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01/23/2003 09:42:29 AM · #1
I see lots of reference to the APS-sized sensor in the D100/D1_ series as a 1.5 "focal length multiplier" or "telephoto factor", and I think that many folks are not clear that the factor is actually a crop factor and not a focal multiplier. There are some important technical reasons that the use of "focal multiplier" rather than the more correct "crop factor" should be discouraged. So, I took some shots this morning that I hope will help clarify the situation for the confused...

First, to illustrate what actually happens to your image in the D100 or D1, we will compare an image with its 1.5 crop. I took a base shot at 50mm, f/6.3, internal flash at 1/60th.

' . substr('//noml.dyndns.org/1.5_explained/50mm_f6.3_1-60th_No_Crop.jpg', strrpos('//noml.dyndns.org/1.5_explained/50mm_f6.3_1-60th_No_Crop.jpg', '/') + 1) . '

Then, illustrating what the reduced size sensor does to the full image inside the D1, I used this crop of 1.5 (from Photoshop)

' . substr('//noml.dyndns.org/1.5_explained/50mm_f6.3_1-60th_1.5_Crop.jpg', strrpos('//noml.dyndns.org/1.5_explained/50mm_f6.3_1-60th_1.5_Crop.jpg', '/') + 1) . '

Notice that none of the elements of the photo have changed: the lighting is the same; the depth of field is the same; the apparent positions of the objects in the field are the same. The only thing that might have changed in the photo is the base resolution (lens resolution in the camera, but pixel resolution for the example) is less for the cropped version of the photo.

Now to isllustrate the difference between "crop factor" and "telephoto factor," we compare the Crop above to a shot made with an actual 1.5 focal length multiplier. I took the lens from 50 to 75mm but remains f/6.3, with internal flash at 1/60th. This image is uncropped.

Notice the blue marker in the mid-ground, and the cup in the background, as well as the "Thermos" label on the far right background. All more out-of-focus than in the Crop photo. The Depth of Field has changed, due to a difference in the focal length. In addition, the apparent position of all the objects in the photo has changed such that they all appear closer together front-to-back due to different "telephoto compression" imparted by different focal lengths. ( I will admit the angle of incidence to the subjects has changed a tad. This was a handheld shot...)

' . substr('//noml.dyndns.org/1.5_explained/75mm_f6.3_1-60th_No_Crop.jpg', strrpos('//noml.dyndns.org/1.5_explained/75mm_f6.3_1-60th_No_Crop.jpg', '/') + 1) . '

If you need further comparison, save the crop and the zoomed image and do a two-picture cycle in your favorite picture viewer. The differences are subtle here, but noticeable. Over a large field, like those in nature photography, or while using long focal lengths as in sports protography, the differences are more apparent.

What does this mean to the shooter? Not much, in practice, as long as the shooter understands what focal lengths and f-stops do to the composition in his or her own camera. If he or she is moving from 35mm, and is very comfortable with those elements in that arena, then the crop factor vs. telephoto factor can prove frustrating, as the photographer expects his or her images to be rendered at one DOF and actually gets another.

Best,

mjc


01/23/2003 09:44:45 AM · #2
Given the size of the photos and the number of hits this page is likely to get, it's probably unwise to host these images on your own internet connection ;o)
01/23/2003 09:47:12 AM · #3
The real issue is that people still talk about the camera focal lengths at their 'effective' 35mm values.

Consider that a medium format camera, that a 100mm lens is considered a 'normal' lens, compared with a 50mm lens on a 35mm camera. Really you want something like a 35mm lens as a normal lens for a smaller digital sensor. All this cropping and multipler factors only come in because people use 35mm film camera sizes but are using it on a smaller sensor. This is were the Nikon DX lenses make a lot of sense, if they never plan on producing a 'full frame' 35mm sensor.

The myth is that 35mm format has some mystical property, when in fact a smaller sensor has a lot of advantages, mostly in the increased optical tolerances that can be exploited to make, cheaper, lighter, equivalently sharp lenses for a smaller sensor.

There are fundamental limits obviously on quality and so on, but other than legacy support,(which _is_ a big deal) this quest for a affordable full frame sensor seems like a wild goose chase
01/23/2003 12:49:29 PM · #4
Originally posted by Gordon:


The myth is that 35mm format has some mystical property, when in fact a smaller sensor has a lot of advantages, mostly in the increased optical tolerances that can be exploited to make, cheaper, lighter, equivalently sharp lenses for a smaller sensor.


