APS Size Digital Sensor: Crop or Multiplier?

[greg_whitten1]

Hi - I have a Canon 10d. This has the 1.6x "crop" / "multiplier" due

to smaller image size. I've always thought of this as a multiplier

effect (i.e., a 50mm lens becomes equivalent to an 80mm lens[50mm x

1.6]). However, I'm not sure if that's accurate. When considering

the image size on the sensor, wouldn't it be the same size on sensor

as it would be on film, so that this is really a "cropping factor"?

If it was a multiplication factor, the image size from a 50mm lens on

film would be smaller than the image size on a sensor from an 80mm

lens (1.6x 50), right? My 70-200 doesn't create a larger image when

used on my 10d than on my EOS film cameras, right?

 

Thanks.

 

Confused in Dallas, a/k/a Greg

[skipd]

You are on the right thought path. Greg. A 50mm lens will project the same image regardless of what's behind it - a full sized piece of film or a smaller digital sensor. The smaller digital sensor only records a portion of the image projected by the lens. Because there is less of the subject matter recorded, it appears as if you had used a longer lens relative to what would have been recorded on a full-sized 35mm sensor or film.

 

Additional thought: "Digital" lenses aren't really digital (except maybe for the electronics that drive the iris and focus mechanism, but that is the same as non-"digital" lenses these days). They are, however, designed to project an image that only covers a typical digital sensor. Use one on a full-frame camera body and you will likely have severe vignetting around the edges of the image.

[mark_goody]

You are right. It's exactly as if you had taken a 35mm neg and cropped out an APS sized portion from the middle.

 

In terms of field of view, it is as if your focal length was multiplied, but I prefer to think in terms of 'crop' because the perspective of the lens (eg 'telephoto compression' effects) doesn't change.

[greg s]

Beyond any shadow of any doubt the effect is a crop. Light that would hit 35mm film (if it had been there) is not hitting the perimeter of the sensor. If you could somehow mask off or suspend film in the same location with the same photographic area as the sensor, you would have a cropped result relative to that of a 'full frame' of film.

[greg_whitten1]

Thanks for helping me with that. Now that I know my 70-200 is still a 70-200 and not a 120-320, I realize I need a 300mm or 400mm prime!

 

The Light is On in Dallas, a/k/a Greg

[mark u]

One of the key things to realise with cropped images is that they end up being enlarged more to create a given size of print. To achieve the same degree of image sharpness from a greater enlargement from any given lens (N.B. not the same angle of view on different formats!) you need to stop down more than if you had enlarged the uncropped image with its wider angle of view to the same print size. This results in a slower shutter speed at any given ISO to maintain the same overall exposure. However, any camera shake will also be magnified by the greater enlargement, so you would need a higher shutter speed, a tripod or image stabilisation or to use higher ISOs when making greater degrees of enlargement from images produced with a given focal length.

 

The other feature of cropped captures is that for the same angle of view as an uncropped image you will of course need a wider angle, shorter focal length lens. However, in order to have the same degree of control over depth of field in the two cases you need a faster lens for the cropped image.

[ellis_vener_photography]

<I>However, I'm not sure if that's accurate. When considering the image size on the

sensor, wouldn't it be the same size on sensor as it would be on film, so that this is really

a "cropping factor"?

 

Ê</I><P>By Jove, I think he's go it!<P>Yes that's it exactly. The angle of view recorded is

smaller than what you get

with the same focal length lens on 24x36mm format media so that is why it appears that

the focal length is multiplied -- but the lens still behaves like what ever focal length you

have it set to. <P>There is no magical tie between focal length and angle of view

recorded. Let's say you have a 100mm lens that fills the format of a 4x5 psheet of film. On

4x5 that lens is a moderate wide angle lens. Switch to a 6x7cm rll film holder o nthe same

camera and without changing anything but the foramt size, you now have a normal focal

length (in terms of angle of view) lens. The angle of view captured would be roughly that

of a 150mm lens if you were shooting 4"x5" film. Switch formats again to 24x36mm and

that focal length is now considered a short telephoto. Switch format size again to the size

of your 10d sensor and the same lens will now be considered a moderate telephoto. All of

this presumes that the distance between camera and subject is the same in all cases.

[daniel_taylor]

"When considering the image size on the sensor, wouldn't it be the same size on sensor as it would be on film, so that this is really a "cropping factor"?"

 

Yes. BUT, because you magnify more to achieve a given print size, there is a "multiplier" effect in play. Where a full frame shooter may need a 300mm lens to capture an athlete, an APS-C shooter will only need 200mm for the same image from the same position. You get away with this "cheat" because digital sensors offer such a clean, high MTF response per unit of surface area.

