I'm very confused about magnification

[HiddenWorlds]

Hello all,

I'm not new to photography and I'd rate my knowledge of microscopy as that of an interested amateur. I'm having great difficulty understanding an apparent discrepancy I've found regarding magnification.

The microscope I use most is an old high school unit from 1982. It's a Swift 960, and it cost me a whopping $80 a couple of years ago. For that price, it's wonderful, and I find its fine focus wheel very easy to use, now that I'm somewhat practiced with it.

I do focus stacking with DSLRs, shooting and focusing manually, and then merging the photos in either Helicon or Zerene. I'm doing eyepiece projection photomicroscopy, which means I have a 50mm prime lens on my camera, and this is mounted onto the eyepiece of the microscope. It works quite well; it eliminates vignetting without wasting too much of the field of view.

Most of my photography these days is using the 10X objective that came with the microscope. The eyepiece is also a standard 10X objective. This should give me a magnification of 100X.

Recently I received a stage micrometer slide. I've taken pictures and recorded the measurements shown on the slide. To calculate the magnification I then divide the sensor width by the value of the micrometer within the field of view.

My sensor is 23.5mm wide, and with my setup configured as I normally use it, the micrometer slide shows a 1mm horizontal field of view in my photos. This makes the calculation very simple. I'm getting 23.5X magnification.

I've also tested this with the 4X objective on my microscope, which with the 10X eyepiece should result in 40X magnification. To my camera I get a field of view displaying approximately 2.4mm, yielding a magnification of approximately 9.79X.

When I remove my camera and adapter and I look through the eyepiece normally at "100X", the field of view that I see with my eye is not significantly larger than what the camera records. The imaginary rectangle representing the camera's field of view would approach the edges of the circle. I'm seeing a very similar magnification compared to what the camera records, hence my confusion.

Why do they call it "100X" when it's not even close? Or 40X when it's really more like 10X? Have I been thinking I've been looking at things on higher scales of magnification than I really have for most of my life due to this?

Thanks in advance for any help.

[tropicolympus]

My sensor is 23.5mm wide, and with my setup configured as I normally use it, the micrometer slide shows a 1mm horizontal field of view in my photos. This makes the calculation very simple. I'm getting 23.5X magnification.

Though by no means an expert, I would suggest that that's only if you view your captured images on a 23.5mm wide display - or a postage stamp-sized print.
What would the apparent magnification be if you viewed the image at a width of, say 100mm?

[HiddenWorlds]

That would be enlargement rather than magnification. You'd have a larger image with the same amount of detail evident, just as if you took your favourite photo and scaled it up. Additional magnification permits the capture of more detail, like finer hairs and particles. I appreciate you taking a stab at it though.

[Scarodactyl]

Your 50mm lens applies its own magnification (or reduction in this case) just as the human eyelens does. The end result is about a 2.3x magnification between the 10x eyepiece and the 50mm lens.

[HiddenWorlds]

The end result is about a 2.3x magnification between the 10x eyepiece and the 50mm lens.

You forgot about the objective lens. What I think you're suggesting would be 10*10*2.3 which would be 230X, and that's not happening.

My 50mm lens applies its own magnification, but it's not reducing 100X to 23.5X because when I look through the eyepiece with my eye I see nearly identical magnification to that which the camera records. I've experimented with different adapters, and adapting my camera lens at different heights from the eyepiece, using other lenses of differing focal lengths, and stuff like that. I've found that having the surface of the eyepiece extremely close to the front element of this lens produces the best results of my available options, and the magnification is most inline with what I see with my eye through the eyepiece. Are you feeling the confusion yet? I still am.

Another fun detail. I've tried this with a 200mm lens, as well. The magnification is certainly increased, well beyond what I can see in the eyepiece with my eye, but the contrast suffered and the depth of field is minimal, and I suspect diffraction was also a problem. It wasn't really usable beyond demonstrating there are limits to how far each part of a setup like this can be pushed. I'm eager to try it with a 100mm macro lens.

[Scarodactyl]

The end result is about a 2.3x magnification between the 10x eyepiece and the 50mm lens.

You forgot about the objective lens. What I think you're suggesting would be 10*10*2.3 which would be 230X, and that's not happening.

