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busting some myths
Myth: Higher crop factor camera (smaller image sensor) means greater magnification for the same focal length lens..

Reason: Magnification has a specific meaning in photography. It means the size (eg in mm) of the subject projected onto the image sensor plane (or film plane) compared to the actual size of the subject itself (eg in mm). The size of the sensor is irrelevant as it doesn't affect the size of the projection. 1:1 magnification (true macro) means that an object that is x mm wide will be projected at a size of the same x mm at the sensor plane. The crop factor of the camera will then determine how much of the sensor/frame is filled by that subject..

In practical terms: All that aside, using a lens of the same FL at the same distance from the subject with a more cropped camera will result in the subject filling more of the captured frame. It's common to state what FL lens would be needed on a FF camera to provide the same framing from the same shooting distance as the lens in question on the cropped body. The issue of magnification is only one of terminology..

Myth: f-stop is the same as aperture..

Reason: The aperture is the size (diameter) of the iris/opening between the aperture blades that lets light through the lens. f-stop or f/ratio is a ratio between the focal length of the lens and the size of the aperture..

In practical terms: f-stop is easy to set and know, whereas aperture has to be calculated, so aperture isn't generally used. However, image sensor pixel density aside, depth of field is largely determined by focus distance and aperture (not f-stop). Larger focus distance mens larger DOF. Larger aperture means smaller DOF. For a given FL, increasing the f-stop reduces the aperture size and increases DOF. Likewise, for a given f-stop, increasing FL increases the size of the aperture and decreases DOF.



Myth: When shooting from the same position, changing the lens FL or changing the camera crop factor change the image perspective..

Reason: Perspective is the relationship between the elements of the image, not how the image is framed. When shooting from the same spot, the elements of the image are exactly the same with respect to each other as each path from the lens to any part of the scene can't change..

In practical terms: A longer lens from the same position won't foreshorten the perspective when compared with using a wider angle lens on a cropped body from the same position. If the f-stop isn't changed, DOF will change, with the longer lens resulting in a shallower DOF. However, the relative sizes of elements in the scene won't change, even if the framing of the scene changes from this same position because the difference in FL doesn't match the change in crop size. Paths of light from the parts of the scene to the camera can't alter when shooting from the same position and change perspective..

Myth: More cropped cameras with the same subject framing don't provide greater DOF for the same f-stop..

Reason: To keep the subject framing the same while changing the camera crop size, you either change the FL while keeping the camera in the same place, or move the camera while keeping the lens FL the same. Keeping the camera in the same place not only retains subject framing, but also retains the perspective in the scene. The shorter FL you need on the more cropped camera for the same framing as the longer lens on the les cropped camera means that for a constant f-stop, the longer lens has a larger aperture, meaning a shallower DOF on the less cropped body. If the same FL lens is used and subject framing is retained by backing up with the more cropped body (which also changes the perspective), the increased focus distance means a greater DOF for the more cropped body..

In practical terms: It's important to specify f-stop and framing (and to some extent perspective) conditions when stating whether DOF is different for different crop cameras..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comments (6)

You are of course technically correct about all this..

But it is going to be 1000' over the heads of 99% of beginners ....

And from a PRACTICAL standpoint I disagree with a couple of your points..

A "beginner" should understand that a smaller (cropped) sensor DOES produce a greater "effective" magnification. That is all he needs to know. That is all he wants to know..

Likewise a smaller sensor DOES result in a greater "effective" DOF..

I appreciate your efforts, but why be "confusing" to a beginner ???.

Thanks for reading .... JoePhoto.

( Do You Ever STOP to THINK and FORGET to START Again ??? )..

Comment #1

JoePhoto wrote:.

You are of course technically correct about all this..

But it is going to be 1000' over the heads of 99% of beginners ....

What's a beginner anyway? The information I've provided will be good for some people, but not others, same as in any forum. It's not just raw beginners who can read this forum and get something useful from it or contribute something to it. There are so many beginners trying to contribute who don't know what they're talking about and they can add to the confusion, despite best intentions. Of course, that situation isn't confined to this forum. Is it better to confuse the issue with incorrect information, or arbitrarily say that correct information is going to be confusing to beginners?.

And from a PRACTICAL standpoint I disagree with a couple of your points..

A "beginner" should understand that a smaller (cropped) sensor DOESproduce a greater "effective" magnification. That is all he needs toknow. That is all he wants to know..

Maybe. However, my opinion is that it's handy to get things right from the beginning. At what stage is someone no longer a beginner and should all of a sudden be expected to get things right? The issue of magnification is a common source of confusion when talking about macro, for example..

Likewise a smaller sensor DOES result in a greater "effective" DOF..