 

"One of the key things to realise with cropped images is that they end up being enlarged more to create a given size of print. To achieve the same degree of image sharpness from a greater enlargement from any given lens (N.B. not the same angle of view on different formats!) you need to stop down more than if you had enlarged the uncropped image with its wider angle of view to the same print size."

 

??? Why? DoF is greater at a given f-stop because you'll be using a shorter focal length to achieve the same framing as on 35mm. You wouldn't need to then stop down more.

 

"This results in a slower shutter speed at any given ISO to maintain the same overall exposure. However, any camera shake will also be magnified by the greater enlargement, so you would need a higher shutter speed, a tripod or image stabilisation or to use higher ISOs when making greater degrees of enlargement from images produced with a given focal length."

 

You do need to use higher shutter speeds, but DSLR's are so clean at high ISO's that they more than make up for the loss.

[cssf]

If you are using a lens made for 35mm film on a camera with a smaller sensor then there is a crop factor because you are cropping out some of the image circle. If you put a lens made for that sensor size on the camera then there is no crop factor because you are not cropping out any of the image circle--that image circle is smaller. This doesn't really matter much though because, on an APS camera, a 50mm lens with an image circle large enough for 35mm film and a 50mm lens with an image circle only large enough for an APS sensor will both result in a field of view similar to that of an 80mm lens on a 35mm film camera--even though the first situation involves cropping and the second does not. That is why I believe that "crop" is not a very useful word for this phenomenon in general.

[jcuknz]

Surely the answer is "It is a cropping factor on the sensor and a multiplying effect to the lens"

[kai_griffin]

JC Uknz... no, there is no magnification factor *at all*, at either sensor or lens... only cropping.

[kai_griffin]

SF CA - that is not correct, either. The so called "crop factor" only relates to the difference between a 35mm frame of film and an APS-sized frame of film using the same lens. It doesn't matter whether the lens used is "designed for digital", "designed for 35mm film" or "designed for 6x9 Medium Format" - all the talk about crop factor is exclusively about the difference between a 35mm frame and an APS frame. There is still a "crop factor" with a "made for digital" lens, because those lenses don't magically increase the size of the sensor to that of a 35mm frame when you mount them on your camera. That's why when you see a 10-20mm made-for digital lens, it is still marketted as "equivalent to" 16-32mm on 35mm -- EVEN THOUGH the lens itself would not cover a 35mm frame well if you tried it.

[kai_griffin]

It's already been said above, and countless times elsewhere for years:

 

The _exact_ equivalent of the "crop factor" concept is in taking a pair of scissors to your 35mm negative. Absolutely nothing else plays a part in this "black magic" - a crop is a crop, no matter what lens or type of lens you use, or what colour hat you happen to be wearing.

 

It's odd that crop factor has only arisen in earnest with digital cameras. I don't recall talk about "crop factor" when APS film cameras were released, and you never hear of "crop factor" when talking about 6 x 4.5 cameras in relation to 6 x 6 or 6 x 7 cameras (or indeed 35mm film cameras), even though the concept is identical. Did owners of the Pentax 110 SLR in the 70's talk about "crop factors", I wonder? For some reason it's only become a topic of great confusion with the advent of digital.

[david_littleboy__tokyo__ja]

"For some reason it's only become a topic of great confusion with the advent of digital."

 

That's because people don't think of APS-C digital as being a different format when it actually is. So both terms are wrong: the correct term is "format conversion factor". You have to multiply your focal lengths by the format conversion factor to find the _name_ of the functionality in the new format.

 

(What I mean by "name" here is that "50mm" can be seen as the name of the functionality provided by the 50mm lens in 35mm. So if you have a 50mm lens, you get the name of its function ("80mm") by multiplying by the format conversion factor.)

 

Anyway, of the two standard terms, "crop factor" is by far the worst. A 50mm lens functions exactly the same on an APS-C camera (except for 1.6x the DOF) as an 80mm lens does on a 35mm camera. Exactly. The images will be identical (except for DOF) when printed at the same size. People who favor the "crop factor" term merely confuse themselves into thinking that the 50mm lens on an APS-C camera won't provide the short telephoto appearance we like in portraits, when actually it does.

[kai_griffin]

"Format Conversion Factor", or FCF... has a definite ring to it, and is the most accurate way of putting it.

 

Consider that I have an 80mm lens designed for medium format, which I sometimes mount on my Canon D60 via an adapter. One could bend the mind trying to figure out all the real & imaginary factors involved, but the truth is that an 80mm lens is an 80mm lens; different camera bodies (read: film formats/sensor sizes) make more or less use of the projected image circle, but they don't change the way the lens itself works - the focal length is fixed and cannot change.

