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07-22-2011, 12:59 AM   #1
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Calculate macro magnification on crop sensor

Searching for detailed info just gave me infos on how to calculate magnification in general.

My question is:

the SMC-F 2.8/135 gives me a 0.25 maximum magnification in closest focussing distance on a film body. If i pair it with a 2x teleconverter I would result in 0.5 magnification.

But if I mount the lens on a digital body with 1.5 crop factor (without teleconverter), do I get 0.25 x 1.5 = 0.375 or is this the wrong way to compute it? The lens behaves like a 200mm lens on the crop camera.

With a tc on digital it would result in a nice IF nearly 1:1 macro lens (0.75), unless I messed up the maths here.

Any suggestions or proof of theory for me?

Many thanks,

Vranx

07-22-2011, 01:42 AM   #2
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your calculations are correct,however the focal length of the lenses haven't changed. Also the minimum focus distance most lenses become slightly wider - internally focusing lenses suffer more from this effect than other lens types. This topic has been brought up before, amazingly many people didn't entirely understand what happens with macro lenses at 1:1 magnification on APS-C format* camera bodies.



The top half of this image was image was taken at 1:1 magnification with a Full frame Nikon D3s - The bottom half was taken with the same lens at 1:1 but with a 1.5X reduced format Nikon D300s



*I do no like to use the term "cropping" because that isn't what is going on here, 1.5X format cameras aren't cropping anything they are just a smaller format and all this affects is the field of view from the lens. Just like 645 is a smaller format than 67, we do not refer to 645 as a cropped format.

Last edited by Digitalis; 07-22-2011 at 06:00 AM.
07-22-2011, 05:35 AM - 1 Like   #3
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The same way a lens keeps its focal length regardless of the sensor, it also keeps its magnification factor. A 1:1 ratiop means the image formed by the lens is the same size as the object you're shooting. Just because your sensor is smaller and the output looks like a cropped image does not mean you are getting a bigger ratio.
07-22-2011, 05:41 AM   #4
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QuoteOriginally posted by bdery Quote
The same way a lens keeps its focal length regardless of the sensor, it also keeps its magnification factor. A 1:1 ratiop means the image formed by the lens is the same size as the object you're shooting. Just because your sensor is smaller and the output looks like a cropped image does not mean you are getting a bigger ratio.
+1

the sensor format does not impact the magnification calculation, specifically
magnification = image size / subject size

the crop factor just confuses the issue,

the teleconverter however does offer improved magnification and this is one useful application of a TC because the TC does not alter the focusing range of the lens,

07-22-2011, 06:44 AM   #5
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1:1 magnification refers to the size of the image on the sensor compared to the original subject regardless of what lens etc you use to get the magnification.

magnification (aka "reproduction ratio"), m=(field.width)/(sensor.width)

This is different from the overall magnification one gets when the image on the sensor is enlarged to a particular display size.

display.magnification = (display.width/sensor.width)

This is where the crop factor enters; for the same display width the crop camera has a smaller sensor hence a larger display magnification.

Dave

PS It is a shame that we use "magnification" to mean different things when talking about photography. It would be nice if our language were more precise (like using "reproduction ratio" and "overall magnification" but ALAS no such luck!

Last edited by newarts; 07-22-2011 at 06:51 AM.
07-22-2011, 10:27 AM   #6
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QuoteOriginally posted by newarts Quote
PS It is a shame that we use "magnification" to mean different things when talking about photography. It would be nice if our language were more precise (like using "reproduction ratio" and "overall magnification" but ALAS no such luck!
actually, I see no shame at all, magnification when we are discussing lenses is very very precise. it means exactly
image size on the recording medium relitive to subject size.

how more accurate do we need to get?

if you think that the crop factor is confusing, then by all means start specifying lenses by the overall reproduction ratio, which the lens maker has absolutely no control over because every one looks at an image on a different size screen or print.

sometimes the desire to simply things adds confusion, your suggestion, while well intended, would do just that.
07-22-2011, 12:49 PM   #7
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QuoteOriginally posted by Lowell Goudge Quote
actually, I see no shame at all, magnification when we are discussing lenses is very very precise. it means exactly
image size on the recording medium relitive to subject size.

how more accurate do we need to get?
Which is exactly what I said..
QuoteQuote:
1:1 magnification refers to the size of the image on the sensor compared to the original subject regardless of what lens etc you use to get the magnification.
You go on to say...

QuoteQuote:
if you think that the crop factor is confusing, then by all means start specifying lenses by the overall reproduction ratio, which the lens maker has absolutely no control over because every one looks at an image on a different size screen or print.
Nowhere did I imply or refer to specifying lenses by overall reproduction ratio! Where did that come from? In fact I went on to explain the effect of looking at different sized displays.

QuoteQuote:
This is where the crop factor enters; for the same display width the crop camera has a smaller sensor hence a larger display magnification.
QuoteQuote:
sometimes the desire to simply things adds confusion, your suggestion, while well intended, would do just that.
All I did was describe the sequence of magnifcations that leads to a displayed image. This was necessary I thought because the OP asked about crop factors and demonstrated using them without displaying an understanding of what was going on.

This confusion was exacerbated by Digitallis' posting of a pair of nice "1:1 magnification" macro photos of the same size - the displayed photos while 1:1 were clearly 50% different in overall magnification. How can "1:1 magnification" be 1.5:1 magnification at the same time?

