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08-08-2010, 06:40 PM   #1
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Crop Factor and Macro Ratio

We are all familiar with the crop factor of the APS-C sensor size that turns a 200mm lens into an effective 300mm lens, even though the real focal length of the lens is still 200mm. My question is does that same issue affect the effective macro ratio of a lens?

I guess what I mean is if a sensor is 2/3 the area of a full frame, then is the magnification ratio proportionately increased so that a 1:2 lens is effectively a 1:1.33 magnification ratio?

08-08-2010, 06:57 PM   #2
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No.

A lens that goes to 1:1 on FF/135 also goes to 1:1 on HF/135 or APS-C. It's not a matter of filling a frame or sensor of arbitrary size. It's a matter of projecting onto a frame an image that's the same size as the subject. A larger sensor just sees a larger amount of the projected image.
08-08-2010, 07:16 PM   #3
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I am not talking about changing the actual magnification ratio, just the effective magnification ratio, just as a 200mm is still a 200mm lens no matter what the sensor size, the effective length is large by 1.5 because of the smaller sensor. I guess I am saying is a macro lens with a 1:2 ratio on an APS-C camera would fill the frame the same as a 1:1.33 macro would on a full frame.
08-08-2010, 07:47 PM   #4
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QuoteOriginally posted by GregK8 Quote
I am not talking about changing the actual magnification ratio, just the effective magnification ratio, just as a 200mm is still a 200mm lens no matter what the sensor size, the effective length is large by 1.5 because of the smaller sensor. I guess I am saying is a macro lens with a 1:2 ratio on an APS-C camera would fill the frame the same as a 1:1.33 macro would on a full frame.
Get over effective length. It's a meaningless term that will only confuse you.

08-08-2010, 08:12 PM   #5
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QuoteOriginally posted by alohadave Quote
Get over effective length. It's a meaningless term that will only confuse you.
Exactly.

Think of it this way: Here is a 100mm lens with 200mm of total extension. That gives it 1:1 magnification, NO MATTER WHAT FRAME ITS IMAGE CIRCLE IS PROJECTED ONTO. Let's say it's a medium-format lens. Project that image onto a piece of photo paper that is 6x9 cm. The size of the subject in the projection is the same size the subject is in real life, ie 1:1 magnification. We develop the print. Now draw a 36x24mm rectangle on the print; that's the size of the 135/FF frame. Inside that, draw a 24x18mm rectangle; that's the size of a 135/HF or APS-C frame (close enough). The subject is still at 1:1, no matter how small a slice of its image we take.

Oh yeah, forget you ever heard the term CROP FACTOR. It is BS. Different-size frames see different-size slices of the same image, that's all. The only 'equivalence' is in the angle/field of view. The 6mm lens on a P&S is NOT effectively 42mm on 135/FF or 28mm on 135/HF or APS-C, no matter what the manual says. A 24mm lens on APS-C is NOT equivalent to 16mm on 135/FF, no matter what anyone says.

Last edited by RioRico; 08-08-2010 at 08:22 PM.
08-08-2010, 08:17 PM - 1 Like   #6
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Here's an exercise for you.
Put a quarter and a nickel side by side.
At 1:1 on a 135 format camera, the quarter fills the frame (more or less) and there is space around the nickel.
At 1:1 on an APS-C format camera, the nickel fills the frame (more or less) and you can no longer see the top and bottom of the quarter.

The details of the above are probably not exactly accurate, but the gist of it is.
Try it yourself, you'll learn more about macro and camera formats than anything I could write here in less than half an hour.
08-09-2010, 01:53 PM   #7
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QuoteQuote:
Oh yeah, forget you ever heard the term CROP FACTOR. It is BS
While I agree with most of what you say, I'm not entirely happy with the above bit.

Your points about different size sensors "seeing" just different sized areas of the image projected onto them is perfectly true.

However, most of us still see that image on our computer screens, or printed, at the same physical size as we would see the image from a full-frame camera.
So, to get to that size, the portion of the image that is "seen" by the sensor is magnified by 1.5x or whatever (1.6x for Canon).

It is this magnification to a "standard" size that causes all the confusion.

