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10-24-2012, 04:05 AM   #1
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What is the crop factor?!

I've heard so many contradictory things about the crop factor of a 645D to FF or APS-C camera.
On THIS very forum, when the 90mm f2.8 was announced, the official post said it was equivalent to a 60mm on APS-C... Um, what? How can that be right. 90mm on 36x24mm is equiv. to a 60mm on a 1.5x crop... so unless the 645D has a full frame sensor... which it doesn't... that's wrong.
Now I realise it's not 'simple' because the 645D is 4:3 not 3:2 but by my maths - I get that a 645D lens, you should halve it, then add a bit, to get an equivalent length on APS-C - that is, the 90mm on 645D would be about 47-48mm for us APS-Cers...

Am I wrong?


Last edited by Tom S.; 03-09-2013 at 05:49 PM.
10-24-2012, 04:28 AM   #2
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Jeffrey Sward, in his post Digital Camera Sensor Size Comparison Chart by Jeffrey Sward rightly refers to the 35mm format multiplier. "Crop factor" is shorthand only if you know the reference is back to the accidental standard that Oscar Barnack created.

Anyway, using that scale, Sward's table gives the 645d's sensor a multiplier of 0.79 for the frame diagonal, which means the 90mm lens for the 645D would have an angle of view equivalent to that of a 71mm lens on a 35mm camera. Based on the longer side of the frames, however, the multiplier is about 0.82, so the equivalent becomes a 74mm lens. Not much in it.

On an APS-C sensor, the equivalent based on the longer side of the sensor becomes a 48mm lens, or, based on the diagonal, becomes a 46mm lens.

Last edited by RobA_Oz; 10-24-2012 at 04:34 AM.
10-24-2012, 06:02 AM   #3
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Correct me if I am wrong, but as I understand it the focal length of a lens is always given in FF 35mm format. This is due to the old film days and is now the gold standard I believe.

Therefore a 90mm lens for a 645D will change its focal length depending on the crop factor of the 645D (which I dont know).
Also a 90mm lens for a 645D will change its focal length for a APS-C (1.5x for pentax).
And a 90mm lens for the 645D will stay 90mm on a FF 35mm film camera.

So its the same concept as for all the lenses specifically designed for APS-C cameras (DA range and so on), where there is always the FF focal length be given even though the actual focal length on the APS-C camera is different.

As I said in the beginning please correct me if I am wrong since I am not familiar with medium format gear at all xD.
10-24-2012, 06:23 AM   #4
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Right - the lens' FL is the same in all places & times but the aps-c sensor intercepts less of the focused image, allowing that outer part that would hit a 36x24 sensor to pass. No action takes place, and the lens does nothing different. 'Crop' is usually an active verb, which is one of the reasons that it confuses.

10-24-2012, 06:28 AM   #5
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That's all you need to know about crop factor. A lens focal length doesn't change. The sensor that reads information from the lens defines the image you get. The size of the sensor will define how much of the image you see. That's all.
10-24-2012, 07:33 AM - 1 Like   #6
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QuoteOriginally posted by Snakeisthestuff Quote
Correct me if I am wrong, but as I understand it the focal length of a lens is always given in FF 35mm format. This is due to the old film days and is now the gold standard I believe.
The focal length is not given in "FF 35mm format" at all, but in millimeters. It is a physical property of the lens, independent of the format of the eventual camera it will be mounted on.

The "crop factor" terminology and comparisons to film 135 format arises from the latter's popularity, and the fact that most DSLR use lenses that were designed for that format but are used with a sensor smaller than the film, thus changing the angle of view.

The "crop factor" or "35mm equivalent" therefore allows people familiar with 135 format to know what angle of view to expect with a given lens on a DSLR. If you've never shot 135 format, you really shouldn't care about the "crop factor", IMHO.
10-24-2012, 08:23 AM   #7
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Why is it then a full frame and an apsc lens give the same view, the apsc lens should give the full view and not the same cropped one as the full frame lens.
Looks like the companies are saving on glass.
10-24-2012, 08:33 AM   #8
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QuoteOriginally posted by bobpur Quote
Why is it then a full frame and an apsc lens give the same view, the apsc lens should give the full view and not the same cropped one as the full frame lens.
Looks like the companies are saving on glass.
You may have it backwards, or something.

Again - see the picture I linked:


The lens has two properties that we are looking at.

Focal length
Image circle

Focal length is a lens property that doesn't care what format it is connected to. It is defined by the size of the front and rear elements, as well as how the distances between these elements.

Image circle is defined by the size of the lens elements, and how they relate to each other. The lens projects an image circle of a certain size onto the sensor. Smaller image circles allow for the lens to be made smaller. In other words, a 50mm made for APS-C image circle will have a smaller body than a 50mm made for FF. Both have will project the same object proportions (object relationships in image), but one has less image coming through.

