Options to increase magnification when the minimum working distance is fixed

Question

I work in a lab which deals with a significant amount of scientific photography. My current situation calls for an increase in magnification, however, there is a physical limit to how close the lens can be to the object. This imposed working distance is far greater than the minimum working distance of the lens itself.

Lens: Nikon AF Micro 60 mm; Working distance at closest focus: 90 mm; Physical minimum working distance: 280 mm; Magnification at physical minimum distance: 0.2

In previous applications with a Nikon 210 mm lens, the close focus distance was several feet, so it was feasible to use a close-up lens and increase magnification by moving the camera closer. This is not the case here. My primary questions are:

1) Would extension tubes be of any benefit here, since the camera/lens cannot be moved closer to the object? Stated in a general way, if the distance between the lens and object remains fixed and an extension tube is added, would the magnification increase? I am aware of the rule that magnification increases by the length of the tube divided by lens focal length, but does this assume that the lens is moved closer to the object?

2) If extension tubes will not help, are teleconverters the only other option? I have used a 3x teleconverter in the past with the above mentioned 210 mm zoom lens.

I would like to get the magnification close to 1:1, and if I was able to fine-tune the magnification, this would be an asset. The camera and lens have plenty of room to move away from the object, if necessary.

Edit: additional details on the application below:

The camera is tasked with capturing laser-illuminated oil droplets suspended in moving air (a technique known as particle image velocimetry). This airflow is contained in a sealed spherical chamber with 4 windows. The camera must be placed on the outside of this chamber for obvious reasons, and therefore it can only be moved up so far before the front lens element bumps into the window. When the camera hits the window, the front lens element is 280 mm away from the "object" (the flow field of interest).

Thanks

Answer 1

1) Would extension tubes be of any benefit here, since the camera/lens cannot be moved closer to the object? Stated in a general way, if the distance between the lens and object remains fixed and an extension tube is added, would the magnification increase? I am aware of the rule that magnification increases by the length of the tube divided by lens focal length, but does this assume that the lens is moved closer to the object?

No, extensions tubes would not help at all. Extension tubes do not change the optical characteristics of the lens, they merely move the camera further away from the lens. This lets the lens focus closer than it could before, which has the corollary effect of increasing magnification. So the extension tubes' magnification works at the expense of subject distance — the exact opposite of what you're trying to achieve.

2) If extension tubes will not help, are teleconverters the only other option? I have used a 3x teleconverter in the past with the above mentioned 210 mm zoom lens.

Theoretically, teleconverters could be an option, but you have to understand that teleconverters also do not change the optical characteristics of the lens they are attached to; they merely magnify the center of the image circle that is projected onto them. So without moving the lens at all, if you added three (3) 2× telconverters to your lens, you could theoretically achieve an 6 * 0.2 = 1.2:1 magnification, at the lens's minimum focus distance of 90mm. If you focus the lens a bit further away (exactly how far away, I can't say), you can bring down the magnification to 1:1, and buy yourself some more working distance. If you had more teleconverter power, that would buy you more magnification, which would result in more leeway to move the system further away from the subject.

However, stacking 3 or more teleconverters will result in pretty substantial image quality loss. Not to mention the light loss of 2 stops of light per 2× teleconverter = 6 stops. Assuming you don't have the luxury of increasing the shutter duration by a factor of 64, and also assuming you're working at or near the widest aperture your lens will allow, the only ways you make up for that loss is to crank the ISO by 6 stops (say, from 100 to 6400), or add a whole lot more illumination to the subject being photographed. But because you said you're capturing laser-illuminated droplets, I assume you can't increase the laser power by a factor of 64.

So really, the best option is simply use a longer focal length macro lens. While it's not cheap, the AF Micro-Nikkor 200mm ƒ/4D IF-ED can be rented for around $100 for 7 days at places such as LensRentals.com. It has a 1:1 reproduction ratio a MFD of 1.6 ft (488mm). You will be much happier with the results than trying to "lego" together a bunch of teleconverters to the back of a smaller lens.

Edit: Even with the AF Micro-Nikkor 200mm lens, you probably won't be able to achieve 1:1 magnification at 288mm. The "minimum focus distance" (MFD) of lenses are the subject-image distance (i.e., from the thing you're capturing, to the image sensor plane). The working distance is the MFD, less the lens's length, less the additional distance between the lens to the sensor (roughly, the "camera body thickness"—this is not a precise description of that distance).

