Crop Factors Explained

[gstark]

Killakoala has kindly prepared a tutorial on Crop Factors for us.

You can find it here.

Many thanx, Steve. We appreciate the work you've put into this.

[dooda]

I'm having trouble with this tutorial. He seems to imply that it is merely a crop, which is sort of true, but not necessarily, because it is cropping but the end result is still a full resolution file--where crop to me implies a reduction in resolution. Like PHD's, with the optical and "digital" zooms, the digital is a crop, the resolution reduces, but the image is bigger.

Am I misunderstanding the tutorial? With sensor crop factor, the resolution is not reduced, which is more similar to optical zooming than cropping IMO.

[gstark]

Dave,

I understand where you're coming from, but the term crop should probably not ever be used in terms of resolution, and only in terms of viewable area.

For instance, let's say that you're looking at a film image, and the neg includes an errant telephone pole that you will be excluding, by selective cropping, from the print of that neg.

Or you have a portrait of Aunt Mabel, again on film, and you're only going to print a very tight crop of her face, chin to forehead, and exclude her hair from the image.

How do you reconcile those scenarios with your supposition that the crop results in a reduction in resolution?

The key here, I think, is to think beyond the digital medium, and revert to the true (traditional) meaning of crop.

Does that help clarify things?

[Alpha_7]

Good explaination thanks Gary, a crop just reduces the viewable area.

[gstark]

Craig,

Good explaination thanks Gary, a crop just reduces the viewable area.

Yep.

And that's all it ever does.

[Greg B]

The other aspect to this is Field of View.

Lenses are described by focal length, and the relevant FoV is generally understood as it applies to full frame 35mm film.

The crop effect could also be considered as a narrower FoV. So the FoV of a 50mm lens on a D70 (say, with a 1.5 crop factor) would be the same as the FoV of a 75mm lens on a 35mm "full frame" camera.

[dooda]

But Gary, that's not ALL a crop ever does. A crop most often changes the resolution of an image. If you're going to crop into a picture of aunty Mabel, and then blow it up to a 16x24, it's resolution is less than a 16x24 uncropped image. I have no problem with the term if it only connotates the reduction of the viewable image, but I think that the term 'crop' has more baggage than that for most.

Greg's term FoV makes more sense to me because it doesn't involve the inherent baggage that 'crop' brings. That is, if I'm understanding you all correctly.

I bring this up because I recently had a conversation with someone who was convinced that the crop factor of DSLR's reduced his resolution because the sensor was smaller. I tried to explain that it didn't, but he couldn't seem to grasp it well, and for my part, couldn't explain it well.

[gstark]

Dave,

But Gary, that's not ALL a crop ever does. A crop most often changes the resolution of an image.

No, never.

If you're going to crop into a picture of aunty Mabel, and then blow it up to a 16x24, it's resolution is less than a 16x24 uncropped image.

You're talking at cross purposes here. Crop should only ever refer to the vieable portion of an image.

The size at which you then print it certainly does being into question the resolution, which might have been adversely affected bya selected crop, but the image size is still not a function of your cropping.


In your example, there are two distinct processes: the cropping, and then the resizing. Consider if you're only printing at 5x4: is the resolution then OK? If so, why is it not adversly affected by your cropping?

I have no problem with the term if it only connotates the reduction of the viewable image, but I think that the term 'crop' has more baggage than that for most.

It's only a reference to the viewable portion of the image.

I don't see any baggage at all with the term crop, because for me it has always referred to just the viewable portion of an image.

Greg's term FoV makes more sense to me because it doesn't involve the inherent baggage that 'crop' brings. That is, if I'm understanding you all correctly.

I believe that you most certainly are, but I suspect you may need to check that baggage.

I bring this up because I recently had a conversation with someone who was convinced that the crop factor of DSLR's reduced his resolution because the sensor was smaller. I tried to explain that it didn't, but he couldn't seem to grasp it well, and for my part, couldn't explain it well.

