Confused about APS-C

[Stingray]

Hi everyone..
I'm looking at getting a wide angle, especially for star and landscape shots.. but budget dictations are being enforced (<$500)
after much reading of the forums and reviews .. I'm leaning towards the Sigma 10-20mm F4-5.6 EX DC (HSM)
However, all this reading has brought my attention to something I hadn't really looked into .. APS-C .. and now I have totally confused myself...
I knew our 60D, was an APS-C format, and never really wondered about it, a lot of pages talk about multiplying the lens by '1.4" to get the "real" length ... ie: my 50mm apparebtly really works out about at 70mm ..
I've searched the forums here for APS-C .. but unless it was way down in the searches, I couldn't find a post that talks more about it, in depth.

What I'd like to know is if I buy an APS-C lens like the sig 10-20 (as an example) - designed for the APS-C sensors - am I getting 10-20mm or is the reality 14-28mm?

[Kym]

Try this http://www.ausphotography.net.au/forum/showlibrary.php?title=Indexes:Appendix_D_-_Full_Frame_vs_APS-C_DX_crop_factor_w_example

You are really getting 10-20mm. Then the crop is applied, but the focal length stays the same

[Stingray]

ahh.. I see, said the blind man!
Thanks Kym,
many pages I read talked about (or mentioned) multiplying the 1.6 (not sure why I said 1.4 .. LOL.. see said I was confused..! ..LOL)
so it's really more about FoV .. not focal length ..
having a plethora of info at one's fingertips is sometimes worse than having too little ;)
Thanks :)

[Lance B]

APS C has a "crop factor" of either 1.5x or 1.6x, not 1.4x as you have indicated. Canon "crop factor" is 1.6x, whereas Nikon, Pentax and Sony (I think) have a "crop factor" of 1.5x. This "crop factor" is simply a comparison of the Field of View (FOV) when compared to that FOV you would have on a FF camera. The reason they talk about crop factor is because it is a carry over from the film days and 35mm film which is the same size as that of the modern FF DSLR's, both the film size and the sensor of a FF DSLR being 24mm x 36mm whereas the APS C sensor is smaller, being 16mm x 24mm for Nikon, Pentax and I think Sony, and the Canon APS C sensor is a tad smaller again being 15mm x 23mm or there abouts. Therefore, the APS C sensor is a cropped portion of the FF sensor, so, if you can imagine, the centre portion of a shot taken from a FF 24mm x 36mm sensor to that of a 15mm x 23mm APS C sensor is sort of like cropping a FF sensor down to that of APS C, therefore your FOV is 1.6x less for APS C. So, your 10-20 lens gives you a Field of View similar to that of a 16-32 lens on a FF camera. Just remember, a 10-20mm lens is still a 10-20mm lens regardless of what camera it is mounted on, just that the FOV changes and therefore the resultant FOV is only like that of a 16-32, but the actual focal length does not change and this is where the confusion can sometimes be.
I hope this makes sense.

[ricktas]

As Kym's link shows the field of view is the thing that changes, Your lens doesn't magically become a different length depending on what camera it is mounted on. The reason these ultra-wide lenses were developed for the smaller sensor formats was for exactly the reasons that smaller sensors needed wider lenses to gain the equivalent field of view of the larger sensors

[fess67]

the crop factor on a Canon 1DmkIV is 1.4

[ROA44]

Could this perspective aspect of the APS-C be described like this.

Get a photo crop the photo by what ever the relevant % on all sides should be then re-size back up to the original size was to end up with the correct perspective of view and compare the original withe the cropped version to see the difference . this may not be worded quite right but is the idea right.

EG. An 8 X10 crop say 20mm or what ever the relevant % should be off each side then re-size back up to 8 X 10 not sure just a thought in trying to get a better understanding myself although I think I understand it.out side of this.

[Kym]

the crop factor on a Canon 1DmkIV is 1.4

That would be 1.3 because it is APS-H format.
http://en.wikipedia.org/wiki/APS-H#Multiplier_factors

[ROA44]

I know the measurement may not be quite correct, but is the concept correct, thought it might be another way to explain and make easier for others to understand.

[arthurking83]

Could this perspective aspect of the APS-C be described like this.

Get a photo crop the photo by what ever the relevant % on all sides should be then re-size back up to the original size was to end up with the correct perspective of view and compare the original withe the cropped version to see the difference . this may not be worded quite right but is the idea right.

EG. An 8 X10 crop say 20mm or what ever the relevant % should be off each side then re-size back up to 8 X 10 not sure just a thought in trying to get a better understanding myself although I think I understand it.out side of this.

I'm not entirely sure what you're trying to describe here ROA.

cropping and resizing are two different aspects of digital imagery.

The APS-C format is a format similar to full frame other than for the fact that it's a smaller one.

What many people believe(or read) is that you can compensate for the smaller format by substituting a shorter focal length to achieve the same result.

That is, using a 10mm lens on APS-C to give you the same look as a 15 or 16 mm lens depending on format differences.
While on the whole, this usually works to a point that it doesn't make a lot of difference, there is a subtle difference. It's just not easily seen.

resizing an image is an aspect that is dependent on the total number of pixels within the image captured, not so much the format.

