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Understanding crop factors

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chrisheathcote
chrisheathcote e2 Member 8241 forum postschrisheathcote vcard United Kingdom
2 Dec 2012 - 10:18 AM

I was reading through a forum earlier (not as good as this one mind Wink ) when I saw the usual what camera for a beginner thread. We all know them. The OP had listed what he liked to take pictures of and asked what lenses etc. this was where someone pointed out the crop factor and this was where my issue started. He was stating that as the 500D was a aps-c body the canon 100-400 would have a reach of 640mm and therefore would be equivalent to a 640mm lens on FF. now my understanding which I am 99.9% certain of is that the magnification will remain 400mm, however the field of view (width of the scene) would be equivalent to a 640mm. I think we have a duty of care to newbies on things like this because I for one wouldn't be happy If I bought a 400mm lens to try and get as close as a 600mm would on FF

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2 Dec 2012 - 10:18 AM

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ourdayphotos
2 Dec 2012 - 10:44 AM

It confused the hell out of me when I started, I thought I'd get extra reach then there was the isssue of is it better to use the long lens on a FF camera & crop it to what it would have been on an APSC, would the more pixels on FF compensate & still give a better quality image? I went FF.Wink

ianrobinson
ianrobinson e2 Member 41107 forum postsianrobinson vcard United Kingdom8 Constructive Critique Points
2 Dec 2012 - 10:46 AM

Well there is a lot of strange info about ff and aps-c, my understanding is that full frame is what it states full frame or the equivelent to 35mm in old money, and aps-c sensors have a crop factor of anything from 1.6 to 1.3 on 1d's .
There is no focal length differences between the full frame or 1.6 crop sensors like some may think, what you do get is a cropped sensor end of.
However saying end of, it does not end there, as a wildlife photographer and landscape photographer i use 2 types of camera for both genres, i use the 7d cropped 1.6 sensor from canon for wildlife, my main reason for this is because you get auto cropped image giving the impression you are closer without loosing pixels if that makes sense, so therefore you can make your subject look like it is closer when actually it is no closer.
Full frame is useful for the wide angle lenses of the l series like the canon 16-35mm f2.8 and 17-40mm f4 lenses and the 5d takes full advantage of lenses like the canon 8-15mm fisheye lens where as a cropped sensor cannot, so therefore for just those reasons plus many more i have not mentioned the full frame sensor is the perfect partner for landscapes and portraits, the 1.6 is perfect for wildlife and sports especially the 7d.

IMHO I think for the money and value for, these 2 cameras are without doubt the best cameras for there genre that canon do and beat many other camera models too.

Last Modified By ianrobinson at 2 Dec 2012 - 10:47 AM
JJGEE
JJGEE  96288 forum posts England18 Constructive Critique Points
2 Dec 2012 - 10:57 AM

My understanding is " equivalent " focal length and perhaps specifically in terms of angle of view.

Back in those days of film I had a Medium Format camera which had a "crop" factor of 3/5

So my 50mm Zenzanon lens on a Bronica ETRSi was "equivalent" ( angle of view ) to a 30mm focal length on say, an Olympus OM4Ti

Last Modified By JJGEE at 2 Dec 2012 - 10:57 AM
ianrobinson
ianrobinson e2 Member 41107 forum postsianrobinson vcard United Kingdom8 Constructive Critique Points
2 Dec 2012 - 11:14 AM

just to add my 7d has 18mega pixels at a cropped frame of 1.6 and my 5d mark iii has a full frame sensor at 22 mega pixels so in theory both cameras should be equal in wildlife or sport right?

If i crop an image of a bird to the same as the 7ds sensor will the pixels become less than the 7D? an interesting one. Also the 5d mark iii is supposed to handle noise much better than the 7d but i see no significant differences between the 2 if any at all.
does any one else have this experience or have i set my 5d mark iii up wrong?

bainsybike
2 Dec 2012 - 11:22 AM

If you were to mark out a crop (APS-C) sized rectangle in the centre of a full-frame sensor, the part of the image recorded there would be exactly the same as that recorded by a crop sensor. Any difference only arises when you compare the image recorded by two different cameras.