How does he explain a 10MP phd camera then? It ostensibly has higher resolution than a D70 or even a 30D, despite having a much smaller sensor.

Yet which of those two types of cameras will be capable of producing a higher quality image?

[Killakoala]

A crop most often changes the resolution of an image

Dave, that's true and i agree with you there.

But i think you may be thinking of the difference between a PRE-process crop and a POST-process crop.

Consider the example of the D70 as in the article.

What you get with a lens at 50mm with the 'crop factor' is an image you would have got with a lens at 75mm with a 6-megapixel FULL-FRAME camera.

So not really a crop, but a crop-factor. The view through the lens is being cropped, not the final image.

[moggy]

Thanks for this Steve, it's made things just that bit easier to explain to people. Love the pics of my favourite food additive! They're the best 'crop' of all.

Bob.

.

[dooda]

Don't get me wrong, I completely understand what is going on with the crop factor, how the 50mm becomes a 75mm FOV 35mm equivalent, I just thought that it may confuse some people, as the crop and the zoom is so close to being the same thing, each with only slight differences.

You're talking at cross purposes here. Crop should only ever refer to the vieable portion of an image.

Now it get's tricky Gary. You see you seem to be taking a prescriptive angle on language, and the word 'crop' where I'm more of the descriptive on definitions. You say that crop SHOULD only ever refer to the viewable portion, but with cropping being totally different now than it was before digital came into play, a crop and resize are becoming the same thing. The millions of people that crop and post on the internet are almost always resized, and not referred to as such, wherein lies my problem that is only noted due to the confusion that I've experienced trying to explain it to others.

Consider if you're only printing at 5x4: is the resolution then OK? If so, why is it not adversly affected by your cropping?

That's not the issue, or the issue that I'm bringing up. The resolution is probably fine, but it IS changed, and whether it's been adversly affected by the crop is beside the point that there has been a change in resolution.

I don't see any baggage at all with the term crop, because for me it has always referred to just the viewable portion of an image.

I think your personal history with the term may differ significantly from those who were haven't been involved professionally in photography since the dinosaurs. The people that tutorial is targeted at is people that are just getting into photography.

How does he explain a 10MP phd camera then? It ostensibly has higher resolution than a D70 or even a 30D, despite having a much smaller sensor

I think that most people just learning this stuff haven't quite thought that far.

[Alpha_7]

I think your personal history with the term may differ significantly from those who were haven't been involved professionally in photography since the dinosaurs. The people that tutorial is targeted at is people that are just getting into photography.

Sorry had to say, this made me chuckle and almost spill my cup of tea.

I am enjoying the debate.

[Aussie Dave]

I see this topic has come around again...

"Crop Factor" - there is so much misinformation around about this. I agree with Gary when he says that it doesn't change the resolution....however....

Which resolution are we talking about ?
The resolution people talk about that is really image dimensions OR the resolution that the image will be printed at (eg. 300dpi)...??

Cropping an image will certainly change the image dimensions (and therefore resolution - if you think of resolution in this way), but cropping a 300dpi image will not change it from being 300dpi.

As for the crop-factor....I believe it is the crop of the image that falls onto the sensor. The image passing thru the (same) lens, projects the same sized image onto the plane of the sensor, regardless of whether the sensor is full frame or APS sized. On the D70, a considerable amount of the image falls outside the boundaries of it's sensor, so the camera can only photograph the part of the image that hits the sensor. This gives the viewer the FOV, equivalent to the focal length x the "crop factor (in the D70's case 50mm x 1.5 = 75mm FOV). The same lens on a camera with a full-frame sensor would see the projected image fill the sensor and so the crop factor would equal 0 (Giving the 50mm lens a FOV of 50mm).

I have read on the net that the crop factor magnifies telephoto lenses....which is a gross misinterpretation. I guess you can't believe everything you read on the net
Which also means you cannot necassarilly believe this....