To clarify, the 50mm lens is giving a 0.23x magnification, so between it and the 10x eyepiece you are getting an additional 2.3x magnification on top of the objective's rated magnification on the sensor.
Your eyelens is something like 17mm. It applies a greater reduction to the image and as such the image doesn't cover your entire field of view.
Your 200mm lens is pushing the magnification up well past what the objective can deliver in resolution, so diffraction is probably the most pressing issue.

[HiddenWorlds]

To clarify, the 50mm lens is giving a 0.23x magnification

Before I try to reply further, can I ask how you're deriving this number? The max magnification of the lens itself is rated at 0.15X.

It's coincidental that the magnification is equal to my sensor width. If I focus the lens to infinity rather than focused all of the way in, I get ~21X.

[Scarodactyl]

It's an empirical measurement, since you're getting ~23x optical magnification with that configuration. Anyway, this is a fairly well known factor since this technique is used both in DIY setups and in some of the older lines from big name brands. Typically you'd want to lock your focus at infinity, though who knows what might work best on any particular system.

[HiddenWorlds]

It's an empirical measurement, since you're getting ~23x optical magnification with that configuration.

It's a measurement of the sensor width which I then divided by the field of view captured in millimeters.

I'm still not grasping why they call it 100X. What is that based on when neither my eye nor my camera can see it? And by that I mean I cannot see a significant difference in magnification whether I look through the eyepiece or use my camera with this configuration.

[Scarodactyl]

Your eyelens's magnification (or rather demagnification) just isn't included in the specs. It's not exactly removeable so it kind of makes sense. When official camera systems are made they do spec the total optical magnification, either by using a special photo eyepiece that projects direct onto the sensor (ie a 2.5x photo eyepiece) or by speccing the demagnification of the lens above the eyepiece (ie the classic 0.32x lenses used on older Leitz and Zeiss systems) so you can calculate the total optical magnification on the sensor from that.

What you're running up against is a big problem in general--you aren't alone in your frustration. Even if you give the correct optical magnification a photo was taken at you'd need to also know the dimensions of the sensor it was taken with, so that isn't universal either. That's why it's important to give the field of view (width of the imaged area) if you want to specify magnification rather than "20x" or whatever. It cuts out all of the potential variations (and in some advertising material outright trickery) and gives all the needed information in one number.

[MichaelG.]

Hello all,

I'm not new to photography and I'd rate my knowledge of microscopy as that of an interested amateur. I'm having great difficulty understanding an apparent discrepancy I've found regarding magnification.

The microscope I use most is an old high school unit from 1982. It's a Swift 960, and it cost me a whopping $80 a couple of years ago. For that price, it's wonderful, and I find its fine focus wheel very easy to use, now that I'm somewhat practiced with it.

I do focus stacking with DSLRs, shooting and focusing manually, and then merging the photos in either Helicon or Zerene. I'm doing eyepiece projection photomicroscopy, which means I have a 50mm prime lens on my camera, and this is mounted onto the eyepiece of the microscope. It works quite well; it eliminates vignetting without wasting too much of the field of view.

.
A couple of points:

First, if I may: The statement that I have emboldened ^^^ is not technically correct.
… ‘eyepiece projection’ is done without a camera lens.
… what you are doing is generally known as ‘afocal imaging’ [because the camera lens should be set to infinity focus, to match the view seen by the relaxed eye]

Second: Although there were some interesting digressions; you may find this recent topic helpful:
viewtopic.php?f=18&t=13110&p=105902&hil ... on#p105902

Enjoy the adventure !
MichaelG.

[Peter]

Hi HiddenWorlds,
You are allowing the camera to confuse the issue. When microscopes were first marked with magnification numbers rather than focal length in fractions of an inch it was decided that the magnification would be the number of times bigger the object appeared when viewed at 10 inches. So if you look at your micrometer scale through the microscope at 100x 0.2 mm should appear to be the same distance as 20 mm on a standard rule heled 10 inches (250 mm) from your eyes.
Hope this helps.
Peter.

[HiddenWorlds]

Thank you for all of the replies. MichaelG, thanks for the correction on my terminology, too. I'm still not sure I grasp why they chose to rate them as they did, but I think this topic helped, overall. The main thing is that I now know what magnification I'm looking at in my own photos. I'll be experimenting with other kinds of setups soon and hopefully eliminating a lot of the glass.