Unfortunately, lenses' overall lengths tends to include the bayonet mount and additional electrical bits sticking out, so it's not as simple as just accounting for flange focus distance (FFD) for a particular lens mount. But for Nikon F-mount systems, the electrical contacts stick into the body about 6 mm, so the the distance between lens and image sensor is about the F-mount's FFD (46.5 mm) less 6, so about 40.5 mm. Thus, the actual 1:1 working distance of the AF Micro-Nikkor 200mm is 488 – 40.5 – 193 (the lens's length) = 254.5 mm

In order to make up for the missing ~34 mm, a low-power teleconverter (or low-diopter close-up lens, such as StephenG talks about in his answer) (such as the Canon 500D, which is just a +2 diopter), would be more than sufficient to give you 1:1 magnification, and also increase your working distance.

Answer 2

If you are unable to obtain a suitable macro lens, you may be able to achieve what you desire by using both teleconverter and extension tube (or diopter filter).

• Extension tube alone would allow you to focus closer, but you specifically state that you are restricted from moving closer to the subject.
• Teleconverter would increase magnification, but you may lose focus at the distance you require. So an extension tube or diopter would be required to compensate. To preserve image quality, use the teleconverter with the least magnification that satisfies your needs.

Answer 3

Close up lenses

I suspect from your comments that you might manage with a good close up lens. These are attached to a normal lens, typically like a filter, and act as magnifying glasses (basically). Good ones have reasonable corrections for aberrations.

While these will magnify, you also end up having to get closer, however the focal length of the lens you can use can be longer so you might still be able to get a useful balance with this combo.

A post on DPReview.com links to a long gone website which it quotes :

If you want to have good working distance, you should go for the 500D mainly on the 70-300. gives you about 1:1 at 40-50 cm working distance.

I've no personal experience of that combo but I used it's sibling the 250D (for shorter focal lenses and giving less working distance !) and it was great.

One I'd suggest looking into wold be a Canon 500D close up lens (note that just to confuse everyone, Canon had a camera model with the same name !). Because these are screw on lenses you can use them with any lens (although you have to either match your filter size or use an adapter to step up to a larger filter). The 500D is 77mm diameter if I recall correctly.

Here are some links about them.

Wikipedia page on close up lenses including some maths

A page about close up lenses includes example shots and figure

I will mention one idea which I am not seriously suggesting, but can sometimes work for some purposes (probably not yours) : Super resolution using computation algorithms and sometimes combining multiple images. These would not be ideal for serious scientific use except in very specific circumstances and for very specific purposes. State of the art algorithms would be ESRGAN and related GAN one, which generate higher resolution by using trained neural networks to make good estimates for the generated data. These all have pros and cons. Of more conventional approaches to squeezing resolution to the maximum, sometimes using a Sinc-type algorithm is better. None of these are perfect, but they have some applications.

Answer 4

Adding extension tubes/increasing FL increases the size of the image circle and the details within it at the sensor plane (it also reduces the light density). It provides the same relative increase in magnification regardless of the focus distance.

However, the primary benefit of using extension tubes is in allowing a shorter MFD. Because it is only at MFD that you get the maximum magnification from the lens. I.e. it would take a really long extension tube to make up for the increased subject distance. And the light loss due to bellows factor (image circle expansion) would be very significant.

Teleconverters do essentially the same thing; they expand the image circle at the image plane, resulting in an increased magnification and loss of light/light density. The disadvantage compared to extension tubes is that they use additional optics which are not (typically) optimized for that particular lens; which causes increased optical errors... stacking multiple TC's only compounds this factor. The advantage compared to extension tubes is that they are comparatively more effective at distances greater than MFD.

Answer 5

Your objective is to achieve magnification 1 = unity sometimes stated as 1:1.

To achieve, the lens placement is centered, object to image distance. The focal length of the lens used is object to image distance divided by 4.

You state the minimum camera to object distance is 280mm. I will bet, this is lens to object distance and not image to object distance. I will assume image to object distance is more like 300mm. I will work the problem for both 280mm and 300mm.

In both cases, to achieve M=1, the lens is positioned at the center of the object to image distance and the focal length of the lens for this task is this distance divided by 4.

For 280mm object to image setup: Lens focal length is 70mm Lens is positioned 140mm from the object and 140mm from the image.

For 300mm object to image setup: Lens focal length is 75 mm Lens is positioned 150mm from the object and 150mm from the image.

Note: The focal length of choice is ¼ of the object to image distance. Suppose, try as you might, this works out to a weird value like 288mm. This calls for a 72mm lens (not likely except as a zoom). Now purchase a 70mm and adjust it's focal length. We covert the 70mm value to diopter units thus 1/70 X 1000 = 14.2857 diopters (power of this lens).

You need a 72mm lens. Convert this value to diopters thus 1/72 X 1000 = 13.8889 diopters.

The difference in power is 14.2857 – 13.889 = 0.4 diopters.

You can procure a + .4 diopter or perhaps a +.5 from an optician. Add this supplemental lens to the 70mm and this will adjust it, making its focal length about 72mm.

Yes – this is a trial and error procedure but you can do this!