Crop factor and lenses

[oldgreygary]

This is just a general question that I'm sure is not really an issue at all but maybe someone can clarify my understanding. On DSLR's which are not full frame then they can have a crop factor e.g. 1.5. When reading about this the info. this suggests that for example this would make a 50mm lens equivalent to approx. 75mm. I guess the bit that I am not understanding is that the characteristics of a 50mm lens are presumably different from a 75mm lens in construction. So, what is it exactly that you are getting when you use a lens on a camera with a crop factor? I understand that it changes the focal length. But, again that slightly confuses me because won't that have an expectation of trying to make the lens perform beyond what it was designed for? Interested to hear responses that will untangle my slightly confused thoughts about this!

Cheers for now

Gary

[jcuknz]

The difference between the 75 and the 50 lense in this respect is the angle of view. The 50mm has the AoV of a 50mm lens on a full frame digital or 35mm film camera. When it is usesd on an APS-C camera the 'crop factor' is either 1.6 or 1.5 in the most common cameras which means it will give you the AoV of an 80mm or 75mm lens when used on the FF/35mm camera.
When you come to Micro Four Thirds the 'crop factor' is 2 so it give you the AoV of a 100mm lens on the FF/35mm camera.

The actual focal length is not changed just the apparent focal length when people think in terms of the 35mm film cameras they may have used in the past. It is a useful way to describe the AoV one gets with various lens ... I doubt really if people in fact think this way but it is a good way to compare lenses in blogs etc.

[AB26]

Hi Gary,

“Crop factor” is just a term used to compare any smaller sensor to a full frame sensor in DSLR’s using a 35mm (FF) camera lens.
The whole thing originated with film camera lenses been used on “crop sensor” cameras.

If you put a FF (FX) lens on a “crop camera” body you get a “cropped image” on the smaller sensor. The image circle on a FF (FX) lens is bigger than that of a DX (crop) lens (therefore the FX lens is thicker and more expensive) the “angle of view” of a thicker lens is greater than that of a thinner lens of the same focal length. The greater “angle of view” of the FF lens causes a smaller area of the scene to fall on a “crop” sensor, hence the term “crop factor”.

Focal length does not change, neither does reach change. When viewed on the same size screen the image from a “crop” camera sensor is enlarged more than that of a FF sensor. The FF sensor captures a larger area of the scene and when viewed on the same screen as the “crop” image, the image captured by the “crop” sensor appears to project a larger image. Crop the FF sensor image to cover the same area of the scene as was captured by the “crop” sensor and you have – CROP FACTOR.
View an image captured on a FF sensor(Nikon) on a screen 1.5X bigger than the screen you are viewing the image from the “crop” sensor(Nikon) and the whole puzzle falls in place.

A 50mm FX lens fitted to a DX body does not become a 75mm lens. On a DX body the image area falling on the sensor (with a 50mm FF lens fitted to the DX body) will cover the same area of the scene as would be covered by a 75mm FF lens on a FF body.
Using a dedicated “crop” camera lens on a FF body will cover the same area of the scène as is covered when fitting that same lens on a “crop” sensor body.

Keep in mind that all images have to be captured from the same spot for it all to make sense.

Does this make sense?

[oldgreygary]

Thanks jcuknz and Andre for your replies. Yes, it does make sense. It's cleared up my misunderstanding that a 50mm lens becomes a 75mm lens on a 'crop camera'. As I understand it the smaller sensor size of a 'crop camera' allows you to use a shorter focal length lens to get the same angle of view as a longer focal length lens equivalent on a full frame camera? So, in essence say using a 50mm lens on a 'crop camera' is actually being 'under used' as the angle of view created by the 'crop camera' is smaller than the lens's actual capacity on a full frame camera? Have I understood all of this correctly!? Thanks for your patience in answering what are probably basic questions of how lens and camera function.

Cheers for now

Gary

[davidedric]

I find it easiest to forget about the particular lens and just think about the focal plane, where the sensor is and the film used to be. On any compatible slr body, the same lens will produce exactly the same image on the focal plane. All that changes is how much of that image the sensor captures.