A "full frame" sensor "sees" a bigger portion of the projected circular image than an APS-C sensor does. And when the APS-C sensor's data is magnified to the same size as that of a standard full frame photo on our screen, it is blowing up a cropped part of the full frame picture - if you see what I mean. Hence the term "crop factor"

Yes, the angle of view of a 50mm lens is the same on all cameras, but the portion of the circular image that the lens projects onto the sensor plane is different for each type of sensor. So while a 50mm lens is said to be a "standard" lens, as it approximates the angle of view of the eye, this only applies to a 35mm or full frame sensor. The equivalent lens, after the smaller image section is magnified to match, would be a 33mm lens. So, either the Pentax 31mm or 35mm lenses would be about as close as we have to a "standard" lens. NOT a 50mm.

The OP was asking how all this nonsense applies to a macro lens, and the first answer is correct - 1:1 is still 1:1, but an APS-C sensor just sees less of the projected image. Yes, it is magnified onto our screens to match the image from a full frame camera, but the term 1:1 actually applies to the image projected onto the sensor, NOT what we see afterwards.

Hope that all makes sense.....

EDIT:

Here is perhaps the best explanation

==== START QUOTE ====
Magnification factor

The crop factor is sometimes referred to as "magnification factor",[3] "focal length factor" or "focal length multiplier".[4] This usage reflects the observation that lenses of a given focal length seem to produce greater magnification on crop-factor cameras than they do on full-frame cameras. This is an advantage in, for example, bird photography, where photographers often strive to get the maximum "reach". A camera with a smaller sensor can be preferable to using a teleconverter, because the latter affects the focal ratio of the lens, which can degrade the performance of the autofocus.

It should be noted that the lens casts the same image no matter what camera it is attached to. The extra "magnification" occurs when the image is enlarged more to produce output (print or screen) that matches the output size of a full-frame camera. That is, the magnification as usually defined, from subject to focal plane, is unchanged, but the system magnification from subject to final output is increased.

==== END QUOTE ====

The whole article, which is actually very good, can be found here:

http://en.wikipedia.org/wiki/Crop_factor

Last edited by Derridale; 08-09-2010 at 02:40 PM.
08-10-2010, 02:35 AM   #8
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QuoteOriginally posted by alohadave Quote
Get over effective length. It's a meaningless term that will only confuse you.
QuoteOriginally posted by RioRico Quote
Exactly.
Oh yeah, forget you ever heard the term CROP FACTOR. It is BS.
+1 for both comments.

For the OP

Magnification ratio is simply the ratio of image size to subject size ON THE RECORDING MEDIUM

It has nothing to do with how yo crop in on a subject either due to the format of the film/sensor or in post processing,

It also has nothing to do with how big we display the image after capture.

This is the same problem with the focal length multiplier. it is all BS.

the only relitive term to consider is angle of view, because that is all that changes when you consider the focal length and the recrding medium size.

The final reproduction ratio of your image is totally independant of the magnification ratio of the optics

08-10-2010, 03:30 AM   #9
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If the same sized display is always used, a photo from the same distance with the same lens results in an image scaled by the crop factor ratio; in common language usage a "magnified image".

Unfortunately, the photographic meaning of "magnification" refers the subject size to the sensor or film, not to the display. This makes no sense to the casual user whose focus is on the enlargement shown in the final display.

Confusion between "enlargement" and "magnification" is at the heart of the problem.

This confusion is not BS - it is real. The person who is looking at a displayed photographic image is not an idiot when s/he confuses "enlargement/magnification" photo terminology.

Perhaps we should be using modifiers like "optical magnification" when referring to what's going on inside the camera system, and "photographic magnification" when referring to the overall system, including the final display.

Dave

PS from the American Heritage dictionary:
mag·ni·fi·ca·tion n.
1. The act of magnifying or the state of being magnified.
2.
a. The process of enlarging the size of something, as an optical image.
b. Something that has been magnified; an enlarged representation, image, or model.
3. The ratio of the size of an image to the size of an object.

Definition #2a precedes #3 implying that for common usage "magnification" should include crop-factor effects.

