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11-01-2012, 08:25 AM   #1
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Magnification with a FF lens on Pentax DSLR

When using a macro lens designed for a full frame format on a Pentax DSLR, I know the effective focal length is multiplied by a factor of 1.5x. What is the impact on the image magnification? For example if the lens is set to a 1:1 ratio is it still 1:1 on the smaller sensor? If not, is the factor to multiply by 1.5X ? I am new to macro. Your help would be appreciated in determining what equipment to use.

11-01-2012, 08:31 AM   #2
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The magnification is still 1:1. You are receiving a cropped image on the APS-C sensor.

11-01-2012, 08:41 AM   #3
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It's more complicated than just sensor size, you also have to take into account sensor pixels per inch, etc. . A sensor with more pixels per inch produces more magnification. Just like in film, if you could use a super fine grain film, it would enlarge cleanly to a bigger print. More pixels per inch= fine grain film.
11-01-2012, 08:45 AM   #4
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The magnification in the traditional sense is still 1:1 regardless of resolution (which is what you're referring to).

QuoteOriginally posted by normhead Quote
It's more complicated than just sensor size, you also have to take into account sensor pixels per inch, etc. . A sensor with more pixels per inch produces more magnification. Just like in film, if you could use a super fine grain film, it would enlarge cleanly to a bigger print. More pixels per inch= fine grain film.



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11-01-2012, 09:02 AM   #5
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All I care about is "how big is the image viewed 1:1 on my computer screen." The rest is interesting as theory. The theory doesn't help you understand why the subject taken with a 50 mm lens is smaller taken on a D700 than the the same subject taken with a 50 mm on your K-5. So maybe that's functional magnification as opposed to magnification as a theoretical concept... but, I'd argue, it's still magnification.
11-01-2012, 09:08 AM   #6
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Let's say you have a Pentax FA 100mm f2.8 macro lens that you've used on a Pentax 35mm film camera. The lens has a 1:1 magnification ratio, so you can take a photo of something that's 36mm X 24mm and it will fill the entire film-negative frame, at the exact size it is in real life. Notice that one part of the magnification ratio is the object size and one part is the image size.

Now you attach that lens to a Pentax DSLR with its smaller sensor, only 16mm X 24mm. Take the same photo as above and you can only fit part of the subject in the image. The image size is 16mm X 24mm. The lens will still have the same magnification ratio, because the object size and image size both change by the same amount.
11-01-2012, 09:13 AM   #7
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I would say that magnification is the ratio between the image projected on the sensor and the actual subject, think of a magnifying glass. The 1:1 on the computer is is a combination of magnification and resolution. If you argue that magnification is actually resolution you also say that a lower resolution camera is way less capable of macro than a high res, even if the acceptable image quality is at a zoom level where the images looks identical.
11-01-2012, 10:33 AM   #8
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Just1moreDave,
Your example is exactly what I am talking about. If the image fills the 24 x 36 frame, and only part of it can fit on the 16 x 24, than is it not effectively magnified on the APS-C? Would it not appear larger than 1:1?
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11-01-2012, 11:04 AM   #9
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QuoteOriginally posted by Pentax Bob Quote
Just1moreDave,
Your example is exactly what I am talking about. If the image fills the 24 x 36 frame, and only part of it can fit on the 16 x 24, than is it not effectively magnified on the APS-C? Would it not appear larger than 1:1?
Pentax Bob
Well no, it will still appear 1:1 because you don't change the image size for the comparison, with APS-C you simply have a smaller image compared to 24 x 36 so the magnification is still the same.

It's when you enlarge the ASP-C to the size of the 24x36 that you see a difference but that differences comes because the enlargement is different.
A higher resolution lens and sensor will also help in that regards.