Yes, that's what I said. I additionally made it clear that equivalence depends on using the same f-stop. If you adjust f-stop the same as you adjust focus distance or FL, as some say you should for real equivalence, then DOF isn't different at all. I didn't go down that path..

I appreciate your efforts, but why be "confusing" to a beginner ???.

Some may be confused, but others won't. Why post nothing in case some people won't benefit from it? How does that make sense? Some of the issues I touched on came up in other posts in this forum, which prompted me to post this thread. They were relevant there and they are relevant here. If the information is confusing to anyone, then they can choose to ignore it..

Are you a beginner? If not, why are you here? Obviously it's because you find it relevant to be here, just like I do from time to time..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #2

John down under wrote:.

JoePhoto wrote:.

You are of course technically correct about all this..

But it is going to be 1000' over the heads of 99% of beginners ....

What's a beginner anyway? The information I've provided will be goodfor some people, but not others, same as in any forum. It's not justraw beginners who can read this forum and get something useful fromit or contribute something to it. There are so many beginners tryingto contribute who don't know what they're talking about and they canadd to the confusion, despite best intentions. Of course, thatsituation isn't confined to this forum. Is it better to confuse theissue with incorrect information, or arbitrarily say that correctinformation is going to be confusing to beginners?.

And from a PRACTICAL standpoint I disagree with a couple of your points..

A "beginner" should understand that a smaller (cropped) sensor DOESproduce a greater "effective" magnification. That is all he needs toknow. That is all he wants to know..

Maybe. However, my opinion is that it's handy to get things rightfrom the beginning. At what stage is someone no longer a beginnerand should all of a sudden be expected to get things right? Theissue of magnification is a common source of confusion when talkingabout macro, for example..

Likewise a smaller sensor DOES result in a greater "effective" DOF..

Yes, that's what I said. I additionally made it clear thatequivalence depends on using the same f-stop. If you adjust f-stopthe same as you adjust focus distance or FL, as some say you shouldfor real equivalence, then DOF isn't different at all. I didn't godown that path..

I appreciate your efforts, but why be "confusing" to a beginner ???.

Some may be confused, but others won't. Why post nothing in casesome people won't benefit from it? How does that make sense? Someof the issues I touched on came up in other posts in this forum,which prompted me to post this thread. They were relevant there andthey are relevant here. If the information is confusing to anyone,then they can choose to ignore it..

Are you a beginner? If not, why are you here? Obviously it'sbecause you find it relevant to be here, just like I do from time totime..

Well ... I was also answering on behalf of what appears to be the level of most of the questions posted here. And most seem very basic..

I understand your feeling that it is best to give them the "most" accurate information ... but I question the result when they can come away more confused and sometimes with the opposite result. (especially referring to your #1 where I feel it is better to give them the PRACTICAL result that the same lens will indeed give them more "magnification").

And Yes .... I am a beginner also ... I have only been in photography for 49 years. (started with a rangefinder and Weston-Master in 1959).

I enjoy helping where I can ... (as I am sure you were also with this very detailed post)..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10.

Thanks for reading .... JoePhoto.

( Do You Ever STOP to THINK and FORGET to START Again ??? )..

Comment #3

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #4

I'm what you'd call a "beginner" too , having started out with my first camera in 1957, but the reason I choose to call myself a beginner is simply that I think we're ALWAYS learning  no matter how long *chronologically* we've been snapping away..

That's why I always check out the 'Beginner's Questions', because there's still stuff I'm learning  often it'll be something subtle, or something I wasn't 100% sure about, or maybe a complex technicality, and even quite often (I hate to admit!) something, after all those years, I simply didn't know about!.

So from me John. I'd like to thank you for posting those detailed explanations of what, even to an "old hand" like me, can be sometimes confusing concepts to nail..

Cheers ..

Comment #5

BlackDraken wrote:.

I'm what you'd call a "beginner" too , having started out with myfirst camera in 1957, but the reason I choose to call myself abeginner is simply that I think we're ALWAYS learning  no matter howlong *chronologically* we've been snapping away..

That's why I always check out the 'Beginner's Questions', becausethere's still stuff I'm learning  often it'll be something subtle,or something I wasn't 100% sure about, or maybe a complextechnicality, and even quite often (I hate to admit!) something,after all those years, I simply didn't know about!.

So from me John. I'd like to thank you for posting those detailedexplanations of what, even to an "old hand" like me, can be sometimesconfusing concepts to nail..

Cheers .

Hi BD. Thanks for your reply. Dead right. We're all learning. Every now and again I start feeling like I know a lot about something, then I realise how much I've learned since the last time I had that thought, which makes me realise how much I don't know. I realise my explanations won't be useful for some people, but for others they might be..

Cheers from John from Adelaide, South AustraliaJohn Harvey Photography http://johnharvey.com.auCanon 40D, Canon 20D & Fuji F10..

Comment #6

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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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