Comparing the 500D (15MP) with the 5D MK 3 (22MP), the APS-C image recorded by the 500D has 15MP, whereas that recorded by the 5D has 22/(1.6 * 1.6) = 8.6MP. So the crop sensor camera (in this example at least) will bring more pixels to bear in the final image, and all other things being equal, more detail. However, all other things are rarely equal. Wink

chrisheathcote
chrisheathcote e2 Member 8241 forum postschrisheathcote vcard United Kingdom
2 Dec 2012 - 11:31 AM

Exactly all a crop sensor does is crop the image from the lens. Save doin it in PP. It doesn't magnify as some people infer

rogerfry
rogerfry e2 Member 8505 forum postsrogerfry vcard United Kingdom
2 Dec 2012 - 1:30 PM


Quote: I think we have a duty of care to newbies on things like this because I for one wouldn't be happy If I bought a 400mm lens to try and get as close as a 600mm would on FF

.
I think you've missed the point, because that's exactly what it does do. If you take a shot from a fixed position using a 400 lens on a crop frame camera, you would have to use a 600 lens on a full frame camera to get the same photograph -(assuming a 1.5 crop sensor....if it was a Canon with a 1.6 crop frame, you would have to use a 640).
A shot taken on a M4/3 camera -( 2x crop sensor) using a 300 lens will give exactly the same result as using a 600 lens on FF.

chrisheathcote
chrisheathcote e2 Member 8241 forum postschrisheathcote vcard United Kingdom
2 Dec 2012 - 1:42 PM

It wouldn't bring the subject any closer though would it only crop the outer edges?

rogerfry
rogerfry e2 Member 8505 forum postsrogerfry vcard United Kingdom
2 Dec 2012 - 2:34 PM

Let's try a different approach!.........Set your Sigma 17-70 at 48mm and take a shot, then set it to 30mm and take the same shot. The 30mm shot represents what you would have got using 48mm on a FF camera. So although you are no closer to the subject in physical terms, your main subject fills more of the frame. ( Effectively bringing you closer to the subject). So your 400 lens on crop frame brings you 'as close' to your subject as a 640 lens on FF.

Paul Morgan
Paul Morgan e2 Member 1315182 forum postsPaul Morgan vcard England6 Constructive Critique Points
2 Dec 2012 - 2:51 PM

I use two different film sizes and there each full frame Smile

One that of 35mm film, well it is 35mm film.

The others M4/3, and its more or less the same as 35mm motion film with its lenses giving pretty much the same reach and dof.

APS C on the other hand is a true crop.

Last Modified By Paul Morgan at 2 Dec 2012 - 2:54 PM
Sooty_1
Sooty_1 Critique Team 41197 forum posts United Kingdom198 Constructive Critique Points
2 Dec 2012 - 8:38 PM

Not taking pixel density etc into account...

If you took a photo with a 50mm lens on a full frame camera (or 35mm film camera), then took a shot from the same place with the same lens on a smaller format (like APS C, with a 1.5 'crop factor'), you would get exactly the same photograph....same perspective etc. what would be different is how much of it you can see. The smaller format crops the image more than the full frame one.

The confusion here is that the images would be different sizes, with the smaller crop being exactly part of the larger image if you overlaid them. Now make them the same size, and the smaller format image appears to be higher magnification. All you've done is to enlarge the centre part of the image to make it the same dimensions as the full frame image.

What is useful is to think of the 1.5 (or whatever) crop factor as being the equivalent field of view rather than magnification...thus a 50mm on full frame will have approx 40 degrees horizontal field of view, whereas on a crop sensor, it will be about 25 degrees, or the equivalent of a 75 mm lens on full frame.