Last edited by newarts; 08-10-2010 at 03:41 AM.
08-10-2010, 04:18 AM   #10
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I do wish people would forget about crop factor. To me, it makes more sense to say that APS C cameras increase resolution of an image than that they increase the magnification of the image. Obviously, this is only because full frame cameras have not yet reached 30-ish megapixels. At some point, you will likely be able to crop your full frame image to APS-C size and not be able to see the difference, but right now on a 12 megapixel full frame, if you crop to APS C size, you lose a good portion of your image and so, the resulting image has lower resolution, but not magnification.
08-10-2010, 05:44 AM   #11
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There should really be a sticky about crop factor because it comes up so often, and the resulting discussion probably confuses people even more!

Somebody did a fantastic post with supporting pics on film and digital and now it's probably 40 pages back.
08-10-2010, 06:22 AM   #12
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I think those who are suggesting that we forget about crop factor are missing the point.

There IS a crop factor - the smaller sensor sees a smaller part of the projected image, so when this is displayed at "normal size" either on computer or print, it is effectively zoomed in. it IS a crop compared to a full frame sensor. And that factor should be applied to the APPARENT focal length of the lens, as it reflects the viewing angle seen by the camera.

So, a 100mm lens on a Pentax K20D is effectively a 150mm lens - in terms of the final image as seen and viewing angle.

This can be an advantage at the tele end of things, but a distinct disadvantage at the wide angle end. My new 8-16mm Sigma is effectively the same as a 12-24mm lens on a full frame camera. In terms of viewing angle!

To deny that is silly. What is perhaps more pertinent is WHY everything must be compared to the old 35mm format in the first place..... (which came about due to standard movie film stock being 35mm film)
08-10-2010, 06:33 AM   #13
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Geez, I didn't think this was such an emotional issue. For what it's worth, I do get the crop factor, angle of view, focal length relationship, and that it is just a crop factor. It just seemed to me the same phenomena could be applied to the macro concept, which apparently it can, even if it is flawed.

Last edited by GregK8; 08-10-2010 at 04:59 PM. Reason: removed the word not because that's not what I meant :p
08-10-2010, 10:40 AM   #14
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QuoteOriginally posted by GregK8 Quote
Geez, I didn't think this was such an emotional issue. For what it's worth, I do not get the crop factor, angle of view, focal length relationship, and that it is just a crop factor. It just seemed to me the same phenomena could be applied to the macro concept, which apparently it can, even if it is flawed.
I was surprised by the responses as well. You are correct in the idea that the crop factor can be applied to macro. The technique is an old one. Unfortunately, there may be peril in doing so. It is easier to fill the frame with the subject, but by doing so you are amplifying the faults of the optical and sensor system. It all depends on the target medium and intended final image size.

Consider the following two images taken with my K10D of the same subject within a few minutes of each other:





One was made with a dedicated macro lens and the other with my FA 77/1.8 Limited. The one from the 77/1.8 was severely cropped to fill the frame. This works pretty good for moderate size on-screen display, but the ability to print the 77 Limited version to more than 4x5" with any quality is limited.

Steve
08-10-2010, 12:19 PM   #15
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QuoteOriginally posted by Derridale Quote
I think those who are suggesting that we forget about crop factor are missing the point.

There IS a crop factor - the smaller sensor sees a smaller part of the projected image, so when this is displayed at "normal size" either on computer or print, it is effectively zoomed in. it IS a crop compared to a full frame sensor. And that factor should be applied to the APPARENT focal length of the lens, as it reflects the viewing angle seen by the camera.

So, a 100mm lens on a Pentax K20D is effectively a 150mm lens - in terms of the final image as seen and viewing angle.

This can be an advantage at the tele end of things, but a distinct disadvantage at the wide angle end. My new 8-16mm Sigma is effectively the same as a 12-24mm lens on a full frame camera. In terms of viewing angle!

To deny that is silly. What is perhaps more pertinent is WHY everything must be compared to the old 35mm format in the first place..... (which came about due to standard movie film stock being 35mm film)
I think you are making up a point. The term crop factor is an invention of the digital photography age.
I would ask you why the term was NEVER bandied about when people shot various formats of film?
What is the crop factor of a 19" screen?
Or a 24" wide screen?
What is really hilarious is that people who have never touched a 135 format camera bandy the term about like as if it means something.
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