[mark u]

For Daniel - read CAREFULLY what I wrote - with the SAME lens if you crop an image, you are magnifiying the degree to which any part of the image is even slightly out of focus when you enlarge it to the same print size as the uncropped image. Therefore you need to use a narrower aperture to reduce the size of the circle of confusion in the capture, so that it will be of similar size when printed.

 

Rougly speaking, at normal shooting distances and excluding hyperfocal conditions, you need to stop down by 1 1/3 stops to get the same depth of field when you use the SAME lens with a 1.6x crop (log(1.6)/log(sqrt(2))), which will increase exposure time by 1.6x. You need to shorten exposure time by the 1.6x crop factor to get a similar degree of sharpness through camera shake compared with the uncropped image. To maintain the same overall sharpness you therefore need to increase ISO by 2 2/3 stops - which means effectively that an uncropped image that might just be hand holdable without image stabilisation would require the almost the full benefit of "3 stop" IS if shot at the same ISO if it is going to be cropped.

[mark u]

I better correct my mistake in the last post. Shortening the exposure to 5/8ths of the duration is equivalent to reducing it by about 2/3 stop, so overall you need 2 stops more ISO speed (or image stabilisation equivalent).

[skipd]

There is NO exposure difference when using a camera with an APS-sized sensor versus one using a full 35mm-sized sensor. Think about it for a moment - if you shot a photo on 35mm film and then cut the negative to provide an APS-sized negative, is there any difference in the exposure? Of course not. The same thing is true in the camera.

 

The only possible difference in the print is sharpness because the image needs to be magnified a bit more for the same size print when using the smaller sensor (or piece of film). This is the SAME THING as the difference between using 35mm vs medium format vs using 4x5 sheet film. Each format requires a different amount of magnification to produce the same size print. The smaller the negative, the sharper the image on it must be to produce the same print quality.

 

The "multiplier" thing is ONLY used because so many of us are used to what a given focal length does for us in a 35mm camera. If you used medium format (6x6 cm, for example), you would have a totally different viewpoint about what focal length does what. 80mm is considered the "normal" lens for 6x6, while 50mm is considered "normal" for 35mm. Taking this into consideration, what is a "normal" lens for a digital camera with APS-sized sensor? It is about 31mm. Now you can figure out what is "wide angle" or "telephoto" based on that. Forget about what works on a 35mm camera (or digital with a 35mm frame-sized sensor) and learn the focal lengths that you need for YOUR digital camera.

[cssf]

If you are using the whole image circle then what exactly is being cropped out? Nothing. There is only cropping going on when you use a lens from a larger format with a smaller one--then you can point to the exact parts of the image circle that are being cropped out. Otherwise there is no actual cropping going on; something else is happening instead.

[mark u]

Skip - I'm not remotely suggesting that exposure changes - just that the optimal combinations of shutter speed and aperture differ when you are cropping an image, which might also lead to choosing a different ISO/film speed to allow a combination of a faster shutter speed and a narrower aperture.

[kai_griffin]

SF CA, sorry - you're on the wrong track - the term "crop factor" has NOTHING to do with image circles. It is purely about the difference between 35mm frame size and APS-C frame size. The lens is irrelevant.

[kai_griffin]

...or, to put it another way, and to answer your question "what exactly is being cropped out" - the answer is that the badly vignetted outer area of the image projected by those "made-for-digital" lenses is what is being cropped out. More directly: if it were not for the "crop factor" of the camera, the lenses would be pointless - the only reason they even manufacture lenses like that is because the target frame is smaller (cropped) relative to a 35mm frame.

[cssf]

This is from dictionary.com:

 

Crop:

 

To cut or bite off the tops or ends of: crop a hedge; sheep cropping grass.

 

To cut (hair, for example) very short.

 

To clip (an animal's ears, for example).

 

To trim (a photograph or picture, for example).

 

I'm confused by some answers; what is being cropped/trimmed when the image circle is the exact same size as the sensor? Thanks for helping to make this clear to me.

[kai_griffin]

SF CA,

You're looking at the crop factor as if it referred to a percentage of the image circle. It does not. It is a percentage of a standard 35mm frame of film. The image circle delivered by the lens has nothing to do with it. In the term "crop factor", the subject of the cropping is not the image circle from the lens, but an imaginary 35mm film frame. A crop factor of 1.6 means that the sensor is 1.6 times smaller than a 35mm frame of film.

 

This is why - I repeat - when Canon market a 10-20mm lens for APS-C sized sensor cameras, they proudly tell you that it is equivalent to a 16-32mm. The crop factor does not just go away because of the lens - that would imply that the lens had the magical power to change the physical dimensions of the sensor! Maybe one day :-))

 

Also, on an unrelated note - the image circle of a lens is not "exactly the same size" as the sensor. If it were, all your pictures would be... cirlces!

[cssf]

So if it is an imaginary crop factor then maybe that is what we should call it. Thanks.