New Tutorial: Macro Lenses

[McQ]

Another new tutorials page has just been (mostly) finished. It's not yet accessible anywhere else on this website, so you'll have to use the direct link below:

Macro Lenses: Magnification, Effective F-Stop & Depth of Field

It's a reasonably technical article that will likely be added as an advanced tutorial. In other words, it's intended more as a background on terminology and how a macro lens works -- as opposed to being a "how to" for taking macro photographs. I'm planning two other articles to accompany this one: one on extension tubes & close-up lenses/filters, and the other will be an intro to macro photography (for the techniques section of the website).

Also, many thanks to Piotr Naskrecki for letting me use some of his macros from exotic locations. He's the author of The Smaller Majority and has a website at www.insectphotography.com

As usual, comments/typos/suggestions/etc are all welcome. I'm still in the process of editing it so there may be some minor changes/fixes over the next few days...

[Dave Humphries]

Hi Sean,

In last sentence of paragraph above diagram in Lens Extension and Effective F-stop, it says
"At 1:1 magnification, the lens becomes twice the focal length from the camera's sensor, in addition to being twice the focal length from the subject:"

It wasn't until I pasted that here, that I now see there is a difference between the two halves; one says "camera's sensor" and the second says "subject" - I think that might warrant a rewording to remove the identical italicised bits.

In the paragraph that follows, it states that "because cameras often still show the uncompensated f-stop setting" - I'm fairly certain my new Nikon Macro lens does compensate; it displays f3.8 at 1:1 rather than f2.8 at infinity (when wide open). However, that doesn't sound like enough compensation when reading; "A rule of thumb is that at 1:1 the effective f-stop becomes about 2 stops greater than the value set using your camera", have Nikon under cooked it?

Another installment later if I find anything else, must go.

Thanks,

[rick55]

Hi, Sean;

You're really on a tear lately! Another wonderful tutorial. I'm delighted to see this one, since I've been fiddling with an extension tube, and I'm convinced I need a real macro lens. So I'm learning a lot from this. I'll mostly only be able to make wordsmithy kinds of comments.

In "Magnification & Sensor Size," it says, "what is the smallest object that I can photograph?" I think it may be more clear to cast that as "smallest object that will fill the frame." Because, of course, it's always a trade-off, since as you clearly show later, the FF sensor will be able to go to a smaller aperture before succumbing to diffraction, giving a better DOF, which can be a big plus, as you also explain later.

In the "Effective F-Stop" section, it says, "For example, most SLR cameras lose the ability to autofocus when the maximum f-stop becomes greater than f/5.6." Wouldn't that be minimum f-stop? Or maximum aperture?

It's wonderful to have all the calculators in there based on the magnification instead of the effective f-stop. For me, wanting to get more into macro, this is much better than trying to double convert. Kudos!

Nitpick: in the "Working Distance" section, there's a slight discrepancy in the definition of "closest focusing distance" compared to the note in the "Magnification" section at the very beginning. In the note, it says some manufacturers "use the sensor to subject distance, while others measure from the lens's front or center" (my bold). Here it says, the closest focusing distance is "measured from the camera's sensor or the optical center of your lens."

Cheers,
Rick

[McQ]

In last sentence of paragraph above diagram in Lens Extension and Effective F-stop, it says
"At 1:1 magnification, the lens becomes twice the focal length from the camera's sensor, in addition to being twice the focal length from the subject:"

It wasn't until I pasted that here, that I now see there is a difference between the two halves; one says "camera's sensor" and the second says "subject" - I think that might warrant a rewording to remove the identical italicised bits.

I'll just remove the second part of that sentence since only serves to dilute the real point I'm making in the first half.

In the paragraph that follows, it states that "because cameras often still show the uncompensated f-stop setting" - I'm fairly certain my new Nikon Macro lens does compensate; it displays f3.8 at 1:1 rather than f2.8 at infinity (when wide open). However, that doesn't sound like enough compensation when reading; "A rule of thumb is that at 1:1 the effective f-stop becomes about 2 stops greater than the value set using your camera", have Nikon under cooked it?

