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Old Mar 4, 2010, 2:00 PM   #1
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Default 'Focal Length' and 'Crop Factor'

The 'Focal Length' of a lens is a measure of how strongly a lens can bend light. A lens with a shorter focal length bends light more than a lens with a longer focal length, so it has a wider angle of view. 'Focal Length' is a physical property of a lens.

The actual angle of view depends on the size of the image sensor. A larger image sensor has a wider angle of view than a smaller one. The term used to refer to the different sizes of image sensors is 'Crop Factor' (sometimes referred to as 'Focal Length Multiplier'). Since the most common and longest lived image sensor is 35mm film, an exposure that is 36mm by 24mm (the size of a 35mm film exposure) is used as the basis for comparison, and therefore has a 'Crop Factor' of 1. Since smaller image sensors do more cropping, they have larger crop factors, and larger image sensors have smaller crop factors.

For instance, Nikon and Sony both make dSLRs with image sensors that are the same size as a 35mm film exposure (commonly referred to as 'Full Frame'), as well as dSLRs with image sensors that are 1/3 smaller (24x18mm). Since a 'Full Frame' image sensor is 1.5 times larger than the smaller image sensor, the smaller image sensor is said to have a 1.5X Crop Factor. Fuji and Pentax only make dSLRs with these smaller, 1.5X crop factor image sensors. Canon makes dSLRs with three different size image sensors. Their top-of-the-line models use 'Full Frame' image sensors, some have 27.5x18.5mm image sensors (1.3X crop factor), and others have still smaller image sensors (22.5x15mm, or 1.6X crop factor.) Sigma's dSLRs use an even smaller image sensor (21x14mm) which has a crop factor of 1.7X. Leica, Olympus and Panasonic dSLRs use still smaller image sensors (17.3x13mm, or 2.0X crop factor.)


(Image courtesy of Wikimedia Foundation)

P&S Digicams have even smaller image sensors, and so have even larger crop factors. On the other hand, traditional medium format cameras can use digital sensors as well, and those sensors are typically 60x45mm, giving them a crop factor of 0.6X.

Fuji's, Pentax' and Sony's 1.5X crop factor image sensors, and Canon's 1.6X crop factor image sensors, together, are often referred to as 'APS-C', because their size closely matches the earlier 'Advanced Photo System' "Classic" film exposure. Similarly, Canon's 1.3X crop factor image sensors are often referred to as 'APS-H' for the 'Advanced Photo System' "High Definition" film exposure. Leica, Olympus and Panasonic dSLRs are members of the Four Thirds System, and so, together, are referred to as Four Thirds, or 4/3 cameras.

'Crop Factor' is useful if you need to move back and forth between two or more different size image sensors, so you can relate the angle of view you'd get with a lens on one camera, to what you'd get with it on a different camera. For instance, a lens with a 30 angle of view on an APS-C dSLR would have a 45 angle of view on a 'Full Frame' dSLR. (1.5 X 30 = 45).

This 'Crop Factor' has given rise to the use of a '35mm equivalent focal length.' For instance, a 100mm lens on an APS-C dSLR would have the same angle of view as a 150mm lens on a 'Full Frame' dSLR. So, on an APS-C dSLR, a 100mm lens would have a '35mm equivalent focal length' of 150mm. It's important to understand, however, that the actual focal length of a lens doesn't change. Its angle of view will change with the size of the image sensor, and that makes the '35mm equivalent focal length' useful for people that move lenses back and forth between dSLRs, but the lens' actual focal length remains the same. The '35mm equivalent focal length' is also useful in determining the angle of view in images captured with P&S Digicams. Because of their much smaller image sensors, and their large crop factors, the actual focal length of a lens in a P&S Digicam and the '35mm equivalent focal length' are very different.

If you never need to move back and forth between two dSLRs with different crop factors, then 'Crop Factor' doesn't mean anything to you. It's mostly a crutch used by people with considerable experience with film SLRs, so they can relate what they know about lenses on their old cameras to what they can expect on their new ones. But if you only have one dSLR, or all your dSLRs have the same size image sensor, then 'Crop Factor' is of little value to you.

One last thing. Aperture is also a physical property of the lens and is unaffected by 'Crop Factor'. An f/2.8 lens on a 'Full Frame' dSLR is still an f/2.8 lens when it's mounted on an APS-C dSLR.
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Last edited by TCav; Mar 7, 2010 at 12:26 PM. Reason: image attribution and discussion of image sensors and lenses in P&S digicams.
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Old Mar 4, 2010, 5:04 PM   #2
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Hi TCav, thanks for posting this, I was honestly going to read and reply to the PMs you sent me with the provisional copy but have been out all day and just back now.