[dooda]

I have read on the net that the crop factor magnifies telephoto lenses....which is a gross misinterpretation

If I took a 50mm and took a picture on a d70. Then I took a 75mm of similar optical quality and took the same picture on a 35mm film plane. Printed them both to 4x6, what would the difference in the image be? Is it really a "gross misinterpretation"? Would a typical person just getting into photography be able to see the difference?

If a 50mm on a d70 looks like 75 mm, acts like, prints out like it, it seems to me that explaining it as such is less confusing than saying "no, it's only cropping the image".

[stubbsy]

a crop and resize are becoming the same thing

Dave

I have to differ with you on this. I'm very new to pgotography - never owned a film camera and started taking pics in 2000. Yet for me these are very different things.

A crop is to take part of an image and remove the area outside that part. So my 3000 x 2000 original may be cropped to 2800 x 1800

A resize is to take an image and blow up the whole things so it's a lot bigger so my 3000 x 2000 original may be resized to 6000 x 4000

I see these unambiguously as different things.

[dooda]

You're probably right. I guess I was thinking that cropping was becoming something different. However just your post illustrates how it might be confusing, when a crop means less pixels, to interpret it as losing resolution...maybe?

I think that a lot has happened since 2000 though...

Sorry if this has been beaten to death, I just love a good debate is all.

[Aussie Dave]

I have read on the net that the crop factor magnifies telephoto lenses....which is a gross misinterpretation

If I took a 50mm and took a picture on a d70. Then I took a 75mm of similar optical quality and took the same picture on a 35mm film plane. Printed them both to 4x6, what would the difference in the image be? Is it really a "gross misinterpretation"? Would a typical person just getting into photography be able to see the difference?

If a 50mm on a d70 looks like 75 mm, acts like, prints out like it, it seems to me that explaining it as such is less confusing than saying "no, it's only cropping the image".

Without actually testing this, I would be inclined to think that although the FOV would look the same, the D70 is not magnifying the image to be 75mm...it just has the same FOV.
The prints would look the same...except possibly the 75mm might have "very" slight perspective differences. However as the 50mm & 75mm lenses are not that much different to each other, in real terms, you probably wouldn't notice it. You could possibly also get the same image with a 200mm lens by standing much further back. The perspective would then look significantly different....but all three prints would look roughly the same. Does that make sense ???

Also, the image from the full-frame camera would "more than likely" need to be resized much more than the image from the D70.

Pulling hypothetical figures from nowhere, let's assume we are printing the 6x4 @ 300dpi. The D70 (3008x2000) would need to be reduced to 1800x1200 (which is the dimensions of a 6x4 @ 300dpi). Say the Full-frame is something like 4368x2912 (comparible to the Canon 5D), you would have to reduce the size of the image much more than you would the file from the D70.

This would not really pose any real concerns, however if you were looking to print at 20"x30", then the D70 has to be resized much more than the full-frame image. This then may become an issue.

All this really has nothing to do with the "crop-factor" though...

[dooda]

But you said it. There would only be very slight differences on the same lens. The 200mm is irrelevent. And I agree with you. the FOV is the same, the actual lens or lens effects will differ, but for the typical person I see this as a relatively simple way of describing the difference, and personally wouldn't call it a gross misinterpretation.

[Aussie Dave]

But you said it. There would only be very slight differences on the same lens. The 200mm is irrelevent. And I agree with you. the FOV is the same, the actual lens or lens effects will differ, but for the typical person I see this as a relatively simple way of describing the difference, and personally wouldn't call it a gross misinterpretation.

Perhaps I went too far with the term "gross misinterpretation". Although, (optical) magnification does not occur....so if someone was trying to convince someone that it was, then this could be seen as misinterpretation (I'll leave out the "gross" part )

[dooda]

Fair enough Dave, and I agree with you.

I'm making too big a deal about it maybe. Guess I'm bored or something.