Yes, I've heard that the nikon cameras do indeed compensate. That was originally why I added the often, but I should probably downplay that to "many cameras" or "some cameras" since Nikon users are a substantial fraction of photographers...

The compensation shown in the Nikon camera might be more accurate depending on how it's estimated. If it measures the decrease in light compared to the max aperture at low magnification then it will be very accurate indeed. It could be that the pupil magnification is having a role here, but it's difficult to say. The rule of thumb for 1:1 magnification is pretty standard though (although standards can still be wrong).

Thanks for the feedback!

[benm]

I was playing with the Macro Depth of Field Calculator and noticed what may be unusual behavior. I plotted the magnification against the DOF. From a magnification of 2.4x down to 1x the DOF increases, as one would expect. At 1x the DOF is 1.2mm (using the 1.6x crop factor and f/2.8). But at magnification 0.95x the DOF is 0.3mm. This trend continues down to about magnification 0.2x where the values seem to as expected (or at least consistent - I don't know what actual values should be). See attached chart; magnification is the vertical axis and DOF is the horizontal axis.

[Andy]

Interesting tutorial Sean. My only gripe, if I can call it that, is that it covers <1x well but doesn't cover higher magnifications. DoF in milimetres? I often have to settle for DoF of about a tenth of a milimetre at 5x but I think it's more like a twentieth when trying to avoid diffraction in a stack, for example. (not that I'm overly fond of stacks personally)

Anyway, I just reckon DoF in microns sounds more impressive

Have tons of shots of springtails and other wee beasties if you're interested? I rarely shoot anything but macro.

(40D, MP-E 65mm and MT-24)

[McQ]

I was playing with the Macro Depth of Field Calculator and noticed what may be unusual behavior.

Thanks for pointing it out. This should be fixed now. The javascript formula was correct, but for some reason it wasn't treating the inputs as proper decimal numbers...

[McQ]

DoF in milimetres? I often have to settle for DoF of about a tenth of a milimetre at 5x but I think it's more like a twentieth when trying to avoid diffraction in a stack, for example. (not that I'm overly fond of stacks personally)

The calculator now outputs in units or microns if the magnification is greater than 1X. I could have just done it this way when the depth of field in mm is less than say 1 or 10 mm, but I figure that this might throw off some of the less than 1:1 readers. If you're doing more than 1:1, you're right, you'd better know what a micron means and this should be how you measure the depth of field

Thanks for the feedback!

[McQ]

In "Magnification & Sensor Size," it says, "what is the smallest object that I can photograph?" I think it may be more clear to cast that as "smallest object that will fill the frame." Because, of course, it's always a trade-off, since as you clearly show later, the FF sensor will be able to go to a smaller aperture before succumbing to diffraction, giving a better DOF, which can be a big plus, as you also explain later.

This has been updated.

In the "Effective F-Stop" section, it says, "For example, most SLR cameras lose the ability to autofocus when the maximum f-stop becomes greater than f/5.6." Wouldn't that be minimum f-stop? Or maximum aperture?

Updated. I really need to stop thinking in terms of aperture diameter when I use f-stops, and vice versa.

Nitpick: in the "Working Distance" section, there's a slight discrepancy in the definition of "closest focusing distance" compared to the note in the "Magnification" section at the very beginning. In the note, it says some manufacturers "use the sensor to subject distance, while others measure from the lens's front or center" (my bold). Here it says, the closest focusing distance is "measured from the camera's sensor or the optical center of your lens."

Woops...fixed as well.

Thanks for the feedback!

[JonathanC]

Picky one this.
Paragraph three "-- which makes sense because closer objects become larger." Perhaps "appear to become"?

[JonathanC]

Another nitpick, beneath the magnification calculator:-
**If using a full frame lens on a cropped sensor, you will need to use a focal length multiplier. Otherwise just use the actual lens focal length (without multipliers).

I think that the image magnification remains the same for cropped sensors, it just doesn't fit as much on it?

