Digital camera sensor sizes

[Pohled]

Hello,

I've read the tutorial Digital Camera Sensor Sizes: How it influences your photography and I still have some doubts.

As for the 35mm equivalent thing, I'm confused about these two parts:

"Similarly, the focal length multiplier relates the focal length of a lens used on a smaller format to a 35 mm lens producing an equivalent angle of view, and is equal to the crop factor. This means that a 50 mm lens used on a sensor with a 1.6X crop factor would produce the same field of view as a 1.6 x 50 = 80 mm lens on a 35 mm full frame sensor."

and

Be warned that both of these terms can be somewhat misleading. The lens focal length does not change just because a lens is used on a different sized sensor — just its angle of view. A 50 mm lens is always a 50 mm lens, regardless of the sensor type. At the same time, "crop factor" may not be appropriate to describe very small sensors because the image is not necessarily cropped out (when using lenses designed for that sensor).

As far as I understand it, a high focal lenght is used to capture distant subjects for example, "telephoto".
My Canon Sx20 has a focal length of 5.0mm to 100.0 mm with 35 mm equiv of 28 to 560mm. So I cannot understand what this equivalent thing really means. Because certainly my telerange doesn't go so far as a real 560mm
The sensor size is 1/2.3-inch type CCD.
So, in which sense I can understand the equivalent thing?

Thank you for the patient. Perhaps my question is not very clear, it's because the matter is really confusing to me.

[revi]

It all comes down to field of view: the image the sensor 'sees' corresponds to a cone with a certain top angle. You can visualise that cone by considering the center of the objective (when focused) as the center of the cone, and drawing lines from the corners of your sensor through this center. The farther the (center of) the objective is removed from the sensor, the narrower this cone will be (try it on a sheet of paper).

Now, having done that for a sensor equivalent to a 35mm diapositive, try and figure out where to put a smaller sensor in that same cone to fill that sensor with the image. You'll find you end up with a much shorter distance between the center of that cone and your smaller sensor. That shorter distance would correspond to the actual focal length you'll have to use to get a sharp image on the small sensor with the same field of view.

In your case, the multiplier for the focal distance is 5.6 (100mm real is 560mm 35mm equivalent), so your sensor is about 6.4*4.3 mm (35mm film gave 36*24 mm images).

As to why we often express focal lengths as 35mm equivalent: many are/were used to 35mm cameras, and have developed a 'feel' for the different focal lengths (perspective and field of view), and nowadays there are many different sensor sizes; so by 'translating' the actual focal lengths to this common standard, the 'old hands' feel they know what they are talking about .

Hope this helps,

Remco

[Pohled]

Merci, revi!

I made a paintbrush, see if it is what you were talking about.


But I still have some doubts, for example, when telling EXIF data, which focal lenght should I use, the real one or the equivalent?

[Boatman]

Pohled, I don't think you have this right, though I confess it is confusing and I may have it wrong. I think in your drawing you should move the smaller green sensor to the back. Now draw angles from the ends of the green sensor through the focal point. You will see that the focal distance is the same but the angle of view has decreased thus increasing the magnification of the lens. So, if this was a true 50mm lens (for a 35mm camera), with the new sensor the magnification would be equivalant to a ~100mm lens (in a 35mm camera) but really it is still a 50mm lens projecting on to a smaller imager.

As for the FX data, it will normally show the actual focal length though some software may convert that and show the 35mm equivalant as well. Most DSLRs have a conversion factor of 1.6 so a DSLR 31mm lens would frame an image like a 50mm lens on a film SLR. And, a 50mm film SLR lens, if used on a DSLR, would look like an 80mm, short telephoto lens.

[revi]

@Jonathan: the drawing illustrates exactly what I described: that the equivalent focal length for 35mm film (compared to actual, smaller, sensor size) has the same viewing angle / field of view...

@Homer: what you suggest comes down to the same thing, but do you realise why?
And yes, of course a 50 mm objective will still focus at 50 mm behind the lens, for a subject at infinity, and any sensor; that was not the question. What I tried to explain whas why we tend to talk about 35mm film equivalences, and why a smaller sensor required a smaller focal length for the same angle of view

[Pohled]

Hi,

Thank you again, Revi and Boatman.

I did another drawing with Boatman's suggestion, actually, it shows the two cases.

Now I'm going to study the rest of the tutorial =D

[pwnage101]

Here is how you should be thinking about it:


black lines trace actual fields of view, whereas blue lines trace the 35mm equivalent FOV. Red curves represent equal angles.

Notice how the blue lines and the inner black lines are parallel and have equal angles. This is because the focal length was multiplied by a predetermined factor that allows one to equate lenses on different formats. The crop factor juts tells you what lens you would need to use on a larger sensor to look the same.

It's like saying a heavier car needs more torque to reach equal speeds.