I think that pretty well covers it. I would add that another time that the 35mm equivalent is useful is in places like this so people with different bodies are aware of what they might need for similar shots, and also the P&S cameras show most things in the 35mm equivalent form.

I've now made this a sticky so we can refer back to it.

Currently I've left it open in case there are others who have anything to add that will help.

Thanks again for taking the time to put this down in words.
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Old Mar 4, 2010, 5:36 PM   #3
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I'm pleased to have have been able to help.
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Old Mar 4, 2010, 5:42 PM   #4
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Originally Posted by Mark1616 View Post
Hi TCav, thanks for posting this, I was honestly going to read and reply to the PMs you sent me with the provisional copy but have been out all day and just back now.

I think that pretty well covers it. I would add that another time that the 35mm equivalent is useful is in places like this so people with different bodies are aware of what they might need for similar shots, and also the P&S cameras show most things in the 35mm equivalent form.

I've now made this a sticky so we can refer back to it.

Currently I've left it open in case there are others who have anything to add that will help.

Thanks again for taking the time to put this down in words.
There is the confusing element to such a discussion in that some P&S cameras have a true focal lenth that, while it's affected by the crop factor, are in actuallity specialized lenses. Some of these little machines with their 500mm or so "equivalent" focal lenth are actually miniature telephoto lenses, and their resolving power far exceeds merely a change in angle of view.

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Old Mar 13, 2010, 2:56 AM   #5
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There is the confusing element to such a discussion in that some P&S cameras have a true focal lenth that, while it's affected by the crop factor, are in actuallity specialized lenses. Some of these little machines with their 500mm or so "equivalent" focal lenth are actually miniature telephoto lenses, and their resolving power far exceeds merely a change in angle of view.

Dave
Hi Dave, I didn't follow what you were referring to here, do you have some examples or something so I can get the 'picture' LOL.

Thanks, Mark.
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Old Mar 13, 2010, 10:41 AM   #6
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Hi Dave, I didn't follow what you were referring to here, do you have some examples or something so I can get the 'picture' LOL.

Thanks, Mark.
Some of these 500mm equivalent cameras have resolving power that far exceeds their 50mm focal lenth. They are in fact specially designed miniature lenses. To get a better answer than that you will have to speak to a technician - perhaps the ones who told me the above...

Dave
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Old Mar 13, 2010, 1:45 PM   #7
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... perhaps the ones who told me the above...
Perhaps you could refer us to them, or vice versa.

To be sure, a 50mm lens for a smaller image sensor will be sharper than a 50mm lens for a larger image sensor, by virtue of the lens' smaller exit pupil, even if the same manufacturing tolerances are used. But that doesn't affect the 'Crop Factor', 'Focal Length Multiplier' or '35mm Equivalent Focal Length'.

Unless I'm mistaken?
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Old Mar 13, 2010, 4:52 PM   #8
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Some of these 500mm equivalent cameras have resolving power that far exceeds their 50mm focal lenth. They are in fact specially designed miniature lenses. To get a better answer than that you will have to speak to a technician - perhaps the ones who told me the above...

Dave
Hi Dave,

I think we are talking cross purposes here, this thread was only about the 'crop factor' rather than image quality, lens resolution etc. It's true, some small lenses are very high quality to be able to get the detail needed for small sensors, but that isn't impacting the field of view and the relation to the 35mm frame.
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Old Mar 13, 2010, 8:04 PM   #9
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Hi Dave,

I think we are talking cross purposes here, this thread was only about the 'crop factor' rather than image quality, lens resolution etc. It's true, some small lenses are very high quality to be able to get the detail needed for small sensors, but that isn't impacting the field of view and the relation to the 35mm frame.
This came about in a discussion on DP review where I was involved in a cropping factor discussion. My position of the cropping factor in so far as it related to dSLR's was on the money. But it was shown and proved that some of the small sensor cameras actually had resolving power far in excess of their 50mm actual FL. DP Review has some highly qualified engineers, and it was shown to me that these were in fact minature lenses, as opposed to a normal lens stuck on a small sensor.

It is simply impossible to resolve the detail captured by these lenses with a true 50mm lens.

Dave
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Old Mar 14, 2010, 4:45 AM   #10
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Focal length has nothing to do with resolving power, you can have a sharp 50mm lens you have had a weak one. That is down to lens design and quality of materials used. So what TCav has said still stands true and this other area is a tangent to the purpose of the tread so to discuss it best I would suggest starting a new thread so this doesn't get confusing for someone using this for reference.
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