If you hand an 8x10" print of a highly magnified flea to a random person s/he is likely to ask about the magnification --- what do you say?

Therefore I thought it important to make the two magnification steps in a displayed macro completely clear with simple math.

Perhaps you took my comment about how it is unfortunate that we use the same word "magnification" to describe two different operations. I think it is unfortunate but alas it is what it is!

I could have just said something like "two 1:1 magnification macros displayed at the same size will have an overall magnification ratio equal to the ratio of crop factors." Which, while precise and true offers little understanding of why it is true.
07-22-2011, 01:09 PM   #8
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How to determine magnification with any lens on any frame (film or digital):

1) Find out how wide your frame is. For FF, that's 36mm. On my K20D, that's 15.6mm.
2) Put a macro lens (or lens setup) on your (d)SLR. Put a metric ruler under a light.
3) At whatever magnification you've chosen, take a sharp picture of the ruler.
4) Count how many mm's of the ruler you've captured. Divide that by the frame width.

And that tells you the magnification ratio. Pretty easy, eh?


Last edited by RioRico; 07-22-2011 at 03:33 PM.
07-22-2011, 01:46 PM   #9
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QuoteOriginally posted by Vranx Quote
the SMC-F 2.8/135 gives me a 0.25 maximum magnification in closest focussing distance on a film body. If i pair it with a 2x teleconverter I would result in 0.5 magnification.
Let me try this again.

What macro photographers call magnification has nothing to do with crop factor or sensor size - it is just the relative size of the image on the sensor to the subject in real life.

Therefore when you mount your FF 28/135 lens on a crop body its maximum mag is 0.25 & when you put on a 2x TC the total mag is 0.5x. (magnifications multiply).

QuoteQuote:
But if I mount the lens on a digital body with 1.5 crop factor (without teleconverter), do I get 0.25 x 1.5 = 0.375 or is this the wrong way to compute it? The lens behaves like a 200mm lens on the crop camera.
With a tc on digital it would result in a nice IF nearly 1:1 macro lens (0.75), unless I messed up the maths here.

This step is wrong - the magnification on the sensor doesn't change - but the answer is right for the following reason:

Remember both crop and ff cameras have the same magnification image on the sensor. Now if you want to print these images at the same size the crop sensor 's image will have to be enlarged more than the ff's image to have the same print size. the ratio of enlargements will be the crop factor.

Therefore your numbers are right; the final print from the crop sensor will show an *overall magnification* of 1.5x that of the ff camera but only because you forced the *prints* to be the same size.

Dave

Last edited by newarts; 07-22-2011 at 01:53 PM.
07-22-2011, 03:01 PM   #10
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Dave. I hope you did not take too much offence to my comments, perhaps you are just part of the collateral damage, if so, sorry for the missile hit

It's just I hate any discussion that touches crop factor and final print size because these Are so far outside the control of lens makers that the whole issue of the simplified summary that was at the end of the post only would apply if we had no control over anything but the initial image on the sensor but regardless of format everything was uncroppable and printed to the same final size
07-22-2011, 03:27 PM   #11
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QuoteOriginally posted by Vranx Quote
the SMC-F 2.8/135 gives me a 0.25 maximum magnification in closest focussing distance on a film body. If i pair it with a 2x teleconverter I would result in 0.5 magnification.

But if I mount the lens on a digital body with 1.5 crop factor (without teleconverter), do I get 0.25 x 1.5 = 0.375 or is this the wrong way to compute it? The lens behaves like a 200mm lens on the crop camera.
If the train wreck above has confused you, just remember this simple rule: Magnification doesn't change with sensor size. If a lens provides 0.25X magnification (1:4 macro) on film or full-frame, it will provide the same magnification on a cropped sensor.

Adding a TC makes a compound lens with higher magnification than the original, but the lens + TC will give the same 0.5X magnification on a cropped sensor as it does on film. Dave and Lowell agree with that, the battle is semantic. ;-)

Last edited by audiobomber; 07-23-2011 at 08:58 AM.
07-22-2011, 03:37 PM   #12
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QuoteOriginally posted by audiobomber Quote
If the train wreck above has confused you, .....;-)
We need a better metaphor . Anyone?????
07-22-2011, 03:49 PM   #13
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QuoteOriginally posted by Lowell Goudge Quote
We need a better metaphor . Anyone?????
It isn't far from a train wreck Lowell, it is confusing stuff as a result of the widely spread oversimplified common usage of "crop factor" as yielding "equivalent focal length" neglecting to specify anything about display or framing.

That, plus the difference between everyday understanding of magnification and its precise meaning in photography.

Go in peace you all,
Dave
07-22-2011, 05:21 PM   #14
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It might be a train wreck, but at least the metaphor could relate more to photography? How about light at the end of the tunnel? But I guess first we need to know if it is an oncoming train (and whether we should catch it or not)

Damm there I go again relating to trains
07-23-2011, 08:45 AM   #15
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QuoteOriginally posted by Lowell Goudge Quote
It might be a train wreck, but at least the metaphor could relate more to photography? How about light at the end of the tunnel? But I guess first we need to know if it is an oncoming train (and whether we should catch it or not)

Damm there I go again relating to trains
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