[ABG]

Great article and thanks for sharing Steve. It certainly cleared up a few misconceptions that I had. I was under the belief that using a 100mm lens on a D70 gave you the same reach as a 150mm lens. I can now see that there's just less of the image falling on the sensor, which brings me to a question. Why didn't/can't Nikon move the sensor closer to the lens so the same amount of image falls on it as 35mm film? I can see it would throw out the distance markings on all lenses not made specifically made for such a camera (ie. focussing at say the 10 feet marking on such a lens will not produce a sharp image of an object 10 feet away), but surely it's still possible to focus the image on a sensor that is further forward than the plane of film on a non digital camera.

[stubbsy]

Dave

Nothing you need to apologise about. We all love a good debate.

[digitor]

...except possibly the 75mm might have "very" slight perspective differences. However as the 50mm & 75mm lenses are not that much different to each other, in real terms, you probably wouldn't notice it. ...

Perspective has got nothing to do with the focal length of the lens. It is purely a function of where the camera is positioned, in relation to the scene being photographed.

(Just thought I'd clear that one up - I like a good debate too!)

Cheers

[gstark]

Dave,

Don't get me wrong, I completely understand what is going on with the crop factor, how the 50mm becomes a 75mm FOV 35mm equivalent, I just thought that it may confuse some people, as the crop and the zoom is so close to being the same thing, each with only slight differences.

You're talking at cross purposes here. Crop should only ever refer to the vieable portion of an image.

Now it get's tricky Gary. You see you seem to be taking a prescriptive angle on language, and the word 'crop' where I'm more of the descriptive on definitions. You say that crop SHOULD only ever refer to the viewable portion, but with cropping being totally different now than it was before digital came into play, a crop and resize are becoming the same thing.

Ok ... in this I think youi're still confused.

it's certainly true that cropping an image will alter the size of the image.

But that is not to say that the image has really been resized: it's only been cropped, and the resizing is merely a side effect of the cropping, and it's not actually a resizing of the image per se.

That's a very important distinction, and while I accept that this may be somewhat confusing, it's important to understand the two different concepts here, and accept that they're two separate and distinct processes.

The millions of people that crop and post on the internet are almost always resized, and not referred to as such, wherein lies my problem that is only noted due to the confusion that I've experienced trying to explain it to others.

Well, that's actually pretty simple to explain, I believe.

The real issue come back to what you're doing to the image. If you're selectively removing a section of the image - say restricting it to just a central portion, or cutting the top and bottom to make it into a pano, then regardless of anything else that happens, you're cropping the image.

Again, the change in size only happens as a result of the crop that you're placing on the image - it's just a side effect.

Let's say that in flying from LA to London the plane stops at NY for refuelling, are you actually visiting NY, or is that just incidental to the trip, and an unavoidable consequene of what you're doing?

[

I don't see any baggage at all with the term crop, because for me it has always referred to just the viewable portion of an image.

I think your personal history with the term may differ significantly from those who were haven't been involved professionally in photography since the dinosaurs.

I hear what you're saying, but with respect, I not only don't accept that premise, but I'll go as far as to say that it's just plain wrong.

That some may hold that misconception doesn't make them correct, does it? More to the point is the fact that yes, crop does hold an historical meaning, and yes, that's what I'm saying it means.

But I have yet to see anybody suggest that cropping an image does not involve the removing of a selected section of that image.

So that brings us back to the fact that crop, with all of its possible side effects, still only relates to the selective cutting out of parts of the image.

How does he explain a 10MP phd camera then? It ostensibly has higher resolution than a D70 or even a 30D, despite having a much smaller sensor

I think that most people just learning this stuff haven't quite thought that far.

Why does that not surprise me?

[Gordon]

Perspective has got nothing to do with the focal length of the lens. It is purely a function of where the camera is positioned, in relation to the scene being photographed.

I was getting all ready to post just that as I read this thread, but you beat me to it ... but now I am here I suppose I better add my 2c worth

Ignoring barrel and pincushion distortion, take a photo from the same place of the same subject with a 50mm lens and a 200 mm lens. The 200 mm image is exactly the same as the centre of the 50mm image, there is no change in perspective at all.

