Home · Register · Join Upload & Sell

Moderated by: Fred Miranda
Username  

FM Forum Rules
Macro World Resource
  

FM Forums | Macro & Still Life | Join Upload & Sell

  

Archive 2014 · Newbie question on relationship between MM and MFD

  
 
lowside67
Offline
• • •
Upload & Sell: Off
p.1 #1 · p.1 #1 · Newbie question on relationship between MM and MFD


Hello everybody,

I am not looking at macro lenses specifically figured this may be the best spot for this question as I know these topics are very relevant in selecting a macro lens.

What I am confused about is the relationship between maximum magnification (MM) and minimum focal distance. I believe I understand the concept of each, that is that maximum magnification is how large an image is on the sensor, typically measured at the MFD and the longest focal length (if a zoom lens). I understand that MFD is how close the sensor can be to the object in question.

The real-life example that I don't understand is the Canon 24-70mm 2.8L I vs II.

Per Canon and many review sites, the original 24-70 2.8L has a MFD of 15" (38cm) and a MM of 0.29x. So presumably, this MM is observed by setting the lens to 70mm, and then moving as close to the object as possible (in theory 1.25' or 0.38m from the sensor item).

However, also per Canon and similar review sites, the revised version II of this lens has an EQUAL MFD of 15" (38cm) but only has a MM of 0.21x. How is it possible that you take a different lens at the same focal length, positioned at the same MFD, and yet the image appears as a different size on the sensor? (I would think the SIZE of the image on the sensor, i.e. the MM, is only affected by those two variables - focal length and distance to the subject)

Thank you for helping me understand! I am looking for a versatile multi-purpose lens but enjoy food photography and while a true macro lens is not needed, having a higher MM would be useful and is why I am trying to understand this.

Regards,
Mark



Dec 15, 2014 at 09:46 AM
Dalantech
Offline
• • • • • •
Upload & Sell: Off
p.1 #2 · p.1 #2 · Newbie question on relationship between MM and MFD


The quick and dirty answer is that it depends on the optics used and how the lens is designed. MFD and focal length are mutually exclusive.


Dec 15, 2014 at 12:10 PM
lowside67
Offline
• • •
Upload & Sell: Off
p.1 #3 · p.1 #3 · Newbie question on relationship between MM and MFD


That still doesnt make sense to me. Taking MFD out of the equation for a moment, wouldnt you agree that if I put a camera on a tripod and shot a stationary object with both versions of the lens at 70mm, they would show the exact same framing of an image? If this is true, then they must render that object with an equal size on the sensor.

Then to bring MFD back into it, if they both have the same MFD, then in theory you could get equally close with both lenses, i.e. 15" from the object to the sensor. If you have two 70mm lenses that are 15" from an object, they should frame the object the same size, right?

Mark



Dec 15, 2014 at 01:11 PM
Dalantech
Offline
• • • • • •
Upload & Sell: Off
p.1 #4 · p.1 #4 · Newbie question on relationship between MM and MFD


lowside67 wrote:
That still doesnt make sense to me. Taking MFD out of the equation for a moment, wouldnt you agree that if I put a camera on a tripod and shot a stationary object with both versions of the lens at 70mm, they would show the exact same framing of an image? If this is true, then they must render that object with an equal size on the sensor.

Then to bring MFD back into it, if they both have the same MFD, then in theory you could get equally close with both lenses, i.e. 15" from the object to the sensor.
...Show more

Nope. Depends on the optics used and how the lens is designed.



Dec 15, 2014 at 01:30 PM
jcolwell
Offline
• • • • • • •
Upload & Sell: On
p.1 #5 · p.1 #5 · Newbie question on relationship between MM and MFD


The lens specs are based on focal length "at infinity" focus distance. As you reduce focus distance, the focal length usually also reduces. So, I expect that the focal length of the Mk II reduces more than that of the Mk I, as you reach MFD. Focal length and distance are key parameters for determining magnification, but it's actual focal length at the working distance, not nominal focal length at infinity.

Google "relationship between focal length and magnification" and you can calculate the actual focal lengths of the two lens versions, from their MFD and magnification at MFD.

Edited on Dec 15, 2014 at 03:53 PM · View previous versions



Dec 15, 2014 at 02:24 PM
lowside67
Offline
• • •
Upload & Sell: Off
p.1 #6 · p.1 #6 · Newbie question on relationship between MM and MFD


Interesting, still trying to wrap my head around it. Since the version II has the lower MM, wouldn't that mean that the focal length actually reduces more than the version I, which would give it a lower effective length at MFD, which would make the image smaller, therefore the lower MM?

Thanks for the input to both of you.

-Mark



Dec 15, 2014 at 02:56 PM
jcolwell
Offline
• • • • • • •
Upload & Sell: On
p.1 #7 · p.1 #7 · Newbie question on relationship between MM and MFD


Yes. I didn't read the MM values correctly. Thanks.


Dec 15, 2014 at 03:53 PM





FM Forums | Macro & Still Life | Join Upload & Sell

    
 

You are not logged in. Login or Register

Username       Or Reset password



This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.