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Understanding crop factors

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chrisheathcote
chrisheathcote e2 Member 8241 forum postschrisheathcote vcard United Kingdom
2 Dec 2012 - 10:40 PM

The point I was trying to make is that when we respond to threads, we need to make sure what we put is correct, this is where confusion starts. To me the subject of crop factors is quite simple. It is like taking an image with FF/35mm and cropping it in PP and then enlarging to original dimensions. It doesn't make the lens any longer.

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2 Dec 2012 - 10:40 PM

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mikehit
mikehit  56312 forum posts United Kingdom9 Constructive Critique Points
2 Dec 2012 - 11:20 PM


Quote: 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.



If the perons asking the question has never used a 35mm film/sensor camera, the concept of 'crop factor' is totally irrelevant because they have nothing to compare it to. All they need to learn is how a lens behaves on their camera.
The question ' how does my 50mm lens act compared to 50mm lens on a 35mm camera' is as relevant as 'how does my 50mm lens act compared to a 50mm lens on a 10x8 plate camera'

rogerfry
rogerfry e2 Member 8505 forum postsrogerfry vcard United Kingdom
2 Dec 2012 - 11:39 PM


Quote:
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. .

.
Mike.....just a couple of points. You say that on APS-C you need 2.56 times as many pixels to get the same pixel density as FF.....it's the other way round. You also say that to print an A4 image of the bird you magnify the image by the same amount in both cases. If you're printing at 300 ppi. the bird from the full frame contains less pixels and will need more magnification to match the crop frame.
I'm sure you will correct me if you think I'm wrong.

mikehit
mikehit  56312 forum posts United Kingdom9 Constructive Critique Points
3 Dec 2012 - 8:06 AM

You are right - you need 2.56 times as many pixels on the 35mm sensor. D'oh!


Quote: If you're printing at 300 ppi. the bird from the full frame contains less pixels and will need more magnification to match the crop frame.

I print to a certain size. I don't limit myself by saying 'I have to print this at 300dpi' and instead I print to what I can get away with in any given situation. In fact if you google the subject many people far more experienced than me say there is little to no difference between 300dpi and 150 dpi or even lower if you know what you are doing.

But we are saying the same thing: I did say in my reply that if you print both images to A4 the 5DII image has fewer pixels making up the image. But the pixel quality is also important (noise etc) and - please correct me if I am wrong - the 5D2 pixels are far superior to the 7D pixels and this will offset a fair amount of the benefits of the 'gain' of pixel density. If that were not the case you could blow up a 7D image to the same size as any 5D image yet no-one claims you can. In other words, the 400mm+7D will not give you a usable image equal to the quality as a 640mm+5D.

bainsybike
3 Dec 2012 - 9:14 AM


Quote: In other words, the 400mm+7D will not give you a usable image equal to the quality as a 640mm+5D.

Exactly.

But will a 400mm + 5D image, cropped to APS-C field of view, give you equal quality to a 400mm + 7D? Obviously resolution will not be the same (fewer pixels) but will the pixel quality make up for that?

Last Modified By bainsybike at 3 Dec 2012 - 9:16 AM
mikehit
mikehit  56312 forum posts United Kingdom9 Constructive Critique Points
3 Dec 2012 - 10:06 AM

This site illustrates exactly what I am talking about.

http://iwishicouldfly.com/iwishicouldfly/journal/html/020112b.html

This is an image in good lighting. if you look at ISO 800, the 7D image is already showing more noise so under less ideal conditions, you would expect the quality of the 7D image to be less than the 5D2.
Those images were all taken with the same 600mm lens on each camera and taken from a fixed distance - imagine putting a 900mm lens on the 5D to 'make up for' the crop factor: is anyone seriously saying that the 7D would equal it?

mikehit
mikehit  56312 forum posts United Kingdom9 Constructive Critique Points
3 Dec 2012 - 10:12 AM


Quote: there is little to no difference between 300dpi and 150 dpi or even lower

That was floating in the back of my mind as I typed. But I posted anyway because the pixels contain the information that go to make up the printers dots, and once you go below a certain level of pixels (without changing print size) the quality of the print starts to break down. So in theory the 5D image has fewer pixels on the bird so risks breaking down in quality of print. But my point was that limiting yourself to a print of 300dpi would never really happen because you can always reduce the dot per inch without compromising image quality.
It sort of made sense to me Tongue

Jestertheclown
3 Dec 2012 - 10:31 AM

By talking about ppi and dpi you'e drifting off of the subject and getting into a completely new debate.
Ppi (pixels per inch) relates to the image as you see it on your monitor and is a measurement of length; pixels per inch along the edge of your image.
Dpi (more correctly dots per square inch) relates to the printed product and is, in a manner of speaking, a measurement of area.
Although the two terms are frequently used to mean the same thing, or to mean each other, they are, in reality, completely different.