Gordon

[gstark]

Dave,

You're probably right. I guess I was thinking that cropping was becoming something different. However just your post illustrates how it might be confusing, when a crop means less pixels, to interpret it as losing resolution...maybe?

I think that a lot has happened since 2000 though...

Sorry if this has been beaten to death, I just love a good debate is all.

Don't apologize. There's nothing wrong with this sort of debate, and especially when it's addressing some fundamental points of possible confusion.

Getting back to your point in thios post, I think it's not correct to interpret less pixels as meaning that you're losing resolution.

It's nothing more than simply .... less pixels.

Resolution, on the other hand, probably should be described with a relationshionship to an area, as in dots per inch, for instance.

So, if we take our 300dpi image that currently enjoys a size of 2000 x 3000 pixels, and we crop it to 1500 x 1500, that image although smaller, will still enjoy it's 300dpi resolution.,

It's inarguably a smaller image, and it's obviously a different size (and shape too), but unless and until you actually do something to alter the resolution, it's still 300dpi.

Is that helping any?

[gstark]

Andrew,

which brings me to a question. Why didn't/can't Nikon move the sensor closer to the lens so the same amount of image falls on it as 35mm film? I can see it would throw out the distance markings on all lenses not made specifically made for such a camera (ie. focussing at say the 10 feet marking on such a lens will not produce a sharp image of an object 10 feet away), but surely it's still possible to focus the image on a sensor that is further forward than the plane of film on a non digital camera.

Great question.

But no, it simply wouldn't work.

Have a look at the top of your camera. On the D70, on the top panel, just to the rear of the LCD, you'll see something like a Plimsol line - a circle with a line through it. That mark indicates where the focal plane of the camera lies, and it shows you precisely where the camera's lenses are designed to focus.

IOW, the lenses cannot focus correctly anywhere else.

Thus, moving a sensor forward would, for starters only result in out of focus images, because the lenses were not working as they should.

But wait, there's more:

Moving the sensor forward would also impart some other issues: as you start to move it forward, you would quickly see that your mirror is in the way. Where would you relocate that to?

Remember too that it needs to be moved up and out of the light path so that you can make your exposure, and you cannot move it any further forward because then it will start to foul the rear elements of your lenses.

That's why some of Canon's digital lenses don't actually fit on their full frame cameras: the mirror mechanism will foul the lens.

And moving the sensor forward also means that you need to relocate the focussing screen lower down into the mirror box.

We're talking about a myriad of physical design issues, apart from the simple fact that optical design of the lens will prevent this from working.

Am I making any sense?

[dooda]

I hear what you're saying, but with respect, I not only don't accept that premise, but I'll go as far as to say that it's just plain wrong.

That some may hold that misconception doesn't make them correct, does it? More to the point is the fact that yes, crop does hold an historical meaning, and yes, that's what I'm saying it means.

I'm not trying to say that you're wrong, but that there's significant confusion regarding the term and it's inherent meanings. Thus explaining it this way to a beginner may be misleading. Whether or not they understand the true meaning of the word. I'm talking about the purpose of the whole thing.

I understand your point. That cropping something merely means that certain parts of the image are gone, not that it's necessarily smaller or bigger (which doesn't happen until you blow it up which is a different activity).

I hope I'm clear. I'm not confused, I understand you perfectly well. I wonder if others will though, and I refer to my confused colleague.

That's a very important distinction, and while I accept that this may be somewhat confusing, it's important to understand the two different concepts here, and accept that they're two separate and distinct processes.

This whole thing is me wondering whether people are going into the tutorial with this understanding, as you said, it's important to understand and accept the two concepts before understanding truly what is taking place between lens and sensor.

[gstark]

This whole thing is me wondering whether people are going into the tutorial with this understanding, as you said, it's important to understand and accept the two concepts before understanding truly what is taking place between lens and sensor.

