Coin photography--My Dumb Questions and Motivation Issues

[Factor]

For about a year I used 7artisans 60mm macro lens for MFT. https://7artisans.store/products/60mm-f2-8

It was very cheap and produced good quality photos but it has two major problems. One, the lens is heavy and when positioned vertically for coin photography it slowly extends under its own weight. So basically after you adjust the focus you only have a few seconds to take the shot, and if it takes more you will need to adjust the focus again. The second issue is the lack of autofocus, which would be okay if the first problem was not so persistent. So eventually I decided to replace it with Olympus 30mm macro. Works reasonable well and much more convenient. Here is an example: https://rpc.ashmus.ox.ac.uk/coin/475574

[Kaleun96]

On 6/5/2024 at 6:14 AM, NathanB said:

Thank you very much, @Kaleun96! You are a good teacher and explainer!  I think I got most of what you were saying, and I'm going to reread it a number of times.  I feel like this is my best chance to understand this stuff!

About the magnification ratio, you helped me to understand something I hadn't understood before when I saw it popping up in various places online.  

The working distance and minimum focusing distance is a little trickier for me to understand how they operate in tandem.  I think I understood you to say that a macro lens that has a longer focal length can see the object even when it is farther away--as opposed to a regular lens which would have trouble seeing it farther away.  On the other hand, a macro lens also can get closer to the object, making lighting easier.  In terms of getting closer to the object, I think that's called increasing the depth of field (or is it decreasing?).

Glad it helped! I still struggle with various concepts myself so it's useful for me to type this out occasionally 😁 

One thing I didn't mention about working distance (WD) and minimum focusing distance (MFD) is that they're closely related but just measuring from slightly different points. The working distance is the distance from the object to the front of the lens at a given magnification. The minimum focus distance is the distance from the object to the camera's sensor, which is approximately equal to: MFD = Working Distance + Length of the Lens. Both assume that the object is in-focus, so at 1x magnification there is a fixed distance the lens and sensor need to be from the object for the object to be in focus. If you dropped the magnification to 0.5x, you would need to move the camera further back and you'd have a different WD and MFD for 0.5x compared to 1x.

source: https://www.sony.co.uk/electronics/support/articles/00267924

The MFD can refer to both the minimum focus distance at a given magnification but also the absolute minimum focus distance for the lens at any magnification. Usually the absolute minimum focus distance of the lens (let's call it aMFD) is the same as the MFD at the highest magnification setting of the lens. So a lens that can do 2x magnification will have its aMFD = MFD at 2x but if you dropped the lens down to 1x magnification, the MFD will be greater than the aMFD, meaning the camera is further away from the object.

The magnification of macro lenses is changed by twisting the focus ring on the lens body. Most camera lenses have a focus ring but on macro lenses it's more closely tied to magnification so often lens manufacturers put the magnification markings on the focus ring (e.g. 0.25x, 0.5x, 0.75x, 1x) in addition to the normal distance markings. The distance markings typically go from infinity ("focused at infinity") down to the aMFD. For non-macro lenses, the magnification of the lens may not change substantially whether you're focused at infinity or focused at the aMFD. For macro lenses, the magnification changes a lot as you change focus from infinity (lowest magnification) to the aMFD (highest magnification). At you change the focus, and thus the magnification, of a macro lens, then the MFD also changes. The reason Working Distance is also used is because it's a bit more practical - we don't really care how long the lens is, we only care how much distance is between the object and the lens, so the WD removes the length of the lens from the MFD.

The depth of field is not necessarily related to the focal length of the lens, rather the depth of field is primarily influenced by the magnification. The higher the magnification, the smaller the depth of field, meaning the smaller the amount of the subject is in focus. So whether you use a 30mm macro lens or 100mm macro lens, it doesn't really matter in terms of the depth of field, the main difference between those two lenses is that the 30mm lens may have a working distance of only ~60mm at 1x magnification while the 100mm lens may have a working distance of 150mm (example numbers). The larger working distance makes it easier for you to light the object, which is generally why people prefer 90-150mm macro lens for coin photography, though it's not strictly necessary.

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In my own photography with my 14-42 mm lens, I like go get as close to the coin as possible (as long as there is adequate lighting) because the magnification ratio is so very low.  But often I find that I can't get close enough because the camera can't focus on the coin when it's too close.  The extension coins definitely helped that a lot, but I want more.

So now, once I can motivate myself to set everything up again, I will try my "new" cheap used Olympus M. Zuiko 12-50 mm lens that I just got.  At first, I was worried--50 mm is only 5 mm larger than my maximum of 45 mm on my kit lens.  I am hoping that because it can be a macro lens, that I will get more than a 10% larger image.  (Unless perhaps it's not a true macro lens?  I see that there is a button on it for a macro setting, but there are also settings on it for other things.)  