Chris_L's the man you need to speak to about this.

mikehit
mikehit  56312 forum posts United Kingdom9 Constructive Critique Points
3 Dec 2012 - 10:50 AM

I was trying not to drift off topic (honest!), Jester but as it had been brought up I was trying to point out that print quality was not as relevant as made out (although roger referred to ppi, I compounded the issue in the way I responded).

rogerfry
rogerfry e2 Member 8505 forum postsrogerfry vcard United Kingdom
3 Dec 2012 - 11:38 AM


Quote: In other words, the 400mm+7D will not give you a usable image equal to the quality as a 640mm+5D.

.
Mike...That was never my argument. My comment -(which you politely referred to as B*ll*cks)- , was that a 400 on a crop frame would bring you 'as close' as a 640 on full frame. You then said that the image projected by a 400 lens onto the sensor of either camera would be the same size, and the fact that the bird you used as your example looked larger in the crop frame camera was irrelevant, and to get your A4 print both images would have to be magnified by the same amount. The reason the bird looks larger in the crop viewfinder is that it's taking up a greater proportion of the sensor, and will indeed be larger in a print made from the whole sensor image of both cameras. All the talk of pixel density,noise,focussing accuracy etc., is a red herring you introduced by talking about image quality, not telephoto effect.

If you and I stand side by side, me with my 7D and you with your 5D, and take a close-up of an animal so that it fills the frame (and fills the print when we use the whole sensor image without any cropping) if I'm using a 400 lens you would have to use a 640. (No doubt your image would be better quality Smile)

So, going back to the post that started all this, a crop sensor does give you an "apparent" increase in focal length.

mikehit
mikehit  56312 forum posts United Kingdom9 Constructive Critique Points
3 Dec 2012 - 12:14 PM


Quote: and will indeed be larger in a print made from the whole sensor image of both cameras.

That is just my point. When photographing a bird,in a vast majority of cases you will crop the image because you are limited by (a) how close you can get and (b) the lens you have with you - the link I posted explains exactly that and I think also shows that despite cropping the 5D2 image you do not lose image quality compared to the 7D.





Quote:
If you and I stand side by side, me with my 7D and you with your 5D, and take a close-up of an animal so that it fills the frame (and fills the print when we use the whole sensor image without any cropping) if I'm using a 400 lens you would have to use a 640. (No doubt your image would be better quality )

And my point is that if you put the 5D2+400 against the 7D+400, the quality of the pixels in the 5D will offset at least some of the gain of the additional pixels.
If you read the link I posted it shows that two images (5D+600 and 7D+600) cropped to the same framing had near-identical image quality. And that is depsite the 5D2 having few pixels covering the feather. Now follow your scenario and compare 5D+1000mm lens and 7D+600mm lens (or more realistically 5D+600 vs 7D+400) - I can imagine only one very clear winner.

Question: where is the focal length advantage?


Quote: was that a 400 on a crop frame would bring you 'as close' as a 640 on full frame.
"apparent" increase in focal length

Surely 'apparent' focal length is irrelevant without discussing image quality. Let's take it to absurd levels: put a 400mm lens in front of a 1/2.5" sensor and that will give you an apparent focal length of about 3,000mm compared to 5D2. Now print it the same size as the 5D2 image...
Absurd only because no-one would dream of buying a 400mm focal length lens to put on a 1/2.5" sensor body precisely because of image quality.

Coming back to the OP: my advice would be that if you are wanting to photograph telephoto images with the image quality of the 5D2 when printing to a maximum of 20x12 and you want a damned good AF system, then the 7D is a bargain. But it has nothing to do with any 'telephoto' effect of the smaller sensor.
With the 5D3 on the market now, its AF means that for me it is a clear front runner if I had the money (Santa, are you listening...?)

Last Modified By mikehit at 3 Dec 2012 - 12:16 PM
rogerfry
rogerfry e2 Member 8505 forum postsrogerfry vcard United Kingdom
3 Dec 2012 - 12:59 PM


Quote: I..... 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

.
Mike....I have no arguments with you regarding image quality, but the inference from the original post was that he was talking about the 'reach' of the lens, not the quality of the final image, which is why I made my comments.

This is where I back out gracefully. Grin

mikehit
mikehit  56312 forum posts United Kingdom9 Constructive Critique Points
3 Dec 2012 - 2:08 PM

I always undedrstood 'reach' to mean the size of the subject (in this case the bird) projected on the recording medium - after image capture is it all about cropping and magnification.


Quote: he was talking about the 'reach' of the lens, not the quality of the final image,

I can understand where you are coming from but the idea of talking about 'lens equivalence' without including consideration of image quality means you have no frame of reference regards the final use of the image. As in many part off digital photography, there are nowadays too many variables for the old mantras to hold.

Last Modified By mikehit at 3 Dec 2012 - 2:09 PM

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