Nick

Last Modified By Sooty_1 at 2 Dec 2012 - 8:43 PM
mikehit
mikehit  56313 forum posts United Kingdom9 Constructive Critique Points
2 Dec 2012 - 8:44 PM


Quote: Let's try a different approach!.........Set your Sigma 17-70 at 48mm and take a shot, then set it to 30mm and take the same shot. The 30mm shot represents what you would have got using 48mm on a FF camera. So although you are no closer to the subject in physical terms, your main subject fills more of the frame. ( Effectively bringing you closer to the subject). So your 400 lens on crop frame brings you 'as close' to your subject as a 640 lens on FF.

B*ll*cks!

Assume that the image of the bird at 30 metres does not fill the APS-C sensor.
If you stood at that 30m distance and put a 400mm lens on APS-C camera and put the 400mm lens on a 35mm sensor, then the image projected onto the sensor is the same for both cameras. The only difference is that the 5D has more 'non-bird' stuff around it.

Now, suppose you wanted to print a full-sheet A4 image of the bird. You magnify the image excatly the same amount in both cases. As you rightly say, there is precisely zero focal length advantage of the APS-C. The fact it looks larger in your viewfinder is irrelevant.

One of the factors is pixel density which will in part govern how much detail you can record. On Canon the rp factor is 1.6, So on a APS-C you need 2.56 times as many pixels to get the same pixel density (difference in sensor area is 1.6x1.6=2.56). The APS-C 7D has 18MP, the 35mm 5D2 has 22MP so the 5D2 has a lesser pixel density. For this reason some people will say if you crop the 5D image to match the 7D image you will have fewer pixels creating the image of the bird which means that you have less detail so the 7D sensor acts like a 'digital zoom'. Except that each 7D pixel is more noisy than each 5D2 pixel which counteracts the additional 'detail' (to some extent).
Now let's take practical matters: many people have said that the 7D is as good as the 5D2 up to 18x20. Then the pixel quality of the 5D2 starts to show their worth and you can continue to print the 5D2 image to larger sizes or at higher ISOs. Now given that printing a bigger image is actually no different to cropping, who will now say the 7D sensor is as good as a 5D2 with a longer lens?
Confused? You will be.

But what if you have a 40D (10MP). This has a lower pixel density as the 5D2 so do all APS-C have a 'focal length advantage'? Nope. Which is why the mantra 'APS-C is like having a longer zoom' can be incredibly misleading.

However, the 7D has a better AF than the 5D2, which is why it will be interesting to hear form people who have used the 7D and the 5D3 (whose AF is said to be as good as the 7D) as to which one gives better images. As it is, the 7D is so good that for most people the expense of the 5D2 or 5D3 is very hard to justify.

Last Modified By mikehit at 2 Dec 2012 - 8:44 PM
bainsybike
2 Dec 2012 - 9:03 PM


Quote: But what if you have a 40D (10MP). This has a lower pixel density as the 5D2

@mikehit, I agree with your post above, so sorry to be pedantic on this point. But the pixel density of the 40D is still higher than that of the 5D2 (or 3 for that matter). 10MP vs 8.2 for the 5D2 in an APS-C sized area.

Nick_w
Nick_w e2 Member 73848 forum postsNick_w vcard England99 Constructive Critique Points
2 Dec 2012 - 9:16 PM

What the op is trying to do, I think is see how it relates to him, when using his camera, I guess he's not interested in an "A" level physics answer, but how to select the correct lens for his camera.

It can seem complicated, the above answer whilst technically correct can confuse someone just trying to get their heads around the basics.

It all relates to old 35mm film days.

35mm cameras are the reference point, a frame of film is exactly the same as a full frame sensor.

A so called cropped sensor is smaller, so to get same field of view multiply the focal length by 1.5 (canon slightly different 1.6).

So a 300mm lens on a cropped sensor would give a field of view that you would get on a full frame camera at 450mm.

Yes perspective etc is not altered by the focal length.

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