[JonathanC]

Sorry for appearing like the biggest pedant ever, under Working Distance and Focal length, "This is different from the closest focusing distance, which is instead (usually) measured from the camera's sensor or the subject." "or" perhaps should become "to"?

[McQ]

Another nitpick, beneath the magnification calculator:-
**If using a full frame lens on a cropped sensor, you will need to use a focal length multiplier. Otherwise just use the actual lens focal length (without multipliers).

I think that the image magnification remains the same for cropped sensors, it just doesn't fit as much on it?

This is a tough one. Yes, technically the imaging circle remains unchanged. On the other hand, the captured image surely does. I guess one could argue the definition of magnification either way, but I decided to keep it consistent with the "real size vs size on sensor" definition as opposed to "real size vs imaging circle size." I think the former is probably more meaningful and practical for use in the field.

Thanks for all the feedback in the other posts! All of this has been changed.

[JonathanC]

This is a tough one. Yes, technically the imaging circle remains unchanged. On the other hand, the captured image surely does. I guess one could argue the definition of magnification either way, but I decided to keep it consistent with the "real size vs size on sensor" definition as opposed to "real size vs imaging circle size." I think the former is probably more meaningful and practical for use in the field.

Thanks for all the feedback in the other posts! All of this has been changed.

Sorry if I was unclear earlier, but what I was trying to point out is that the magnification (image size : actual size) has nothing to do with the sensor at all. The sensor format makes what you can capture different, but that relates to FOV not magnification. If we are relating the "effective" focal length using a multiplication factor for crop sensors, then that again relates to FOV, not the dimensions of the projected image.

If we use an example of a 50mm lens at 100mm from the focal plane to give a 1:1 correspondence, then the in-focus bit of the target (at 100 mm from the lens) will be the same size as it is in real life. If we allow a crop factor of 1.5, then there's no way that the image size will vary because nobody told the lens what sensor size we are using. Hope that makes sense?

BTW keep up the good work, these tutorials are excellent.

[McQ]

Sorry if I was unclear earlier, but what I was trying to point out is that the magnification (image size : actual size) has nothing to do with the sensor at all. The sensor format makes what you can capture different, but that relates to FOV not magnification. If we are relating the "effective" focal length using a multiplication factor for crop sensors, then that again relates to FOV, not the dimensions of the projected image.

If we use an example of a 50mm lens at 100mm from the focal plane to give a 1:1 correspondence, then the in-focus bit of the target (at 100 mm from the lens) will be the same size as it is in real life. If we allow a crop factor of 1.5, then there's no way that the image size will vary because nobody told the lens what sensor size we are using. Hope that makes sense?

BTW keep up the good work, these tutorials are excellent.

Yes, I understand what you're saying. I understand that magnification described the size of the imaging circle vs the actual object size. My only concern is that this can be misleading, because the image captured at the sensor has *effectively* been magnified when using a full frame lens on a cropped sensor. How about this:

I've removed the qualifier sentence below the calculator on focal length, but added the following technical note at the end of the section:

Note on cropped sensors: If you're using a full frame lens on a cropped sensor, the light captured at the sensor will appear more magnified than if it were captured using a full frame 35*mm sensor -- even though the focal length is the same. This is just because the smaller sensor crops out the exterior regions of the image -- not because the lens has magnified the image. However, if you want to know the effective magnification above, then a focal length multiplier can be used -- but only for full frame lenses on cropped sensors.

All of this requires a lot more sentences than I'd like (especially for the introductory section on magnification), but it's definitely now technically accurate.

Thanks for all the careful reading.

[Peter Ryan]

From someone who enjoys macro photography but is not as technical as others here I did very much appreciate the article and it helps explain and reinforce my understanding of this topic. I use a 180m Tamron macro lens on a Nikon body and I love it. I use it for small creatures but love it in the rainforest with fungi and small growth on the forest floor and the side of tree ferns, etc.
This is one of the reasons I joined this form and I am never disappointed.
Thanks.