Just another way of expressing what's already been said (and it may not be a universal truth) but it's how I got my head around it

[chauncey]

The word "crop" more than anything, is a physical sensor size comparison. The results are that the smaller sensor utilizes only the centermost portion of the lens as compared to that same lens mounted on a full frame body. And that is a good thing because your utilizing only the "sweet spot" of that lens (another subject).

Another consideration is pixel density on that sensor...generally speaking a "crop body sensor" is more densely packed with pixels (more pixels per square inch) than is a FF body sensor. The end result of that is that when you use a crop sensor camera you end up with "more pixels on target" which is giving the "illusion of more reach". That 18 MP 1.5 crop sensor...were you to make it full size and maintain the same pixel density, you would have a 40.5 MP full frame sensor.

The downside to a crop sensor is that they, generally speaking, because of those densely packed pixels, have poorer low light performance than their full frame brothers.

Your choice...crop sensor with more pixels on target...that 300 mm lens now performs as if it is a 450 mm lens using the sweet spot of the 300mm lens, or a full frame sensor with an expensive 450mm lens.

I would submit that, were the two sensors of the same technical generations and, they were being used by the same photographer and, not in a low light environment, one would not be able to discern a difference in the final print. IMHO

[Manfred M]

Thanks jcuknz and Andre for your replies. Yes, it does make sense. It's cleared up my misunderstanding that a 50mm lens becomes a 75mm lens on a 'crop camera'. As I understand it the smaller sensor size of a 'crop camera' allows you to use a shorter focal length lens to get the same angle of view as a longer focal length lens equivalent on a full frame camera? So, in essence say using a 50mm lens on a 'crop camera' is actually being 'under used' as the angle of view created by the 'crop camera' is smaller than the lens's actual capacity on a full frame camera? Have I understood all of this correctly!? Thanks for your patience in answering what are probably basic questions of how lens and camera function.

Cheers for now

Gary

Gary - there is no such thing as "under used" in the way you are describing focal length.

Ignoring the image circle for a moment, a lens of a specific focal length will always give exactly the same coverage. The 50mm lens in a full-frame camera is a "normal" lens and roughly shows things the way we see. Move to the 1.5 crop factor, and you have a short telephoto lens, move to a medium format camera, and your have a wide angle lens.

The only thing that changes in any of these cameras is the size of the sensor. The reason we see the different views is that we will tend to enlarge the final images to the same size; whether than be on a computer screen or a traditional paper print. This additional enlargement of a smaller image is really all we are really seeing here.

[DanK]

Focal length does not change, neither does reach change.

I agree with the first clause, but I think the second is confusing, at least as I use the word "reach."

For a given focal length and distance, the image of a given object will fill a larger proportion of a small sensor than of a large sensor. So, if by reach you mean something like "how far away can I stand and fill the frame with a given subject?", a given focal length provides more reach on a smaller-sensor camera.

[inkista]

Same lens. Different bodies.

Lens: Sigma 8mm f/3.5 EX DG circular fisheye. Which is designed to project its full image circle inside the frame.

On a full frame body (5DMkII):



On a 1.6x crop body (Canon 350D):

[jcuknz]

The 50mm lens in a full-frame camera is a "normal" lens and roughly shows things the way we see.

To split hairs I would suggest that this statement is completely wrong. If it had been expressed as "The perspective of a 50mm lens on full frame is the most pleasing to the eye" I would not argue.

As I view this monitor from a distance of 18 inches at any one time all I see is "all I see", or two fingers of my outstretched hand, which suggests an angle of view of just a few degrees. But the eye is incrediblely fast at re-focusing and our appreciation of any scene is a combination of countless images put together by the brain.

Getting my cheap school protractor out I find ' two fingers ' is about 10 degrees or less.
There is a Nikon UK link http://www.europe-nikon.com/en_GB/pr...nses/simulator which can be used to turn degrees into focal length and reverse but it doesn't seem to be working for me today

[oldgreygary]

Excellent responses and thanks for clarifying. In a much better way, the two pictures from the 'fisheye' lens show one of the issues that was in my thoughts. In that although the lens will work on both bodies the original function of the lens can be lost when used on a crop body. I guess the 'fisheye' is at the extreme end of lens choice but nevertheless illustrates that you need to be clear in your thoughts about what you are going to use the lens for (obvious point, I know!).

