Question for the Technical Gurus

[Mike Buckley]

Please see the screenshot shown below of an online calculator. The macro lens I use is capable of producing a 1:1 magnification ratio. However, I made a photo with the lens not fully extended, which would have resulted in a ratio of about 1:1.6 if the extension tube had not been mounted. So, I completed the third field in the calculator with the number 0.6 (1:1.6=0.6). In that situation, is it likely that the results produced by the online calculator are reasonably close to accurate?

I'm only curious about the resulting magnification ratio. It's not a factor in anything about how I choose to make a photo.

[george013]

Please see the screenshot shown below of an online calculator. The macro lens I use is capable of producing a 1:1 magnification ratio. However, I made a photo with the lens not fully extended, which would have resulted in a ratio of about 1:1.6 if the extension tube had not been mounted. So, I completed the third field in the calculator with the number 0.6 (1:1.6=0.6). In that situation, is it likely that the results produced by the online calculator are reasonably close to accurate?

I'm only curious about the resulting magnification ratio. It's not a factor in anything about how I choose to make a photo.

The macro reproduction ratio is the maximal magnification your lens can archive at the closet distance. I don't know this terminology, but I read that from the magnification numbers behind it.
Even if you focus on a longer distance and archive a smaller magnification, the 90mm stays 90mm.
What you calculated is for a lens of 90mm and a max. magnification of 0.6, and than adding 36mm.

Don't rely on those calculators blind. Macro lenses can change focal length at max magnification.

So if you've a 90mm lens with a max magnification of 1, then you should fill 1 in the field.

Using tubes places the focal point before the sensor when focused at infinity. It's quite easy explained with the simple lens formula and a diagram.

George

[Shadowman]

In reading my Kenko mini manual there is a formula for calculating the magnification ratio: length of Ex-tube + Master Lens Advance volume Image/Master Lens Focal Length.

The Master Lens Volume is defined ad Master Lens Focal Length2/Master Lens Distance Reading-Master Lens Focal Length

[Mike Buckley]

George: If I change the third field to a value of 1, the resulting ratio is 1.4:1. Are you saying that calculation is likely to be more accurate (recognizing of course the caveat you mentioned in your first post)?

[Mike Buckley]

John: I would appreciate it if you could point me to the mini manual that you mentioned. I can't find it on the Internet and, if one was provided with my product, I apparently threw it away, which would have been extraordinarily unusual.

[george013]

I borrowed a picture from Wikipedia. https://en.wikipedia.org/wiki/Lens_(optics)


What's meant with the max magnification of a macro lens is how far can the lens in it's barrel been extended. If you've a M=1, then that means the lens can be extended with a distance equal with the focal length. In that case you've an object distance = image distance =2*focal length. Adding extra length between lens and camera extends the image distance. Your object distance must be shorter now to place the image on the sensor. It's all in the formula 1/f=1/o+1/i. The letters are different.

Another example. M=i/o, image distance / object distance. If you enlarge i, then M is growing.

George

Forgot the answer on your question. Yes. Not only more accurate, but the only right answer.

[Shadowman]

John: I would appreciate it if you could point me to the mini manual that you mentioned. I can't find it on the Internet and, if one was provided with my product, I apparently threw it away, which would have been extraordinarily unusual.

[Mike Buckley]

Thank you to George and John!

John: The document you provided seemed familiar, so I checked the one place it might be in my home and, indeed, it was there: it was attached to my warranty card and sales receipt.

[Shadowman]

Thank you to George and John!

John: The document you provided seemed familiar, so I checked the one place it might be in my home and, indeed, it was there: it was attached to my warranty card and sales receipt.

Whenever I purchase an accessory, lenses included; I usually just attach and go or look for online tutorials. Those tucked away documents can sometimes hold the key to fully utilizing your gear.

[Mike Buckley]

Absolutely, John!

This is the only time I've bought a product when I didn't store the manual shipped with it in the box. That's because it's also the only product that I store in the box. I store it there because the box is so handy for storing the extension tubes in my makeshift studio. Now that I have thought about it, I think I decided not to store the mini manual in the box because I feared doing so would cause problems when taking the tubes from and returning them to the box; it's a small box with little room to spare, as you know.

[Shadowman]

Absolutely, John!

This is the only time I've bought a product when I didn't store the manual shipped with it in the box. That's because it's also the only product that I store in the box. I store it there because the box is so handy for storing the extension tubes in my makeshift studio. Now that I have thought about it, I think I decided not to store the mini manual in the box because I feared doing so would cause problems when taking the tubes from and returning them to the box; it's a small box, as you know.

The box has been where I've been keeping mine, now I have a copy on my computer and on this forum.

[Mike Buckley]

When using the online calculator correctly as George explained how to do and when using the formula provided in the Kenko mini manual, the resulting magnification ratios are the same. I'll create a spreadsheet that makes it easy to use the formula in the mini manual and compare the two methods going forward for at least awhile.

Thanks again to John and George!

