Newbie Macro Question re: MFD and MWD

[canoli]

I was looking at the reviews for a couple Macro lenses, and they all spec out an MFD and a Minimum Working Distance.


I understand the MFD, as all lenses have that, but I don't understand what a working distance is. It's usually quoted as approximately half of the MFD. How does that work? I thought the MFD is the limit - meaning you the glass won't focus any closer. Obviously I'm missing something...


Thanks for any help you can give me. I can wait to try some macro shots, but I need a good lens first, and this MWD seems to be an important part of a lens' capabilities.


Thanks again!

[STL]

MFD is the distance between the sensor and the subject. Minimum Working Distance is the distance between your lens' front element and your subject when the lens is set at 1:1 magnification.


For longer lenses, MWD might indeed be half the MFD, because the longer reach of the lens will allow you to get further away from your subject.


For example, my Sigma 50mm Macro MWD is about 2 inches, while the MFD is around 9 inches (all of these figures at 1:1 magnification). I won't be able to snap a bug's picture with such a small MWD, but it is more than enough for flowers and electronic circuits (the reason why I bought the lens in the first place, as I am an Electrical Engineering student).


Take for example this macro shot of a tulip :


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f/8, 1/400 s, ISO 200, Sigma 50mm f/2.8 DG EX Macro on Rebel XTi


Even if this was taken at about 1:2 magnification on a 1.6x body (that makes the 50mm behave like a 80mm), I was CLOSE to the flower. No insect would have stayed there with me coming this close to it with the lens.

[canoli]

wow, nice shot! Those are the type of shots I want to start getting.


So thanks for the explanation - I think it makes perfect sense now - it's the length of the lens, which I completely ignored while trying to figure it out on my own. I forgot that the MFD is from the sensor to the subject - duh.


So the minimum focus distance is always going to be greater than the minimum working distance - because of the lens barrel. Got it. Thank you!


One thing I don't get though is - why did you specify 1:1 magnification? Doesn't this formula hold true for different magnifications?


sorry if that opens up a whole different discussion that you'd rather not get into. Magnification numbers like 1:1 are easy - subject is life size on the sensor - but magnifications like 1:5.8 (the mag spec for the 70-200 2.8) baffle me.


Thanks again for your help - I'm looking forward to buying one of the macros tomorrow!

[STL]

One thing I don't get though is - why did you specify 1:1 magnification? Doesn't this formula hold true for different magnifications?


sorry if that opens up a whole different discussion that you'd rather not get into. Magnification numbers like 1:1 are easy - subject is life size on the sensor - but magnifications like 1:5.8 (the mag spec for the 70-200 2.8) baffle me.





At 1:1 magnification, the lens is focussing as close as it can. This is why the working distance is so low. When you reduce magnification, you in essence "reduce" a bigger subject so that its image is now the size of the sensor. You must step back to allow the lens to focus on this bigger subject, so the working distance increases. The MWD is the minimum working distance, this is why I stated the lens is at maximum magnification. The working distance of a lens focussed at infinity (generally past 5-10 meters) is... infinity !


1:5.8 means the size of the subject will be reduced 6 times at most, so you can expect to fill the frame with a subject that is around 144 x 216mm on a full-frame camera body (5D, 1Ds) when the lens is focussing as close as its optics allows. You will not be able to fill the frame with a smaller subject; you will need to crop the picture somewhat if you wish to do so, losing resolution in the process.


You can cheat a little bit by adding extension tubes (empty tubes that present a male and a female EF mount at each end and that is put between the body and the lens). These tubes allow the lens to focus closer, but the difference in magnification depends on the focal length of the lens they are used with.

[Jon Ruyle]

Even if this was taken at about 1:2 magnification on a 1.6x body (that makes the 50mm behave like a 80mm), I was CLOSE to the flower. No insect would have stayed there with me coming this close to it with the lens.


You might be surprised. Some bugs let you get *very* close, some don't.

[STL]

You might be surprised. Some bugs let you get *very* close, some don't.


...dead ones ? [:P]


My girlfriend is looking to buy the Canon 100mm Macro. When she does, I might venture in bugs photography, though. For now, I prefer flowers.

[Jon Ruyle]

...dead ones


[:)]


Sure, dead ones let you get close. But I was actually talking about living bugs. Sometimes they won't move unless you hit them over the head with the lens. (And if you do that, they don't move either- as you cleverly pointed out).

[canoli]

whew! thanks for that.


I'll need to read that over a few times - but I really thank you - finally, after reading that short paragraph a couple more times I'll understand the magnification factor.


grazie!

[canoli]

Thanks STL, I think I'm getting a handle on the focusing distance questions I had.


I'd like to ask you about this idea. You wrote:


"you will need to crop the picture somewhat if you wish to do so, losing resolution in the process."


I've seen this phrase a number of times, and it always seems to imply the image is degraded somehow.


I always think of resolution as a camera system's ability to capture detail, but you're not referring to that here, are you? You mean "lose resolution" in the sense that you're throwing away pixels, right?


The lp/mm - the level of detail that was captured - stays the same. There's no loss in the image's quality, only in the quantity of pixels. Is that true?


Thanks again for sharing your knowledge - it is much appreciated!

[STL]

I always think of resolution as a camera system's ability to capture detail, but you're not referring to that here, are you? You mean "lose resolution" in the sense that you're throwing away pixels, right?


Yes. If you crop, you lose pixels. You could use bicubic interpolation in PS to increase the resolution by generating additional pixels between the original ones, but the resulting picture will never be as sharp and clear as a full resolution image.






The lp/mm - the level of detail that was captured - stays the same. There's no loss in the image's quality, only in the quantity of pixels. Is that true?





The captured level of detail, yes. However, if you print your full resolution image (say on a 4x6 or a 8x10), you will be able to get a much higher PPI (Pixels Per Inch) rate than if you use a cropped image. If you print a small picture, the difference may not show up, but the picture will appear more degraded as you move to a bigger paper.


As I never printed my pictures so far (I put them on my website), I don't know the optimal PPI rate. Maybe another photographer here will be able to tell you.

[canoli]

Ah! Good, thank you, that explains it.