As an (Nikon)example: The D800 has 36Mp and the D7000 has 15Mp. D800 is a full frame, D7000 is APS-C(and the approximate difference is to use the 1.5x rule)
So, for the same FOV equivalence between D7000 and D800 you need to multiply/divide 1.5 for the focal length of the lens between the two.

so if you choose a 10mm lens for the D7000, and a 15mm lens for the D800 you would see an image that is almost indistinguishable between the two camera/lens setups.
If you crop either of those images, they would then look different .. as they already look similar.

So, to make the example more reflective of reality, we'll now change the focal lengths to 200mm(for APS-C) and 300mm (for full frame) .. and again the images will look pretty much similar again if compared.
But if you decide to use a 200m lens on the D800, which will have a much wider perspective compared to the D7000+200mm lens, and then crop the D800 image to the same FOV as the D7000+200mm lens, the images will again look pretty much similar with a few pixels difference.
This is because the D800 cropped to the APS-C format is now 15Mp .. all due to the number of pixels available.

Resizing of the images has no bearing or relevance to FOV and perspective. resizing is basically only relevant when the need to display the image is important .. whether printed or on screen.

[Stingray]

As Kym's link shows the field of view is the thing that changes, Your lens doesn't magically become a different length depending on what camera it is mounted on. The reason these ultra-wide lenses were developed for the smaller sensor formats was for exactly the reasons that smaller sensors needed wider lenses to gain the equivalent field of view of the larger sensors

Thanks Rick
That was the confusing part, some web pages "talked" about the 50mm becoming equivalent to 80mm (and same for other pages and other Focal lengths.. ) hence my confusion and need to get some real input ..
I would love a FF camera, but reality says .. "NO" at this time.. lol

thanks to everyone who replied.. I have learnt something new about photography *yay*

[Kym]

So with a Pentax 645D (Medium format) ... the crop factor relative to 35mm is a divider ... i.e. 0.79

So 50mm becomes (cropped to) 39.5mm :D

[Tannin]

the crop factor on a Canon 1DmkIV is 1.4

No, it is about 1.3. To be exact, the 1D III has a 1.28 factor, the 1D IV is 1.29.

As a matter of detail, other "standard" crop factors are seldom exact. Most "1.5 crop" Pentax cameras are actually 1.52, with some at 1.53 and 1.54. The Canon "1.6" 7D is actually 1.61, though nearly all other Canon crop cameras are exactly 1.6; Nikon's "1.5 crop" cameras are mostly 1.52 and 1.53; Sony is the same as Nikon, mostly 1.54 crop.

[arthurking83]

....

So 50mm becomes (cropped to) 39.5mm :D

expanded too? :p

[ROA44]

Not sure why I put the enlarge part in doing to many things at once I think. But thank you Arthur yes have got it.

[swifty]

Adding a little more confusion, the earlier Sigma DSLRs had a 1.7x crop and was still considered APS-C.
APS-C itself is a legacy film format size that became a common DSLR sensor size largely due to the costs of manufacturing FF digital sensors, esp. in the early days of digital photography.

The only reason we apply this so-called crop factor is that 135 film was the most popular film format for decades and focal lengths expressed in 135 film (FF digital) format meant something to people. So it's become a standard for comparison, especially given the large variety of digital sensor sizes.
Even point and shoot cameras express the 135 equiv focal lengths of their lenses even when the crop factor is often more than 5x.

[Kym]

So how come Medium is > Full ? :D

[Tannin]

So how come Medium is > Full ? :D

^ Good point! I'll explain it after you explain how come Burra is in the "mid-north". :eek:

[Kym]

^ Good point! I'll explain it after you explain how come Burra is in the "mid-north". :eek:

Mid North of SOUTH Australia!! Simple :p
Not to be confused with the Far North of SA which is South of the NT border. :confused013

[fess67]

Ahhh 1.3 true, true. Still learning :D Thanks guys

[Tannin]

South Australia: 26° (NT border) to 38° (southern-most point), meaning that its centre is at almost exactly 31°. Burra - a representative point in the "mid-north" - is at 33°40' or 33.667 (decimal) degrees south. This point in the "mid-north", then, is not just in the south of South Australia, it is very nearly 300 kilometres south of the mid-point!

But what does this have to do with crop factors? Well, quite a lot actually, insofar as we can have lots and lots of fun arguing about the correct way to consider the details and write them down exactly so, and (in general) achieve absolutely nothing of any practical use in doing so. If you are going to be any good at your craft you need to learn the basics correctly, but that said, in practical reality it is often easier to ignore the fact that the popular usage is completely wrong. I too call Burra the "mid-north" even though I know it isn't, and we all call Cobar the "far-west" of New South Wales even though it's not even west, never mind far! And we all know that translating focal lengths into "35mm equivalents" is often as misleading as it is helpful, but we generally just go along with the silly common convention because it's easier not to argue. Besides, I'd much rather be wrong about a focal length and have to change lenses than be wrong about my geography and have to walk 300 kilometres to find an open servo!