I think you got your facts backwards on this one; A smaller sensor needs HIGHER optical quality:

You have a D60 - Take one of the cheapest zoom-lenses Canon makes (28-80 seems like a good example), make sure it is slightly dusty and has a few scratches. Then put it on your camera. Take a picture, and have it printed LARGE (A2-size). Do the same with a 35mm camera. You will be shocked at how much better the film-based picture looks. That is not because of the extra data existant in the film, but because the lens is shit. BUT - all the little defects register a lot better on your imaging chip, because each defect will have a larger impact - relatively.

On the other hand, this also means that a D60 fitted with a high-quality lens (Take a 28-135 IS or a 16-35 L), will gain MORE from the increased optical quality than a 35mm film camera, because of the relative quality difference; In practice, a D60 RAW image contains as more data than a 35mm print (a little less than a scan done with a high-quality film (or drum-) scanner, but that is a different discussion altogether), and the benefits from a better lens are directly visible.

Allow me to shamelessly plug a few articles I have written, to illustrate further (if you are interested):

CCD (imaging chip, also largely counts for the CMOS chips in D60s)
Chromatic Aberrations (lens design problem)
Lens Flare (more about lens design problems etc)

I know a lot about these topics, but I am hardly an expert. If you have corrections, I'd be more than happy to be corrected.

Haje
01/23/2003 12:53:02 PM · #5
Originally posted by Gordon:

The myth is that 35mm format has some mystical property, when in fact a smaller sensor has a lot of advantages, mostly in the increased optical tolerances that can be exploited to make, cheaper, lighter, equivalently sharp lenses for a smaller sensor.



By the way; You are right in the statement above, but that only goes for compact digital cameras. D-SLR's are shaped to be as similar to 35mm cameras, and unless the lens design changes dramatically (not impossible - Nikon just launched their first DX series lens which is engineered especially for smaller sensors.) I can't see that change. The whole point of D-SLRs is that they are supposed to be able to use the high-quality optics available to 35mm systems.

HJ
01/24/2003 10:05:44 AM · #6
Originally posted by SharQ:

Originally posted by Gordon:

The myth is that 35mm format has some mystical property, when in fact a smaller sensor has a lot of advantages, mostly in the increased optical tolerances that can be exploited to make, cheaper, lighter, equivalently sharp lenses for a smaller sensor.



By the way; You are right in the statement above, but that only goes for compact digital cameras. D-SLR's are shaped to be as similar to 35mm cameras, and unless the lens design changes dramatically (not impossible - Nikon just launched their first DX series lens which is engineered especially for smaller sensors.) I can't see that change. The whole point of D-SLRs is that they are supposed to be able to use the high-quality optics available to 35mm systems.

HJ



But we are putting these lenses that are designed to resolve a sharp image with limited aberations over the entire area of a piece of 35mm film. The sensors in most current digital cameras do not cover this sensor area, hence a lot of additional engineering effort, optical quality and lens construction is used to produce a perfect image, that we don't actually care about. We are effectively 'throwing away' a lot of that image at the edges, but are still paying for it in glass weight, construction complexity etc.

If we reduce the design tolerences (by reducing the area we wish to have good image production upon) then the design complexity for the lens is reduced, allowing you to make a cheaper, lighter lens with exactly the same quality on the same sensor.

I'm not saying change the sensor, I'm saying it makes sense to design the lenses for the digital SLR sensor, rather than using something that is effectively overengineered to be correct on a much larger sensor space. As such, there isn't anything magical about the 35mm film size/ ratio - it was after all just two bits of movie film spliced together originally.
01/24/2003 10:08:15 AM · #7
Originally posted by SharQ:

Originally posted by Gordon:


The myth is that 35mm format has some mystical property, when in fact a smaller sensor has a lot of advantages, mostly in the increased optical tolerances that can be exploited to make, cheaper, lighter, equivalently sharp lenses for a smaller sensor.


I think you got your facts backwards on this one; A smaller sensor needs HIGHER optical quality


It does, for the area that it is resolving. But the challenge when doing lens optics is to keep the resultant image even across the whole film plane (digital or not) Smaller sensor area == reduced tolerences across the lens design. So you need improved optical quality within a smaller area, if your sensor is better than film at recording details, but it should still be easier to make a lens with those qualities and the result should be lighter too.
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