Ah I see perhaps where there is some confusion. It's definitely true that the closer you get to an object, typically the higher the magnification you can achieve. So I can see why you want to get as close to the object as possible. However, the advantage of a macro lens with a longer focal length (e.g. 100mm) is that you can achieve the same level of magnification while being further away from the coin. So let's say your 14-42mm lens with extension tubes allows you to achieve 1x magnification when the lens is 40mm from the coin, the benefit of the 100mm macro lens is that it may let you achieve 1x magnification while keeping the lens 150mm away from the coin. The images from the two cameras will be nearly identical (some differences due to perspective distortion) as the magnification is the same for both (1x), the main benefit is just the greater working distance allowing you to have more space between the camera and the coin for adequate lighting.

As you increase the magnification, the depth of field will decrease, so you will find that a single image isn't able to capture the coin fully in-focus. Focus stacking is a way to fix this. You take multiple photos of the coin, each photo with a slightly different part of the coin in-focus, and then use software to merge the images together so that only the in-focus parts from each image are kept.

If I recall correctly, there is a relationship between focal lengths and extension tubes where the lower the focal length, the less extension you need to achieve a given magnification. Extension tube increase the lens magnification by an amount equal to the extension distance divided by the lens focal length, so if your extension distance is 30mm then for a 50mm lens the added magnification would be 30/50 = 0.6x, while if the focal length was 30mm the added magnification will be 30/30 = 1x magnification. The reason you can't just use shorter and shorter focal length lenses with extension tubes (e.g. the 12mm focal length of your 12-50mm lens) is because the focus plane ends up shifting closer and closer to the lens, reducing the working distance, so at some point the focus plane actually ends up inside the lens, making it impossible to focus on the object.

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One other thing I am still confused about is focal length.  A macro lens at 60 mm and a non-macro lens at 60 mm are so different--like the proverbial apples and oranges.  So how is the focal length of the lens useful to a photographer when it means two such very different things?  

Feel free not to answer--I don't want to abuse your generosity.  But if you are willing, I'm definitely all ears!

 

Definitely happy to help answer! I got all this information from the countless other people who take the time to put it on the internet so the least I can do is repay the favour 🙂

For photographers, the focal length is mainly affecting the field of view and that's what they're thinking about when deciding which focal length to use. A 24mm focal length is great for landscape photography because it has a very wide field of view. You can go ever wider, and a lens at 12mm focal length might give you a "fisheye" perspective where the field of view is incredibly wide. I believe the field of view for the human eye is equivalent to about a 22mm focal length lens.

Wide focal lengths like 12-28mm lenses have quite strong perspective distortion, which makes them less suitable for portrait photography. A great example of that is this GIF:

So a portrait photographer probably wants to stick to a lens with a focal length of around 40-80mm. They could use a much higher focal length lens, say 200mm, but other factors begin to become relevant: long focal length lenses are typically bigger and heavier and let in less light than a smaller focal length lens. There's also some other factors but less relevant to the current topic. Going back to landscape photography, let's say you're standing on a hill overlooking a valley a few hundred meters away and down in the valley is a small farm house. A wide angle lens like a 24mm lens may have a field of view that gives you an image the shows the valley, part of the hill you're standing on, the sky, the horizon in the distance, and the farm house way down there in the valley. If you change to a 400mm lens, the field of view will narrow considerably. We can no longer see the sky or the horizon, or the hill we're standing on, the field of view is now so narrow that we can only see the house in the valley and 20m either side of the house. So the house now takes up most of the image, whereas with the 24mm lens it only took up a small spot of the image. Therefore, we've essentially zoomed in on the house by narrowing the field of view.

So that's a brief explanation of what different focal lengths are used for but field of view isn't something you may need to think about when it comes to macro photography. For macro photography, the main consideration of different focal lengths is the working distance that I mentioned above. Wide focal length = short working distance, long focal length = long working distance. And the difference between a 60mm non-macro lens and a 60mm macro lens is the ability to focus more closely (minimum focusing distance) and the magnification ratio that the lens can achieve.

 

[maridvnvm]

The primary difference between s standard lens and a macro lens is the minimum focusing distance. So for a 60mm lens for example. What this means is how close you can get to an object with the lens whilst still maintaining focus. A Macro lens gives you a 1:1 image magnification at the sensor. A standard lens is not able to focus in as closely and thus the magnification at the sensor is less (depending on the minimum focus distance). The reason for the increased cost of the macro lens is the increased complexity in the optics to allow this close focus distance.

[Ten-Speed]

Since I don't have a camera, with a lot of trying different back lights and moving the coin around, I can sometimes get a fair picture from my camera in my laptop (Mac/Photo Booth). However, one has to then transfer this photo and make another file so that the image can be inserted Into the posting.