03-09-2013, 01:00 PM   #9
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Jin Is Right

Good tip Jin! The only difference is sensor size. The 645D is much larger than a full-frame camera as it is considered Medium Format. Full-Frame is considered 35mm and APS-C is considered Prosumer size as it's the smallest of SLR style cameras made. A 90mm lens on an APS-C camera would be a long (telephoto) lens on this camera however put a 90mm lens on a 4x5 film camera and it becomes wide angle! I have a Fuji GX617 with a 105mm lens and that's considered wide for that camera; 90mm is as wide as you can go on that camera and still produce an image from edge-to-edge. At 105 on this format, at times I get light-fall-off, even with a center ND filter which tells you just how far you are stretching the frame at 105mm. Many 4x5 Cameras can shoot as small as a 65mm lens however at this wide angle on this format, you will have very little movement of the bellows as the lens in this instance would need to be very close to the film plane to stretch the image across 5 inches of film.

I tend to prefer the older Pentax M,A,F,K and FA lenses because they were made in Japan and well crafted. The new Pentax lenses are mostly plastic and made in Vietnam and even China. A DA 15mm lens for example is really 22mm on a Pentax K5, K5II or IIs model camera. The Pentax DA 12-24mm lens is actually 18-36mm on the APS-C camera format as the Pentax crop factor is 1.5 on the newer cameras. Keep in mind, when you get down below (true) 18mm, distortion becomes quite evident, that is why most lenses below 12mm in the DA line up are all fisheye lenses. The fisheye is the only way to keep barrel distortion in check when you are that wide. Most lenses, especially zooms, when used at their widest point are not very sharp away from the center for this reason; there's just too much stretching to fill the image plane to be sharp.
03-09-2013, 05:23 PM   #10
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QuoteOriginally posted by JinDesu Quote
It is defined by the size of the front and rear elements
When referring to focal length,this is Incorrect.


QuoteOriginally posted by jcasey52 Quote
The fisheye is the only way to keep barrel distortion in check when you are that wide.
Actually the designers leave distortion uncorrected on purpose. They let barrel distortion run wild. Very wide rectilinear lenses can and have been made, especially with the use of aspherical surfaces.
03-09-2013, 05:33 PM   #11
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QuoteOriginally posted by Snakeisthestuff Quote
Correct me if I am wrong, but as I understand it the focal length of a lens is always given in FF 35mm format. This is due to the old film days and is now the gold standard I believe.

Therefore a 90mm lens for a 645D will change its focal length depending on the crop factor of the 645D (which I dont know).
Also a 90mm lens for a 645D will change its focal length for a APS-C (1.5x for pentax).
And a 90mm lens for the 645D will stay 90mm on a FF 35mm film camera.

So its the same concept as for all the lenses specifically designed for APS-C cameras (DA range and so on), where there is always the FF focal length be given even though the actual focal length on the APS-C camera is different.

As I said in the beginning please correct me if I am wrong since I am not familiar with medium format gear at all xD.
Yes, you are wrong. Lenses don't change focal lengths, ever, just as your height does not change when you walk into a room with a taller ceiling. The field of view changes, and the only reason it does that it because it is cropped. Like the pictures in this thread show, depending on its size, the sensor will capture more or less of the image circle created by the lens. That's it -- smaller sensors crop more (thus the moniker "crop sensor") than bigger ones.
03-09-2013, 08:28 PM   #12
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QuoteOriginally posted by desertscape Quote
Actually the designers leave distortion uncorrected on purpose.
Not quite true. The difference between a rectilinear wide angle and a fisheye is the a matter of intentional design. Rectilinear projection retains straight lines at the cost of increased geometric distortion at the edges of the frame. Fisheyes use some form of circular projection (there are several kinds) with a result that proportional geometry is retained throughout the frame, but at the cost of (some, not all) straight lines. The situation is analogous to Mercator vs. other map projections. Mercator produces straight lines, but at the cost of accuracy.


Steve
03-10-2013, 07:41 AM   #13
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QuoteOriginally posted by JinDesu Quote

That's all you need to know about crop factor. A lens focal length doesn't change. The sensor that reads information from the lens defines the image you get. The size of the sensor will define how much of the image you see. That's all.
To add to this, the different rectanges represent the "field of view" for that format.
03-10-2013, 10:30 AM   #14
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Stevebrot--- yes I agree that there are two different forms of distortion and only one of the two can be corrected for a lens. The fisheye corrects for geometric distortion and not linear. But, seeing the differential image magnification across the frame that causes barrel distortion, one wonders how the geometric distortion can be corrected?
03-22-2013, 09:24 AM   #15
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QuoteOriginally posted by vonBaloney Quote
Yes, you are wrong. Lenses don't change focal lengths, ever, just as your height does not change when you walk into a room with a taller ceiling. The field of view changes, and the only reason it does that it because it is cropped. Like the pictures in this thread show, depending on its size, the sensor will capture more or less of the image circle created by the lens. That's it -- smaller sensors crop more (thus the moniker "crop sensor") than bigger ones.
True. What I think they were trying to say is when you put a 20mm lens on a 1.5 cropped sensor, the focal length would be equivalent to a 30mm lens on a Full-Frame digital camera. The lens is still 20mm and even the editing software you might use will say it was shot at 20mm though the cropped sensor will give you a 30mm field of view. 20mm on a 1.5 cropped sensor is still a wide angle of view and I find using anything wider causes too much distortion for my taste, therefore I much prefer the 30mm equivalent FOV my K5IIs provides. If I need wider, I simply mount my camera on a panoramic head and pan several shots in incremented degrees, then stitch. Much more effective for me than using a 15mm lens or worse, a fisheye which really distorts things.
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