that lens resolution isn't as good. Are you saying that lenses with shorter focal length can't have as good a resolution as those of longer focal length? If so, I think that questionable as a broad statement; can you provide more information?.

Dave.

Http://www.pbase.com/dsjtecserv..

Comment #5

Dsjtecserv wrote:.

But I'm a little mystified by your assertion that lens resolutionisn't as good. Are you saying that lenses with shorter focal lengthcan't have as good a resolution as those of longer focal length? Ifso, I think that questionable as a broad statement; can you providemore information?.

No its much more straightforward than that. Its simply that a real FF + 160mm lens setup uses the entire intended FOV of the lens (and hence it's full ability to resolve,) as oppose to using a 1.6x crop + 100mm lens (of exactly the same quality), which is now using only a portion of it's available FOV to resolve the same scene you got with the 160mm lens. So you get less resolving power simply because some of it goes unused..

Thats all...nothing at all about quality or abilities of different focal lengths. ..

Comment #6

So you get less resolving power simply because some of it goes unused.>>>.

It may be helpful if you were to discuss what gives a lens it's resolving power....

John.Please visit me at:http://www.pbase.com/johnfr/backtothebridgehttp://www.pbase.com/johnfr/digital_dartmoor..

Comment #7

Graystar wrote:.

No its much more straightforward than that. Its simply that a realFF + 160mm lens setup uses the entire intended FOV of the lens (andhence it's full ability to resolve,) as oppose to using a 1.6x crop +100mm lens (of exactly the same quality), which is now using only aportion of it's available FOV to resolve the same scene you got withthe 160mm lens. So you get less resolving power simply because someof it goes unused..

Thats all...nothing at all about quality or abilities of differentfocal lengths. .

I'm not sure I buy that, but I'm not an optical expert so I can't refute it. But it seems you are saying that the light actually striking the smaller sensor has passed through less of the total surface of the lens elements only the central portion of the lens diameter and thus the full glass wasn't used to create the image. I don't belive that matters. If the fabrication of the lens surfaces was consistent across the diameter, it's resolving power should be consistent. In practice, I think the central portion would be more likely to have been fabricated with greater precision than the edges, and thus might have be slightly better resolving capability. That is certainly suggested by various lens tests..

I don't know, I think you need to make a clearer case before I accept that there is any resolution advantage to using a longer lens on a smaller larger sensor, everything else being equal..

Dave.

Http://www.pbase.com/dsjtecserv..

Comment #8

I'll think about it some more and try to rephrase ..

Comment #9

Ok, well I tried... and failed! sorry..

My example, although I failed to mention it explicitly, demonstrated that the full sensor and the smaller crop sensor have the same resolution. resolution being pixel elements per unit area squared in my example. you could theoretically take a picture with a full and crop sensor with the same resolution (NOT megapixels) withthe same prime lens. cut ("crop") the sides off the full frame sensor's image by 20% and you (theoretically) have the same "print" as the smaller sensor...

Comment #10

PhotonFiend wrote:.

Ok, well I tried... and failed!.

Not entirely. I think the problem is, one has to understand on beforehand to see the logic, i.e. "reason backwards"...

Comment #11

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This question was taken from a support group/message board and re-posted here so others can learn from it.

 

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