But this is something loose from lens magnification.
11-01-2012, 11:13 AM   #10
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QuoteOriginally posted by Pentax Bob Quote
Just1moreDave,
Your example is exactly what I am talking about. If the image fills the 24 x 36 frame, and only part of it can fit on the 16 x 24, than is it not effectively magnified on the APS-C? Would it not appear larger than 1:1?
Pentax Bob
At 1:1, a 1.75mm long insect will cover 1.75 mm on the sensor or film regardless of the sensor or film size. Printing an image or projection a color reversal (aka slide) is another story.
11-01-2012, 12:01 PM   #11
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Think of it as using a magnification glass. Magnification is the strength magnification of the glass and crop factor/sensor size is how big the glass is. Resolution is simply the optical the resolution quality of the glass.
11-01-2012, 12:40 PM   #12
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QuoteOriginally posted by Pentax Bob Quote
Just1moreDave,
Your example is exactly what I am talking about. If the image fills the 24 x 36 frame, and only part of it can fit on the 16 x 24, than is it not effectively magnified on the APS-C? Would it not appear larger than 1:1?
Pentax Bob
you need to be careful about the difference between absolute magnification, i.e. the image size on the sensor or film, and the enlargement magnification when viewing.

macro lenses are based on the image magnification to the sensor/film, and this is not at all relevant to any one format. it is that simple. Magnification, like focal length and F stop are physical lens properties independent of format.

It is only when you enter into the realm of field of view, that format begins to matter. and it is only when printing and viewing that the final picture size enters the equation.
11-01-2012, 12:52 PM   #13
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That's right: the image size on the sensor has nothing to do with final picture size. If I use the same lens on a 24x36 camera and on an APS-C camera, I can print both on 16x20 paper with as little cropping as necessary. In doing so, I enlarge the APS-C image more to fill the print. (That places more demands on the lens for the APS-C print.) I could also enlarge the center of the image from the 24x36 sensor more and get the same result as the APS-C image on 16x20 paper.
That's why FF lenses are at their best on FF sensors: the lens resolution doesn't have to be "stretched" as much on prints. (You do still make prints, don't you?)
11-01-2012, 03:39 PM   #14
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Good grief! This argument with Normhead all depends on whether we are talking capture or display. I believe we are talking capture in this thread. In which case pixel count doesn't matter.

If we WERE talking display then our delightfully irritating friend's points take on more validity. Mr. Norm I personnally would LOVE spending an evening with you at the pub sparing over a pint or three. I think it would be entertaining for both of us. We might even learn a thing or two from each other-at least before we hit the third pint.

If you ever make it to my neck of the woods Mr. Norm, there is an Irish pub a few blocks away. The first glass is on me.
11-01-2012, 05:32 PM   #15
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And I would say, if you have a 3000 pixel wide image of coin taken on a large sensor, and a 3000 wide pixel of the same coin taken on a sensor half it's size you have exactly the same functional magnification. And as long as you haven't exceeded the resolving power of the lens you would have indestinguishable images. The magnifying power of the lenses involved is irrelevant. One might be a 150 ff lens mm lens. The other might be a 100 mmm on APS-c. As long as the image doesn't exceed 8-10 MP the magnification would be the same for both lenses. And the output image would be exactly the same.


QuoteQuote:
If we WERE talking display then our delightfully irritating friend's points take on more validity. Mr. Norm I personnally would LOVE spending an evening with you at the pub sparing over a pint or three.
After two I'm pretty much comatose.

QuoteQuote:
If I use the same lens on a 24x36 camera and on an APS-C camera, I can print both on 16x20 paper with as little cropping as necessary. In doing so, I enlarge the APS-C image more to fill the print.
That would be true in film. In film you are enlarging circles of confusion as you enlarge your print. In digital, as long as the circles of confusion are smaller than one pixel, your focus is functionally in sharp focus... something that is theoretically impossible on film. If it's in sharp focus, it should remain sharp even as it's enlarged.

But even in film, you used finer grained film to capture more detail and to be able to have them look good blown up. You didn't use Tri-x 400 for copy film, you used 32 ISO whatever it was. Smaller pixels is somewhat the same.

In essence what I'm saying is capture is irrelevant, if it makes no difference to the display. You can print your D800 files at 600 dpi if you want, but you won't see any more than k-5 file printed at 300 dpi. There will be a lot of detail you won't see in either of them.
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