[george013]

When using the online calculator correctly as George explained how to do and when using the formula provided in the Kenko mini manual, the resulting magnification ratios are the same. I'll create a spreadsheet that makes it easy to use the formula in the mini manual and compare the two methods going forward for at least awhile.

Thanks again to John and George!

Just keep in mind that when mentioning the max magnification of a macro lens, that is done on the shortest focus distance, so at the maximal extension of that lens. That's what the calculator needs to know.
If that's known, than it can calculate with extension tubes.

If you add extension tubes not only the image distance is enlarged, but also the focal point is moved away from the sensor. You can't focus at infinity anymore.

When adding extension tubes also the aov is changed. That aov is not only important for what image the sensor can capture, but also for the exposure, or dof, or diffraction. Many calculators will ask for and/or calculate with the effective F-number. Basicly that's a recalculation of that F-number but than based on the actual image distance in stead of the focal length.

If you buy a macro lens, than mostly is mentioned the shortest focus distance and the magnification. The focus distance is the distance from subject to the film/sensor plane. With a M=1, that should be 4xfocal length.

Your working distance is the distance between lens and subject. It's the space that"s left to move a box between them as I mentioned ones before. When using a lens hood that's changing.

Good luck.

George

[pnodrog]

I am a bit doubtful about the above calculator as it maybe based on a thin lens. Your 90mm macro at 1:1 is probably 300mm but should be 360mm using thin lens calculations.

Here is a link that may give you better results but you will have to know the actual focus distance of your when working at x0.6 rather than the 300mm at x1.0

I am outside my technical comfort zone but I think it is worth you checking.

Tested link and it did not seem to work will test again but here is the ref www.flybacon.com/cameras/macro.aspx.

[george013]

I am a bit doubtful about the above calculator as it maybe based on a thin lens. Your 90mm macro at 1:1 is probably 300mm but should be 360mm using thin lens calculations.

Here is a link that may give you better results but you will have to know the actual focus distance of your when working at x0.6 rather than the 300mm at x1.0

I am outside my technical comfort zone but I think it is worth you checking.

Tested link and it did not seem to work will test again but here is the ref www.flybacon.com/cameras/macro.aspx.

All calculators are based on the thin lens formula. So is the one you mention.

Don't forget the main numbers in those formulas are 1)the focal length and 2) the minimum focus distance as it's mentioned in your link. These two figures are properties of the lens, not selected values by the photographer. Adding the magnification you can check if the nominal focal length still is used.
I don't know what it means: dof at infinity? It's physical impossible to focus at infinity when using an extension tube. Or magnification at infinity?????

In Mikes example focal length = 90, extension tube = 36. At infinity the focus point would be on the sensor plane, but with that extension tube it's 36mm before it. So the image distance will be 126.
Thin lens formula 1/f=1/o+1/i. Or 1/90=1/o+1/126. This gives an object distance of 315mm from the optical middle of the lens. From the sensor plane it will be 315+126=441mm. Far away from infinity.

Or is meant the lens focused physical at infinity, so working with a fixed imaged distance of 36mm? Looking at the list again it gives the values of the lens when turned in max distance and minimum distance.

George

[pnodrog]

All calculators are based on the thin lens formula. So is the one you mention.

Don't forget the main numbers in those formulas are 1)the focal length and 2) the minimum focus distance as it's mentioned in your link. These two figures are properties of the lens, not selected values by the photographer. Adding the magnification you can check if the nominal focal length still is used.
I don't know what it means: dof at infinity? It's physical impossible to focus at infinity when using an extension tube. Or magnification at infinity?????

In Mikes example focal length = 90, extension tube = 36. At infinity the focus point would be on the sensor plane, but with that extension tube it's 36mm before it. So the image distance will be 126.
Thin lens formula 1/f=1/o+1/i. Or 1/90=1/o+1/126. This gives an object distance of 315mm from the optical middle of the lens. From the sensor plane it will be 315+126=441mm. Far away from infinity.

Or is meant the lens focused physical at infinity, so working with a fixed imaged distance of 36mm? Looking at the list again it gives the values of the lens when turned in max distance and minimum distance.

George

You're probably right. I just preferred it because you can enter a bit more information and the results looked good. If it was something I was particularly interested in I would take the time to delve into it and do some tests.

[Mike Buckley]

Paul: I wouldn't use the online calculator you found because the Nikon cameras to choose from were discontinued so long ago that neither of my cameras, which have also been discontinued, are on the list.

For anyone interested in the online calculator displayed in the first post of the thread, you can find it here.

[Shadowman]

Thanks for the link.

[DanK]

The standard equations aren't accurate with macro lenses because the nominal focal length is incorrect. You will generally obtain higher magnification than the equation gives you. You can find an interested experiment and explanation from "Lord V"--Brian Valentine, a superb macro photographer--here.

[Mike Buckley]

I'm not hoping to calculate the exact magnification ratio; instead, I'm looking for something that is just an approximate guideline. As an example, I made a photo when the macro lens was not at all zoomed to its largest magnification. Then when I mounted the extension tube, I wondered if the resulting magnification would be greater than 1:1. Using the calculator, it's easy to determine that it is, which is the only information that interested me.