Lens Crop Factor

I both agree and disagree Wilt. But in this case, with someone specifically asking about it, I think it is appropriate to provide both answers and not to disparage the "opposing" one.

The key fact is that most new photographers here (typically using an APS-C format DSLR as their first and only "real" camera) do not need to concern themselves with "crop factor" math except for when they may be directly comparing other camera formats to their own. However, they are often apparently made to honestly believe that they need to multiply all focal lengths used on their cameras by the 1.6 "crop factor" or "magnification factor". Many of these photography newbies get hoodwinked into believing, for example, that their 50mm lens is "actually an 80mm lens" when it's used on their so-called "crop" camera (which, of course, does not actually crop anything - especially when used with lenses designed specifically to cover only the APS-C format sensors).

This garbage information is fed to the newbies from a lot of places, including some of the sales people in some camera stores. After the newbies have "learned" the false information, it's often difficult to get them to actually understand the truths involved. I wish I could get to the sources of the bad information to get them straightened out before they infect more newbies.

I think that most people upgrading from a point and shoot camera are initially interested in how to cover the same (or better) range on a DSLR. For that, the "crop factor" is entirely relevant, as people usually know the "35mm equivalent" focal length of their P&S cameras.

It's even useful going the other way, for more experienced shooters. For example, a while back I decided to get a decent P&S for when I'm hiking and don't want to carry the weight of a DSLR. Now I knew that I needed no worse than about 17-18mm at the wide end on an APS-C camera, which works out as around 27-29mm in "35mm equivalent". As a result, I could tell that the old Canon G9 was no good (35mm equivalent), but the G10 or S90 was probably OK (28mm equivalent). Similarly, I worked out that the Panasonic LX3 was not suitable due to it's restricted range at the long end. (Before anyone points it out, I know the issue is complicated by the different aspect ratios, but it still gets you in the ballpark). Yes, I could have worked it out from the ratios of the sensor sizes of the cameras, but the "35mm equivalent" technique made things easier as the information is readily available.

It similarly helped when I bought a micro 4/3 camera, as I initially converted both APS-C and micro 4/3 focal lengths to 35mm equivalents. (Now, though, I think in terms of my Panasonic GH1 having a 1.25x "crop factor" compared to my Canon 40D )

SkipD wrote in post #11901595
The key fact is that most new photographers here (typically using an APS-C format DSLR as their first and only "real" camera) do not need to concern themselves with "crop factor" math except for when they may be directly comparing other camera formats to their own. However, they are often apparently made to honestly believe that they need to multiply all focal lengths used on their cameras by the 1.6 "crop factor" or "magnification factor". Many of these photography newbies get hoodwinked into believing, for example, that their 50mm lens is "actually an 80mm lens" when it's used on their so-called "crop" camera (which, of course, does not actually crop anything - especially when used with lenses designed specifically to cover only the APS-C format sensors).

This garbage information is fed to the newbies from a lot of places, including some of the sales people in some camera stores. After the newbies have "learned" the false information, it's often difficult to get them to actually understand the truths involved. I wish I could get to the sources of the bad information to get them straightened out before they infect more newbies.

Excellent point. Based on this we clearly need to explain it fully here because otherwise they are going to get the wrong info elsewhere.

SkipD wrote in post #11901595
This garbage information is fed to the newbies from a lot of places, including some of the sales people in some camera stores. After the newbies have "learned" the false information, it's often difficult to get them to actually understand the truths involved. I wish I could get to the sources of the bad information to get them straightened out before they infect more newbies.

The problem is a society where it is considered normal or acceptable to learn such things from sales people. It really seems very strange to me, as a non-American. It's a bit like many Americans' desire to sit 18 hours in a plane and drink coffee from a US chain instead of the better local stuff. At the root of both is consumerism.

Of course the salesman is going to claim the 50mm is "effectively 80mm", because if he doesn't the one next door will be able to sell an 80mm because "it's longer for almost the same price".

And I strongly suspect many of the US people posting don't actually know that mm is just a unit of length. (You guys know, right, that a 50mm lens used to be known as a 2 inch lens?) So the focal length just becomes a cipher, prone to misunderstanding.

There's plenty of wrong information emanating from online forums, including this one. The subjects of perspective and depth of field seem to be widely misunderstood, and I for one lack the energy to try to correct the falsehoods. Anyone can plug values into the depth of field formulae and see what is being posted is wrong, yet there are pages of textual handwaving arguments. When I see that others have posted those formulae and are ignored I really am not going to bother making an argument with both hands tied behind my back. I just accept that some people can't or won't understand, will never be as effective craftsmen as others, and move on.

Yes, the terminology bothers me too. I think of my 1.3x camera as a "3/4 frame", and you could call the 7D a "5/8 frame". Of course Canon won't ever call them that because it makes them sound inferior. As for their APS-* terminology - ugh.

I do personally find "crop factor" useful for some things. For example, when researching African safaris most of the information about shooting distance is given in 35mm equivalent focal length, so I'd better be able to do that division by 1.3. On the other hand, when scouting my own locations I estimate distance and subject size and end up with an angle of view, which tells me which lens to take next time. I also don't find "crop factor" useful for a different aspect ratio.

"Crop factor" and "35mm equivalent focal length" are just terminology. If it were a more complex subject, I'd agree with Skip that existing terminology should be discarded (any mathematician will tell you that most of the work is choosing the right definitions), but it really isn't hard and the existing definitions are entrenched.

melcat wrote in post #11903631
And I strongly suspect many of the US people posting don't actually know that mm is just a unit of length. (You guys know, right, that a 50mm lens used to be known as a 2 inch lens?)

This is the most ignorant and closed-minded thing I've seen all day, and that's saying something considering how I spent my morning.

i'm going to start converting all my lenses from mm to inches...just to feed into that stereotype

sad part is it makes it seem so tiny now...with my sigma 5.9-19.7OS zoom....i like the bigger numbers...500mm sounds way better than 19.7

krb wrote in post #11905557
This is the most ignorant and closed-minded thing I've seen all day, and that's saying something considering how I spent my morning.

This is the most ignorant and closed-minded thing I've EVER seen!

Of course, all US citizens KNOW FOR A FACT that in Australia, everyone has koalas and kangaroos for household pets, and Paul Hogan (Crocodile Dundee) is your Prime Minister!