[PopsPhotos]

This is a pretty good explanation of the confusion.

http://www.kenrockwell.com/tech/crop-factor.htm

The sensor in the camera is at the same distance from the lens in all cases. Thus, an APSC sensor gets to see only a portion of what the lens sees and less than what the larger sensor/film sees.

Pops

[Dave Humphries]

~ an APS-C sensor gets to see only a portion of what the lens sees and less than what the larger sensor/film sees.

That's how I think of it too Pops, the lens of a given focal length makes a certain size image circle which the sensor, either FF or APS-C sits inside.
Since the APS-C sensor is smaller, some of that gets wasted around the edges, the sensor only registers a (virtual) crop from the whole image circle, thus it appears as if you have used a longer focal length lens (compared to a FF/film camera).

To slightly complicate matters, a DX lens 'doesn't bother' to make the large image circle, but crucially, the focal length isn't different, so you still apply the same thinking to it.

That also neatly allows you to figure why vignetting isn't an issue with a FF lens on a DX body and why the performance is better - because you're only using the good bit in the middle. Someone with FF lenses, moving from DX body to FF body may notice this loss of quality (in addition to the change of angle of view).

It may help you visualise this if you prove it to yourself as follows;

a) Get a sheet of white paper
b) Draw two boxes on it; one 24mm x 36mm and one centrally inside that, 16mm x 24mm (use a pen to aid visibility) - these are your sensors
c) Point a bright desk lamp at a patterned wall covering or magazine cover propped against a wall - this is your subject
d) Using 50mm f/1.8 or similar (ideally prime) lens
e) Turn off other room lights, shield the paper from light spill
f) Move paper and lens to focus an image the wall covering (or magazine, etc.) onto the paper
g) If you can hold it still enough, observe how the scene captured by the inside box is 'more zoomed in' than the outside box

That's why it is called crop factor

Notes to improve effectiveness
Adapt experiment for what you have available
Make sure the lens is not stopped down; do what ever you have to to get its aperture wide open
Keep 'sensor' paper parallel to 'subject'
Get someone with steady hands to help, so you can concentrate on looking at the results.

Cheers,

[bambleweeney]

That's how I think of it too Pops, the lens of a given focal length makes a certain size image circle which the sensor, either FF or APS-C sits inside.
Since the APS-C sensor is smaller, some of that gets wasted around the edges, the sensor only registers a (virtual) crop from the whole image circle, thus it appears as if you have used a longer focal length lens (compared to a FF/film camera).

To slightly complicate matters, a DX lens 'doesn't bother' to make the large image circle, but crucially, the focal length isn't different, so you still apply the same thinking to it.

That also neatly allows you to figure why vignetting isn't an issue with a FF lens on a DX body and why the performance is better - because you're only using the good bit in the middle.

I use a couple of the lenses from my medium format Mamiya on my Nikon. Big/Heavy/Manual Focus but the quality is astounding with one or two of them, not only no vignetting but pretty much edge to edge sharpness as well.
The 80mm f2.8 gives me the equivalent of a 120mm with great colour and tonality and the 80mm f4 macro is also pretty good but maybe just a tad flat on the colours.

[Glenn NK]

The term focal length multiplier is really incorrect. Nothing gets multiplied.

Here's how it works (I'm saying the same things as Dave Humphries, but wording it differently):

Let's put the same lens on two cameras with different sensor sizes, and let's mount the cameras on tripods side by side, focused/aimed a the same subject.

The sensor size DOES NOT affect how the light rays come through and are focused by the lens. The light rays are entering the lens, but any light that falls beyond the edges of the smaller sensor are lost.

I'll say it again - the optical properties of the lens are not changed by the camera.

The smaller sensor will not capture all of the width and height of the image that the large sensor will capture. This should seem logical, and is a fact.

For a Canon 1.6 crop body the dimensions of the image captured will be 5/8ths of the dimensions of the image captured by a so-called FF sensor. Note that the inverse of 1.6 = 5/8. The 5/8 ratio is applied to both width/height, so the area of a 1.6 crop sensor is 5/8 x 5/8 = 0.39 of a FF sensor.

This "focal length multiplier" terminology drives me nuts because the multiplier makes the image smaller - IOW, the smaller sensor doesn't take as much picture.

Glenn

PS - I think sales people coined the term to make buyers think that smaller sensors have an advantage.

[Pohled]

Hello,

Thank you all for the drawings, corrections, explanations and so on. It's much clear now for me.

But on fixed lens camera, as my compact CanonSx20, I think that the lens are designed to avoid this 'waste' of light out of the edges, aren't they?

And now I understand why full frame cameras are much more expensive

[Dave Humphries]

But on fixed lens camera, as my compact CanonSx20, I think that the lens are designed to avoid this 'waste' of light out of the edges, aren't they?

Yes, absolutely correct - it is just 'more of the same' as I mentioned about DX lenses not producing the large image circle.

Your SX20 lens will produce a tiny image circle, not much bigger than the sensor.