Cheers for now

Gary

[AB26]

To split hairs I would suggest that this statement is completely wrong. If it had been expressed as "The perspective of a 50mm lens on full frame is the most pleasing to the eye" I would not argue.

Manfred is right. How do you determine "normal" focal length. Fit a zoom lens with focal length from below 35mm to above 50mm. Turn your camera to portrait mode look trough the viewfinder with your right eye keeping your left eye open. Zoom until the image in your right eye matches the image as seen by your left eye. That is NORMAL FOCAL LENGTH.

[Manfred M]

To split hairs I would suggest that this statement is completely wrong. (

I would agree that it is not a "perfect" answer as a normal lens is usually defined as one having the same focal length as the diagonal of the negative or sensor, and in the case of a full frame camera, this works out to a tad more than 43mm. It really has nothing to do with being "pleasing to the eye".

The current definition of the normal lens for full-frame being the 50mm lens really comes from the Leica I, courtesy of work by Oskar Barnack and Max Berek of Leitz back in 1924. Their choice of focal length stuck, even though they are technically off by just under 7mm. This was the f/3.5 50mm ELMAX, for E Leitz and MAX Berek. Most camera manufacturers followed suit ever since and the view of 50mm being the normal lens for this format has stuck.

[tao2]

I would agree that it is not a "perfect" answer as a normal lens is usually defined as one having the same focal length as the diagonal of the negative or sensor, and in the case of a full frame camera, this works out to a tad more than 43mm. It really has nothing to do with being "pleasing to the eye".

The current definition of the normal lens for full-frame being the 50mm lens really comes from the Leica I, courtesy of work by Oskar Barnack and Max Berek of Leitz back in 1924. Their choice of focal length stuck, even though they are technically off by just under 7mm. This was the f/3.5 50mm ELMAX, for E Leitz and MAX Berek. Most camera manufacturers followed suit ever since and the view of 50mm being the normal lens for this format has stuck.

+1...Thanks Manfred, saved me writing all that. Though some folk still regard anything from 35mm-50mm in non-digital as "normal". Mr Leitz wouldn't pony up for a 42/3 lens so Leica went down a side road, which they've never recovered from...

[rpcrowe]

Perhaps this illustration might help...



Note, the size of the mountains are the same because a lens of a certain focal length doesn't change because it is used on a full frame or crop camera.

Two thngs do change:
1. The useable image circle is larger when using a lens designed for a full frame camera
2. The side by side coverage and top to bottom coverage is greater with a full frame than with a crop camera

In reality, we often shoot from a closer distance (portraits are a great example) when using a full frame camera than when using a crop camera if we are using the same focal length on either camera. Or, when shooting from the same distance we often will use a wider focal length lens on a crop camera than on a full frame camera. That is why we often talk of "equivalent focal lengths".

As an example, our cropping will be approximately the same when shooting with a 50mm lens on a 1.6x crop camera and an 80mm lens on a full frame camera thus, we will most likely shoot from the same camera to subject distance.

NOTE: In the world of Canon, lenses designed for full frame cameras (EF lenses) can be used on both full frame and crop cameras while lenses designed for crop cameras (EF-S) can only be used on Canon 20D and later crop cameras. The D30, D60 and 10D cameras are 1.6x crop models but cannot use EF-S lenses.

[Glenn NK]

The optics of a lens are determined by the lens, not the camera. Don't get sidetracked by anything else.

If the lens is a 50 mm lens, it will be a 50 mm lens no matter what body it is attached to.

The physical characteristics of a lens can't be changed by the camera body (when I sit is a very small automobile, I don't become smaller - my size is fixed).

I've noticed that most lenses are round, not square, not rectangular, and not triangular. It seems logical that the shape of the focused light rays coming out of the back of a lens will be circular. I believe this to be a truism.