I thought this was clear in what Steve has written. If you disagree, please feel free to suggest improvements and/or changes to Steve. He's a nice guy and won't bite your head off.

Perhaps an ear.

But in all seriousness, the tutorials are meant to be living documents, and something is less than clear, we certainly want to improve that.

As to your colleague, again, just because somebody misunderstands something doesn't make their version correct.

The easiest way to approach him about this might be with one print and a pair of scissors. Add some gridlines to the print before you cut -er, I mean crop - it. There's no easier, nor more direct, way of showing him what a crop is.

Explain to him that the gridlines represent pixlks, and all that you're doing when yoiu're "cropping" is reducing the pixel count - as will be obvious - but that the resolution has not been changed.

If he is still of the belief that the resolution has changed, ask him to demonstrate that to you with that very same print.

Which hasn't actually been changed.

[ABG]

Am I making any sense?

Perfect sense Gary - thank you. It was a question that popped into my head and I asked without thinking it through. Lazy on my behalf, I know.

Another lazy question and I wouldn't be offended if you told me to get off my arse and look at the Canon site. How do Canon achieve their full frame sensors? You indicated that some of their digital lenses won't suit as they foul the mirror. Are their sensors physically the same size as a 35mm slide/film frame, or are they just slightly biger than the Nikon sensors, but further forward? If they are further forward, how do the old Canon lenses focus properly on the sensor?

[phillipb]

I would guess that they are in the exact same position as the nikons only bigger, so no image wasted outside the sensor.

[Mitchell]

I would guess that they are in the exact same position as the nikons only bigger, so no image wasted outside the sensor.

Precisely.

Relevant to this is the link John posted to 'digital darrell', who explains why he thinks that a "full size" sensor is suboptimal and Nikon is doing the right thing by refusing to go there...

http://www.dslrusers.net/viewtopic.php?t=14233

[gstark]

Andrew,

How do Canon achieve their full frame sensors? You indicated that some of their digital lenses won't suit as they foul the mirror. Are their sensors physically the same size as a 35mm slide/film frame, or are they just slightly biger than the Nikon sensors, but further forward? If they are further forward, how do the old Canon lenses focus properly on the sensor?

The Canon FF sensors are, effectively, the same size as a 35mm film frame, and their standard range of lenses will comfortably fit their digicams and film cameras.

The concept behind the so-called digital lenses is that they are smaller and lighter than 35mm lenses, yet they're built to similar optical dimensions as 35 mm lenses sharing such things as focal length nomenclature and some aspects of oiptical design. Being physically smaller, they should also be lighter, however, and also cheaper to manufacture (and buy), but they also will throw a smaller image circle.

This is fine for a smaller digital sensor, but will cause vignetting on a FF sensor or film frame.

Here's where the fun begins though: some of the Canon lenses have a protuding rear element as a part of their design (no, I don't know why ) and it's this facet of their design/construction that causes problems in the FF cameras: the rear element may intrude into the mirror box chamber, and can foul the mirror in its movements to get out of the light path during the making of an exposure.

Understand that we're talking very fast movement, vibration, and just millimeters of intrusion here though, so it may not look like much, if any at all, but in certain circumstances it is an issue.

I can't recall which cameras or lenses specifically, but I believe that the mounts have been slightly modified in order to help preclude incorrect fitment.

[ABG]

Gary,

Here's where the fun begins though: some of the Canon lenses have a protuding rear element as a part of their design (no, I don't know why ) and it's this facet of their design/construction that causes problems in the FF cameras: the rear element may intrude into the mirror box chamber, and can foul the mirror in its movements to get out of the light path during the making of an exposure.

Thanks, once again you've cleared things up for me.

[Mj]

I shall refrain from saying much on this... seems plenty of ground already been covered. But I'll certainly say 'thankyou' to Steve for taking the time to write up a tut on an issue that is often misunderstood and always badly abused by the marketeers and sales folk out there touting this as some magical benefit of the digital age !!!