Ever wonder what happens to the round image projected by the lens onto the sensor? The circular image shape is coming through the lens, but the image you get is always rectangular/square.

What happened to the light rays outside the rectangular image? They don't hit the sensor, and are absorbed by the black area in the camera.

The smaller sensor of a "crop" camera doesn't capture as much of the potential image coming through the lens that a larger sensor will.

The area of the scene captured by the crop sensor is smaller, so it looks as though the image was shot through a telescope - the lens hasn't changed, and it hasn't changed anything.

Fortunately, the pixel density of crop sensors is usually higher than on so-called FF bodies, so when the sensor captures a smaller image area than a FF sensor does, the higher resolution (more pixels per unit area) makes up for the "enlarging" that occurs.

Glenn

PS - I started writing this post before Richard made his post - I think we've pretty said the same thing by different means.

[Inkanyezi]

The optics of a lens are determined by the lens, not the camera. Don't get sidetracked by anything else.

If the lens is a 50 mm lens, it will be a 50 mm lens no matter what body it is attached to. /.../

I wonder if I confuse too much by throwing a spanner into the works here.

There is (at least) one camera, the Nikon E, that indeed does change the focal length of the optics that are attached to it. Instead of having a crop factor, the Nikon E has an optical system before the sensor, which decreases the focal length of the lens (at the same time increasing its relative aperture). So although it has a tiny sensor, the lens behaves as were it used on a full frame camera.

But of course, on all cameras without such an optical system, the focal length will remain the same regardless of sensor size.

[oldgreygary]

Thanks for the further analysis and images which have added to my understanding. The illustrations with the explanations have clarified and cleared my confusion. I think that part of my misunderstanding has been created when I have read articles that have been written about lenses that say a 50mm lens becomes a 75mm lens on a 1.5 crop camera. I will read future articles with my newly acquired knowledge to have a better understanding of what is being talked about. As has been explained the amount of information coming through the lens is the same. But, it is the sensor size that is determining what gets recorded onto the card. I hope that I have understood that right!?

Cheers for now

Gary

[DanK]

As has been explained the amount of information coming through the lens is the same. But, it is the sensor size that is determining what gets recorded onto the card.

Yes, that's right.

think that part of my misunderstanding has been created when I have read articles that have been written about lenses that say a 50mm lens becomes a 75mm lens on a 1.5 crop camera.

When people write that--if they understand it themselves--they re referring to reach, and in that respect (only), they are correct. For example, suppose you shoot images of a sport that requires that you be a minimum distance from the action, and your question is: how long a lens do I need to have a person at midfield fill the frame? If you need a 200mm to fill the frame with a Canon 1.6 crop, you will need 320mm to fill the frame of a 'full frame' camera.

The difference in FOV, and therefore the difference in reach (as I have used the term above) is not affected by pixel density. A 50mm lens had the same reach on my old XTi as on my current 50D, which has the same sensor size but a different density. The same size of subject would fill the frame on both cameras, but the 50D covers the image with more pixels. FOV is just trigonometry and depends on only focal length and sensor size. Specifically, FOV=2arctan(d/2FL), where d is the size of the sensor. However, the typical difference in pixel density between full-frame and crop sensors creates a number of the other advantages and disadvantages of each size

[Manfred M]

I wonder if I confuse too much by throwing a spanner into the works here.

There is (at least) one camera, the Nikon E, that indeed does change the focal length of the optics that are attached to it. Instead of having a crop factor, the Nikon E has an optical system before the sensor, which decreases the focal length of the lens (at the same time increasing its relative aperture). So although it has a tiny sensor, the lens behaves as were it used on a full frame camera.

But of course, on all cameras without such an optical system, the focal length will remain the same regardless of sensor size.

I wouldn't call it changing the focal length of the lens at all. This is really supplemental lens that has been built into the camera body that is really an inverse teleconverter, so the lens itself is still operating as per the focal length(s), but so far as the sensor is concerned, the supplementary element is decreasing the focal length and increasing the maximum speed of the lens by 4 stops. An interesting approach that allowed a user to use F-mount lenses on a 2/3" sensor.

We can see how well this worked, based on the numerous models that followed it...